PAINTING MATERIALS

oT Short Encyclopaedia

BY

RUTHERFORD J. GETTENS

Chemist, Department of Conservation, ana
Fellow for Technical Research, Fogg Museum of Art

AND

GEORGE L. STOUT

Lecturer on Fine Arts and Head of the
Department of Conservation, Fogg Museum of Art

fc ~

WITH AN INTRODUCTION

^ EDWARD W. FORBES

Director, Fogg Museum of Arf ^ •

It

NEW YORK

D. VAN NOSTRAND COMPANY, Inc.

250 Fourth Avenue

Copyright, ig 42 , hv

2 ). %Jan V'^^trand Qompany^ Inc,

All Rights Reserved

This book^ or any part thereof^ may not be reproduced
in any form without permission in writing from the
authors and the publisher.

First Published April 1942

Reprinted February 1943^ December 1946^
November 1947

PRINTED IN THE TJ N I T E D S T A T E S OF AMERICA

I«ANCASTER press, INC,, LANCASTER, PA.

INTRODUCTION

The Department of Conservation of the Fogg Museum of Art has
for many years made a study of the materials and processes of paint¬
ing. These studies have included the fields of chemistry, microscopy,
physics, and the use of infra-red and ultra-violet rays. Also special
investigation has been made of the use of the x-ray in the examina¬
tion of paintings and to a lesser extent of sculpture and bronze. All
these methods of research have been useful in dealing with problems
of restoration and of conservation and in the detection of forgeries.

We feel that such research is valuable in many ways: in the his¬
torical examination of the processes and materials of the past; in
the study and detection of forgeries in the present; and in the inquiry
into the scientific care and restoration of works of art.

Finally it is important for the creative artists of today, who
must understand sound processes and know how to choose permanent
materials if their work is to endure. The various scientific approaches
supply information and data bearing on all of these fields.

Mr George L. Stout has for many years been the head of the
Department of Conservation, and associated with him has been Mr
Rutherford J. Gettens, chemist and Fellow for Technical Research in
the Fogg Art Museum. Mr Stout has been the editor of Technical
Studies in the Field of the Fine Arts. He and Mr Gettens have both
written many articles in this magazine embodying the results of
their work.

It is encouraging to see that so many artists are beginning to take
a real interest in technical problems. We feel that there is a need for
a book which will co-ordinate in easily available form a large amount
of knowledge and research in methods of painting. This field is at¬
tracting increasing attention among the art lovers of the world, and
it is hoped that the growing number of inquiring minds which are
eager for information will find this encyclopaedia valuable.

Edward W. Forbes

PREFACE

This was not started as a book. It was begun as a series of notes
and was published as separate sections in Technical Studies in the
Field of the Fine Arts from 193^ until 1941- At the start those data
were assembled about which little information was available, par¬
ticularly those on supports and mediums. As more were put together
and a book was suggested, a question came up about discarding the
sectional arrangement and-putting all of the entries in a single alpha¬
betical sequence. In the end it was decided to keep the five sections
intact and to print them as they are. The grouping is, perhaps,
slightly awkward and is certainly unusual in any volume that calls
itself an encyclopaedia,’ but that word seems open to some variety
of definition, and practically there appeared to be good reasons for
leaving the data arranged as they were.

Chief among those reasons is that custom has made such an
arrangement habitual. Painters and all workers in the materials of
paint have grown familiar with handbooks and texts in which pig¬
ments, mediums, and the others are treated separately. Individual
names are apt to be unknown and information about a general kind
of material can probably be got more handily when that kind is
segregated. Time and trial will show whether or not a change might
have been better and whether or not it should be considered at some
later date.

Those who have occasion to use this book will find it uneven as
to quantities of information set down. That is because so much study
has been made of certain kinds, and so little of others. Pigments, for
example, have been explored by painters since the beginning of the
art and by scientists for many generations. Solvents, on the contrary,
are most of them new things, recent developments in industry and in
the painting trade. Their utility is limited and knowledge about them
is only beginning to work its way into the arts. The section on tools
and equipment has only a small amount of previously published
reference data. Much of it, in contrast to other sections, is assembled
directly from sources. Headings or titles of the sections may need
some explanation. The word, support, as defined in a publication on
museum records by a committee of the American Association of
Museums {Technical Studies, III [1935], p. 204) means ‘the physical
structure which holds or carries the ground or paint film,’ This would

VI

Preface

include panels, canvas, paper, and even the masonry of walls. The
word, inert, is still strange to the artist-painter but has a common
application in industrial painting to materials mixed with a medium,
as is a pigment, but which, unlike pigments, have little or no tinting
or hiding power.

In a broad sense, these data were put together for workers in the
art of painting, for all who do work in the art—painters, teachers of
painting, students, museum curators and conservators, paint chemists,
and analysts. There is much that will concern the museum worker
and the paint analyst more than others—distinctions among chemical
and physical properties, problems of conservation, and history of
materials. These details, however, may be of some interest to painters,
and surely they will have a value for students and teachers. Because
this encyclopaedia is for those who work in the arts, the information
has been made selective rather than exhaustive. Many more materials
could have been listed if the aim had been to produce a thorough,
scientific compilation. As it is, most artists will find here facts about
materials that are not familiar to them and that they may never use.
Yet each entry may have its practical worth in the problem of some
painter or worker with paint at some time in his professional life. It
is only hoped that omissions are not too many. Facts about materials
have to be put in the terms in which such facts have their most exact
meaning. Often that requires using the terms of chemistry and physics,
and for the artist who finds these baffling a short glossary has been
added for the purpose of defining some of them.

Recent years have seen an increase in demand on the part of
painters for more information about materials they use, and as this
demand can be satisfied the art will be'enriched. With a wider range
of technical means, a wider scope of expression will become possible.
Many excellent publications have led in that direction. This one is
added not to take the place of any others but to take a somewhat

different place and one that has not been filled:

Much of the work of collecting this information was made possible
by grants for research by the Carnegie Corporation. Revision of the
periodical publication _ has been done through a gift from Robert
Treat Paine 11. This aid is gratefully acknowledged.

Cambridge ^Massachusetts
November 4^

TABLE OF CONTENTS

Introduction.. •

Preface. ^

Mediums, Adhesives, and Film Substances. 3

Pigments and Inert Materials. 9^

Solvents, Diluents, and Detergents.. • ■ • 185

Supports...

Tools and Equipment. ^77

Glossary. 3^5

MEDIUMS, ADHESIVES, AND FILM SUBSTANCES

Acrylic Resins (see also Synthetic Resins). The polyacrylic resins have been
recently developed. Neher has outlined the history of the work on this class
of compounds and he credits their industrial development to Otto Rohm of
Darmstadt. Chemically, they are closely related to the vinyl resins (see Vinyl
Resins), for they have a CH2 = CH—group in common. Although solid polymers
can be made from acrylic acid, CH2 = CH-COOH, and from methacrylic acid,
CH2:C(CH3)C00H, it has been found that the esters of these acids lend them¬
selves better to the formation of useful resins. Most useful is that made by the
polymerization of methyl methacrylate, CH2:C(CH3)C00CH35 often referred to
as methacrylate resin.

Methyl methacrylate monomer is a volatile liquid of low viscosity which
boils at 100.3° C. Polymerization is autocatalytic and is easily effected by
light, heat, and oxygen. The polymer is a hard, strong resin which has the
clarity of glass. It is a linear polymer and is thermoplastic, although its softening
temperature is high (12,5° C.), Now it is used chiefly as a plastic for clear or light-
colored, molded articles. For these it is more suitable than polyvinyl acetate,
because it is harder, is less rubbery, and has little cold flow. It can be worked
well mechanically. The solid resin is so clear that printed matter can be read
through masses of it several inches thick with perfect visibility. It is insoluble
in water, alcohols, and petroleum hydrocarbons (Anonymous, ‘Methacrylate
Resins,’ P« ^ 1 63), and is soluble in esters, in ketones, in aromatic and in chlorinated
hydrocarbons. Lacquers and protective coatings may be made by dissolving the
clear resin in these solvents singly or in combination. In general, the solubility
is lower than that of polyvinyl acetate. The acrylic resins are characterized by
their strong adhesion to most surfaces, and advantage may be taken of their
thermoplastic properties to effect good adhesion. Ultra-violet transmissibility and
stability to light are high. The refractive index is 1.48a to 1.521. Polymerized
methyl methacrylate is supplied as a molding powder and in made-up forms
under the trade name, ‘Lucite.’

In addition to methyl methacrylate, other methacrylic ester polymers are
available, including ethyl, 72-propyl, isobutyl, and fi-butyl. These have become
commercially important as materials for protective coatings and lacquers. Strain,
Kennelly and Dittmar supply data on their physical properties, solubilities, and
compatibilities with resins and plastics. As the molecular weight of the esterified
alcohol radical increases, the polymers become softer and more plastic. Film¬
forming and adhesive properties, as well as solubility and compatibility, also
change markedly along the series from methyl esters to the higher esters. The
higher esters become increasingly more miscible with aliphatic type solvents, the
butyl.and isobutyl esters being soluble in petroleum solvents. Strain presents
data which show wide variations in viscosities of methyl methacrylate polymers
made from different solvents in the same concentrations. Toluene gives io.wer
viscosity for the polymer than any other single solvent'tested.

3

4

Painting Materials

It has been suggested (‘ Methacrylate Resins,’ p. 1163) that the monomeric
ester, since it has such low viscosity and can be polymerized so easily, may be
used as an impregnating agent which can be polymerized in situ. Porous, fibrous,
and cellular materials, which are ordinarily difficult to impregnate because of
the viscosity of the organic solutions of the polymers, may be treated for pro¬
tection and stiffening in this way. It is also reported {ibid^ that ‘ monomeric
methyl methacrylate has been used to protect wood to give a final product con¬
taining as much as 60 per cent by weight of resin.’

Albumen (see Egg White).

Alkyd Resins (see also Synthetic Resins). The alkyd resins are obtained by
the elimination of water from polyhydric alcohols (glycol and glycerol) with
dibasic acids (phthalic, etc.). These resins have been prepared from a number of
different ingredients leading to widely differing properties. There are many so-
called ‘ alkyd resins.’ Combined with drying oils, they are now much used in the
industrial preparation of paints, lacquers, and enamels which are durable and
flexible and do not yellow. Some of the resins are thermosetting and are used for
making molded articles. The alkyd resins are the most important of the synthetic
resins in the industrial paint and lacquer field today. Incorporation of alkyd resins
in cellulose nitrate and cellulose ester coatings has helped to overcome some of
the disadvantages of the latter.

Amber (see also Resins). The name ‘ amber ’ in early times was given to many
hard resins. It is, properly, a fossil resin found chiefly on the shores of the Baltic
Sea but also in Denmark, Sweden, Norway, France, and along the coast of
England. A dark variety has been found near Catania, Sicily. Aristotle was the
first to record that amber was not a mineral but a fossil tree resin. It is mostly
known in its natural state as jewelry. Beads of it have been found in early English
graves and good specimens are still highly valued for ornamental purposes. It
has been used, also, as a varnish ingredient, undoubtedly when adulterated with
other hard resins.

The chief distinguishing feature of true amber is its yield of succinic acid
when heated, and the name, ‘ succinite,’ is now commonly used in scientific
writings to denote the real Prussian amber. There are several ways to distinguish
between amber and copal with which it is often confused or adulterated. One is
the presence of succinic acid in the distillate of amber; another is the insolubility
of amber iri cajuput oil which completely dissolves copal; amber, when heated
quickly, splits up and then fuses into a viscous liquid, the drops of which rebound
when falling on a cold surface ; copal resin does not have this characteristic.

Amber is practically insoluble in ordinary resin solvents. When made into a
varnish, it is melted or distilled and the residue is dissolved in amber oil, oil of
turpentine, or a fatty oil. It makes a very dark, slow-drying varnish, unsuitable
for paintings, and there is doubt that it was ever employed alone for this purpose.

Animal Waxes (see also Waxes and Vegetable Waxes). These are obtained
frorn a great variety of sources and have little in common, except their absence
of glycerides. Small deposits may be found in many parts of animals and are

Mediums and Adhesives

5

also present in the cell contents of their tissues. Hydrocarbons do not seem to be
of so frequent occurrence as in the vegetable kingdom; among the alcohols there
are cholesterol and allied substances, which replace the phytosterols of the plants,
and higher aliphatic alcohols containing, as a rule, fewer carbon atoms than the
aliphatic plant alcohols. They have, in fact, the same carbon content (i6, i8, ao)
as the most common fatty acids (Hilditch, p. 12.7).

Balsam (see also Resins). This general term has been used to designate the
resinous exudate from trees of the order Coniferae. It is also spoken of as oleo-
resln, turpentine, or gemme. The flow of balsam is quite profuse from shallow
incisions, except for larch balsam, and for that the heart of the tree is pierced.
The composition of balsams varies with the habitat of the tree. Those containing
the largest amount of essential oil come from trees growing in sandy soil near the
sea. Balsam is a soft, semi-liquid consisting of terpenes associated with bodies
of resinous character. By distillation, turpentine and the residue, colophony, are
obtained. The balsams most used in varnishes or as paint mediums are Venice
turpentine, Strasbourg turpentine, Canada balsam, and copaiba balsam. Bal¬
sams flow easily on a surface and give a lustrous, pleasing quality when first
applied. Unless a harder resin Is mixed with them, however, they deteriorate
easily.

Beeswax (see also Waxes) is produced by the common bee. Apis mellifica,
and also by some allied species. It is not collected by the bee, but is the
secretion of organs situated on the underside of the abdomen of the neuter or
working bees, and is used by them in forming the cells of the honeycomb. They
are said to consume about ten pounds of honey in order to secrete one pound of
wax. The wax may be obtained by melting the combs in hot water and by strain¬
ing to free it from impurities, or by pressure extraction. A further yield may be
obtained by the use of volatile solvents. The industry is carried on in many parts
of the world and, naturally, the waxes from widely different localities vary con¬
siderably in texture, color, and, to some extent, in chemical composition. The
color ranges from light yellow to dark, greenish brown. Those of light color are
used directly in many cases but the darker colored varieties are more frequently
bleached. This may be done by treatment with bleaching earths or charcoal, or
by chemical means such as simple exposure to light and air, or by treatment with
ozonized air or hydrogen peroxide; the use of oxidizing acids such as chromic
acid tends to cause deterioration. Beeswax is fairly brittle, but is plastic when
warm; bleached beeswax, white wax,’ is heavier, more brittle, and has a smoother
fracture. Like other waxes, beeswax Is somewhat complex in composition and
contains about 10 per cent of hydrocarbons in addition to alcohols, acids, and
esters. It consists principally of melissyl (myricyl) palmitate (CisHsiCOOCsoHm)
and there are also present small proportions of a number of other alcohols and
acids, including ceryl and melissyl alcohols, palmitic, cerotic, melissic, and prob¬
ably other higher fatty acids. Beeswax is very likely to be adulterated. In some
districts it is the custom to place artificial combs in the hives. These are fre¬
quently composed of paraffin wax or stearic acid, or a mixture of the two, and

6

Pahstting Materials

the resulting wax will thus be largely adulterated. Besides its use in the arts (see
Waxes, history in painting), and it has doubtless been the principal wax used^by
painters, beeswax is mainly used in candle manufacture and in the preparation

of wax polishes. ^ , • r

Benzoin (see also Resins) is a dark, resinous substance obtained from trees

(Styrax Beitzoin and other species) growing in Siam and in Sumatra. Siamese
benzoin has a characteristic odor which results partly from the presence of i per
cent vanillin. It has frequently been used as a plasticizer for varnishes and lac¬
quers. It was imported into Europe at an early period, but Merrifield (I, cclx)
says that it does not appear to have been used as an ingredient in varnish until
the middle of the XVI century when it became a spirit varnish, but did not figure
in the preparation of oil varnishes. It is mentioned in various mediaeval MSS,

Binding Medium (see Medium).

Bitumen Waxes form a link between the vegetable waxes and the mineral
waxes. In this respect they resemble lignite and peat, the parent substances which
are bodies intermediate between vegetable and mineral in character (see also
Waxes and Montan Wax).

Blown Oil. The usual procedure for preparing blown oil is to pass an air
current through the oil (see Oils and Fats), at about iao° C., in the presence of
traces of cobalt driers. Blown linseed oil is used somewhat instead of stand or
poiymerized oils which are more expensive to manufacture. By prolonged blow¬
ing, drying oils yield jelly-like or even solid, elastic masses. Fatty oils belonging
to the class of semi-drying oils lend themselves especially to the manufacture
of blown oils. Rape oil and cotton-seed oifare blown in order that the products
may be mixed with mineral oils to produce specific lubricants, while other blown
oils find various technical applications.

Boiled Oil is oil which has been heated with the addition of lead, manganese,
or cobalt oxides, or other suitable compounds of these elements, such as the
linoleates or resinates. Formerly it was usual to heat the oil at a6o° to 290° C.,
to add a metallic oxide, and to continue heating for a few hours until a homo¬
geneous solution was obtained. The modern practice is to operate at lower tem¬
peratures (130® to 150° C.) and to employ ' soluble driers ’ such as the metallic
resinates or linoleates. If the oil is blown with air, the driers may be incorporated
at temperatures as low as 100° C., for slight oxidation of the oil facilitates dis¬
persion of the driers. These are probably colloidally dispersed, not truly dissolved.
Boiled oils have the property of absorbing oxygen from the air at a much more
rapid rate than does raw linseed oil, and the time required for the formation of
a skin is thereby much shortened (see Oils, drying process). They are used largely
for industrial paints, varnishes, and enamels, and for waterproofing, for electrical
insulation, and for patent leather. Doerner (pp. 105-106) says that commercial
boiled oil is not of much use for artistic purposes because it dries with a sleek,
greasy sheen and easily forms a skin.

Mediums and Adhesives

7

Bone Glue is impure gelatin prepared from bones (see also Gelatin and Glue).

Canada Balsam (see also Balsam) is derived from a fir {Abies balsamea Mill.)
which grows widely in the eastern United States and Canada. It is obtained from
small blisters in the bark and only a small amount can be collected at a time.
The balsam is relatively pure and is valuable for its transparency and its high
refractive index (1.5194 to 1.5213 at ao° C.). It was introduced into Europe in
the XVIII century.

Candelilla Wax (see also Waxes) is obtained from the stem of the leafless
Mexican plant, Pedilanthus pavinia^ and from other Mexican genera of the Eu-‘
phorbiaceae. It is a brownish, brittle mass which may be bleached. Although of a
lower melting point than carnauba wax, It finds application in similar industries.

Candlenut Oil is obtained from the seeds of Aleurites moluccana^ a tree cover¬
ing large areas in the western tropics. For use in paints and varnishes, it is rec¬
ommended by some and condemned by others. It is closely related to tung oil.

Carnauba Wax (see also Waxes) is obtained from the Brazilian palm, Corypha
cerifera (the carnauba tree), on the leaves of which it forms a deposit. The young
leaves are cut and dried and the wax powder is scraped off and melted in boiling
water. It is bleached with fuller’s earth or charcoal or by a chemical oxidant
such as chromic acid. It is a yellowish, hard, brittle material of exceptionally
high melting point (83® to 86° C.) which increases somewhat with age. The major
component of the wax is mellssyl (myricyl) cerotate (C25H51COOCS0H61) with
minor amounts of hydrocarbons, wax alcohols, and higher fatty acids. Owing to
its hardness and high melting point, it takes a fine, hard gloss when rubbed.
It has been recommended (Rosen, p. 115) as a coating material for paintings,
when mixed with other waxes.

Casein (see also Casein Tempera), usually referred to as a glue, is an organic
compound belonging to the class known as proteins, the most complex compounds
with which chemists have to deal. Furthermore, it belongs to one of the more
complex subdivisions, the phosphoproteins. It consists of carbon, hydrogen,
oxygen, nitrogen, sulphur, and phosphorus, and, although it has been the subject
of many investigations, a great deal of information is still lacking with regard
to the amino-acids of which it is composed. Like all proteins, it is amphoteric,
it functions both as an acid and as a base. It has, however, decided acid
properties and exists in milk as calcium caseinate. Casein is prepared from
skimmed milk by heating it at 34.5° to 35° C. and adding hyi'ochloric acid
till the mixture reaches a pH of 4.8. It is then allowed to settle and, after sepa¬
ration from the supernatant liquid, is washed with hydrochloric acid, also with
a pH of 4.8. Casein so prepared is technically pure, and is a snow-white, slightly
hygroscopic powder with a specific gravity of 1.259. It reacts as a weak acid, is
insoluble in water, alcohol, and other neutral organic solvents, and is soluble in
the carbonates and hydroxides of the alkali and alkaline earth metals and in
ammonia.

8

Painting Materials

The curd of milk with nearly any alkali like borax, trisodium phosphate, or
sodium carbonate, will yield an adhesive. If the alkali^ is lime, the adhesive is
hio-hly water-resistant. Nowadays hydrated lime (calcium hydroxide) may be

more convenient to use than quicklime. Sutermeister (c. VII) discusses the
theory and practice of casein glue formulation and says (p. 190) that a casein
alue capable of giving excellent dry strength and water-resistance may be pre¬
pared from 100 grams of casein, 300 grams of water, and 16 grams of calcium
hydroxide. The casein must be finely ground and must be allowed to soak thor¬
oughly before the lime is added in order that solution may take place as readily
as possible. Since the working life of such a glue is limited to 10 to 45 minutes,
it must be used immediately. Prepared casein glues are now on the market, which
have only to be mixed with water.

Casein yields one of the strongest glues known and has been used for centuries
by joiners and cabinet makers. It has served extensively as a binding medium
for cold-water house paints, and, to a limited extent, for pictorial painting, both
as a binding medium and in the preparation of grounds. Craftsmen of ancient
Egypt, Greece, Rome, and China are considered to have used it. Without doubt
it was a joining adhesive in the cabinet work of the Middle Ages. MSS of the
time give directions for preparing an adhesive out of lime and cheese, very similar
to an adhesive that Is now used for putting together the wooden parts of an aero¬
plane. (A large part of the casein used as a glue today is consumed by the wood¬
working industries.) Ancient Hebrew texts mention the use of curd (casein) in
house painting and decoration. Michelangelo is said to have used a combination
of sour milk, oil, and pigments to produce highlight effects on walls (Sutermeister,
p. 105). The material used in the many well preserved XVIII century ceiling
paintings in upper Bavarian and Tyrolean peasant houses is lime-casein. It is
little used as a painting medium by modern artists, except, possibly, for mural
decorating. The casein film is hard, brittle, and insoluble, and lends itself poorly
to handling and to correction.

Casein Tempera (see also Casein). This medium, made from skim milk and
lime, has been used since very early times. It has great adhesive power and has
long served as a joining glue, as well as for painting on walls. Unless properly
thinned, lime casein is not considered to be suitable for easel painting. It is occa¬
sionally used, at present, to make oil color short, and has even been added as a
medium with oil colors. With the addition of one fifth of its volume of slaked
lime, casein becomes liquid, is easily emulsified, and can be thinned with water.
Three to five parts of water, or more, can be added, and the emulsion should be
freshly made before it is used. Lime-casein sets quickly and becomes very hard.

For easel painting, Doerner (p. ai8) recommends powdered casein which is
insoluble in water but is soluble in ammonia. Forty grams of casein are mixed
with a small amount of water, and then 250 cc. of warm water are added. After
the lumps have been pressed out, 10 grams of ammonium carbonate, dissolved
in a few drops of water, are added. The solution is ready for use after the carbonic

Mediums and Adhesives

9

acid has escaped through effervescence. Ammonia casein may be kept in a corked
bottle and diluted in water before it is used. The adhesive power, though not so
great as that of lime-casein, is good. It has the additional advantage that its solvent
is harmless. Commercial caseins are often prepared with potash or soda, and, as
these lyes destroy certain colors, the litmus test (red litmus should not turn blue)
can be applied. It is difficult to keep casein colors in tubes without their harden¬
ing and crumbling. Glycerine may be added, but, although this keeps the paint
moist, it destroys the insolubility of casein in water. A great difficulty with this
medium, besides its brittleness in the film, is its tendency to encourage mold
growth.

Castor Oil (see also Non-Drying Oils) is an oil from the seeds of Ricinus
communis which is grown in India and in most hot countries. It is the heaviest
of all the fatty oils, is almost colorless, and is very viscous. Chemically, it is
quite different from the other fatty oils (see Oils and Fats), consisting largely of
the glyceride of ricinoleic acid (C18HS4O3); a small quantity of hydroxystearic
acid and stearic acid also occurs. It is largely used as a plasticizer and in practice
is distinguished from many oils by its ready solubility in alcohol.

Celluloid (see also Cellulose Nitrate) is a pyroxylin plastic that is plasticized
with camphor. In time the camphor disappears and the film becomes brittle.
Celluloid lacquers have been extensively used during the past fifty years in re¬
storing and repairing objects of art. Celluloid clippings could easily be dissolved
in acetone or in some solvent mixture.

Cellulose Acetate (see also Cellulose Coatings and Cellulose Nitrate). Cellu¬
lose acetate is a white, bulky solid that now finds extensive use as a coating and
lacquer material and as a molding compound. Compared with cellulose nitrate,
it has some advantages and some disadvantages. Although acetylated carbo¬
hydrates were known as early as 1865, it was not until 1910—1911 that mention
of products which were like those now called ' lacquers ’ and which contained
cellulose acetate, began to appear in the patent literature. Cellulose acetate is
prepared from some form of cellulose, like cotton linters or paper, with a mixture
of glacial acetic acid, acetic anhydride, and concentrated sulphuric acid. The
product is an ester of cellulose (which may be considered to be a polyhydric
alcohol) and acetic acid. Cellulose acetates of widely different properties may be
made. The low-viscosity acetates are best for lacquers. The chief difficulty in the
way of the commercial development of cellulose acetate has been its limited
solubility. It is dissolved by fewer organic solvents than cellulose nitrate, and
these few are strong—acetone, diacetone alcohol, ethylene dichloride, and the
glycol ether acetates. Even with such solvents, the dilute solutions of cellulose
acetate are viscous. In general, the lacquers have too low solids content for wide
commercial application. Hofmann and Reid have made an exhaustive study of
the solubility of cellulose acetate in single solvents and in solvent mixtures, and,
on the basis of their experimental data, have been able to work out some very
satisfactory lacquer formulas. There has been difficulty from the tendency of cellu-

lO

Painting Materials

lose acetate lacquers to blush in humid weather because of the rapid evaporation
of such solvents as acetone and methyl acetate, but it is now possible, by proper
choice of high-boiling solvents, to prepare lacquers with a high degree of blush

resistance.

Most cellulose nitrate plasticizers are incompatible with cellulose acetate and
it is hard to prepare a satisfactory plasticizer. Moreover, few natural resins are
compatible with it and that has prevented the development of a cellulose acetate
lacquer wnth good adhesion. Recently, however, it has been found that some of
the alkyd synthetic resins (glycol phthalate) may be used with cellulose acetate
in the combined role of resin and plasticizer. Cellulose acetate is superior to
cellulose nitrate in that it does not yellow or become so much degraded in sun¬
light. It is chemically more stable and, also, the solid cellulose acetate is nearly
non-inflammable, in contrast with cellulose nitrate. Hill and Weber have recently
made a study of the comparative stability of cellulose nitrate and cellulose acetate
motion picture films. From their oven-aging tests they found that cellulose ace¬
tate retains its flexibility and weight much better than cellulose nitrate. On
artificial aging, cellulose acetate remains neutral but cellulose nitrate increases
greatly in acidity. They conclude that a cellulose acetate film appears to be a
stable substance.

As an impregnating material, cellulose acetate has little value because solu¬
tions are too viscous and have too low solids content. Advantage may be taken,
however, of this high viscosity in impregnating and stiffening old fabrics, because
the cellulose acetate does not appreciably penetrate and darken. This is important
in the conservation of old textiles. Plenderleith (p. 12) suggests that a i per cent
solution of cellulose acetate in acetone, applied in several coats, be used for
strengthening brittle fabrics. He also recommends it (p. 19) as a cement for re¬
pairing old ivories. A film of cellulose acetate may be used in place of glue for the
sizing of artists' canvas. It has long been used as a 'dope' for aeroplane wing
fabrics. Care must be taken in the application of cellulose acetate—and, for that
matter, of almost any cellulose ester coating—not to have solutions that are too
thick or viscous, especially on smooth surfaces. Such coatings have poor adher¬
ence and are liable to peel. It has been observed that films of cellulose acetate
applied as a thick lacquer to a smooth paper base can be stripped as intact films
from the paper without any difiiculty. The incorporation of synthetic resins with
the film helps to alleviate this shortcoming.

CeUttlose Coatings (see also Cellulose Acetate and CeEulose Nitrate). Several
plastic and coating materials are derived from cellulose which is the principal
carbohydrate constituent of many woody plants and vegetable fibres. Cotton
fibre and delignified wood are the most important raw materials for the produc¬
tion of these derivatives, many of which are esters. The cellulose coating materials
are colloidal in nature. They may be dispersed in organic solvents and in this way
used as lacquers. Wilson says (p. ii): ‘ For practical purposes cellulose may be

Mediums and Adhesives

-4

considered as a complex alcoholj with three hydroxyl groups for each unit
cule. These alcoholic radicals can be esterified by acids and the acetic and nitric
acid esters have tremendous importance in industry/ These cellulosic coating
materials can not properly be called ‘ synthetic-resins/ although they may be
used for lacquers and molding compounds in a similar way. The rawj modified
cellulose materials are^ for the most part^ light-colored or white, powdery or flaky
materials that do not have a resinous lustre or fracture. Moreover, they are
natural products prepared by dissimilar chemical processes. Cellulose acetate and
cellulose nitrate are frequently classified as " plastics."

In recent years there have been developed some new cellulose materials which
are similar to cellulose acetate and which are said to be superior in many respects.
Among these is cellulose acetobutyrate, which is more highly miscible with resins
and plasticizers than is cellulose acetate. Lacquers can be made from it which
are tough, flexible, and resistant even to out-of-door weathering. Cellulose aceto-
butyrate is a white, flaky material; it gives a colorless film which transmits all
visible and ultra-violet light in the solar spectrum and does not yellow or discolor.
The refractive index of the pure film is 1.47 at 25° C. Another derivative is ethyl
cellulose, a cellulose ether, which is softer and more extensible than the cellulose
esters and, hence, requires little or no plasticizer. Benzyl cellulose is another
cellulose ester, suitable for lacquer formulation.

Cellulose Nitrate (see also Cellulose Coatings and. Cellulose Acetate). Cellu¬
lose nitrate, also known as gun cotton or pyroxylin, has been known for nearly
one hundred years. (Wilson [p. ii] says that the cellulose nitrates are broadly
and incorrectly termed ' nitrocelluloses.") It was not until after 1920, however,
that its manufacture became important through the demands of the wood- and
metal-finishing industries. It is made by treating cotton linters or high-grade
tissue paper with a mixture of concentrated sulphuric and nitric acids, which is
partially diluted with water. Dry cellulose nitrate is a voluminous, white or
faintly yellow solid which is readily flammable and deflagrates if brought near
a naked flame. For shipping purposes it is usually moistened with alcohol or
some other organic liquid. It is sold on the basis of its viscosity in standard
solution; for example, a specification that the cellulose nitrate is ‘ R. S. one half
second cotton " indicates that when it is made up in a standard solution (regular
solvents), one half second is the time required for a standard steel ball to fall
through ten inches of the solution contained in a one-inch-diameter, vertical
column at 25® C. (A. S, T. M. method). One half second cotton is used extensively
for preparing lacquers, but cellulose nitrate is prepared commercially with a vis¬
cosity as high as 200 seconds.

The best solvents for cellulose nitrate are the organic esters, ethyl acetate,
butyl acetate, amyl acetate, and ketones, like acetone and diacetone alcohol.
Paraffin hydrocarbons, coal-tar hydrocarbons, and even the lower alcohols have
little or no solvent effect, although these solvents may be used as diluents along

12

Paintin-g Materials

with the esters and ketones. In recent years the glycol ethers have become impor¬
tant celiiilose nitrate solvents. Solutions of pyroxylin in simple solvent mixtures
do not make very good surface coatings. Well compounded cellulose nitrate
lacquers are complex in composition. Pure cellulose nitrate solution^ like pharma¬
ceutical collodion, dries out to a brittle film which shrinks as it hardens. For this
reason it is necessary to incorporate with the solution liquid or plastic materials
which are retained in the film and keep it flexible. Camphor and castor oil have
long been used with cellulose nitrate. The former, 'however, is readily lost from
the film since its vapor pressure (for this purpose) is high. Castor oil has a ten¬
dency to develop rancidity and an unpleasant odor on standing, and it makes the
film too soft if used in slight excess. In recent years synthetic plasticizers, like the
triphenyl or tricresyl phosphates or dibutyl phthalate, have come into favor.

In addition to plasticizers, nearly all pyroxylin lacquers contain certain
amounts of resin, either natural or synthetic. Resins increase the body of the
film, enhance the gloss (where this is desirable), and improve the adhesion, par¬
ticularly to metal and to glass. Dewaxed dammar is used where a pale lacquer
is required. Shellac, copal, elemi, mastic, sandarac, the phenolic and the vinyl
resins, and others are compatible with cellulose nitrate. Besides the solvents used
for taking the cellulose nitrate into solution, it is usually necessary to add small
quantities of solvents which have a higher boiling point. Such solvents are known
as blush resistants.’ If the main solvent or solvents evaporate too rapidly, they
may chill the surface to which a lacquer is applied and cause water to condense
in the film; this, in turn, causes the film to turn white (blush or bloom). Small
amounts of such solvents as diacetone alcohol, the glycol ethers, and the lactates
are commonly used for this purpose. These high-boiling solvents also improve the
brushing and spraying qualities.

Cellulose nitrate has two main shortcomings. In the first place, it is not stable
to light, particularly strong sunlight. Devore, Pfund, and Cofman say (p. 1836):

e action of sunlight or ultra-violet light on an unpigmented nitrocellulose film is
accompanied by a variety of phenomena in addition to the gaseous decomposition.
The film becomes acid, its^ brittleness increases, its tensile strength decreases, and
after prolonged exposure the film becomes yellow. The viscosity of a solution pre-

pared by redissolving an irradiated film is lower than that of the solution from which
tne mm was cast.

In their experimental work they found that there is a sharp peak in the curve
indicating a strong maximum^ of decomposition per unit energy in the region
represented by lines near 3130 A. Gloor found that sunlight not only subjects a film

Pfriitrocellulose lacquer to stresses incidental to normal temperatLe change but

^at It also proinotes photochemical changes in the film itself. His data indicate

“ ' pronounced local denitration and
degradation, wide the effect of heat I, the same but more general. The second

Mediums and Adhesives

13

shortcoming of cellulose nitrate^ the inadequacy of plasticizing materials now
available for itj has already been touched upon. Loss of plasticizers^ however,
may not be secondary to the effects of light and heat. Camphor and such plasti¬
cizers escape eventually because of their inherent vapor pressures. The incorpora¬
tion of natural and synthetic resins tends to lessen some of these shortcomings.
The high flammability of cellulose nitrate compositions is well known. There is
much greater danger attendant upon application of the lacquer than there is
from any possibility that the dried film will ignite.

Cellulose nitrate, particularly in the form of a celluloid lacquer, has played
some part in the restoration of museum objects in the last quarter century,
principally as an adhesive and as an impregnating agent (see Lucas, Antiques^ etc.,
index). In the Third Report of the British Museum on the Cleaning and Restoration
of Museum Exhibits (p. ao) is a record of the employment of a celluloid varnish
for coating baked clay tablets prior to washing them in distilled water. Plender-
leith (p. 15) says that a celluloid lacquer is useful for coating the powdery surface
of decayed wood; advantage is taken of the great contraction of the celluloid to
re-enforce the surface.

Cement, Frequently adhesives, and film materials generally, are referred to
by this name if they are used for the purpose of joining objects or parts of objects.
For such a purpose, a number of types of film material may be used (see Glnej
Resins, and Synthetic Resins)*.

Ceresin (see also Waxes and Ozokerite) is obtained from Galician earth
wax,’ ozokerite. It is harder than paraffin, is dazzling white in appearance,
inodorous, and transparent at the edges. It consists of a mixture of hydrocarbons
and differs from paraffin wax in being plastic and non-crystalline in character
(Fryer and Weston, p. 208), The melting point varies between 65° and 80° C.
It is not attacked by acids, either cold or hot, or by alkalis, which do not saponify
a trace of it. It is entirely volatilized at a high temperature without alteration.
It is employed as a substitute for beeswax which it resembles in plasticity. It is
often adulterated with paraffin wax, many so-called ‘ ceresins ’ being, in fact,
entirely paraffin.

Cherry Gum (see also Gums) is from the cherry, mahaleb-cherry, apricot, and
plum trees. It swells in water, and about 10 per cent is enough to form a thick
substance. The solution is pressed through a cloth. It may be emulsified with
fatty oils and balsams. It gives great transparency to color, but is inclined to
chip easily if used alone or if applied in thin glazes. When added to an egg or
casein emulsion, it is said (Doerner, pp. 223-224) to give a brilliant, enamel-like
effect. It is mentioned as a painting medium in some treatises, particularly of
northern origin, and probably had occasional use as late as the XIX century.

Chinese Insect Wax (see also Waxes) is the deposit of an insect, Coccus
ceriferusy which is a parasite on certain Asiatic trees. The wax is obtained by
placing: the larvae of the insect on certain selected trees ud which it creens., and

14

Painting Materials

on the twigs and leaves of which the wax is deposited. Wax is removed first by
scraping, and finally by skimming water in which the scraped leaves and branches
are boiled. Insect wax is pale-colored and resembles spermaceti but has a more
fibrous structure and is more opaque. Chemically, it consists largely of ceryl
cerotate (C25H51COOC26H53) together with other wax. esters and a small propor¬
tion of hydrocarbons. It contains very little free fatty acid. It is employed in the
East for much the same purpose as beeswax, but it is not largely exported.

CMnese Wood Oil (see Tung Oil).

Collagen (see also Gelatin and Glue) is the organic material which largely
comprises the bones^ the tendons^ the cartilage, and the skin of animals. Xhere
is no tissue which consists exclusively of collagen, and it is invariably associated
with other protein material such as keratin, elastin, mucin, chondrin, etc., In
addition to other non-protein organic material and inorganic salts. When collagen
is heated in water to 8o° or 90^ C., it is slowly converted into the protein, gelatin.

Collodion (see also Cellulose Hitrate). It is said (Wilson, p. 140) that pharma¬
ceutical collodion still consists of 8 ounces of pyroxylin dissolved in 3 parts ether
and I part alcohol. Proprietary substitutes are made up in amyl and butyl acetate
solutions and give a better product. As a plasticizer for flexible collodion, 3
ounces of camphor and a ounces of castor oil are used.

Colophony (see also Balsam and Resins), or rosin, is the residue which remains
after spirits of turpentine has been distilled off from the balsam or crude tur¬
pentine produced by various species of pine. A large amount of colophony comes
from the long-leaf pine of the southern United States, and, in France, from the
Pinus maritima. The proportion of colophony to turpentine seems to be related
to the condition of the trees' habitat. The usual ratio is about three to one. There
is a larger amount of essential oil in the balsam from trees near the coast. After
distillation, the residue, which is rather dark, must be purified. Colophony has
a low melting point (100® to 130° C.) and is very soluble. It facilitates the running
of harder resins, and is supposed to improve the flowing qualities of a varnish.
In industry it is often used as a clarifier for dammar and other natural resin
varnishes (see Dammar). Its acid value is between 165 and 175, which corre¬
sponds to about 89 to 97 per cent of abietic acid. Being so strongly acidic, it
probably acts by combining with basic substances which would otherwise be
precipitated.

As a varnish resin, colophony has many defects. The pale color and brilliant
gloss evident when it is freshly applied disappear rapidly on exposure. The film
becomes permanently whitened by the action of water and is easily destroyed
by abrasion. The introduction of Chinese wood oil to rosin varnish has materially
raised its value for industrial use. Colophony retards the gelation of the oil, and
the rapid drying and hard film of the oil reduce the weakness of the resin. A
synthetic resin called ester gum,’ made by esterifying colophony with glycerine,
is now widely used in the varnish industry. From ancient recipes for oil varnishes

Mediums and Adhesives

15

requiring a large amount of pica greca^ with or without the addition of a soft resin,
it is known that colophony was used in Italy as early as the IX century.

Congo Copal (see also Copal and Resins) is derived from the tree, Copaifera
Demeusi Harms., in the Belgian Congo. It is found as a fossil resin in deposits
from six inches to three feet underground, although some is still obtained by
tapping the trees. It is considered the standard fossil resin. Except for colophony
and shellac, Congo copal and resins from the Dutch East Indies constitute the
bulk of natural resins in present-day manufacture of varnishes for general use.
Like all the copals, it has no definite melting point but is fused at from 180° to
200® C. It must be heated before it will dissolve in oil, thus forming an oil varnish,
and this process makes the resin darker. It has probably not been much employed
as a picture varnish.

Copaiba (Copaiva) Balsam (see also Balsam and Resins) is an oleo-resin
obtained from the tree, Copaifera landsdorfii^ in South America. It is a deep
brown, viscous liquid with a peculiarly fruity odor and a high content of essential
oil. It is soluble in fatty and essential oils as well as in alcohol. It was formerly
much esteemed by restorers. Max von Pettenkofer (see Doerner, p. 125) used
this balsam in the so-called ‘ Pettenkofer treatment' for the brittle, dried-out
^ linoxyn ’ skin of old oil paint. When combined with ammonia, it is less harmful
to a paint film than is a strong alkali solution (see Varnish). Helmut Ruhemann
(‘ A Record of Restoration,' Technical Studies^ III [1934], p. 7) mentions using
it to make a mixture of petroleum spirit and ethyl alcohol in the process of re¬
moving surface varnish from a Flemish painting.

Copal (see also Resins) is the general name given to a large variety of hard
resins. They are obtained as fossils and are also taken directly from living trees.
The fossil resin, found three or four feet underground, is the harder kind and is
the most valuable and widely used of all the resins. The copals vary much in their
origin, in their degrees of hardness, and in their solubility. They are products,
also, of many different species and even genera of tree. It appears, from the re¬
searches of Tschirch and his associates, that copals consist of ‘ resenes,’ neutral
compounds containing oxygen, and of resin acids. The oxidation of these resenes
by contact with the air, and the resultant increase in the acid number and de¬
crease in iodine absorption, have been illustrated by experiment. The finer the
particles of the resin and the more porous they are, the higher will be their acid
number.

There is a wide range in the solubility and fusibility of copals according to
their origin and age. The melting points vary from 180° to 340° C. For conversion
into a soluble form, they are heated at a temperature of 200° to 220® C. for several
days, or are distilled dry, at a temperature of 380° to 400° C., or until 25 per cent
of copal oil has passed over. The benefits from the increased solubility by dis¬
tillation are counteracted by the color which darkens in proportion to the tem¬
perature or time of heating.

i6

Painting Materials

x4s the term ' copal' is so commonly used for a variety of resins^ tests have
been made to distinguish the true or fossil copal from such resins as dammar^
colophony, and Kauri and Manilla copals. True copal is insoluble in an 8o per
cent solution of chloral hydrate, but the other resins are partially or completely
soluble. The hard and soft varieties may be distinguished by treating a sample
with boiling water. After standing for half an hour, the hard copal remains un¬
changed, whereas the soft copal becomes milky and opaque. The hardest copal
resin is Zanzibar; Sierra Leone and Kauri are of medium hardness; Manilla is a
soft copaL'

Congo copal is the chief copal resin used in general commercial varnish manu¬
facture today. It is practically the standard fossil resin. Copals appear in the
market in a variety of forms and colors. They may be had in large lumps or
pea-size ‘ tears,’ and they range from an almost colorless, transparent mass to a
bright, yellow-brown. They have a conchoidal fracture.

Copal resins make a thick, hard, dark, oil varnish. From varnish recipes of
the Middle Ages, it may be assumed that amber was often confused with copal.
The trade in copal probably began in the X century with the Arabs, but it is
mentioned only infrequently until the latter part of the XVIII century. It has
been used chiefly as a furniture and coach varnish. The old coach painters appar¬
ently executed their designs in bright oil colors, freely mixed with turpentine.
This was coated with several layers of a spirit varnish, well rubbed down, and
over that was spread a copal oil varnish. Because of its tendency to become yellow
and dark, however, and because of the difficulty of dissolving and removing it,
copal varnish is not practical or useful as a varnish for paintings.

Crude Turpentine (see Balsam and Turpentine). This is another term for the
balsam in its natural state as it exudes from the pine tree. More commonly, that
is called ‘ turpentine,* / balsam,’ or ^ oleo-resin.’ The last two names are prefer¬
able, for the common use of ‘ turpentine ’ applies to ‘ spirits of turpentine.*

Dammar (see also Resins and Varnish.) is derived from a certain family of
trees {Dipterocarfaceae) growing in the Malay States and in the East Indies.
The tree, Agathis Dammara^ sometimes grows to a height of eighty or even a
hundred feet. From incisions the resin oozes readily in a soft, viscous state, with
a highly aromatic odor which it loses on hardening. ' Dammar Mata Kuching,*
from Malaya, is known as ‘ cat’s eye resin,* and is of a very high quality. It is
used for incense in the Orient, but appears in the European market in trans¬
parent, brittle, odorless lumps for the manufacture of a spirit varnish. Its dis¬
tinguishing characteristic is that it is completely soluble in coal-tar hydrocarbons
and in turpentine, and is almost completely insoluble in alcohol. It is light in
color, lustrous, and adherent. The film is soft, however, is less durable than that
made from copal resin, and has a tendency to remain slightly tacky. Its paleness
and the ease with which it may be used have caused it to be very popular, and
it is regarded by some as the best varnish for pictures (see Maximilian Toch,

Mediums and Adhesives

17

‘ Dammar as a Picture Varnish/ Technical Studies, II [1934L pp. 149 S.). The
proportions sometimes given are, i part of resin to 3 parts of turpentine. Sabin
(p. 135) suggests 5 or 6 pounds of dammar resin dissolved in i gallon of turpentine,
and allowed to settle for sixty days. In tempera emulsions, i part of resin is
dissolved in 2 parts of oil of turpentine. Two per cent dammar in petroleum spirit
may be used as a pastel fixative.

Often the varnish is cloudy, probably owing to the presence of insoluble
resenes. According to Barry (p. 94), cloudiness does not necessarily mean reduc¬
tion in durability. If the resin is ‘ run ’ before it is dissolved in the turpentine,
a clear varnish results, but it is naturally darker. It can also be cleared by adding
rosin, though this detracts from its quality. There is recent evidence that dammar
varnish is very resistant to blooming, even in a moisture-laden atmosphere. It
is used to some extent with nitrocellulose in making clear lacquers. Reid and
Hofmann (p. 498) have given a formula for dewaxing dammar resin for commercial
purposes. The resin solution obtained is clear and not milky. Their formula is:

Dissolve 80 pounds of Batavia dammar in a mixture composed of 20 pounds of
ethyl acetate and 40 pounds of petroleum distillate having a boiling range, 80-
180° C. When completely dissolved (in a mixer equipped with a mechanical agi¬
tator), add 100 pounds of denatured alcohol, agitate for a time, and then allow to
settle overnight. The waxy precipitate forms a cake in the bottom of the vessel,
and when the clear supernatant solution has been drawn off, the wax cake is re¬
moved. This wax has not yet found any special use and is generally burned.

Dammar is mainly composed of dammarolic acid (C64H7703(C00H)2) and two
resenes. Its melting point is from 100® to 1^0° C.; its specific gravity, 1.062; and its
acid number, 18 to 16.

Dextrin (see also Starches) is commonly prepared from starch by heating the
dry material at 200° to 250° C. It is less commonly prepared by moistening starch
with dilute nitric or hydrochloric acid and heating it, when air-dried, to about
I C. Dextrin, as prepared, is a mixture of soluble starch, at least three varieties
of true dextrin, and sugar (maltose and dextrose). It dissolves in water and yields
a syrupy solution with strong adhesive properties. With iodine it gives a color
which varies from red to violet. Its discovery is said to have followed the obser¬
vation that starch, which had been roasted during a fire in a Manchester ware¬
house, yielded a sticky, gummy solution when wetted with water.

Distemper (see also Tempera) is a term common in the painting trades,"^
particularly in England, and indicates a paint made with a glutinous medium.
It is ordinarily used on walls or’in scene painting.

Dragon's Blood (see also Resins) was known by this name in mediaeval times
when it was used as a pigment in manuscript illumination. ‘ Leave it alone/ says
Cennini, ‘for it is not of a condition to do you much honor’ (Thompson, The
Materials of Medieval Tainting, p. 124). It seems to have been used for medicinal

i8

Painting Materials

purposes being mentioned in this connection by writers from Dioscorides to
the XVl’century. In the XVIII century the Italian violin makers used it as an
ingredient in their varnishes. Today it is used to some extent in colored spirit
varnishes and for lacquering metals. Laurie {Materials of the Painter s Craft,
p. 203) quotes an auripetrum recipe, a yellow varnish for coating tin foil, which
calls for an ‘ oil varnish coloured by saffron, aloes, the inner bark of black plum
or dragon’s blood ’ (St Audemar MS., Merrifield, I, 115). These substances are
all easfly dissolved in hot pine balsam, which can then be diluted with boiled oil

and turpentine. _ 7 , u- 1

Dragon^s blood comes from a species of rattan palm^ Ccildfyius aTdcOy which

grows in Further India and in the Eastern Archipelago. The variety from Su¬
matra, which appears in commerce in the form of eighth-inch sticks wrapped in
fibre, is considered the best. On the surface, the resin appears brown but it gives
a red, lustrous fracture and a light red powder. It is soluble in alcohol, in ether,
and in fixed and volatile oils, and, if heated, it gives off benzoic acid. An inferior
resin comes in lump form from Socotra and is the product of the tree, Dracaena
cinnahari.

Drier (see Siccative and Oils, drying process).

Drying Oils are oils (see Oils and Fats) which have the property of forming
a solid, elastic substance when exposed to the air in thin layers (see Oils, oxida¬
tion). This ‘ drying ’ power decreases as the iodine absorption diminishes, i.e.y it
is proportional to the total amount of unsaturated fatty .acids present. The
iodine values of these oils (see Linseed Oil, Walnut Oil, Poppy-Seed Oil, Tung
Oa, Soya Bean 01 , Perilla Oti, Sunflower Oil, Hempseed Oil, Candlenut Oil,
and Safflower Ofl) range from about 200 to 120. They find their chief use in
commerce as the vehicles for pigments in paints and in varnishes.

Egg Tempera (see also Egg Yolk and Egg White). The whole egg, the yolk, or
the white may be used as a tempera medium. Doerner (p. 213) gives a recipe for
using a whole egg, which requires with it an equal measure of oil, or stand oil, or
oil varnish, and two measures of water added separately with thorough shaking.
According to him, the freshness of the egg is important for the quality and the
permanence of the emulsion. He says that pigments containing sulphur, such as
cadmium, vermilion, and artificial ultramarine, when used with an egg emulsion,
may decompose by combining with the nitrogen and sulphur compounds in the
egg to form hydrogen sulphide, and he finds that the addition of vinegar or phenol
*is inadvisable because they discolor some pigments, and he prefers a drop of oil of
cloves or small amounts of alcohol.

Among many other present-day recipes for egg tempera is that of Kurt Wehlte
in Ei-Tempera und ihre Anwen dungs arten (Dresden: Herrmann Neisch, 1931),
pp. 28-29. He requires: I part of whole egg, ^ part of linseed oil varnish,part
of dammar resin in turpentine, and i part of water. For a somewhat different
tempera, he suggests substituting oil for the amount of resin in this one. These are

Mediums and Adhesives

19

complicated emulsions^ possibly with oil as the continuous phase (see Emulsions).
A mole simple medium which makes use of the whole egg is that described by
Cennino Cennini, c. LXXII (Thompson, The Craftsman's Handbook, p. 51). He
speaks of a tempera for wall painting, made of the white and yolk of an egg into
which are put some cuttings of young shoots of a fig tree. These are beaten well
together. A very rare form of egg tempera was developed by the Indians of Canada
(see Douglas Leechman, ‘ Native Paints of the Canadian West Coast,’ Technical
Studies, V [1937]? PP- 206—207). They used, among other mediums, eggs from
various species of salmon, sometimes taken fresh, sometimes dried, and some¬
times worked up by being chewed in the mouth together with a piece of red cedar
bark.

The egg tempera which is traditional and reflects the practice of many cen¬
turies is that made simply with yolk of egg. It is described by Thompson in The
Practice of Tempera Painting (p. 96):

Take a raw fresh hen’s egg, and crack it on the side of a bowl. Lift off half of the
shell, keeping the yolk in the lower half, and letting the white run into the bowl.
Pass the yolk back and forth from one half shell to the other several times without
breaking it, so as to get rid of as much of the white as possible; and pinch off between
the shells the little white knots which adhere to the yolk. Put the yolk into a cup,
and break it, stirring up with it one or two tablespoonfuls of cold water. It does
not much matter how much water you add; a little more or less makes no difference.
You will probably develop a preference for a thick egg mixture or a thin one as you
get used to it, and either is all right. The main point of adding the water is to cut
the greasiness of the yolk a little, and make it fairly liquid. Pour it into a four-ounce,
glass-stoppered, wide-mouthed bottle.

He recommends adding to this two or three drops of vinegar or 3 per cent acetic
acid as a preservative and to make the medium less greasy. Into the egg yolk as
prepared, the colors are mixed. They have already been ground in water and about
equal parts of pigment paste and prepared yolk are put together, proportions being
adjusted to the needs of each pigment, and the whole thinned out with water.

White of egg or glair has probably been most used as a medium for illuminating
books, and for powdered or ‘shell’ gold, and for bole. The traditional use of it is
described particularly in two MSS. One of these is in Naples, Biblioteca Nazionale
MS. XII.E 27; it is translated with notes by Thompson and Hamilton, De Arte
Illuminandi (New Haven: Yale University Press, 1933). The other is published
also by Thompson, ‘ The De Clarea of the So-Called “ Anonymus Bernensis,” ’
Technical Studies, I (1932), pp. 8—19, and 69—81. The former is of the XIV century
and the De Clarea is described by this translator (p. 11) as ‘ a fragmentary extract
from a lost work of the second half of the eleventh century.’ There is little to be
added to that treatise so far as preparation of the glair is concerned. The author
distinguishes two kinds—one made by beating and the other by pressing. The
latter sort is squeezed through cloth and is contaminated in the process. The

20

Painting Materials

beaten glair is better. The white is separated from the yolk and is thoroughly
beaten in a platter with a wooden whisk until it sticks to the platter even when
that is turned bottom-side up. Then the platter with the froth is left in a cool
place, tilted slightly, until the glair liquid has settled out. With this the colors are
tempered. Of this medium Thompson says {The Materials of Medieval Painting,
pp. 55-56) :

It is a delicate binder, very modest and retiring and inconspicuous; and it preserves
the individual quality of a pigment beautifully. . . . Glair is rather weak and
brittle, especially when newly made, and partly for this reason (which militated
against its use in strong concentrations), partly because it was not dense enough to
bring out the full quality of some pigments, it was often supplemented in book
painting by gum arabic.

Egg White (see also Egg Tempera and Egg Yolk) contains in quite different
amounts the same substances found in the yolk. Church (p. 72) gives the per¬
centage proportions of these as follows:

Water.. 84.8

Albumen, vitellin, etc. 12.0

Fat or oil. 0.2

Lecithin. trace

Mineral matter. 0.7

Other substances. 2.3

The albumen is the adhesive substance of egg white and is complex, containing,
besides carbon, hydrogen, nitrogen, and oxygen, about 1.6 per cent of sulphur.
As a pure film it is clear and brittle and is readily dissolved by water. Church has
suggested (p. 73) that if paintings in tempera, before they are quite dry, were
heated to 70° or 75° C. the albumen would be changed to an insoluble form, or
that treatment with tannin would serve the same purpose. Egg white as a medium
is called also by its other name, ‘ glair.’

Egg Yolk (see also Egg Tempera and Egg White) is an oily emulsion in which
the oil particles are suspended in a solution of albumen. Its average composition
is given by Church (p. 72) in percentage proportions:

Water...... 51.5

Albumen, vitellin, etc.... 15.0

Fat or oil. ....... aa.o

Lecithin.. 9.0

Mineral matter.. i.o

Other substances. . .. 1.5

The lecithin is a fatty substance to which has been given the empirical formula,
C42H84NPO9, but it differs from most fats (see Oils and Fats) in containing nitro¬
gen and phosphorus and in being very hygroscopic. It evidently acts as an emulsi-

Mediums and Adhesives

21

fying agent. When egg yolk is used as a painting medium, it dries to a strong film,
first by evaporation of the water and then by a slow hardening of the oil which
remains suspended in the albuminous matrix. This oil content is greater than that
of the albumen and, in consequence, the ultimate film is very little affected by
water. ^

Elem. This is a generic term applied to a large number of resins obtained from
trees of the Burseraceae family. Manilla elemi, or ‘ soft elemi,’ the only kind that
has been closely examined, comes from a species of Canarium, C. commune grow-
mg m the Philippines. Other varieties come from South America, Africa, and the
East and West Indies. American or West Indian, Yucatan elemi, is generally
found in commerce. Manilla elemi is a soft, semi-crystalline, yellow resin, with a
fennel-hke odor. It is usually viscous like a balsam, but may be quite hard. The
true elemis have comparatively low acid and saponification values, and one per
cent of ash is the highest limit for a good sample. All varieties are easily soluble-
ether, alcohol, chloroform, carbon disulphide, and benzol are effective solvents’
benzine and petroleum ether being less so. Elemi is used chiefly to modify the’
consistency of varnish. It is not employed as a paint medium, and, when added to
recently ground colors, gives them the appearance of being covered with frost.
Since^ the so-called Dutch Process ’ of relining pictures on canvas came into
pramice early in the XX century, gum elemi has been used in this process as an
addition to waxes, its effect being to increase the tackiness of the wax. It is in-
clu(^d in a number of formulas given by Plenderleith and Cursiter (pp. 02 and qa)

Emissions (see also Egg Yolk, Ofls and Fats, Oils, history in parting, and
Waxes, history in pamting). An emulsion consists of drops of one liquid suspended
in another liquid. In most cases there is an actual film around the globules which
keeps them from coalescing. With any pair of non-miscible liquids, such as oil
and water, there may be two kinds of emulsions, one with drops of oil suspended
in water and one with drops of water suspended in oil. The necessary conditions
for forming a stable emulsion are that the drops shall be so small that they will
stay suspended and that there shall be a sufficiendy yiscous or plastic film around
each to keep the drops from coalescing. An emulsifying agent is a substance which
goes into the interface and produces a film having satisfactory physical properties.
According to Bancroft’s theory, an oil-in-water emulsion is formed if the emulsi¬
fying agent at the interface is chiefly in the water phase, and a water-in-oil
emulsicm is formed if the emulsifying agent at the interface is chiefly in the oil
phase. For example, sodium and potassium oleates are water-soluble colloids and
they are excellent for emulsifying oils in water. The gums are also water-soluble
colloids and certain ones are much used in pharmaceutical work for emulsifying
oils in water. Calcium and magnesium oleates form colloidal solutions in oil and
can, therefore, be used to emulsify water in oil; rosin and the resinates behave in
the same way. Since sodium oleate emulsifies oil in water and calcium oleate
emulsifies water in oil, a mixture will behave according to the relative amounts of

Wai:eT Molecules ^
Sodium Cleate- Molecules

Oil

.Water

An emulsion of oil and water, made mechanically by stirring, has a pattern
approximately like that in and a similar emulsion, made by an electric mixer,
is indicated at k These, as magnified to about 150 diameters, are taken from
photomicrographs in'Paints, Famishes, Lacquers and Colors, 9th ed. (Washing¬
ton, D. C.: Institute of Paint and Varnish Research, 1939), by H. A. Gardner,
figure 350? page 202. At y is illustrated the action of an emulsifying agent in
dispersing oil particles in a continuous phase of water. This is taken from a
diagram, figure 128, page 3^7> Colloid Chemistry (Boston: Houghton Mifflin
Co., 1939), by Robert J. Hartman. Ati^is diagrammed an oil-in-water emulsion;
and at ^ a water-in-oil emulsion. Both are adapted from Hartman, figure 127,
page326.

Mediums and Adhesives 23

each present. There will be some ratio of calcium to sodium at which the two
okates will practically balance each other and the slightest relative change will
change the type of emulsion. Although most emulsions are made with gelatinous
CO Olds as emulsifying agents, theoretically, this is not necessary. Anything that
will go into the interface and make it sufficiently viscous will give the same result.
If enough of a fine powder is put into the interface, a plastic mass is formed there
which will stabilize the emulsion. It is not always easy to tell by inspection whether
water is the external phase or the internal phase in a given emulsion. One way is
o exaniine the emulsion under a microscope while a little water or a little oil is
being added. The one that is the external phase will mix readily with the emulsion
and the other will not. If the emulsion is not deeply colored, its type may be
recognized by means of a few minute crystals of a fat-soluble dye, such as Sudan
III or Scar et R, which are dropped on the surface and give a spreading color to a
water-in-oil emulsion but not to an oil-in-water type,

^ It is not possible to say to what extent emulsions have been used as mediums
in painting. Berger interpreted the description of Punic wax given by Pliny (XXI
49 ) and by Dioscorides (II, 105) as an emulsion. His theory, however, has been
overwhelmingly refuted by the studies of Eibner, Laurie, Schmid, and others.
Modern attempts to explain the Flemish method of the XV century, particularly
that of the Van Eycks, have brought about, among others, the view that both oil
and tempera were used. There is a further difference of opinion, however, as to
whether the medium was an emulsion of the two or whether the two were used
either alternately or in juxtaposition to produce a final result. Maroger has
strongly argued for emulsion as the explanation. It is, of course, possible to emul¬
sify either wax or oil, and many experiments have been made in recent times with
both emulsions as painting mediums.

Encaustic (see also Waxes) was a method of painting with wax common in
ancient times. The word refers, literally, to the process of melting or burning the
color into the surface on which it was applied.

Fatty Acids are the organic, aliphatic acids which are combined with glycerine
to form fats and oils (see Oils and Fats). In most fats and oils there is a small

per cent of uncombined, free fatty acid.

Fish Glue is impure gelatin prepared from fish heads, bones, and skins. The
pure gelatin from fish bladders is known as isinglass (see also GelatiHj Glue, and
Isinglass). Glue made from the skins is clearest and best. Usually, fish glue is
marketed in liquid form but it can sometimes be obtained in the form of cakes or
broken sheets which are hygroscopic and readily soluble in water. As a liquid It
contains a preservative and sometimes an essential oil like wintergreen or cinna¬
mon to mask the odor. This glue Is inferior to animal glues as an adhesive and is
more easily spoiled by bacterial decomposition. Alexander says (p. aiS) that the
joint strength of a common commercial fish glue was only 260 pounds per square
inch, but, according to the Forest Products Laboratory {Technical Note^ F-a),
high-grade skin glue should average 1,700 to 1,800 pounds.

24

Painting Materials

Fisii Oil shows wide varis-tion. The oil s-nd the lilnis made from it have bad
odors and the films suffer from non-uniformity. Although it is ordinarily called a
non-drying oil, it will dry when boiled with a siccative. Ooerner (p. 114) says that^
in spite of all efforts at prevention, it occurs here and there in artists colors.

Fixative. Any film material, which can be dissolved in low concentration and
low viscosity, may be sprayed upon drawings or pastels for the purpose of holding
the pigment granules in place. Such a sprayed film, or material capable of being
sprayed, is called a ' fixative.' As fixatives are commonly used, however, they
include only the natural resins such as mastic, dammar, and bleached shellac
(see also Resins).

Flour Paste (see also Starches). Since flour consists of a mixture of gluten and
starch, flour pastes differ materially in their working properties from starch pastes,
and pastes made from different varieties of flour also differ among themselves.
Wheat flour and rye flour are the ones most commonly used, but rice flour is
often made into paste and some mixtures contain corn, barley, or buckwheat.
Besides the simple preparation of paste by cooking flour in water, there are other
methods which result in a partial breaking down of the flour molecules by means
of fermentation or heat. For example, in one process mentioned by Alexander
(Walton, p. 177) flour and water are mixed to form a dough which is fermented
at 110° F. and then is cooked, dried, and pulverized. The powder may be kept
indefinitely without deterioration and be used for a paste when desired. Ferment¬
ing pastes of flour are known to be used for mounting in Japan. In another process
flour Is heated under steam pressure with about five times its weight of water
and, when partly cooled, has a quantity of raw flour added. Dextrin may be used
in place of the cooked flour.

For certain purposes, the flour can often be advantageously combined with
other materials. For securing paper, leather, etc., to metals, Alexander {loc. dt.)
gives the following directions. Ten pounds of animal glue are melted in 3 gallons
of water at a moderate heat. Twenty pounds of rye flour are then mixed with
gallons of cold water and 8 pounds of acetic acid are added; the whole is poured
into the melted glue and boiled. Doerner (p. 226) says that many tempera recipes
used in commercial art are based on rye-paste emulsion, which, for such a purpose,
is combined with a glue solution and boiled linseed oil. He gives a trade recipe
which has been found useful by different artists in the painting of large, decorative
surfaces: Rye flour, 125 g., is mixed with 50 cc. warm water and to this are added
100 cc. of cold water. After these are thoroughly mixed, 300 cc. of boiling water are
put in and then 125 cc. of boiled linseed oil. This is followed by loo cc. of cold
water and 125 cc* of boiled linseed oil. The whole mixture is then given an op¬
tional thinning with water.

Foimogelatm (see Gelatin). When gelatin solutions, especially concentrated
solutions or those containing free alkali, are treatedwith formaldehyde, the gelatin
IS converted, upon drying, into an insoluble substance known as formogelatin.

Mediums and Adhesives

25

which seems to be a compound of formaldehyde with gelatin. The formogelatin is
decomposed by repeated washing with boiling water, by heating to 110° C, and
by cold 15 per cent hydrochloric acid. Although formogelatin is nearly insoluble
and swells much less in water than does untreated gelatin, it is not, strictly
speaking, a very water-resistant adhesive. Bogue (p. 319) says that joints made
from it fail to retain their strength when subjected to drastic treatment with either
cold or hot water. Casein and blood albumin glues are much more highly water-
resistant.

Gelatin (see also Glue and Tempera) belongs to the complicated class of or¬
ganic compounds known as proteins and is composed of carbon, hydrogen, oxygen,
and nitrogen. Its exact composition is not known but, like all proteins, it is made
up of large molecules of high molecular weight. Though an animal product, it is
not itself found in the animal organism, except under pathological conditions.
The parent substance of gelatin Is collagen, of which the organic material of the
bones, the tendons, the cartilage, and the skin is largely comprised. Gelatin is
slowly formed from collagen by heating that in water to 80'' or 90'' C. It Is a nearly
colorless, transparent, amorphous substance. In its normal, dry state, in which it
still retains 15 to 18 per cent of water, it is flexible and horny, and has a slight
yellow cast. Precipitated from alcohol or from salts, it is pure white and nearly
water-free.

Gelatin is a typical colloid of the emulsoid type, and the viscosity of its solu¬
tions is high and variable with slight alterations in temperature, concentration,
hydrogen-ion concentration, etc. It swells to many times its normal volume when
immersed in cold water or in dilute acids or alkalis; a slightly acid solution Is the
most favorable for maximum swelling. Above 35° C., the swollen jelly goes readily
into solution. A firm jelly is formed when a solution containing i or more per
cent of gelatin is allowed to stand at 10° C. Gelatin, either pure or in the impure
form known as glue, is used extensively as an adhesive. •

Glair (see also Tempera and Egg Tempera) is the white of egg (see Egg White)
and is a term now largely employed with reference to the painting medium pre¬
pared from this substance.

Glaze. When the quantity of medium is so great in relation to the quantity of
pigment that light Is refracted through the film produced by the mixture of these
two and is reflected from the surface beneath it, such a film is commonly called a
‘ glaze.’ The term has no precise meaning but usually indicates a coating in which
there is some pigment content. Its main characteristic is transparency. When the
pigment used is opaque and pale, films of this general type are called ‘ scumbles.’

Glue (see also Gelatin and Tempera) is an adhesive consisting largely of
gelatin, but the collagen from which gelatin or glue is prepared is invariably
associated with other protein material such as keratin, elastin, mucin, chondrin,
etc., in addition to non-protein, organic material and inorganic salts which may
or may not remain in the glue. Glue and gelatin merge into one another by im-

*26

Painting Materials

perceptible degrees. The difference Is one of purity: the more impure form is
termed ^ glue ^ and is used only as an adhesive; the purer form, termed ^ gelatin ^
or ^size; is used when an especially fine adhesive or a medium is required, and it
has other uses—in foods, in photography, etc. Generally,^ it is customary to use
the word ' size' to indicate a nearly pure gelatin. Glue is an organic, colloidal
substance of varying appearance, chemical constitution, and physical properties.
It occurs in commerce in a wide variety of forms and colors. The colors range
through all shades of white, yellow, and brown and glue may be transparent, trans¬
lucent, or opaque. Gelatinous or glue-forming tissues occur in the bones, skins, and
intestines of all animals. These agglutinating materials are removed by extraction
with hot water, and the solution, on evaporation and cooling, yields a jelly-like
substance—gelatin or glue. Glue Is prepared from fish bones, skins, or bladders
which give impure forms of bone gelatin, skin gelatin, and isinglass. Parchment
size is made from parchment waste.

When glue is soaked for some time in cold water, it softens and swells without
dissolving, and, when again dried, should resume its original properties. When
gently heated, it dissolves entirely in water, forming a thick, syrupy liquid with a
characteristic but not disagreeable odor. Remelted glue Is not so strong as that
which is freshly prepared; and newly manufactured glue is inferior to a glue which
has been in stock for some time. Glue loses strength continuously under the action
of heat, and it is better to heat successive small amounts rather than to have a
large lot cooking for a relatively long period. All glue solutions putrefy with time
and lose their adhesive power. The following table ((/., Encyclopedia Britannica^
nth ed., p. 143) shows the holding power of glued joints with various kinds of
wood:

WOOD POUNDS PER SQ.UARE INCH

With grain Against grain

Beech. 85a 434.5

Maple. 484 346

Oak.. 704 302

Fir... 605 152

The word ‘ glue ’ has been extended to many other substances that are not
glue at all. Solutions of gums, dextrins, converted starches, etc,, are often called
glues, generally modified by the adjective, ‘ vegetable.’ Silicate of soda is called
‘ mineral glue ’; solutions of rubber, asphaltum, and the like, in benzene or
naphtha, are called ' marine glue ’; and those of casein are called * casein glue/

^ Glue k used extensively in painting grounds. Gesso is a thick, white, water
paint, with chalk or gypsum as the inert material, and glue or gelatin as the
binding medium. In mediaeval painting, size was sometimes used as a medium in
books, especially for blues which often had to be laid quite thickly, and it was
important to have a strong binding medium which would not be too brittle when

Mediums and Adhesives

27

used in quantity. Doerner (p. 303) says that glue-color painting is very useful
for decorative work. Glue-color is easily soluble in water, but spraying it with a 4
per cent formalin solution will render it less soluble (see Formogelatin). Wax
soap is often applied for the same purpose. For use in painting, glue can be
emulsified alone or it can be used as an addition to egg and gum tempera.

Among the stone carvings of the ancient city of Thebes, at the time of
Thothmes III, the Pharaoh of Exodus, on a stone at least 3,300 years old, there
is a representation of the process of gluing a thin piece to a yellow plank of syca¬
more. A glue pot and brush are shown, together with a chunk of glue that has a
characteristic concave fracture. Such a piece of glue, which had originally been
rectangular but had dried, was found in the tomb of Queen Hatshepsut (XVIII
dynasty). Glue is mentioned in the Bihle in Ecclesiastes, XXII, 7: ‘He that
teacheth a fool is like one that glueth a potsherd together.’ Pliny refers to it with
gums, milk, eggs, and wax, as a vehicle for the paints of the ancient Egyptians.
During the Middle Ages glue was used extensively, and old MSS give directions
for its preparation. In the MS. of Jehan le Begue (Merrifield, I, 148) is given a
detailed recipe for the preparation of glue from the skin of an ox or cow. It is
described as a mordant for powdered tin. In the Bolognese MS. (Merrifield, II,
466) it is mentioned in connection with gilding. Cennino Cennini (see Thompson,
The Craftsman'Handbook, p. 67) gives recipes for its preparation and speaks
of a glue which is made of the clippings of the muzzles of goats, feet sinews, and
many clippings of skins.

The earliest practical manufacture of glue that can be directly traced from the
present day was in Holland at the time of William III. It was evidently made there
in 1690, and shortly after was introduced into England and established as one of
the permanent industries by about 1700. In France, the industry started in the
vicinity of Lyons, and for many years these factories were the most important
of their kind in Europe.

Glycerides are esters of glycerol. Oils and fats are glycerine esters of the
fatty acids, i.e., they are glycerides (see Oils and Fats).

Glycerine, or Glycerol, is the trihydroxy alcohol (C3H5(OH)s) which is com¬
bined with various fatty acids to form oils and fats, i molecule of glycerine
being combined with 3 molecules of fatty acid to form i molecule of an oil or fat
(see Oils and Fats). Free glycerine, or glycerol, is a syrupy, hygroscopic liquid
used as a plasticizer for aqueous mediums (see also Gum Arabic).

Gouache (see also Gums) is actually a water color or a gum tempera and the
word is used more to describe the opacity obtained with such paints than to
define any particular difference of original material. Like water color, it is ordi¬
narily applied on a paper support, but less thinly than water color and with
mixed tints of white and color rather than with transparencies of color.

Gum Arabic (see also Gums and Tempera) is produced by several species of
Acacia growing in Africa, India, and Australia. The gum from Acacia arabica

Painting Materials

28

Willd. is inferior to that produced by Acacia Senegal^ and is little used for artistic
purposes. The good gum is variously known as Kordofan^ picked Turkey, white
Senaar, and Senegal gum. The tree, called by the natives, 'hashab/ grows chiefly
in the Sudan and Senegal, sometimes attaining a height of twenty feet. The gum
exudes naturally and is slightly darker and less valuable than is that from the
cultivated trees. Gum arable—Kordofan or Senegal—appears on the market in the
form of rounded Tears/ either colorless or slightly yellow. The lumps are brittle
and break with a vitreous fracture, exposing a transparent interior, colorless in the
finer grades. Senegal gum, which Church (p. 79) considers the only sort that
ought to be employed in painting, should leave no appreciable residue when dis¬
solved in cold water. It should be clear, giving no color with tincture of iodine.
If a reddish purple color results, the gum has probably been adulterated with
dextrin.

To prepare gum arable for use, according to Church (p. 80), it is finely pow¬
dered and slowly stirred into boiling, distilled water, the proportions being one
measure of the powdered gum to two of water. The solution should stand for at
least a day, and then be decanted from any sediment into a wide-mouthed bottle
covered with a glass cap. The addition of a lump of camphor, a tew drops of
eugenol, or jS-naphthol, makes an effective preservative. Gum solutions may be
emulsified by a fatty oil (see Emulsions), and a small amount of glycerine (not
more than 5 per cent) may be added to the emulsion to eliminate brittleness. Gum
emulsions are sometimes used in the manufacture of water colors.

Gum Tragacanth (see also Gums) is produced by leguminous shrubs belonging
to the genus Astragalus. It consists of a small quantity of gum soluble in water, a
little starch and cellulose, and a large proportion of a mucilaginous substance
which swells in cold water but does not dissolve. This latter constituent is a com¬
plex compound of carbon,hydrogen, and oxygen, called‘bassorin.' Gum tragacanth
contains 12 to 15 per cent of water, and leaves 2 to 3 per cent of ash when burned.
It may be prepared for use by placing some finely powdered gum in a bottle, wet¬
ting it with alcohol, and then adding the required amount of water, with shaking
at intervals. Only 2 or 3 per cent of the gum makes a thick solution. Unlike gum
arable, which dissolves in water, gum tragacanth swells, forming a mucilaginous
mass which must be strained through a cloth. A uniform consistency is difficult
to obtain. Gum tragacanth may be used as a medium for painting on linen. It
thinly and the painting left unvarnished for some time. Its
principal use, however, is as a binder in the manufacture of pastel crayons.

Gums (see also Tempera). A group of non-crystalline, structureless materials,
occurring widely in plants, composed mainly of carbon, hydrogen, and oxygen,
and forming viscous solutions or mucilages, is given this general name. Their chief
characteristic is that they dissolve in water, forming a clear solution, or swell
when they are soaked in water. This differentiates them from resins which are
sometimes misleadingly called by the same name. Gums differ, also, from gelatins,

Mediums and Adhesives

29

glues, and protans, which form similar mucilaginous solutions, in that the latter
are definitely nitrogenous bodies, while the gums contain practically no nitrogen.
1 hey are insoluble in alcohol, do not melt but char on heating, and do not give
off a nitrogenous odor. The one chiefly used for a painting medium is gum arabic;
gum tragacanth and cherry gum are of less importance.

With the exception of gum arabic, the chemistry of plant gums has not been
very thoroughly studied. The plant gums are salts of complex, organic acids,
usually with calcium, magnesium, and potassium. The complex acids are built
combination with the acidic part of the molecule. C. L. Butler
and L. H. Cretcher (‘ The Composition of Gum Arabic,’ Journal of the American
Chemical Society, LI [1929], pp. 1519-1525) have identified rhamnose,^f-galactose,
and /-arabinose m the sugar fraction of the hydrolysis product of gum arabic.
Calcium,^ which is the principal metal of this gum, is present as the salt of aldo-
bionic acid. The solution of gum arabic may be precipitated by basic lead acetate,
and It IS thickened or rendered turbid by the addition of solutions of borates,
ternc salts, or alkaline silicates. Mixtures of copper sulphate and so dium^hy dr ox¬
ide, and of neutral ferric chloride and alcohol are valuable confirmatory tests
Gum arabic yields about 3 per cent ash, consisting of calcium, magnesium, and
potassium carbonates. The percentage of moisture varies with the diflPerent
varieties of the gum. Senegal gum contains more moisture (i6.i per cent) than
the Sudan gums, which may make it a superior product, although the Sudan gums
are lighter in color.

As sizing materials and as tempera mediums, gums have probably been used
for a long time. In chapter XXVII of the Schedula diversarum artium of The-
ophilus, early XII century, there is an account of the use of gum in place of sun-
dried oil (see Laurie, Materials of the Painter's Craft, p. 164). Jehan le Begue
(Merrifield., I, 284) describes the preparation of a rose color in which powdered
brixilhum is ground with a gum water made of two thirds gum arabic and one
third clear water. And there is good evidence that long before this it had been a
medium common in the practice of classical painting.

Hempseed Oil. Hemp {Cannabis sativa) is cultivated in Western Europe, in
North America, and in Japan. The oil from its seed is greenish in color and is
slow-drying; it also wrinkles badly and has a somewhat greasy texture.

Honey (see also Tempera) has been used from early times as an addition to
water-soluble mediums, such as gum arabic or size or glair, and as late as the XIX
century was a common ingredient in moist water colors. By retaining a certain
amount of water, it had the effect of keeping these materials from becoming
brittle through extreme dryness. It is made up of two sugars, dextrose and levulose,
with other compounds, and about 20 per cent of water. Church (pp. 82-83)
recommends that a solution of levulose be prepared from pure honey and used
instead of the more complex substance. In modern practice, small quantities of

glycerine take the place of honey in aqueous mediums.

30

Painting Materials

Hydrolyzed Oils are oils or fats (see Oils and Fats) which have been split up
by the action of water under suitable conditions, glycerine and fatty acids being

formed in the process. j ^ t. *

Isinglass is a very pure fish gelatin (see also Gelatin and Fish. Glue). It is

yielded by the sounds (swim bladders) of a limited number of fishj chiefly the
sturgeon, which, for centuries, has been the main source of the celebrated Russian
isinglass.' North American isinglass comes mainly from the hake, but some is
obtained from the cod. It is nearly pure collagen. Alexander says (p. 221) that,
when soaked in cold water, it swells greatly without losing its organized, fibrous,
thread-like structure, but that boiling converts it into gelatin which, probably
because of the ease of its formation, yields a very strong jelly. It was formerly
used as an adhesive much more generally than it is at present. Thin, transparent
sheets of the mineral, mica, are often wrongly called isinglass.

Japan Wax is prepared from the berries of various species of the sumach tree
(^KJius^ which is cultivated in Japan and China for the lacc^uer it yields. The wax
is a by-product. The berries yield 15 to 25 per cent of wax which occurs on them
as a greenish coating. They are gathered and stored until ripe, and are then
crushed and the kernels are separated. The remaining crude wax is pressed in
wedge presses and is purified by remelting and sun-bleaching, the wax being kept
moist during the process. It is pale yellow or light brown in color with a pro¬
nounced and characteristic odor. It acquires a white, powdery surface. Its melting
point varies from 48*" to 55° C., that usually found being 53°; the wax solidifies
again at about 41'', and, when recently solidified, melts at 42° and only slowly
regains its original melting point. Its iodine value is low; Fryer and Weston (p.
156) give the average value of 6. It is readily soluble in benzene and petroleum
ether, and is sparingly soluble in cold ether; it is insoluble in cold alcohol, but
dissolves on warming and separates again on cooling. It is of hard and brittle
consistency with a conchoidal fracture. Physically, it resembles bleached beeswax.
Since it is similar to the waxes in physical properties, it is called a wax, and some
writers classify it with them. From a chemical standpoint, however, the term
‘ wax ’ is, of course, a misnomer for it, as it consists largely of palmitin and free
palmitic acid together with a small proportion (less than i per cent) of dibasic
acids (^.1-., japanic acid, €22114202)5 and probably some soluble acids. (See also
Oils and Fats.)

Kauri (see also Copal). The tree which produces this resin is a conifer, Agathis
australis^ growing to an immense height and age on the north island of New
Zealand. It is easily Vmn’ and combined with oil, and the better grades make a
very good varnish for industrial purposes. It is believed by some to be inferior in
hardness and durability to Sierra Leone or Zanzibar copal varnishes. On the other
hand, certain investigations on ‘ blooming V tend to show that varnishes cor¬
rectly made from Kauri do not bloom in the most humid atmosphere.

Mediums and Adhesives

31

L&c (see also Resins and Shellac). This resin is not a natural exudate from a
tree but is produced by an insect belonging to the Coccidae or scale insect family,
Tacchardia lacca. Like the cochineal insect, these produce a red dye which was
formerly valued both in India and in Europe. The tree, Butea Jrondosa Roxb.,
which is still one of the most important hosts, is referred to in Sanskrit writings as
'Lakshatarn, the tree which nourishes a hundred thousand insects,’ and in the
writings of ^lian, about 250 A.D., there is reference to the insect yielding a red
coloring matter. Mention of the use of lac in varnish is made in the ‘Ami-Akbari,’
1590 A.D., which tells of the decorating of the imperial palace. The dye was used
in Spain and in Provence as early as 1220, and is mentioned in recipes for rose-
colored lake. The Paduan MS. (Merrifield, 11 , 688) speaks of the colorless resin
left after the extraction of the dye, but apparently its use as a varnish ingredient
was unknown in Europe until the XVI century.

^ The insect attaches itself to various trees of the acacia family (particularly
Ficus religiosa Linn.) for feeding and breeding. Both hard- and soft-woods may act
as hosts, but generally soft, non-resinous trees are preferred. The insects fasten
themselves to twigs and branches and produce a scaly covering consisting of an
amber-like material which is the basis of shellac varnish. The secretion forms a
kind of cocoon, and as the separate exudations coalesce, a continuous layer is
deposited over the twig. This resinous secretion is collected in June and November
and is dried and ground. It is then washed free of coloring matter and is filtered
and dried (quickly, for it grows dark on exposure. The lac is graded as to size and
the largest particles, seed-lac,’ go to make the best grade of shellac varnish (see
Shellac). Lac, dyed yellow, is used in the East Indies for ornaments. It is an
important ingredient in sealing-wax. At present, the largest portion of the total
lac production is taken by the electrical and gramophone industries.

Lacquer (see also Resins) is a broadly used term. It has been applied to paints
or varnishes that dry with a high gloss. It has been applied, also, to coatings made
from shellac. In modern commercial usage it indicates a coating material that
dries by evaporation of the solvent. Coatings made from synthetic resins and
cellulose derivatives are commonly known in the trade as lacquers. Such coatings
are usually clear but they may be pigmented or dyed.

More strictly, and in the fine arts, ‘ lacquer ’ is used for a natural resinous
exudation from the tree, Rhus Vernicifera DC. In China, where the art of lac¬
quering originated, the tree is called Tsichou (varnish tree) and in Japan, where
the art was imported and reached its highest development, it is known as Urushi
No-Ki. There are other lacquers from Burma, Indo-China, and Formosa. These
are like the Chinese and Japanese variety, though Burmese lacquer dries more
slowly and is said to be free from the irritating effect on the skin, which is the
European worker’s objection to the Japanese product. The tree, indigenous to
China and cultivated in Japan at least since the VI century, is tapped when
about ten years old, horizontal incisions being made in the bark. The resin is a

32

Painting Materials

milky liquid which rapidly thickens and darkens on exposure to air. It can be
kept in closed containers without change for a considerable length of time. On
standing, it separates into two layers, the top, of superior quality, being filtered,
slightly heated, and thinned before it is used.

Secrecy surrounded the methods of making and working lacquer, and it-is only
recently that its composition and the art of its manufacture and application have
been generally known and studied. When used as a furniture varnish, it can be
applied in extremely thin coats. It has the unique quality of attaining extreme
hardness in the presence of moisture. It does not become brittle, and can be highly
polished. It affords permanent protection as it is unaffected by acids, alkalis,
alcohol, or heat up to about i6o° C. It may be dried either in a moist chamber at
temperatures below 2 o° C., which takes several days, or at ioo° to aoo° C,, which
takes only a few hours. Moisture and temperature are both important. In drying
at low temperatures, evaporation is increased with the amount of moisture present.
The loss of weight from evaporation occurs before the film begins to set, and then
an increase in weight, from oxidation, is noticed. As moisture accelerates the
drying process, it has been suggested that it acts as an oxygen carrier and also
provides a suitable medium for the action of the oxidase enzyme, " laccase.' At a
temperature above 63° C., laccase ceases to be active, and the drying occurs by
oxidation. When lacquer is mixed with Chinese wood oil (80 per cent lacquer to 20
per cent bodied oil) it dries more slowly than lacquer alone, but is satisfactory,
for the oil gives the mixture a high gloss, and the lacquer prevents the oil from
forming a wrinkled film.

According to the findings of Majima and Tahara (see Barry, pp. 141-142),
lacquer contains 90 per cent urushiol and 10 per cent hydro-urushiol. Chinese
lacquer was found by Majima to have a lower percentage, possibly because of
inferior methods of cultivation or of climatic differences. Yoshida lists the com¬
position as follows:

Urushic acid..85.15 per cent

Gum . ... 3.15 “

Nitrogenous matter. 2.28 ‘‘

Volatile and water. 9.42

Its density is 1.002. Tschirch, in his investigations, found that an oily, non-volatile
substance, which was soluble in petroleum ether, was responsible for the skin
poisoning which affects lacquer workers.

Lanolin (see Wool Wax).

^ Linoxyn (see also Oils and Fats) is oxidized linseed oil. This comprises a col¬
loidal system which is by no means a simple mixture of organic compounds. In
its simplest terms it must be looked upon as a conglomerate of unsaturated
glycerides which have partially or completely undergone addition of a molecule
of oxygen at each double bond, together with the unsaturated glycerides which

Mediums and Adhesives

33

have not added oxygenj any saturated glycerides originally present in the olh
and small amounts of other compounds formed by slight decomposition and
further oxidation of the first oxygen-addition product. At presentj it is widely
held that linoxyn consistSj physically, of a continuous lattice-work of oxidized
glyceride molecules enclosing the liquid (unchanged) glycerides, the whole forming
a perfectly homogeneous, solid jelly (Hilditch, p. 391).

Linseed Oil, the most important of the vegetable drying oils, is obtained from
the seeds of the flax {Linum usitatisstmurn)^ the same plant that furnishes linen
fibre. The content of oil varies with the source and the season. An average is 35
to 40 per cent. The oil is obtained almost entirely by expression rather than by
solvents, for the oil cake is of great value in cattle-feeding. Cold expression yields
the better oil. This is edible, has a pleasant flavor, and a bright, golden yellow
color. Most of the oil in commerce, however, is hot-pressed. This method yields
a light brown oil which is slightly turbid, owing to albuminous and extractive
substances and to moisture. Artists’ oils are generally obtained by cold expression.
Linseed oil may be refined chemically, by washing, or by simply allowing impuri¬
ties to settle. The three standard methods of chemical refining are; (i) with
concentrated sulphuric acid, (2) with alkali (sodium hydroxide or carbonate), and
(3) with brine (see Oils, refining). The alkali refined oils are more desirable and
more expensive. If an oil is used as a grinding medium, free fatty acid should be
present, for the neutral oils wet pigments with difficulty (see Oils, relation to
pigments). Artists’ oils are usually bleached by exposure to sunlight. Doerner
(p. loi) objects to bleaching, for he says that it does not last and that it is better
if the inevitable yellow tone is taken into account from the outset. Further
bleaching can be obtained by chemical means.

Linseed oil has a faint but distinctive odor. Its iodine value (see Oils and
Fats) varies from 170 to 195, being highest in Baltic oil which is from the purest
seeds. The iodine number in this is the highest of those of known fatty oils, except
perilla. The chemical composition of linseed oil, like that of the other vegetable
drying oils, is not fully established. It is essentially composed of mixed triglyc¬
erides of linolenic, linoleic, oleic, and stearic acids with small amounts of other
acids, e.g.y palmitic. It also contains approximately i to 1.5 per cent of materials
classified as unsaponifiable matter. Proportions of the four main types of acids
vary according to origin and to conditions of growth. Long (' Drying Oils ’) gives
the following percentages in North American and South American seeds: saturated
acids (stearic and palmitic), 4.8 to 9.0 per cent; oleic, 13.2 to 16.0 per cent;
linoleic, 37.9 to 45.0 per cent; linolenic, 36.4 to 40.3 per cent; unsaponifiable
matter, 1.05 to 1.4 per cent. Linseed oil is the one commonly used for grinding oil
colors, as an additional painting medium, and as an ingredient in emulsions and
in varnishes.

Manilla (see also Copal). This name came into use because the Philippines
was the first principal region to ship copal resin to the European varnish manu-

Painting Materials

facturers. Now oniy a small amount is collected in these islands. The bulk of it
comes from the Dutch East Indies, more than 75 Pft cent being estimated as
shipped from Macassar. It passes through many hands before its final shipment,
and it is probably adulterated to a considerable degree. This would accourit for
the many different properties ascribed by authorities to Manilla copal and the
difficulties in tracing a sample’s origin. The wide varieties of commercial grades
come from the tree, Agathis alba (Lam.) Foxw., growing to a height of two hun¬
dred feet, nearly that of the Kauri pine of New Zealand. Manilla is a soft copal,
and makes a less durable varnish than the hard varieties.

Mastic (see also Resins and Varnish) is a resin used chiefly in varnishes for oil
paintings. It comes from the tree, Pistacia Lentiscus, which grows in the Greek
Archipelago. It is also found in Portugal, Morocco, and the Canary Islands,_but,
since the days of Dioscorides, commercial production has been almost exclusively
confined to the island of Chios. The trees are small and the resin, contained in the
bark rather than in the wood, is collected from numerous vertical incisions. Mastic
appears commercially in small, pea-like, transparent tears, of a pale sti aw color.
It is very fragile, and has an aromatic odor. Its melting point is very low (95 C.),
its specific gravity is 1.074, and, besides resinous constituents, it has a small
quantity of essential oil and moisture. It is almost entirely soluble in alcohol,
ether, chloroform, and ethereal oils, but the greater part is insoluble in petroleum

spirit. . . . r

Mastic, as well as dammar, may be used as a varnish ingredient, for certain

purposes as a paint medium, and as an addition to oil colors. The varnish produced
with mastic is light-colored, glossy, and elastic. It yellows with age, becomes
fragile and fissured, and blooms readily in a moist atmosphere. From old recipes
it seems probable that the varnish most widely used from the IX till the late XV
century was made by dissolving mastic, or both mastic and sandarac, in linseed
oil, often with the addition of a considerable quantity of pica greca, or colophony.
Today mastic is primarily used in spirit varnishes. A mastic-turpentine solution
is most common because it does not dissolve dry paint. To increase its elasticity,
elemi resin, linseed oil, or Venice turpentine is sometimes added (see Megilp).
Doerner (p. 130) recommends the following proportions: for a picture varnish
and painting medium, i part of mastic is dissolved in 3 parts rectified or doubly
rectified oil of turpentine. For a tempera emulsion, the proportions are I : 2. The
commercial mastic varnishes are usually prepared by dissolving the resin in hot
oil of turpentine. They are sticky and very yellow, characteristics which, according
to Doerner, come from the method of preparation as well as from the probable
use of inferior resin. He maintains that cold-prepared varnishes are almost color¬
less and remain so. Mastic and sandarac are easily confused. Mastic, however,
softens when chewed, whereas sandarac powders. The colors, also, are different,
sandarac becoming darker and redder with age.

Mediums and Adhesives

Mat Varnish (see also Resins and Waxes) is usually prepared by adding white
damitr resir""’ turpentine, in the proportions 0/1:3, to mastic or

Medium is the word usually applied to the binding material or vehicle that
holds together pigment particles in paint. The relation of the quantities oTt^e
two principal ingredients of the paint film has been studied, particularly with

Ta d,W f‘ i b-t ia added actuall,

med with tmperr ” «'“»

_ stency, has been used by artists on account of its excellent working qualities
enameUike effect but becomes brittle and yellow with age.
Methacrylate Resins (see Acrylic Resins).

are (see Oils and

hydrocarbons (compounds of carbon and hydrogen) obtained in

the distillation of petroleum Thev ^ ^utamea in

plasticizers for some resins. ‘ pamting, except rarely as

conmb'^f It I T "'^^-P-ih-hle on treatment with caustic alkalis; they
contain no alcohols, but consist entirely of hydrocarbons. They occur in the

earth s crust and may be divided into (i) those obtained by distilktion—paraffin

Montan Wax (see also Waxes) is obtained from lignite or peat by extraction
wi petroleum ether or similar solvents. It is a material of high but variable
me ing point and is largely used for the same purposes as carnauba wax. Samples

ToTs vet beL ill K “ ’ the free acid. The alcoholic components have
carbon .»oun. of hydro-

m/;T/ 7 7L f- the origin of mordant gilding, Thompson

{The Materials of Medieval Paintings p. 202) says:

Quite early in the Middle Ages, certainly before the twelfth century (when docu
mentary evidence first begins to be common), someone found that if he made a
mark with gum or glair on parchment, and clapped a bit of gold leaf on it before
kiTT ®tick, and when it was dry it could be burnished

IdlflT tould write a letter with some sticky material

writing wTgdd ink

36

Painting Materials

The o-um or glair would make a water mordant. Frequently with them an inert
material, like bole or slaked plaster of Paris, was added to give bulk. Later, mix¬
tures of oil and resin with pigment were used for the same purpose (see Thompson,
op. «V., pp-226-228).

Non-Drying Oils (see also Oils and Fats). Vegetable, non-drying oils are
characterized by the preponderance of glycerides of acids absorbing two atoms of
iodine to the practical exclusion of glycerides of acids absorbing 4 or 6 atoms
of iodine. The iodine numbers range from about 100 to 80. These oils thicken at
elevated temperatures but they do not dry to a skin, even on^ long exposure.
Fryer and Weston (pp. 135-143) list the following oils as non-drying: ravison oil,
rape oil, mustard oil, jamba oil, the kernel oils, almond oil, arachis oil, rice oil,
olive oil, and castor oil.

Oils and Fats. Oils and fats belong to the class of chemical compounds known
as esters. They are essentially the glycerol esters of aliphatic acids (see Fatty
Acids) and, to a large extent, acids of the eighteen carbon series. Fats and oils,
however, being natural products, contain varying amounts of impurities, sub¬
stances that occur in the seeds along with the fats; small amounts of these are
squeezed out or extracted along with the fat or oil. Most of such impurities are,
of course, removed during the refining process (see Oils, refining), but small
quantities are retained. The impurities are called ‘ unsaponifiable matter.’ Most
important are phytosterol (C27H45OH) and its isomer, cholesterol. The former
occurs in all oils and fats of vegetable origin; the latter is characteristic of all oils
and fats of animal origin. Chemically, there is no distinction between oils and fats
but, popularly, the term ‘oil’ is used to denote the substances that are liquid at
ordinary temperatures and the term ‘ fat ’ is used to denote those that are semi¬
solid or solid, and typically greasy to the touch.

When oils and fats are treated with steam under pressure, they split up,
glycerine and a mixture of fatty acids are produced, and the elements of water
are absorbed. This process of splitting and absorption of water is termed ‘ hy¬
drolysis ’ (see also Hydrolyzed Oils). There are five principal series of fatty acids:

1. The stearic acid series, all saturated.

2. The oleic acid series, one double bond.

3. The linoleic acid series, two double bonds.

4. The linolenic acid series, three double bonds.

5. The clupanodonic acid series, more than three double bonds.

Practically, the saturated fatty acids are distinguished from the unsaturated by
the fact that the unsaturated acids can combine directly with iodine while the
saturated can not. One molecule of an unsaturated fatty acid is able to combine
directly with 2 , 4, 6 or more atoms of iodine according as it contains I, 2 , 3 or
more double linkages. As oils and fats differ chiefly in the amount of unsaturated
fatty acids which they yield, the most discriminative results are obtained by a
determination of this figure. The iodine value or number is usually the percentage

Mediums and Adhesives

of iodine chloride, calculated as iodine, which is capable of being absorbed by the
substance. ^

The principal acids occurring in fats and oils are:

OCCURRENCE

Widely distributed; probably the most
abundant of the natural fatty acids.
Widely distributed but not present in
large amounts in most fats and oils.
Present in most fats and oils.

Present in most fats and oils.

Present mainly in tung oil.

Present mainly in linseed and perilla oil.

The saturated and unsaturated fatty acids consist of long chains of carbon atoms
strung together with an acidic or carboxyl (-COOH) group at one end of the
molecule. In nearly all cases each carbon atom is attached only to two other
carbon atoms, the remaining valences being satisfied by hydrogen atoms or left
m some cases unsatisfied (ethylenic linkage or double bond). The majority of
the fatty acids are thus ‘ straight-chain,’ aliphatic compounds; either branched-
chain or closed-chain fatty acids are exceptional. Another curious and striking
feature of the natural fatty acids is that, almost, and perhaps entirely, without
exception, they contain an even number of carbon atoms. Since one molecule of
glycerine is combined with three molecules of fatty acid, either simple triglycerides
where all the fatty acid radicals are alike, or mixed triglycerides, consisting of
two or three different fatty acid constituents, are possible. It is unusual to find
any simple triglyceride in a vegetable oil unless a considerable excess of one fatty
acid IS present. Various mixed triglycerides, dilinoleo-linolenin, oleo-linoleo-
linolenin, etc., have been obtained from linseed oil but there is no evidence of the
presence of simple glycerides such as triolein. In the case of tung oil, which con¬
tains 75 to 85 per cent of eleostearic acid, the triglyceride of this acid can be
separated, the remaining acids presumably being present in the form of mixed
glycerides.

The accompanying table gives the source and some of the more important
analytical characteristics of the principal drying oils. The specific gravity is the
ratio of the mass (weight) of the oil to the mass of an equal volume of water at
4 C. The oils are all lighter than water; their specific gravities do not vary widely.
The refractive power of oils varies more and is chiefly governed by the proportion
and degree of unsaturated matter present. Both the saponification number and
the acid number depend on the amount of acid present; the saponification number
IS a measure of the amount of combined acid, i.e., glyceride, and the acid number
is a measure of the amount of free acid. Numerically, the saponification number
IS the number of milligrams of caustic alkali (KOH) required to saponify (see

NAME

FORMULA

DOUBLE BONDS

Palmitic

O16H32O2

0

Stearic

C1SH3SO2

0

Oleic

C18H34O2

I

Linoleic

O18H32O2

2

Eleostearic

C18H32O2

2

Linolenic

O18H30O2

3

38

Painting Materials

Saponified Oils) one gram of the fatty material; and the acid number is the f^^ber
!f milligrams of caustic alkali required exactly to neutralize the
I crram of the material. The meaning of the iodine number is explained above.

^ing process. Drying oils (see Oils and Fats) are suitable vehicles for
pigments and for mixture with resins because of two distinct processes which
Lcur in them when they are exposed to heat or to the action of atmospheric
Tygen: (i) A thickening occurs, and, in certain cases, the oil becomes a jelly
Stion . This is undoubtedly from some kind of association or polymerization
ofthe molecules, and takes place when the oil is heated for a time at a temperature
of about a to” C. (See Polymerized Oil.) (a) When a drying oil is exposed to air
it becomes a solid, rubbery mass or, if exposed in thin layers, it becomes a clear,
hard solid. There are two principal processes recognized in this drying of an
oil namely, oxidation and subsequent polymerization of the oxidation product
to’form aggregates of high molecular weight. The oxidation is effected by the
addition of oxygen to the unsaturated glycerides without any great amount of
molecular disruption. The ‘ drying ’ of oils is imperfectly understood. For linseed
oil, there exists the largest amount of experimental data, but the process for all

‘ drying ’ oils is essentially the same. , , , r

When exposed to air, linseed oil absorbs oxygen, first slowly, then, for a time,
more rapidly. After that, the rate progressively diminishes as the process nears
completion. At ordinary temperatures the period of induction is from one to three
days, and the process is complete in about twenty to thirty days; at loo C. the
whole process occupies only six or seven hours and the induction period is less
than half an hour. If a small percentage (about o.l to 0.3 per cent as metal) of
certain salts such as lead, manganese, or cobalt linoleates or resinates (abietates)
are present in the linseed oil, the oxidation period is greatly shortened; this is
mainly because the induction period is eliminated and oxygen absorption sets m
immediately at its maximum rate. The weight increase of the oil film on drying
is from oxygen absorption less the diminution resulting from the escape of volatile
products of decomposition. The true oxygen absorption is about 28.7 per cent
of the original weight of the oil and the primary process is one of addition of a
molecule of oxygen at each ethylenic linkage or double bond. In the case of trilin-
olein, the first product of the action is apparently a substance of the formula,

[ 0 - 0 ,

I°-°l

(CH,(CH,),- CH • CH ■ CH, ■ CH • CH ■ (CHJrCOO) ,CjH

Trilinolenin yields a similar triperoxidic compound:

^0-0^ jO-O^ j0-0|

(CHs- CHr CH- CH- CHa-CH- CH- CHr CH- CH- (CH2)7COO)3C3H6.

These compounds may be further oxidized and break down into carbon dioxide,
acetic acid, acrolein, and non-volatile oxidation products of lower molecular

SOURCE AND AVERAGE ANALYTICAL CHARACTERISTICS OF THE PRINCIPAL DRYING OILS

Mediums and Adhesives

39

The weight increase of drying linseed oil, as a result of combination with oxygen is shown graphically. In these curves which
have been adapted from A. Eibner, tJberFette 6 /e, p. 30, table i, per cent increase in weight is plotted against drying time
in days. Curve a shows it for linseed oil, cold-pressed and prepared as a film in the dark and weighed in red light. Curve
b is for the same oil prepared and dried in diffuse daylight, i = point of initial set; 2 = tacky stage; 3 = tack-free dryne

40

Painting Materials

weight. In no case, however, is the proportion of non-volatile products very large,
and undoubtedly the constituent to which a paint film owes its peculiar tenacity
and transparency is the addition product of the unsaturated glyceride with oxygen.

Chemically, there seems little doubt that the initial product is of the nature of
an organic peroxide; on the other hand, it seems that the peroxide is only a transi¬
tional phase and does not exist to any degree in the final product. Many schemes
have been put forward in an effort to account for what takes place after the
initial formation of the peroxide. Although there is little or no experimental evi¬
dence for the opinion, the most likely explanation is that rearrangement to a
hydroxyketone takes place, followed by subsequent polymerization:

--CH-CH-

-C(OH)=C(OH)-

-CHOH-CO-.

Studies on the mechanism of the formation of synthetic resins have recently
offered a new approach to the drying phenomenon in oils and natural resins.
Bradley maintains, from theoretical considerations, that the drying of oils is but
a typical manifestation of a more general phenomenon which consists of the
transformation of an organic substance from an essentially linear structure to the
so-called three-dimensional polymeric form (see Synthetic Resins). He has shown
that the oxygen convertibility of drying oils is closely related to their heat con¬
vertibility and that convertibility, in general, is governed by the specific nature
and number of the reactive or functional groups of the oil molecules. The function
of the catalyst lead, manganese, or cobalt linoleates—or resinates) is not
thoroughly understood either. It can be assumed that the metallic driers and the
initial peroxide product, in the absence of the driers, are both simple oxygen
carriers or catalysts for the reaction. It can also be assumed that the ' induction
period,’ in the absence of a drier, represents the time taken to produce sufficient
' peroxide ’-product in equilibrium with the rest of the system to act as the oxida¬
tion catalyst. Long and his associates have found that little oxidation takes place
in a linseed oil film after it has set to a hard gel. The ultimate failure of these films
is not caused by continued oxidation; aging consists of a gradual transition of polar
liquid phase to solid phase of substantially the same ultimate analysis. Embrittle¬
ment and failure of drying oil films is essentially a matter of reduction of the
percentage of liquid phase to low values.

Clewell has recently found that linseed oil surfaces, both dry and liquid, can
be studied by electron diffraction. He has followed polymerized oil through
different stages of drying and finds that the surface structure of a wet film is
completely amorphous. As the oil absorbs oxygen in the drying process, a gradual
orientation of carbon chain molecules, normal to the film surface, occurs. Com¬
plete orientation does not exist until the film has completely dried. Alignment

Mediums and Adhesives

41

is better explained on the basis of a three-dimensional polymerization than by
the simple splitting off of fatty acids.

The oxidized product from linseed oil is known as linoxyn, and this refers more
often than not to the final material obtained. That must be looked upon as
a conglomerate of unsaturated glycerides which have partially or completely
undergone addition of a molecule of oxygen at each double bond, the un-
which have not been attacked, any saturated glycerides
originally present m the oil, and perhaps a small amount of other compounds
formed by slight decomposition and further oxidation of the first oxygen-
addition product. Oxidized linseed oil, or linoxyn, has certain physico-chemical
resemblances to the typical colloid gelatin. For example, the viscosity relationships
of linoxyn and of gelatin are qualitatively similar and both materials swell when
treated with suitable liquids, water in the case of gelatin and hydrocarbon solvents
in the case of linoxyn. It is supposed that linoxyn consists physically of a solid
lattice-work of oxidized glyceride molecules enclosing the liquid (unchanged)
glycerides, the whole forming a perfectly homogeneous, solid jelly. Most drying
oils other than linseed oil do not give so satisfactory a film on exposure to air;
linseed oil itself, if intensively oxidized, becomes thick and finally crumbles into
soft fragments. In these cases, separation of the colloidal solid phase from the
homogeneous medium has taken place, and, instead of a clear, solid film or jelly,
a more or less coagulated and heterogeneous system of colloidal solid interspersed
with clear jelly is produced.

The utility of the oxidized products from various fatty oils differs widely and
bears little relationship to the original s'tate of unsaturation of the oil. Linseed oil
is the most suitable vehicle for paints; perilla oil is more unsaturated but gives an
irregular film; tung oil is equally unsaturated but has more tendency to separate in
the heterogeneous phase, so that the films produced are frequently dull or mat.
Soya bean oil, safflower oil, sunflower-seed oil, and other oils of fairly high iodine
numbers and pronounced drying properties do not yield such satisfactory films as
linseed oil; the products tend to be softer and more gummy. Fish oils oxidize very
readily but the products also tend to be gummy. These variations are doubtless
conditioned to a large extent by the general type of unsaturation present. Thus,
in oils of the poppy-seed class, there is little linolenic acid present, but there is an
abundance of linoleic acid; tung oil contains a linoleic acid (eleostearic acid)
structurally different from the ordinary variety; the fish oils contain only traces
of linolenic acid, as a rule, but do contain fair quantities of oleic and linoleic acids
and marked amounts of unsaturated acids with twenty and twenty-two carbon
atoms and the equivalent of four or five ethylenic linkages. On the other hand,
perilla oil contains more linolenic acid than linseed oil and yet does not give so
satisfactory a product. The most abundant constituent of linseed oil is believed
to be dilinoleo-linolenin (Hilditch, pp. 388-396):

42

Painting Materials

(CHs ■ (CH2) 4 • CH = CH • CH2 • CH = CH • (CH2) 7 • C00)2^^ ^

CHa • CH2 • CH = CH • CH2 • CH = CH ■ CH2 ■ CH = CH(CH2) iCOO'^

Oils, history in painting (see also Resins, history in painting). A great amount
has been written on this subj ect but many problems still remain obscure, and many
questions are still unanswered. For poverty of evidence to the contrary, oil
painting is considered to belong to relatively modern times. Oil has never played
an extensive role in Oriental art, but, before it can be eliminated entirely from the
ancient art of the East, further study must be undertaken. ^

Whenever the early classical writers on history or medicine speak of oils, it is
always with reference to their medicinal, cosmetic, or culinary uses. Dioscorides,
who is supposed to have lived in the age of Augustus, mentions two drying oils,
walnut oil and poppy oil; the use of bruised linseed is recommended medicinally
by Hippocrates and other Greek writers on medicine; the medicinal oils enumer¬
ated by Pliny include walnut oil; he also speaks of the juice of linseed; Galen,
writing in the II century, speaks of the drying properties of linseed and hempseed
oils and he also mentions specifically the expression of walnut oil. The first mention
of a drying oil in connection with works of art is by Aetius, a medical writer of the
V and the beginning of the VI century. He describes the preparation of walnut oil
and says that it is used by gilders and encaustic painters to preserve their work
owing to its property of drying. He mentions linseed oil on the same page in con¬
nection with its medicinal uses, yet he speaks of nut oil as though it were the only
one employed in the arts. Leonardo, writing a thousand years after Aetius, recom¬
mends nut oil, thickened in the sun, as a varnish.

The first descriptions of the preparation of an oil varnish, by dissolving resins
in a drying oil, are found in the Lucca MS., supposed to be of the VIII century,
and in the Mappae Clavicula, of doubtful though early date (see Berger and
Merrifield). It is not until the time of Theophilus, XII century, and of the MS.
of Eraclius, supposed to be of about the same date, that the use of a drying oil as
a paint medium is described. The earliest writers who distinctly mention the
admixture of solid colors with oil for the purpose of painting are thus Eraclius,
Theophilus, Peter de St Audemar, and the unknown author of a similar treatise
preserved in the British Museum (see Merrifield). It has been supposed that both
Eraclius and Theophilus were of some country north of the Alps, but there is
ample evidence that oil painting of a limited kind was practiced in Italy at an early
period. Lorenzo Ghiberti says that Giotto occasionally painted in oil. Again, accord¬
ing to a document found by Vernazzain the archives of Turin, a Florentine painter,
named Giorgio d’Aquila, contemporary with Giotto, was employed in 1325, by
the Duke of Savoy, to paint a chapel at Pinarolo. The artist was furnished with a
large quantity of nut oil for the purpose, but the oil, from some cause or other, did
not answer his purpose, and was sent to the ducal kitchen (Eastlake, I, 46).
In England early documents relate to the use of oil in art. By 1239 it is mentioned

Mediums and Adhesives

4J

in connection with painting. Similar notices appear in account rolls belonging
to the reign of Edward I (1274-1295), and in others dated 1307 (Edward II)
Another series exists in the records of Ely Cathedral, from 1325 to 1351, and a

preserved in accounts belonging to the reign
of Edward III, with regard to the decoration of St Stephen’s chapel (1352 to
1358). Prom a study of such evidence, Eastlake concludes that oil painting was
employed in Germany, France, Italy, and England, during the XIV century, if
not before. He says however (I, 58), that proofs of its having been employed for
pictures in the modern sense of the term, are less distinct, and are not numerous.

M.r of oil in painting, Thompson {The Materials of

P PP* ^5 — ^7) SH.y*S I

Without entering upon the controversial questions of “oil painting” as it is
generally understood, some mention must be made of the uses of drying oils in
medieval painting. Long before the elaboration of the developed techniques of
fifteenth-_ and sixteenth-century oil painting, oils were used in connection with
painting in other media. Transparent colours are much more transparent and rich
in od than in water colour or egg tempera, and a certain amount of oil glazing was
certainly used along with tempera painting from quite early times. We do not
know accurately how much it was used, and we are not always sure of being able
o recognize it in paintings now. One connection in which oil media were quite
regularly employed was the glazing of metal surfaces, particularly red over gold
(as may be seen in the Paolo Uccello Battlepiece in the National Gallery), and over
silver and tin as well, and green too over these metals. We know that from remote
ages oil and varnish glazes of yellow colour were applied to tin to make it look like
gold, to silver also, and even to gold itself, to make it look more like gold. Medieval
examples of glazes of this sort are known, though they are not common until the
Renaissance. The inside of the dome in the central panel of Giovanni Bellini’s
ran Altarpiece was gilded, and then shaded down with rich, warm oil glazes; and
_this_ technical device, which is quite common in Renaissance painting, especially
in sixteenth-century Germany, must look back to the ancient tradition of gold-
coloured lacquering in oil varnishes on metal.

_ How hr oil glazes were used over tempera painting, and how far tempera
painting glazed with oil media was repainted in tempera colours, and what the
materials of these combined operations may have been, are questions still to be
settled. A vast amount of evidence will have to be weighed and sifted laboriously
before any attempt to solve these problems can be regarded as in-any degree
authoritative. The modern tendency is to regard the development of oil techniques
as an evolution of manipulative methods primarily, rather than a sudden adoption
of new materials. We may be quite sure that the “Secret of the Van Eycks” was
not merely something which could be kept in a bottle; but we cannot pretend to
adequate knowledge of the physical elements of Flemish, or, for that matter, of
any niteenth-century oil painting, at the present time.

This agrees with the earlier comment of Eastlake (I, 88) that in about 1400 the
practice of oil pamtmg had been confirmed by the habit of at least two centuries.

44

Painting Materials

He points out its inconvenience, as compared with tempera, for works that
reouired careful design, precision, and completeness, and assumes that the Van
Eycks (traditionally credited with the discovery of oil painting) had aimed to
overcome the stigma attached to it as a process fit only for ordinary purposes and
mechanical decorations.

Laurie thinks that stand oil (see Polymerized Oil) was known and used at the
time of the Van Eycks (see ‘ Notes on the Medium of Flemish Painters ’)• Maroger
and Ruhrmann (Walther Ruhrmann, ‘ Das Bindemittel der alten Meister,’
Technische Mitteilungen fur Malerei, L [i934]3 PP- 43"47> 60-67, 74“7^>

81-84) have both written recendy on the Van Eyck medium and both explain it
as an emulsion.

Whatever may have been the particular developments of the XV century in
the use of oil as a painting medium, it is apparent that during the XVI and XVII
centuries it became the prevalent film material. Then and for some time later it
was ground with pigment in the painter’s workshop. During the XIX century
various means were devised for storing this paint. Bags of skin or small bladders
had already served as containers. Rigid metal tubes with pistons were occasionally
employed for this purpose and then these gave place to the collapsible tube in
which artists’ oil paint is now sold.

Oils, refining. In the commercial manufacture and refining of oils from seeds,
the first process is to clean the seeds themselves, and, with modern devices for
this purpose, foreign material can be reduced to a few tenths of one per cent.
The seed is then ground and expressed, either hot or cold, or the oil is extracted
by suitable solvents. Such oil is always more or less impure and must be further
refined. The impurities present consist of suspended matter, including mucilage,
albumenoid matter, and resinous bodies, which may be dispersed as relatively
coarse matter or in exceedingly fine suspensions of colloidally dispersed material.
The impurities also include natural coloring matter, free fatty acids produced by
hydrolysis, and semi-volatile compounds dissolved in the oil and giving it odor
and taste. There are three principal methods of refining linseed oil: (i) with con¬
centrated sulphuric acid, (2) with alkali (sodium carbonate or caustic soda), and
(3) with brine. In the first, the oil is agitated in lead-lined tanks with approxi¬
mately two per cent of sulphuric acid. The acid dehydrates and coagulates the
albuminous and carbohydrate material which settles out and allows the clear oil
to be drawn off, washed, and dried. The acid value of oil refined in this way is
higher than the acid value of the original oils. This is desirable in those that are
to be used for grinding certain pigments, for the free fatty acids facilitate wetting.
In the second process the oil is agitated with a hot, aqueous solution of sodium
hydroxide or sodium carbonate. The soap formed by the action of the alkali and
the coagulated albuminous and carbohydrate material settles out and the clear
oil is then washed and filtered with the aid of fuller’s earth. If the oil is for paints
or varnishes, only enough alkali is used to neutralize most of the free acidity,
about 0.3 to 0.5 per cent of free fatty acid being left. The alkali process is the

Mediums and Adhesives

45

most costly but yields a product of superior clarity and color. In the third method,
oil is boiled in tanks containing strong brine with a lo per cent solution of crude
aluminum sulphate or with a lo per cent solution of sulphuric acid, and then is
allowed to settle. The settled oil is treated with dry fuller’s earth which adsorbs
any remaining mucilage and also bleaches the oil to some extent. Besides these
methods industrially used, there is an old workshop practice, still somewhat
followed, of washing oil with water. The two fluids are shaken together and when
the water, and any water-soluble impurities it carries, has settled out, it is drained .
off. Frequently, also, the whole container, after shaking, is put in a freezing
temperature and when the water part is frozen, the oil is poured off. Laurie
{The Painter's Methods and Materials, p. 128) describes some of these methods
in detail:

Linseed oil is prepared by grinding, heating and pressing the seeds of the common
cultivated flax. As it comes from the press it requires to be refined. This can be done
in various ways. The oldest and the most satisfactory manner for artists’ purposes
is to expose it to light and air in covered glass vessels. A variation of this method is
to float it on salt water, introducing as well a certain amount of sand. Large glass
flasks are filled one-third of salt water, one-third of oil, and are then loosely corked
and placed outside. Every day for the first two or three weeks the contents are
vigorously shaken up. The oil is then left for a few weeks to clarify and bleach. By
this process mucilaginous and albumenoid substances are removed from the oil,
and the final product, pale and clear, dries quite quickly enough.. . . The same result
is obtained on a large scale by the addition of a small quantity of sulphuric acid,
which chars and removes impurities, and subsequent washing.

Linseed oil can also be bleached by the action of ozonized air. The bleached
oils are almost colorless, but a certain amount of oxygen is inevitably absorbed
by the glycerides during the oxidation of the non-fatty coloring matter to color¬
less derivatives. Linseed oil which has received one or the other of the foregoing
treatments may be designated as refined, pale, or bleached linseed oil, but in the
paint and varnish trades these oils which have not been further treated (see
Polymerized, Boiled, and Blown Oils) are frequently classed as raw linseed oil,
in spite of any preliminary refining.

Oils, relation to pigments. The oil required to give a stiff paste with a pigment,
i.e., when each pigment particle is thoroughly wetted by the liquid, is known
as the oil absorption of the pigment and its numerical value is given by the volume
of a standard oil (linseed) required for 100 grams of pigment. There are differences
of opinion on this subject. Thorpe and Whitely (II, 103) say: 'The oil required to
give a stiff paste with a pigment depends chiefly on the specific gravity of the pig¬
ment and, also, on its physical condition, e.g., the shape and size of its particles.’
According to Gardner and Levy (p. 531), ‘ The amount of oil required for pigment
saturation or wetting is directly proportional to the specific surface of the pig¬
ment mass existing at the point of saturation. As the specific surface of the mass
is relative to its degree of particle subdivision or fineness, it also measures to a

46

Painting Materials

great extent the fineness of the pigment. The oil absorption factor being relative
to the surface conditions of the pigment is independent of its chemical composi¬
tion or specific gravity.’ Williamson observes: ^ The^fact that no simple relation¬
ship exists between oil absorption and consistency is explained as follows: Two
factors which act in opposition to each other—degree of wetting and soap
formation—control the values obtained from oil-absorption measurements,
whereas the consistency of the ground pastes is controlled by soap formation
alone.’ By empirical methods Wolff and his collaborators have derived a mathe¬
matical expression for the consistency of a paint as a function of pigment con¬
centration, and from this they calculate the critical oil content of paints. They
have found that this critical point indicates the pigment-fixed binder ratio which
yields the optimum paint properties from the point of view of drying, water
resistance, etc. Elm (Tundamental Studies of Paints’) has studied experiment¬
ally the relationship which exists between the critical point of a paint as deter¬
mined according to Wolff’s method and the durability of the same system on
exterior exposure. Results showed that the critical point falls within the range
of pigment concentrations yielding good durability, and the conclusion that paint
durability is a function of paint consistency seems justified. There are other
factors besides pigment-vehicle ratio which affect the durability of a paint.
J. Schmidt says that for each pigment there is an optimum acid value for the
oil used in grinding and that this affects working quality and permanence.

Oils, yellowing. All fatty oils (see Oils and Fats) have a tendency to yellow
with time; darkness and dampness increase this tendency and it is also acceler¬
ated by certain pigments. It varies with different oils and somewhat, also, with
the particular sample and the treatment it has received. Poppy and walnut oil
have, in general, less tendency to yellow than has linseed oil. Cold-pressed linseed
oil yellows more than oil that has been thickened in the sun, but stand oil (see
Polymerized Oil) is superior to both. Impurities may tend to increase yellowing,
but complete removal of them does not remove the tendency, for pure, synthetic
trilinolenic glyceride turns color badly. Furthermore, the yellowing of an oil is
independent of the free acid content. Various vegetable oils turn yellow, to some
extent, according to degree of unsaturation. Other things being equal, the least
unsaturated oil, that is, the oil with the lowest iodine number, has the best color
retention. Polymerization of oils reduces their iodine numbers and makes them
less susceptible to yellowing. This is especially true if they are heated in an inert
atmosphere, such as nitrogen or carbon dioxide, so as to prevent the formation
of oxidation products. The nature of the chemical change that causes yellowing
is not known. It is well known, however, that drying oils yellow more readily
in the dark than when exposed to light. In fact, films that have become badly
yellowed after storage in the dark may be partially bleached when exposed to
the light. Werthan, Elm, and Wien (p. 775) found that white linseed oil paints
yellowed more in red light than in blue light and, hence, wave-length of light
seems to be a factor.

Mediums and Adhesives

47

Oieo-Resin is a natural combination of resinous substances and essential oils
occurring in or exuding from plants. It is usually a soft semi-liquid in which the
resin is in solution in the essential oil. Deitrich (p. i^) classifies oleo-resins into
four sub-groups: (a) the varnish groups derived mostly from plants of the Anacar-
diaceae family; (b) the copaiba group, sweet-smelling resins similar to the balsams;

(c) the turpentine group, derived from Coniferae which comprise soft resins; and

(d) the elemi group, which are soft resins containing above lo per cent of ethereal
oil. Among the oleo-resins most common in pictorial painting are Venice turpen¬
tine and copaiba, both used in the older practice of picture restoration and in the
compounding of some surface films. The term, oleo-resin, has been used occa¬
sionally to define mktures of drying oil and resin, but the definition given here
is the one common in standard works on the technology of resins. (See Balsam.)

Ozokerite (see also Waxes). The origin of ozokerite is still, like that of petro¬
leum, a matter of controversy. It is regarded by some as an intermediate product
between natural fat and petroleum, but the more common view is that it is an
oxidation and condensation product of petroleum. It is a variable substance. It
may be quite soft or as hard as gypsum. In color, it varies from a light yellow to
a dark, greenish brown. The refined product is known as ceresin.

Paraffin Wax (see also Waxes and Mineral Waxes) is obtained chiefly in the
distillation of shale oil, lignite, and American and East Indian petroleum. It is
a mixture of saturated hydrocarbons of the C7iH2n+2 series. Its melting point has
the wide range, from 35 to 75° C.; it is available on the market in samples melting
at 48 to 50^ 50 to 52°, 52 to 54°, 54 to 56°, 56 to 58°, 58 to 6o^ and 60 to 62° C.
The higher the melting point the harder, the heavier, and the less crystalline is
the material. The softer varieties contain not only lower members of the usual
constituents but also more or less of the liquid members which have not been
removed during the process of manufacture. Commercial paraffin wax is a white
to bluish white, translucent material of lamlno-crystalline structure. It is an
extremely indifferent substance, being attacked only slowly by the strongest re¬
agents. It is freely soluble in mineral oils, ether, and benzene, but is only sparingly
soluble in hot alcohol. Because of its inert nature, it finds many applications in
the arts and industry.

Parchment Size (see also Glue and Tempera) is a nearly pure gelatin'made
from parchment waste. Its preparation is simple: cuttings are soaked in water
until they are soft, and then are heated for to 2 hours in a double boiler with
enough fresh water to cover them. When the size has been taken into solution, the
waste is strained off and the liquor is allowed to cool to a gel or is used while
warm. The jelly that forms can be sliced and allowed to dry. The finished product
may be kept indefinitely and used as desired.

Paste (see Starch and Flour Paste).

Pastel (see also Gums). This kind of painting material is a chalk or crayon
made from pigments and fillers held together in stick form by a weak gum me¬
dium, usually gum tragacanth.

Painting Materials

Periila Oil is obtained from the seeds of the Perilla ocimoideSy an annual plant,
occurring in China, Japan, and the East Indies; it is of the mint family, and is
closely related to the highly colored Coleus seen in gardens. In appearance and
odor, perilla resembles linseed oil. It is highly unsaturated and is characterized
by its iodine number (190 to 205) which is the highest in any of the known oils.
It dries quickly but gives a dried film which is somewhat marred by irregular
markings and spots. During the last few years perilla oil has become of industrial
importance.

Phenol-Formaldehyde Resins (see also Synthetic Resins). Although the phenol-
formaldehyde resins (bakelite) are much used commercially, because of their
yellow to deep brown color they have found little application in the treatment of
works of art. Certain of the phenolic resins are soluble in oil and can be added
in oil varnishes. Stamm and Seborg have recently described how the shrinkage and
swelling of wood may be lessened by impregnating it with a phenol-formaldehyde
mixture and condensing the mixture directly in the cell structure.

Pitch Is a term which is often improperly applied to the resin or crude tur¬
pentine that exudes from pine and fir trees. It may mean the residuum from the
distillation of turpentine (see Turpentine and Colophony), or it may mean asphalt
or bitumen. More broadly, pitch is any tenacious, resinous substance, black or
dark brown in color, which is hard when cold and is a thick, viscid, semi-liquid
substance when heated.

Plasticizers are non-volatile, or little volatile, liquids or solids that are incor¬
porated, usually in small amounts, in a lacquer or varnish and are retained in the
film after escape of volatile solvents for the purpose of keeping the film adhesive,
elastic, and flexible. A plasticizer is a necessary component of most cellulosic
coatings (see Cellulose Acetate and Cellulose Nitrate). Although such natural
products as camphor and castor oil have been used for this purpose, the modern
trend is toward ‘ chemical' plasticizers which are usually high-boiling esters like
dibutyl phthalate and tributyl phosphate. Some of the synthetic resins serve as
plasticizers for cellulose coatings. (See also p. 199.)

Polymerized Oil or Stand Oil (see also Polymerized Resins). If air is excluded,
most oils can be heated to a temperature of about 250° C. without undergoing
any appreciable chemical change. Some oils {e.g.y linseed oil) become pale in
consequence of the destruction of the dissolved coloring matter. W^hen heated
above 250^ and up to 300° most drying oils (see Oils and Fats) undergo a
change which is essentially one of polymerization. The iodine number falls
idly, i.e,y a certain number of the ethylenic linkages become saturated, not by the
addition of hydrogen or oxygen but by polymerization. When the iodine number
has fallen to about 100 ( thin stand oil ’), the density has increased from about
0.935 to 0.966 and the oil has become somewhat more viscous. On further heating
at the same temperature, the iodine number falls lower, though more slowly, the
viscosity increases rapidly, and the oil becomes very thick but remains clear. In

Mediums and Adhesives

49

certain cases (as in tung oil and safflower oil) the oil is converted into a gelatinous
or rubberdike material. The effect of heat is thus to diminish the iodine number
(unsaturation) and to increase the viscosity, but in the absence of air there is no
oxidation. Typical figures for various commercial polymerized oils are given by
Leeds as follows (see Hilditch, p. 400):

Raw oil

PER CENT

LOSS ON

THICKENING

SPECIFIC

GRAVITY

15° c.

0.9321

IODINE

NUMBER

169

SAPONIFICA¬

TION

NUMBER

194.8

Thin oil

3

0.9661 100

196.9

Middle oil

6

0.9721

91

^ 97-5

Strong oil

12

0.9741

86

190.9

Such polymerized oils are often known as ‘ lithographic varnishes.’ Not much is
known of the nature of the constituents of the polymerized oils. There is a little
loss of volatile products of decomposition but the chemical structure remains
essentially the same. During the heating polymerization takes place—molecules
of linseed oil unite to form larger molecules, and these larger molecules are less
liable to the chemical changes that produce yellowing, cracking, and disintegra¬
tion. Such oils dry more slowly but the resulting film is much more durable than
the film of raw or boiled or blown oil, and white lead ground in stand oil yellows
very slightly. From the physical standpoint, mainly in view of the observed
viscosity relationships, it is now thought that, like the oxidation product (linoxyn),
the fully polymerized glyceride must be a solid, colloidal structure and that the
thickened or stand oils are systems in which the colloidal polymerides are dis¬
persed in the unchanged portion of the fatty oil.

Polymerized Resins (see Synthetic Resins and Polymerized Ohs). Polymerized
resins are synthetic resins that are formed from simple, monomeric compounds
by the chemical process of polymerization. Bender, Wakefield, and Hoffman say
(p. 125): ‘ Polymerization is the term we use to denote change of a substance
without loss or gain of material, but generally with a transfer of energy to a less
fusible, less chemically active form of higher average molecular weight.’ The
vinyl, acrylic, and styrene artificial resins are formed by this process. An example
will serve to explain it. The unsaturated, organic compound, styrene, has the
simple formula, CH = CH2. Styrene is a clear, sweet-smelling liquid that boils at

CcHb

146° C. and has a molecular weight of 104. In this form it is known as the ‘ mono¬
mer.’ Under suitable conditions, however, as when warmed or when a catalyst is
added, or even when allowed to stand for a long time, the monomeric units com¬
bine to form solid polystyrene which may consist of thousands of monomeric
units and have a molecular weight estimated to be in the hundreds of thousands
(Ellis, ‘Tailoring the Long Molecule,’ p. 1135). These polymeric molecules are

50

Painting Materials

often termed ‘macromolecules.’ From the work of Staudinger and co-workers
It IS generally agreed that the formation of polystyrene from monomeric styrene
may be represented thus (Ellis, loc. cit .): ^

/CH = CHA . CH-CH2- CH- CH.- CH- CHa- CH- CH^ - - .

VCeHs / CsHs CeHs CsHs CgHs

This type of polymer is known as a linear polymer. Ellis {loc. cit.) says about if
The long Cham of polystyrene is, in effect, a single molecule and the propert es'
of various po ymeric styrenes are a function of the size. Relatively shmt chain!
dissolve readily (in solvents) to yield solutions of low viscosity this^lnw ' ^

IS not affected by heat.’ Under certain conditions orroWrizatiin^ h^

the polymer may form in branched chains or in threldLensionnl ’

The molecular polymeric structure ran Kp ^^olecules.

structure in which the linear chains are fused too- th ° T “^’Pact and complex
formation of a network design. Such polymers oflcyllle T"'r "l'

r«ins built from linear maoromolecules °me mo^e ^sSttWe'for' mSi*')

t^an are those bu.lt from three-dimensional molecules. Ellis (/or, r,V., p"

micelles considered to be elongated valence forces yielding

between the linear chains and sdubiiit, reidtl"" ‘rhlT" i"“,

be ,n random arrangement in solution but they ne™ kse thSntTtf.Tmch"'’'

tion of such plant products Ts'cdlulrae anP'th *”'r *” "**“''*• forma-

Polyii.nction.Ii.y.- p. ="‘>"■003 C- Polymers and

Hymers is that they alL amonAtLr'cTa'teArm

■iepee such mechanical properties as steLn ■ “ “Bo'^oant

and hardness.’ ® elasticity, toughness, pliability,

Polyvinyl Acetate (see Vinyl Resins).

Po yvmy Alcohol (see Vinyl Resins).

Pnl^^J (see Vinyl Resins).

Polyvinylchloride (see Vinyl Resins).
which\s'gmwnTargdyln*h^

The seeds contain 45 to 50 per cent of oil Minor,

light golden yellow in color Znd is the ^wh’ite n P^^^

ot-pressed oil is reddish in color. It can be sun ®°”^®erce; the

r. It can be sun-bleached to a nearly water-dear

Mediums and Adhesives

51

oil, but Doerner (p. 109) recommends using it in its natural state, for he says that
the bleaching does not last. Poppy oil has been known from classical times (see
Oils, history in painting), but it was not until the XVII century that it came,
in Holland, into general use for painting. It is used today in the preparation of
tube paints, especially with the light pigments, because of its pale color. Some
authorities (e.g., Eibner) object to its use because of numerous disadvantages,
chiefly, poor drying. Owing to its high linoleic acid content (its iodine number
is in the neighborhood of 150), thin layers of poppy-seed oil dry, but, linolenic
acid being absent or present only in small amount, the film formed melts at about
100° C., and is softer and more soluble in ether than is a linseed oil film. It does
not yellow much on aging but has a tendency, especially in a closed space, to
resoften (‘ synaeresis ’)• Poppy oil has a greater tendency to crack than has lin¬
seed oil, especially if it is not thoroughly dried or if it is too quickly painted over.
Its properties as a paint oil are improved by polymerization.

Protective Varnish (see also Resins and Waxes). The problem of coating a
paint film in order to seal it from destructive agencies of all kinds is one that has
challenged painters, restorers, and technologists for many centuries. In spite of
innumerable studies and experiments, no single solution has been found, and, in
view of the diversity of paint materials, it is doubtful if any one film substance
could be expected to provide safely for the covering of all pictures. The difficulty
is to find a film material which is highly impermeable, enduring in itself, harmless
in application to paint, capable of safe removal from paint, and possessed of such
optical properties that it does not distort the subtle tone relations of a pictorial
design.

Permeability to moisture has been studied experimentally by Gettens and
Bigelow with concluding evidence in favor of waxes, natural soft resins being
next the waxes in having this property. A comparison of these results with other
properties of a series of film materials used for pictures was recently made by
Stout and Cross. Their observations tended to support a practical suggestion
made by Helmut Ruhemann (see Stout and Cross, p. 249) that a thin coating of
resin followed by a coating of wax be taken as an effective means of protection
for the usual type of European painting. It is an old rule, and one supported by
theoretical studies, that a resinous film should not be put over paint until the
paint has had many months in which to become thoroughly dry.

Pyroxylin (see Cellulose Nitrate).

Resins (see also Synthetic Resins) are secretions or excretions of certain
plants. Most of them are the products of living trees, but some copals are from
trees long dead, and amber is from a plant known only as a fossil. According to
researches by Tschirch, Stock and others (I, 93), nearly all resins and balsams
are formed in special secretory glands. The resin may exude naturally to some
extent onto the surface of the bark, but it is generally collected by wounding the
tree. The trunk is ‘ tapped ’—small incisions, either vertical slashes or triangular

TABULAR COMPARISON OF SOME NATURAL RESINS

Painting Materials

52

PHYSICAL AND CHEMICAL

CHARACTERISTICS

jsqranu

uorq'EogixiodBg

00 0 « ,

C7\ Cl M T 00 M o> CD d

Mrii

0 0 CDmM 3 W^OcS>-<-^

CO Lo 00 ^ tT CS cl 00 OS

loqinnu ppv

00

’^MDCOO ^

coooi~iM •ooMV'sor^htvo

ll!l

vohhO*^ dwCPs.cooOoo

x^pai 3AT:i3-eij3'a;

'O 0 '-o 0 tT h-ovO VO vd

Tj-codO

Vo VO VO VO VO j j Vo VO VO

WMWM MM MMMM

'Do gupppi

’^ 0 Q 0 0 M 0 0 0 0

d VO OO^Odvodvod

CO.M, ClH<<~^H^MMMM

0 ‘ 0 ‘ OOOT^O'^»vovo

VO d oodcor^OOCTscow

d M KH M M MM

SOLUBILITY

([on|o:j)

|c/Dco| |i-M|co^cn'^^”<

stipuscijnx

COCOGOCO COCOOOCOC/DCOCOCO

00

CO

(|ou-Bq;3ra)

COOOOOCQ G0»--'000QC/500C0CO

COpn < PU ^ (U pLi (Li

spuopprp ausjAqtjg;

j fCOl ffiCOlOOCOCOCOlT'

i i 1 00 * (^05

Oh CO

t—1

plipa

s

s

1 AS

PS

AS

PS-I

PS-S

S-PS

I

(JSipa

jaqig

cocococo COCOCOCOCOCOCO^-I

CO Ph Ph fL,

<

jOqOOlB SIIOPOEIQ

1 |W| COCO|»-Ht--.cOCOCO

apijojqo

uoqjBQ

S

S

PS

PS-I

PS

PS

S

PS-S

PS-I

PS-I

SAiosoipD

S

S

PS-I

SS

S

S

(piTBinq)
pqootB lA^riQ

1 j^l |g|CO^“iCOCOCO

(BqqqdEumnaiopad) cncococo coco|cncOHH>MHH

amznse: ^<<<C

(pznsq) auBzuaa;

^COCOCO ^COCOCOCOCOhmm-i

P-) P -1 P-I

loqooiBxAmy

^cocoi cocococococococo

^ ‘ Ph < < < Ph

. ' 00

pqoop lAqt^a

m ^ ^ cpcocpcococococo

cop^ <j P^pqpH

cocococococococo

P^ 1 P-, < P^ < ffn Pq

(-1 ui

Amber

Canada balsam
Colophony (rosin)
Copaiba balsam
(Para)

Ckjpalsi

Congo

Manilla

Sierra Leone
Elemi

Dammar (Batavia)
Mastic

Sandarac

Shellac

The symbols of solubility are as followst S = soluble; AS == almost soluble; PS = partly soluble; SS = slightly soluble or almost insoluble;
PSH ~ partly soluble hot; I — insoluble. The resins and solvents listed are selected because relatively full information is available about them.
The table is not complete and can not be entirely accurate; since the natural resins are not pure substances and, hence, are not easily reproducible,
authorities commonly disagree on their properties. The solubility table has been compiled from several sources, but, chiefly, from A. Tschirch and
E, Stock, Die Harze^ I, no. The data are based upon the crude resin and not upon applied thin films.

Mediums and Adhesives

53

holes, are cut in the bark. In some cases the Incisions pierce to the heart of the
tree. At first, the flow of resin is very small, giving at most, a ‘ tear,’ as in mastic
and sandarac. In the conifers, producing colophony and turpentine, however, the
injury seems to stimulate the growth of the canals and resin flows profusely at
certain seasons.

Resins are considerably modified on exposure to air and light so that the
collected product rapidly changes in consistency and color from its natural state
within the tree. Handling and adulteration often change it still more; a wide
variety of resins, though all from the same species of tree, may be found on
the market. Much early research on the properties of resins has had to be dis¬
carded because the samples could not be accurately traced. This difficulty of
obtaining pure, more or less uniform, samples still presents a problem to inves¬
tigators and accounts for differences in many of their results. Several general
characteristics are found in resins as a whole: they exhibit an amorphous struc¬
ture, rarely crystalline but often glassy; they soften with heat, and finally melt
to a more or less sticky fluid; they burn with a smoky flame. Ail resins are
insoluble in water, but generally they can be fairly easily dissolved in alcohol,
ether, and carbon disulphide. Unlike fats and oils, they do not leave a greasy
mark on paper, and they are resistant to reagents and to decay.

Resins form the basis of all natural varnishes. They may be used in the raw
state fresh from the tree, or in mixtures (see Varnish). With the exception of
amber, the fossil and hard resins called by the generic name ‘ copal ’ come from
Africa, South America, New Zealand, and the East Indies. Amber is found chiefly
on the shores of the Baltic Sea, and in northern Europe. These hard resins are
combined with a drying oil to make oil varnishes, and are extremely strong,
resistant, and usually dark in color. From the East Indies and the eastern and
southern shores of the Mediterranean Sea, come the soft resins, dammar, mastic,
and sandarac. These are the spirit-varnish resins, the resin or a combination of
resins being dissolved usually in turpentine, and they constitute the most gen¬
erally used picture varnishes. The balsams or oleo-resins come from coniferous
trees widely distributed throughout Europe and America. Lacquer is the exudate
of a tree indigenous to' China and Japan. Shellac, the most common resin in
commercial varnishes for woodwork, is not a product of a particular tree, but
is the excretion of an insect feeding and breeding on a variety of trees in India,
Siam, and Burma.

Chemically, the natural resins differ from one another very widely, and
inquiry has not been carried far enough yet to make possible a satisfectory
arrangement of the resins from a chemical viewpoint. Barry (p. 2.5) classifies the
essential constituents as;

54

Painting Materials

I* Aromatic acids.

2. Aliphatic acids.

3. Resinols and resino-tannols. (Alcoholic Constituents.)

4. Resin acids. (Acidic Constituents.)

5. Resenes,

6. Essential oils.

Aliphatic and aromatic acids, aside from the characteristic resin acids, are not
common. Acids of the benzoic and cinnamic acid series occur in a few resins but
mainly in those that are of minor importance commercially; succinic acid, which
occurs in amber, seems to be the only aliphatic acid found. The resin acids may
occur free or combined with resinous bodies which Tschirch divides into two
groups: the resino-tannols, which give the tannin reaction, and the resinols,
which do not. The acids which have been most investigated are abietic acid,
C20H30O2, and its congeners. The resenes are inert substances and are very re¬
sistant to chemical reagents. They appear to be of an aromatic nature. In some
cases, the oxygen content is so small that they might be hydrocarbons of high
molecular weight. Contrary to expectation, their chemical inactivity seems to
have no direct relation to the hardness and durability of varnish films. Zanzibar
copal, for instance, a very hard resin, has 10 per cent resene; dammar, of medium
hardness, has 60 per cent; and colophony, a very soft resin, also has 10 per cent.
Two investigators, Haber and Von Weiman (see Barry, p. 29), agree that, as the
resenes interfere with crystallization, they tend to keep the resins in an amorphous
condition.

The amount of essential oil in resin varies. The considerable amount present
in the fresh exudation from the tree either evaporates or changes chemically on
exposure to air. Thus, the balsams are practically solutions of resin in essential
oil, the elemis and soft copals contain sufficient oil to make a pasty consistency,
and most of the fossil resins contain only an extremely small amount. Following
is an arrangement of some of the natural resins according to hardness.

COPALS—OIL VARNISH RESINS

Hard

Medium

Soft

Zanzibar

Sierra Leone

Kauri

Manilla

SPIRIT VARNISH RESINS

Dammar

Sandarac

Shellac

Mastic

Colophony

Resins do not have definite melting points because the fusing is a continuous
process and may extend over a considerable range of temperature. Hard resins,
particularly, have a long transition stage, the ^ upper melting point * being reached

Mediums and Adhesives

55

as the resin decomposes and an oil distills off. As samples of the same variety of
resin may give different results, the melting point can not be used to identify

a particular resin.

When exposed to ultra-violet light, resins tend to decompose and to yellow.
Generally, the natural gums and resins are said to be less sensitive to ultra-violet
rays and to be more transparent, especially in the shorter wave-lengths, than are
synthetic resins. The relatively transparent resins remain stable under ultra¬
violet radiation, Kauri being found the most stable.

From x-ray diffraction studies resins have been found to be amorphous and
to give patterns that are similar to those obtained from liquids. The intermolecu-
lar spaces are larger in the soft resins than in the hard ones. It is possible that
the hardness of resins like Zanzibar copal and Congo copal depends upon the
strength of the forces preventing the larger particles from becoming scattered.
This theory would also explain the wide variations in solubility of different resins.
Kauri, for instance, easily dissolves in drying oils, and shows a weak inner ring
which contracts and becomes diffuse when the resin has been melted; Zanzibar
copal, which is soluble only with difficulty, shows a sharply defined, inner halo.

The solubility of a particular resin varies with its age and with its handling
after collection. In general, the hard resins are most difficult to dissolve. No
resins are soluble in water. All may be classified according to solubility: those
which are completely soluble in one or more organic liquids, and those which are
partially or completely insoluble. The balsam resins, such as colophony, are
entirely soluble in alcohol. The soft resins, such as dammar, mastic, and sandarac,
are more or less readily dissolved in alcohol; but the fossil resins, amber and
copal, are soluble only with difficulty, though more readily so if they are first
melted. Prolonged contact of the resin with the solvent results in a dispersion
rather than in a true solution. Some of the hard resins, however, may be com¬
pletely dissolved in alcohol on standing for several weeks. It does not necessarily
follow that, upon evaporation of a dissolved resin, a satisfactory varnish film
results. Often an uneven and opaque mass is obtained, as many resins are pre¬
cipitated when a certain concentration is reached.

There appears to be little available information concerning the autoxidation
of resins. No doubt oxidation plays an important part in the drying and hardening
of oleo-resins and balsams, and autoxidation may take place, to a limited extent,
in the drying of soft resins. Tschirch and co-workers (I, 309) have found that
when such resins as colophony, mastic, and sandarac are powdered and exposed
freely to the air there is a definite amount of oxygen absorption which is accom¬
panied by a lessened solubility of the resins in petroleum ether. The aging,
embrittlement, and final destruction of soft resin films, however, do not appear to
be caused primarily by autoxidation but rather to be the result of molecular
re-arrangement and association and loss of volatile essential oil. Tschirch and
Stock (loc. at., pp. 315-317) report that many of the resins which have been found

56

Painting Materials

with mummies in various Egyptian and Carthaginian sarcophagi are little altered
with respect to chemical or physical behavior. Oxidation^ although it may play
some part in the formation of fossil resins, does not seem to be an important factor
in ultimate disintegration.

Resins, history in painting. Either as natural exudates from the tree or as
fossils mixed with an oil, resins have been used from very ancient times. Laurie
{Materials of the ’Painter's Crafty p. 27) examined a fragment of a coffin of the
XIX Egyptian dynasty, and found a reddish varnish which dissolved easily in
alcohol, leaving the gesso and the black painting beneath unaffected. It was too
transparent to have been a mixture with beeswax, and, as alcohol and petroleum
were unknown at the time of its application, it seems likely to have been a natural,
semi-liquid resin laid on while warm. A solution of resin in oil is said to have
been found in specimens taken from the temple of Jupiter Ammon. As the Egyp¬
tians were unacquainted with linseed oil, the conclusion is that this was oil of
cedar which was used also for embalming. Mummy cases of the first millennium
B.C., examined by Tschirch (see Barry, p. i), revealed the use of mastic, storax,
and Aleppo resin; sandarac was found in Carthaginian varnishes of the same
period. According to Lucas {Antiques: Their Restoration and Preservation^ p. 56),
the black, varnish-like coating on many wooden funerary objects from ancient
Egypt, wrongly termed bitumen, pitch, or tar, is a black resin possibly such as is
found and used in India, China, and Japan today. It was applied directly to
the wood. It dissolves in alcohol and acetone, and, sprayed with these, it will
soften and re-adhere. Laurie thinks that the use of varnish in Egypt was limited
in time to the XIX and XX dynasties (1300 B.C.), being abandoned shortly
after. Two embalming resins, used by the Incas of South America and examined
by Reutter (see Barry, p. i), appear to contain, among other things, Tolu and
Peru balsam, Phoenician mummies show the use of amber.

Jn China, p early as the Chou dynasty (1169 255 B.C.), lacquer, a natural

resin from a living tree, was in use as a carriage varnish. Covers for jars, coated
with red lacquer and belonging to the Han dynasty (266 B.C. to 250 A.D.) have
been found.^The art of applying and carving lacquer reached its height in China
technically in the Ming dynasty (1368 to 1644 A.D.). The Japanese learned the
husbandry of the lacquer tree and the use of the resin from the Chinese. The
earliest specimens of lacquer work in Japan are of the VI century, although it is
mentioned in records which date two centuries earlier. The process of working
lacquer was kept a carefully guarded secret, and it is only recently that investi¬
gators have examined the constituents of lacquer and the methods by which it
was employed (see Lacquer).

SabinTp. 437)mefers to Gulick and Timbs who, writing in 1859, were of the
opinion that varnishe^composed of resins dissolved in oil must have been used

China before the height of Greek art, and they concluded
t the Greeks, also, must have been acquainted with the varnish process. It is

Mediums and Adhesives

57

a natural inferencCj for pitch and crude varnish were used in ship-buildingj and
the Egyptians used pure resins. The only historical reference, however, is made
by Pliny (John, p. 36), who, in speaking of Apelles, tells how that artist covered
his pictures with a layer of ^ atramentum.'' This was evidently some kind of a
varnishing process; the layer was said to be so transparent that, while it did not
affect the colors and served to protect them from dust, it was invisible, except
under close examination. According to Pliny, no one was able to imitate this
process. It is hard to understand why there were difficulties, for, from Pliny^s
intimations, it can be assumed that the varnish was a semi-liquid resin, dissolved
in a little fluid bitumen. He lists resins obtained from many varieties of pine
and others from Syria and Africa, such as mastic and terebinth; but, although
resins dissolved in olive oil were used pharmaceutically, there is no mention of a
varnish being prepared in this way. Varnishes that can be applied with a brush
require the use of a thinner or of a solvent. The solvent power of turpentine was
known as early as 460 B.C. and was referred to by Pliny, but apparently spirits
of turpentine, or other volatile thinners, were not employed in the manufacture
of varnish.

Historical records about resins and their use in varnish making up to the
XVIII century are largely recipes and statements of early writers. These have
to do with oil varnishes, but it is often impossible to know the exact nature of
the resin called for. The term, ‘ amber,’ was used to cover a great variety of hard
resins, and it referred often to their color regardless of their origin. In the same
way, ^ mastic ’ and ' sandarac ’ are used indiscriminately, and other resins are
called by names which are difficult to identify today. Many of the varnish recipes
given in the Lucca MS. in the VIII century and those given during the following
centuries, however, may be interpreted in the light of present knowledge about
the resins probably available at those times. Both mastic and sandarac were
used and many pine balsams were known. Of the hard resins, amber was certainly
obtainable at an early date, and African copals probably could be found in the
markets. Barry (p. 44) quotes Fr, Stuhlman {Deutsch OsL Afrika) as believing
that the trade in copals began as early as the X century when Arabs exchanged
it with the Indians for cotton fabric. From isolated statements it appears to
have been used in the XVI century, but it is not mentioned to any extent until
the latter half of the XVIII century. The Lucca MS. (Berger, III, 15) describes
the preparation of a varnish by dissolving resin in linseed oil, and one recipe,
de lucide ad lucidas^ calls for amber, mastic, three kinds of turpentine, resin,
galbanum, myrrh, two gums, a little linseed oil, and florae fupplu This would
make an extremely stiff varnish which, while still hot, would have to be rubbed
onto the picture. Here there is confusion again about the term, amber. True
amber could only have been dissolved by first fusing the turpentines and by
adding the amber before the oil. Several recipes call for a hard resin, and, although
this is called * glassa ^ or ' karabe,^ and has, therefore, been assumed to be amber,

?8

Painting Materials

it is more likely to have been a hard copal^ possibly indistinguishable from amber
even to the varnish makers. Large quantities of balsams were sometimes used,
as is shown by a recipe consisting of 3 parts Venice turpentine, 3 parts oil, and
I part mastic.

Theophilus, in the early XII century {Schedula diversarum artium) (see Berger,
III, 57), describes the process of varnish making. According to him, the resin was
melted to a clear liquid and was then poured into hot oiL The solution was beaten
and stirred until a drop, after cooling, remained clear and was of the desired
consistency. These same principles are followed today in the manufacture of oil
varnishes. Laurie {Materials oj the Painter's Crafty p. 287) is of the opinion that
the common varnish used from the IX till late in the XV century consisted of a
fairly soluble resin, either sandarac or mastic, or both, dissolved in linseed oil
and having pica greca^ which is now called 'colophony,’ added in considerable
quantities. In the early XVII century, oil gives way as a constituent, and spirit
varnishes, that is, resins dissolved not in oil but in turpentine or natural naphtha,
and later in alcohol, are mentioned. They originated, apparently, in Italy, and
a recipe is given in a treatise supposedly published about 1740 (Barry, p. 4), for
an oil varnish thinned with turpentine.

In the five centuries between Theophilus and Alberti there developed an
appreciation of the different varieties of resins. Matthioli, in his commentary on
Dioscorides published in 1544, makes the observation (Barry, p. 5): . the

juniper produces a resin similar to mastic called (though improperly) san¬
darac. With this resin and linseed oil is prepared liquid varnish, which is used
for giving lustre to pictures, and varnishing iron.’ Pine balsams are frequently
mentioned, and probably the Flemish painters used balsam more than the painters
of Italy where the climate was much drier. De Mayerne (Berger, IV, 185, 259,
and 269), writing in the XVII century, gives recipes used by Flemish painters
and speaks of Venice turpentine as a suitable substance for the preparation of
varnishes, advising that it should be dissolved in spirits of turpentine, with the
occasional addition of sandarac or mastic or a few drops of oil to give it toughness.
Early Flemish paintings have been supposed to owe their durability to the fact
that the vehicle was an oil varnish which fixed the painting without the need of
a further coating. Records of the methods of the Van Eycks (XV century) and
» their followers are so scanty that it is impossible to speak with certainty of the
ingredients they used. Many authorities on this period affirm that the vehicle
was oleo-resinous. Laurie considers it improbable that amber varnish was serL
ouriy employed m painting, although Rembrandt is said, by his contemporaries,
to have used it. It is likely that the so-called amber varnish purchased in shops

wScrlrfos badly"^ ^

Modern literature on resins and their use in painting may be said to start
with Watin, who published The Jrt of the Painter, Gilder and Varnisher in 1778.

Mediums and Adhesives

59

He describes amber and copal varnishes and the use of turpentine as a thinner
(pp. 221^ 240^ and 259). In giving the proportions of oil to resin^ he comments
on the varying effect which different proportions had on the hardness and rate of
drying of the varnish (pp. 232 and 237). In the last century there has been much
investigation, notably by Tschirch and his co-workers, into the nature of specific
resins.

Rosin (see also ColophLony and Balsam) is the residue left after spirits of
turpentine has been distilled off from the balsam collected from pine trees. Rosin
is a resin. The similarity of the two words has caused much confusion, and it is
preferable to call rosin by its other name, colophony.

Rubber, cMorinated. Chlorinated rubber is an odorless, non-inflammable,
yellow-to-whlte solid; it Is soluble in the coal-tar (toluene) and chlorinated ali¬
phatic hydrocarbons (ethylene dichloride), and in these solvents it may be applied
as a coating. ElHs {Chemistry of Synthetic Resins^ p. 116) says that the film is
hard, tough, glossy, and translucent with a light yellow color, and that it exhibits
a high resistance to the action of acids, alkalis, and oxidizing agents. Where
decorative or protective coatings are to be exposed to a very unfavorable environ¬
ment, such as smoke, salt spray, and fumes, or extreme weathering, the use of
chlorinated rubber as a medium or surface film might be considered.

Safflower Oil occurs in the seeds of the Carthamus tinctorius to the extent of
30 to 32 per cent. It is a relatively slow-drying oil with an iodine number of 130
to 147. Polymerized safflower oil (see Polymerized Oils) ia prepared by heating
the oil to about 250° C., a treatment which turns it into a jelly-like mass. In
India this has been prepared by natives for many centuries and is known as
" Roghan ’ or ' Afridi wax.’ (See also OEs and Fats.)

Sandarac (see also Resins) is produced by a coniferous plant, Callitris quadrU
valvisy which grows In Africa on the Mediterranean coast and also in Australia.
It now comes chiefly from Algiers, but in ancient rimes it was apparently ex¬
ported in considerable quantities from Benghazi, then known as Berenice. (See
Varnish.) The resin first called ' sandarac ’ was probably juniper resin, having a
dull reddish color and yielding a dark brown varnish. The term was given, also,
to a variety of red pigments. The present sandarac resin occurs in somewhat
elongated, pale yellow lumps, being dusty on the surface and crumbling into
reddish powder when chewed. It is soft and brittle and similar to mastic. Its
melting point is 135° to 145^ C. and its specific gravity, 1.078 to 1.088. It is soluble
in alcohol and in ether, but is only partially soluble in chloroform, oil of turpen¬
tine, and petroleum ether. Like the hard resins, it is readily soluble in hot tur¬
pentine, if it is first fused; if it is gradually heated with turpentine, it will soften
into a stringy mass without dissolving. With alcohol or turpentine, sandarac
gives a white, hard, spirit varnish, the resulting film being harder than a mastic
film. With age, however, it becomes darker and redder. Sandarac varnish is
used particularly for coating metals, as it gives a lustre when applied thinly. Its

6o

Painting Materials

tendency to brittleness may be reduced by the addition of Venice turpentine,
elemij and other substances. It has been observed that films from its alcohol
solution develop fine, hair-like cracks which give the surface a silky sheen.

Sandarac was commonly used as a varnish ingredient in the Middle Ages.
A recipe is quoted by Laurie {Materials of the Painter's Crafty p. a86) in which
sandarac and pica greca (colophony) are added to an almost equal amount of oil.
This would make an extremely thick varnish, and one that could be used only
if rubbed on while hot or if thinned in some way. In many of these old recipes
the proportion of resin to oil is very high, indicating the use of a soft resin.

Saponified Oils are oils or fats (see Oils and Fats) which have been split up
by the action of an alkali, glycerine and a soap being formed in the process:

(Ci5H3iCOO) 3C3H5 + 3NaOH = 3Ci5H3iCOONa + C3H5(OH)3

tripalmitin sodium soap glycerine

hydroxide

Sarcocolla is a resin from Penaea sarcocolla^ an African Penaeaceae.

Semi-Drying Oils (see also Oils and Fats). Vegetable semi-drying oils are
those containing notable proportions of glycerides of acids which absorb 4
atoms of iodine, and only small amounts (if any) of glycerides of acids absorbing
6 atoms of iodine. These oils have iodine numbers ranging from about lao to
100. They are characterized by the fact that they form a skin when exposed to
the air at somewhat glevated temperatures. Some very slow-drying oils may be
classed by one writer as drying oils and by another as semi-drying oils. Those
classed as vegetable semi-drying oils by Fryer and Weston (pp. I26“i3a) are
maize oil, kapok oil, cotton-seed oil, sesame oil, croton oil, and curcas oil. The
semi-drying oils lend themselves especially to the preparation of blown oils.

Shellac is the resinous secretion of the lac insect (see also Lac and Resins).
After the crude lac is gathered from the tree, it is crushed and graded and the
largest particles, called ^ seed-lac,’ are selected for making the best grade of
shellac varnishes. The lac is heated, squeezed through a cotton bag, and worked
into a^plastic mass ready for stretching. It is stretched, either on rollers or by
hand, into a thin sheet about four feet square. The sheet is slowly cooled and is
broken into the flake-like pieces which appear on the market. Shellac comes
almost entirely from India, although it is also produced in small quantities in
Burma, Indo-China, and Siam. Its constitution has been investigated, but results
vary because the origin of samples is often indefinite or unknown. Tschirch and
Farner (see Barry, p. 258) give the following data on a sample of unknown origin.

Coloring matter..........

Wax.,....

......... 6.5 “ “

fsr^

Moisture............

or** **

Residue.............

Mediums and Adhesives

6i

They found it to consist largely of esters of an acid to which they assigned the
formula, CH3CH2CH2(CHOH)(CH2)7CHOH*COOH, i,e., dihydroxyficocerylic
acid. The melting point is between 77° and 82"^ C. Gardner and Whitmore have
experimented with shellac in different organic solvents. They conclude that
hydroxyl, carboxyl, and carbonyl groups are probably present.

Shellac is a spirit-varnish resin. When it is commercially prepared, 5 to
7 pounds are dissolved in i gallon of alcohol. Often, oleo-resins are added to
increase the elasticity. Sandarac, mastic, and Manilla copal are sometimes mixed
with it, and dragon’s blood or gamboge* is occasionally put in for coloring. Shellac
varnish gives a smooth finish and takes a high polish. The film is tough but not
completely water-resistant. It is used as a primer for wood because it prevents
any resin escaping and affecting the paint film and because it is impervious to
the solvents ordinarily used for fresh oil paint. In restoration, it is sometimes
applied as a weak priming over filling gesso in the losses of a paint film, for it
has the property of being wetted with water and so will take an aqueous medium.
Its color and slow solubility keep it from being much employed in connection
with paint.

Shellac Wax (see also Waxes). The wax content of shellac generally varies
between 3>^ and 6 per cent. Pure shellac wax is of a light, rich, yellow color and
resembles carnauba in strength and hardness. Its melting point lies between 78
and 82"^ C.; its mean iodine number is 9.2. It is marketed in small quantities,
and is usually derived from bleaching processes of the resin (see Shellac). In
such processes, it has been subjected to saponification by alkalis with detriment
to its color, melting point, and hardness.

Siccative or Drier. Any metallic salts or solutions of them which are added to
drying oils for the purpose of accelerating the rate of drying or oxidation go under
this name. Usually they are derived from lead, cobalt, or manganese (see Oils,

drying process). ^ ^ 1 • j 1

Sierra Leone Copal (see also Copal and Resins). TJiis resin is obtained by
tapping the tree, Copaifera guibourtianay which grows in the British colony in
Africa. The quality is more uniform than that of many copals. Tapping is
permitted every three years, and the resin is collected five or six months after the
incisions are made. It is brittle, hard (next in hardness to Zanzibar copal), has
a light yellow color, and produces a pale, durable, and elastic varnish. Formerly,
a fossil variety was found, but now Sierra Leone resin comes from the living tree.

Silica (see Water-Glass).

Silicon Ester, which is usually tetraethyl silicate, is a clear, fluid, organic
compound of silicon. It has been used slightly as a medium for painting. When
exposed to the air in thin films, this ester hydrolyzes with the formation of
colloidal, hydrated silica that is the film material. It is a modern development and
is still in the experimental stage. It is related to water-glass.

62

Painting Materials

Size (see also Gelatin^ Glue, and Parchment Size) is a term frequently applied
to gelatin or to very pure glue. Herringham^ in ' Notes on Mediaeval Methods '
in her translation of the Book of the Art of Cennino Cennini (p. 243) says that,
except in English, there are not distinct words for size and glue, and the word,
‘glue,' is constantly used in translating where ‘size' would be more nearly
correct. Church (p. 63) maintains that the term, ‘ size,' should be synonymous
with ‘ gelatin' derived from skins and bones. It should not be used for ordinary
glue, especially that from cartilages and sinews, which contains chondrin. It
has been a custom, however, to use the term more broadly for various materials,
like starches, gums, and albumen, that are used to stiffen fabrics and to give a
smooth surface to writing paper. In paper manufacture, much ‘ rosin size ' is used
for that purpose. In its broadest sense, the term, ‘ size,' is used to mean any
material that fills or dresses a porous surface. Glue size is frequently used in
preparing wood surfaces for painting. Thompson {The Practice of Tempera
Paintingy pp. 18-20) gives directions for the making and application of gelatin
size in the preparation of a panel for tempera painting.

Skin Glue is impure gelatin prepared from the skins of animals (see also
Gelatin and Glue).

Soap is any metallic salt of a fatty acid. Ordinary soap is the sodium salt but
soaps can also be formed by lead, manganese, cobalt, and other compounds
combining with fats and oils. When a fat or oil combines with a metallic hydroxide
to form a soap, glycerine is set free. (See Saponified Oils and p. 200.)

Sodium Silicate (see Water-Glass).

Soya Bean Oil, The soya bean {Glycine hispida and varieties) is native to
China, Manchuria, and Japan, and the plant is being cultivated in other countries.
The seeds contain about 18 per cent of oil. A typical analysis of soya bean oil
gives 14 per cent of palmitic acid, 26 per cent of oleic acid, 57 per cent of linoleic
acid, and 3 per cent of linolenic acid. It Is a slow-drying oil (its iodine number is in
the neighborhood of 130), and it forms a soft and not very durable film. It is used
in some tube colors to meet the demands of the painter for more slow-drying
colors. (See also Oils and Fats.)

Spermaceti (see also Waxes) is obtained as a solid precipitate from the head
oil of the sperm and bottlenose whales {Physeter and Hyperoodon). It occurs in
glistening, white, crystalline masses and is very brittle.

Spirits of Turpentine (see Turpentine) is the distillate of crude turpentine or
balsam.^ The balsam collected from conifers is allowed to settle and spirits of
turpentine is distilled off, leaving the residue, colophony or rosin. The word,
turpentine,' is now commonly used instead of the longer term.

Spirit Varnish, a resin dissolved in a volatile solvent (see Varnish).

Stod Oil. According to J. G. Bearn {The Chemistry of Paints, Pigments and
Famishes [London: Ernest Benn, Ltd, 1923], p. aa6), ‘ stand oil ’ is derived from
e German word, Standole. This gets its name from the fact that, on standing.

Mediums and Adhesives 63

the mucilage coagulates and separates out from drying oils. (See Oils and Fats
and Polymerized Oil.)

Starches (see also Dextrin and Flour Paste) are carbohydrates occurring in
plants and synthesized by them from carbon dioxide and water by means of
energy derived from sunlight and absorbed by chlorophyll, their green coloring
matter. This process is known as photosynthesis. Starch occurs as white granules
in nearly all plants, the granules from different plants differing in size and shape.
Because of the difference, a microscopic examination will reveal the source of the
starch. The size ranges from a diameter of i/x (/x = i/iooo millimeter) or less to
one of i50jLi, and, although in some starches the granules are nearly all large (^.^.,
canna), and in some nearly all small (<?.^., rice), there are starches consisting of
both large and small granules. The granules of most starches exhibit diversity of
form. They are classified by Eynon and Lane as round, lenticular, elliptical, oval
or ovate, truncated, and polygonal. The percentage of starch present varies from
plant to plant. Rice is about 75 per cent starch; corn, 50 per cent; and potatoes,
20 per cent. Starch is composed of carbon, hydrogen, and oxygen and has the
empirical formula, (C6Hio05)n5 where the exact value of ' ;z ’ is unknown. It is
prepared commercially from potatoes, corn, rice, etc., by the simple, mechanical
process of grinding and washing. When a suspension of impure starch in water is
allowed to settle slowly, the impurities settle out first and can be removed. It
is almost insoluble in cold water but, when a suspension in water is heated, the
granules swell and form a viscous solution which sets to a jelly on cooling. If
sufficiently diluted, it will remain liquid at room temperatures. Dextrin is pre¬
pared by heating dry starch at 200° to 250° C. On hydrolysis in the presence of
dilute hydrochloric acid, starch is converted to the sugar, dextrose (C6H12O6);
in the presence of the enzyme, diastase, it is converted to the sugar, maltose
(C12H22O11). A characteristic and delicate test for starch is the deep blue color which
it gives with iodine.

For the preparation of starch paste the starch from rice, wheat, corn, potatoes,
or arrowroot may be employed. It is mixed with enough cold water to make
a creamy liquid which is then poured into the requisite amount of boiling water.
Starch, being free from nitrogen and sulphur, has no effect on pigments but it
holds poorly on fat grounds and is, therefore, not to be recommended in connec¬
tion with oil colors. Ordinary starch paste is too viscous to be very suitable as a
binding material for pigments.

Since paste contains a very large percentage of water, it tends to soften or to
* wet up ' the material to which it is applied, and to shrink or swell cloth or paper.
The moist, organic film encourages mold growth, and the changing moisture con¬
tent of the atmosphere makes for instability. These disadvantages can, however,
be largely eliminated by proper handling. The paste films become brittle with age,
probably from a gradual loss of moisture and from development of mold and
micro-organisms. An added hygroscopic material would be advantageous if it
did not increase the tendency to mold growth. Stout and Horwitz say:

64

Painting Materials

The aqueous adhesives have not proved themselves inferior. . . . They need
further and more exact investigation, but evidently if their flexibility can be some¬
what increased and if their composition can include a permanently effective fungi-
cide, the traditional use they have had with paper can be confidently kept in
the future.

Soluble starch can be prepared from starch by treating it with a dilute acid
under controlled conditions. It can be prepared, also, by treating starch with
various oxidizing agents, with glycerine, with enzymes, or with an alkali. Accord¬
ing to the conditions, starch may be modified or converted so as to give all degrees
of viscosity or adhesiveness. The mildest treatments give rise to soluble starch'
more complete conversion results in dextrin or sugar (maltose or dextrose). After
the process is completed, the product should be washed and the excess acid or
alkali should be neutralized. The preparations obtained by various methods
are mixtures, in different proportions, of products of the more or less complete
disaggregation of starch. The product dissolves to a clear solution in hot water
and a 2 per cent solution will remain clear or only faintly opalescent for some
dap. Soluble starch can be used in the preliminary priming of canvas instead of
animaj size. According to Church (p. 95), it is also admirably fitted as a binding
material m water color painting, for the various preparations of soluble starch
become, to a high degree, insoluble in cold water after they have dried.

The preparation of starch from cereal grains dates back to antiquity A
description is given by Cato, c 170 B.C., in his treatise on Roman agriculture (see
Walton, p. 236). According to Pliny, the process of extracting starch from grain
was discovered by the inhabitants of the island of Chios. Its history in E^ypt
dates from very early times. Walton (p. 235) says, ‘ Strips of Egyptian papyrus,
some speciinens of which date back to about 3500 B.C., were cemented together
with a starchy adhesive.’ There is, however, doubt as to whether starch was used
Schubarrpapyrus Partington (p. 206) records an observation by

sufficLt althn T T of the plant being

sufficient, although a paste from the sifted crumbs of sour bread, or flour with

tLment Z “ight have been used. It has been reported that a Chinese

of grain is the tm^ld with starch. Paste made from the flour

(Thomp^n,

wiedge^hr^ughr

Stefeochromy (see Water-Glass).

','T “<I OI.0-Re.in) comes largely from

blreSly ob^ ^rt ‘I I' « *

sedimentation before being filtered It diff^ ” partially clarified by

coagulated by magiilsAL hai c°'“ ” tkat it »

y magnesia and has less color. It is much more expensive, however.

65

Mediums and Adhesives

and on this account Venice turpentine has almost completely replaced it in com¬
merce Neither of these oleo-resins is easily obtainable, particularly in an unadul¬
terated state.

Strasbourg turpentine was largely used in the XVI century. According to
Church (p. 65), It IS undoubtedly the olio d’Mezzo mentioned by early Italian
writers. The best quality is said to come from Miespectinata DC., a silver fir grow¬
ing on the Italian side of the Tyrolese Alps. Dissolved in a terpene, this oleo-resin
was used as a varnish for pictures in tempera and oil, affording special protection
to verdigris and to some other easily decomposed pigments. Strasbourg turpentine
contains about 57 per cent of resinous acids (different from those in Venice
turpentine), 28 per cent of terpenes, and 13 per cent of resins.

Styrene Resins (see also Synthetic Resins and Polymerized Resins). A clear
M oriess resin may be made by polymerizing styrene or phenyl ethylene (CsHgCH
CH2) Elhs ( The Newer Chemistry of Coatings,’ p. 129) says that it is
one of the most promising of the plastic materials suitable for coatings. The solid
resin is colorless, like glass, and it is very tough. It is insoluble in alcohol, acetone
petroleum hydrocarbons, and the glycol ethers, but it is soluble in coal-tar hydro-
carbons, chlorinated hydrocarbons, esters, and turpentine. It is also reported
that polystyrene lacquers are quick-drying and that, even when baked for a long
time, the films do not become insoluble. The resin is thermoplastic and softens
above 150° C. It may be plasticized with dibutyl phthalate or with triphenyl

phosphate. The refractive index is 1.50 to 1.75 (Ellis, Chemistry of Synthetic

Restnsy p. 236).

Sunflower Oil is obtained from the common sunflower {Helianthus annuus),
cultivated in Russia, China, Hungary, and India for food. The seeds contain about
50^per cent of a pale yellow, very fluid, slow-drying oil with a pleasant flavor.
It is used in the preparation of oil varnish. It is said to be used in Russia for grind¬
ing colors (Doemer, p. 113). It has an iodine number of about 130 and pronounced
drying properties, but does not yield such a satisfactory film as linseed oil; there
IS a tendency to produce softer, gummier products. (See also Oils and Fats.)

Sun-Tluckened Ofl (see also Blown Oil, Boiled Oil, Oils and Fats, and Poly¬
merized Oil) is oil which has been exposed to the action of light and air in shallow
containers. During exposure it is stirred from time to time to prevent the forma¬
tion of a skin. It thickens in a few days and, when it has acquired the consistency
of honey, it is ready for use. It dries with a gloss and has been used for centuries
as a painting medium and with resin as a varnish. It has already absorbed some
oxygen and dries more quickly than ordinary linseed oil. The old masters further
accelerated drying by putting the oil in leaden vessels. It is probable that no
polymerization takes place on such exposure (Laurie, ‘ Notes on the Medium of

Flemish Painters, p. 125)3 3 .nd sun-thickened oil is more like a blown oil than a
stand oil.

Painting Materials

Surface Film (see also Resins and Synthetic Resins) is sometimes used as a
sliiJy Tore general definition than ‘ varnish ’ of Ae layer or ayers of film
material applied over a completed painting (see also Protective Varnish).

S^S Resins. These are complex, amorphous, organic semi-sohds or
solids^that are made by chemical reactions from a variety of raw materials.
They approximate the natural resins in many physical propemes: lustre,
comparative brittleness at ordinary temperatures, insolubility m water, and
fusibility and plasticity when heated or exposed to heat and pressure. They com¬
monly differ from the natural resins in chemical constitution and in their
Z chLical reagents (see Ellis, Synthetic Resins and Thetr PlasHcs, p. 13). Most of
the synthetic resins that have been developed for commercial purposes are pre¬
pared from unsaturated, organic compounds by the chemical process known as
‘polymerization ’ (see Polymerized Resins) or from oxygen-containing compounds
(particularly hydroxy) by condensation (Ellis, op. cit., p. 27) Some resins are
produced by the combined effect of both polymerization and condensation. In both
these processes, simple and usually liquid substances are changed into solid but
generally plastic substances that are useful for molding or for the preparation of
paints, varnishes, and lacquers. The polyvinyl resins (see Vinyl Resms) , the acrylic
resins (see Acrylic Resins), the styrene resins (see Styrene Resins) and some of
the alkyd resins (see Alkyd Resins) are good examples of linear or additive
polymer resins that dissolve in organic solvents and, hence, have become com¬
mercially practicable for the preparation of colorless lacquers and varnishes.

Many of the most important synthetic resins, historically and^ comrnercially,
are prepared by a chemical process known as condensation. This is similar to
polymerization, except that the primary reaction of simple molecules to form
macromolecules is accompanied by dehydration, and the condensed product is
not a multiple of the molecular weight of the simple starting substance (or
monomer). Condensations may also take place between unlike molecules. In
the formation of some resinous substances, both polymerization and condensa¬
tion are supposed to take place and this process is known as polymerization
condensation/ or ‘ multicondensation/ Although in some cases linear molecules
are supposed to be formed in the early stages of condensation, the products
usually end as three-dimensional molecules and, hence, are usually infusible
and insoluble in organic solvents. The condensation resins are of minor im¬
portance in the preparation of paints, varnishes, and lacquers, but are of major
importance in the preparation of molding compounds. The most important are
made from formaldehyde and phenolic compounds. Bakellte is a typical example.
Certain of the phenolic resins are soluble in oil and can be used to make oil
varnishes directly (Ellis, rit, p. 1141), but most of these products are dark in
color. Light-colored resins from the condensation of urea and formaldehyde are
of commercial importance in the preparation of molding compounds and coating
compositions.

Mediums and Adhesives

67

The manufacture of resins by the synthesis of macromolecules through the
processes of polymerization and condensation is not far removed from the pro¬
cesses that nature uses in building up resins^ drying oils, cellulose, rubber, and
proteins. Carothers Polymers and Polyfunctionality ’) has touched upon the
role of polymerization in the growth of living organisms. He says (p. 42): ^ Reac¬
tions of polymerization appear to be uniquely adapted to the chemistry of vital
growth because they are the only reactions that are capable of indefinite structural
propagation in space.’

The synthetic resins formed by polymerization and condensation appear to
have very good chemical stability. In the process of their formation, all unsatu¬
rated groups are stabilized by the formation of cross linkages. Many of them are
stable, even to strong acids, bases, and oxidizing agents. Among the cellulose
derivatives only cellulose nitrate is known to be unstable and to have poor aging
qualities (see CeEtilose Nitrate). Cellulose acetate, on the other hand, appears
to have very good chemical stability (see CeEulose Acetate). The permanence
of artificial coating materials is usually considered from two points of view: first,
the tendency to turn yellow, and, secondly, the tendency to become brittle with
age. The first of these seems to be associated with ultra-violet light absorption
and the second is attended by complete loss of solvent and plasticizer. The syn¬
thetic resins proper—the vinyl, alkyd, and the methacrylate resins—appear to be
inherently more plastic than the cellulose derivatives. In the technical literature
there has been found no mention of continued polymerization as a factor that
might cause the embrittlement of polymer resins.

Synthetic Resins, physical properties. The usefulness of the synthetic resins
depends upon various physical properties like hardness, clarity, transparency to
ultra-violet light, adhesiveness, refractive index, moisture permeability, and
thermal behavior. Although hardness is not a primary requisite for protective
coatings on works of art, in special cases it may be a desirable property. Kline
and Axilrod have recently made a study of the hardness of clear, synthetic
plastics in connection with their use in aircraft windshields. In their tabulated
results on scratch resistance, an acrylate resin led the list. The vinyl and styrene
resins and cellulose acetate and nitrate gave intermediate results, and were much
alike. Vicker’s indentation-hardness tests placed these materials in different order.
Here a styrene resin led the list and the other synthetic resins were less easily
indented than cellulose derivatives. Synthetic resins and natural resins, like many
other substances, owe their exceptional physical properties of strength, elasticity,
hardness, and plasticity to their peculiar molecular structure.

Cellulose plastic coatings and synthetic resins, in this order, have high moisture
permeability as compared with coatings made from natural resins and waxes.
A study made by Gettens and Bigelow on the moisture permeability of an ex¬
tensive list of materials that have been suggested or used for protective coatings
allows the comparison of the moisture permeability of synthetic resins and plastics

68

Painting Materials

with natural resin and wax coatings. Recent studies of moisture permeability of
coatings usually have been limited to coatings for special purposes, or to the
effect of adding various resins or plasticizers to certain types of coating bases.
The methods of measuring permeability of industrial products have not been
standardized, and so it is impossible to compare the values obtained. Natural
and synthetic resins, when added to cellulosic coatings, usually reduce moisture
permeability, but the addition of liquid plasticizers increases it. There are many
other influences on permeability. Within limits it is held to be inversely propor¬
tional to film thickness, and conditions under which films are applied and dried
no doubt affect it. Solvents retained in the film, as well as liquid plasticizers
that are hygroscopic, appreciably increase moisture permeability. The age of
a lacquer film, up to the point where it begins to break down as a result of natural
causes, would be expected to have an effect on moisture permeability. Wray
and Van Vorst have found that the moisture permeability of cellulose nitrate
and vinyl resin lacquers decreases with age. Their findings were based on measure¬
ments carried out over a period of one year. Kline, who has recently made a study
(p. 236) of synthetic resin finishes for aircraft, believes that water vapor is absorbed
and transmitted through the films by a process of chemical diffusion. He found
that an aircraft finish made with a.cellulose nitrate base is very permeable to water
vapor. He also found (p. 244): ‘ The additional protection afforded by the thin film
of wax (carnauba) against passage of moisture was found to be considerable, being
in some cases equivalent to doubling the thickness of the original film.’ It’seems
that the use of wax coatings may help to overcome one of the inherent disadvantages
of synthetic resin coatings, namely, a high moisture permeability. Where lacquer
films with high moisture impedance are required, the use of chlorinated rubber
coatings (see Rubber, chlorinated) may be considered.

• ^ furnished by the study of Gettens and Bigelow, it has been

impossible to find data by which the moisture permeability of a large number of
synthetic resins may be directly compared. The Hercules Powder Company, in a
ooklet entitled Ethyl Cellulose, furnishes the following comparative figures for
moisture permeability of some plastic materials. These data are in terms of aK
value which represents the grams of water permeated per hour/sq. cm./cm. thick¬
ness of film by a modified Gardner method.

Ethy cellulose... 2.7a X io-«

Cellulose nitrate. 1.68 X io-«

Benzyl cellulose. I_j2 x io-«

Tornesit (chlorinated rubber).'. o.'so X 10-®

necL'latl^oSlTntoa^'"'fJb P'^^poses, it is sometimes

necessary to take into account the refractive index of the coating base Coatinvs

wi high index of refraction sometimes cause changes in value^of colors and^a

Mediums and Adhesives

69

general darkening of surfaces. Among the synthetic resins, there is a wide choice in
respect to refraaive index. Polymerbed vinyl acetate and cellulose acetate have
very low refractive properties. If in viscous solution so that they will not penetrate
deeply, they may be used to coat paper, textiles, and granular surfaces so that
very little darkening or change in value results. The refractive indices of several
clear, synthetic resins and cellulose derivatives are given in the table below. These
data have been collected from various sources: manufacturer’s bulletins and
journal articles.

SYNTHETIC RESINS, INDICES OF REFRACTION

Polyvinyl acetate.

Cellulose acetobutyrate.....

Ethyl cellulose.

Cellulose acetate.

Cellulose acetate—plasticized
Cellulose nitrate—plasticized.

Cellulose nitrate.

Polymethacrylate.

Poly vinylchioracetate.

Glycerol phthalate.

Polystyrene.

n — 1.46-1.47
1.47

1.47

1.48

1.48- 1.51
1.45-1.50
1.51

1.48- 1.52
^•53

1.56-1.58
1.50-1.75

Conspicuous among the properties of many of the synthetic resins is their

behavior when subjected to heat and pressure. On the basis of their response to
heat, they may be divided into two main classes: (a) those that are ‘ thermoplas¬
tic,’ and (b) those that are ‘ thermosetting.’ For molding purposes, the powdered
or granulated resin is placed in a form and when this is heated (usually with
pressure) the resin fuses and flows; when the mold is cooled, it sets to a hard, solid
substance. A thermoplastic resin is permanently plastic when repeatedly heated
and cooled. The thermoplastic resins are, for the most part, linear polymers like
the polyvinyl, polystyrene, polymethacrylate, and alkyd resins. On the other
hand, certain of the synthetic resins, when heated and formed in a mold, set to
hard and permanently infusible compounds. These are called ‘ thermosetting
resins. The resins that belong to this class are three-dimensional polymers, or
condensates, and the phenol-formaldehyde resins are good examples. Houwink has
discussed the colloidal structure of linear and three-dimensional polymers in their
various stages of formation, and he has made a comparison between the thermo¬
plastic and thermosetting resins on the basis of their intermolecular bonds.

Tempera (see also Egg Tempera). The meanings of the word, ‘ tempera,’ as
used to define a painting medium, have been many and have changed from time
to time in the history of the art. As late as the XV century this term probably
included all mediums but, with the gradual prevalence of oil, its limits were
narrowed until it has often meant only a medium prepared from egg. A broader
definition which allows it to include albuminous, gelatinous, and colloidal mate-

70

Painting Materials

rials is also in use. For specification this requires a second ternij and the whole
would be, for example, ^ glue tempera,^ ^ gum tempera,^ or ^ egg tempera."

If glues and gums as well as egg are to be considered as tempera mediums, the
history of this general group would go along with the history of all painting until
the late Middle Ages. Then, although oil came to displace the aqueous materials
in some degree, it did not crowd them out of use altogether. Before the advent of
oil, wax was about the only other medium as such, and fresco gave a place to a
crystalline binder for purposes of wall painting. It can still be said, however, that
the greater part of pictorial painting, the world over, has been done with tempera
—with some kind of aqueous medium.

Recently, studies have shown results which indicate that the fresco method
was used in India as early as the XI or XII century A.D. (see S. Paramasivan,

‘ The Mural Paintings in the Brihadisvara Temple at Tanjore—An Investigation
into the Method," Technical Studiesy V [1937], pp. aai-aqo), but glue was the
general medium, it appears, for panel pictures and illuminations, and also for
painting on walls. In a treatise compiled by Somesvara of the XII century and
containing material corresponding to that of the VII century, a glue made from
buffalo skin is spoken of as " the adamantine medium " and is said to be ‘ uni¬
versally approved for painting " (see Ananda K. Coomaraswamy,' The Technique
and Theory of Indian Painting," Technical StudieSy III [1934], pp. 60-61). Glue
has been the traditional medium also for the painting of China and Japan. Even
for walls, there seems little doubt that pigments were mixed into a thin size.
The nature of the size probably differed from place to place, and included any
of the animal tissues from which it can be made.

With the exception of the salmon egg medium of the Canadian West Coast
Indians (see Egg Tempera), there is little to suggest that egg was used much out¬
side of Europe. Laurie {Materials of the Painteds Crafty pp. ao-21) considers it
quite probable that egg, both the white and the yolk, was used as a medium in
Egypt, but reports one analysis of paint from a tomb of the XIX dynasty in
which the medium was found to be gum. Lucas {Antiques: Their Restoration and
Preservationy ^. 140) says that all Egyptian mural paintings are in tempera, but
does not distinguish the particular kind. In his Ancient Egyptian Materials (p.

149), he had already said that if egg white were ever used, it must have been at a
late period, because the domestic fowl was not indigenous but was an importation.

The complete disappearance of the easel painting and practically of the wall
painting of classical Greece has made the inquiry into the medium of that art
depend on a few isolated references by classical writers. Certainly it appears that
more than one tempera was familiar to them. Laurie {Materials of the Painteds
Crafty p. 67) makes the following translation of a passage in Pliny (XXXV, 26):

Painters put on sandyx as a ground colour; thereafter, laying on purpurissum with

egg, they produce the brilliance of vermilion. If they prefer to produce the brilliance

Mediums and Adhesives

IT-

of purple, they put on caeruleum as the ground colour, and then lay on purpurissum

with egg.

The translator, {pp. cit., pp. 68-69) says that gum, egg, and glue were all at the
command of the classical painter and refers, also, to a mention, in a papyrus of
Thebes, of a medium made of egg and gum to which was added bile, to make the
color flow easily.

The mixture of egg white and gum was evidently common at later times.
Cennino Cennini {c 1400) speaks of it (Thompson, The Craftsman’s Handbook,
p. 102) as a medium for ‘ mosaic gold ’ and it appears in the Naples MS. of the
late XIV century (Thompson and Hamilton). Occasional strange mixtures of
tempera for paint are found in treatises on materials that reflect medieval practice.
Among these are two in the MSS of Jehan le Begue. One (Merrifield, I, 306) is
made of lime, ashes, wax, fish glue, and mastic. Another (Merrifield, I, 316) is
made of pieces of flesh boiled in water with the root of the plant called, ‘ stipatum.’
Gums and glues (including size) must have been used extensively in European
tempera besides their employment in illumination. Gums, particularly, are often
mentioned in the Strasbourg MS. (Berger, III, 155 ff.). Yet it is probable that
egg was the common tempera of European paint during the Middle Ages and the
early Renaissance. It also, either as glair or whole egg, is frequently mentioned in
the Strasbourg MS., a document of the XV century and definitely northern in
origin, and it is clearly the main tempera medium treated by Cennino Cennini,
the chief recorder of Italian practice. For panel painting this writer prescribes the
yolk alone (Thompson, The Craftsman’s Handbook, p. 91): ‘ . . . you must always
temper your colors with yolk of egg, and get them tempered thoroughly—always
as much yolk as the color you are tempering.’

Analysis of paint from a portrait by Holbein (unpublished report by Ruther¬
ford J. Gettens) indicated that the medium was tempera. It is probable, however,
that egg tempera did not long survive in general practice after the XVI century.
By certain slight changes the transparent inks and thin gum temperas used in
connection with writing, drawing, and illuminating were ultimately worked into
a standard paint having a medium of gum with additions of hygroscopic materials.
This paint, called ‘ water color,’ is very common in present-day practice and is
used thinly on a support of paper (see Gums).

Tung Oil, also called ‘ Chinese wood oil,’ is obtained from the seeds of Aleurites
cordata, A. fordii, and A. montana, contained in a nut, and growing in China
and surrounding countries. A similar tree is native to Japan and yields Japanese
tung oil which varies slightly from the Chinese oil and is not so satisfactory. The
seeds contain about 50 per cent of oil. The native method for separating it is very
crude. The seeds are roasted over a naked flame and are ground between stones
before expression. Wooden presses are used. The cold-pressed oil is light in color
and is mainly exported; the hot-pressed oil has a very dark color. The former is

Painting Materials

72

called ' white tung oil ’ and the latter, ‘ black tung oil.’ An experimental cultiva¬
tion of Aleurites fordii in Florida has succeeded, and oil is now being produced
there on a commercial scale. The Florida kernels yield a high quality, very pale oil,
of low acid value (0.3 to 1.4) which is free from the unpleasant odor associated
with the Chinese oil. Tung oil contains a large proportion (75 to 85 per cent) of
eleostearic acid (Ci8ff32C2), a stereo-isomeride of linoieic acid. This acid contains
two unsaturated double bonds and gives the oil its drying property. The rest is
largely olein with some saturated acid. Its average iodine value is 165. Its specific
gravity (about 0.941) is high, and it has, also, a high refractive index (1.522 at
15° C.). Tung oil appears to dry in about two days in moist air, but the resulting
film is always wrinkled or cracked and uneven. In dry air about fourteen to
twenty-one days are required and a smooth, coherent film is obtained. In either
case it takes twenty-one to thirty days for the full gain in weight (12.9 to 13.3
per cent). From this it appears that tung oil is really a slow-drying oil, and that
the rapid rate of drying in moist air is not ' drying ’ in the usual sense {i.e.,
oxidation and polymerization), but is a colloidal change in which moisture acts
as the coagulant (W. Schmidt, ‘ Zum Trockenvergang des Chinesischen Holzol’s,’
Farben-Zeitung, XXIX [1924], pp. 1261-1262). A characteristic property of tung
oil is that it forms a jelly on being heated. If it is ‘ bodied ’ or polymerized (see
Polymerized Oils) by heating at 270° to 290° C. until it thickens, it will dry to a
smooth, glossy film which is highly resistant to the action of water. It is not much
used by artists, although it has various favorable qualities. It is as unsaturated
as linseed oil, but has considerably more tendency to gelatinize or separate in the
heterogeneous phase, so that the films produced are frequently dull or mat. It
also yellows badly and may cause skin diseases. (See also Oils and Fats.)

Turpentine (see also Balsam and Oleo-Resin) is a semi-liquid, natural exudate,
containing terpenes associated with bodies of resinous character, from pine trees
widely distributed throughout the world. The raw or crude product from the tree
is also called balsam, or oleo-resin, and the word, ‘ turpentine,’ is commonly used
for the spirits of turpentine, the distillate of the crude balsam. The residue, after
distillation, is colophony or rosin.

Turpentine is commonly used as a thinner or diluent for oil paints and var¬
nishes. There is no indication that it was employed in this way by the ancients,
although its solvent power was known and referred to by Pliny. Turpentine and
alcohol are now the usual solvents for spirit-varnish resins. It appears that spirit
varnishes were introduced into Italy in the XVI century and were widely used
by the Flemish painters in the XVII century. At that time, also, turpentine was
used to dissolve verdigris and to prepare lake color (see Resins, history in painting).

Varnish (see also Resins, Oils and Fats, and Waxes). A picture varnish is the
protective coating, usually with a resin content, laid over a paint film. The use of
such a coating was probably introduced at a remote time in the history of the art
for the purpose of protecting water-soluble mediums and fugitive pigments and.

Mediums and Adhesives

73

also, to give a uniform surface to the work. As the variety of resins increased,
during antiquity, and as the differences among them were recognized, it became
necessary to make distinctions. At one time amber and other hard resins had been
called ^ herenice' By the XIV century Italian writers referred to resin as vernix
and to the varnish as vernice liquida. It has been suggested that the origin of the
name is based on the story of Queen Berenice of Cyrene whose golden, or amber-
colored hair was accepted by Venus as a thank offering for the safe return of the
Queen’s husband, and was transformed by the goddess into what is now known
as the Milky Way. Laurie offers a less romantic suggestion: that the name came
from that of sandarac resin which was formerly called ‘ berenice^' being exported
from Berenice on the African coast. In any event, the name, varnish, came from
such a word.

There are two kinds of varnish in common use: the simplest consists of a solu¬
tion of a resin in a volatile solvent; the second is made of a resin dissolved in a
drying oil to which a thinner is generally added. The spirit varnishes are of a soft
resin, such as mastic or sandarac, dissolved in turpentine or in alcohol; they are
brittle and not very durable. An oil varnish in modern manufacture is made by
heating a hard resin, such as a copal, and then dissolving it in linseed oil, with or
without tung oil. The harder the resin, the higher is the temperature to which it
must be heated. The mixture is thinned with turpentine or with alcohol. Lead
and manganese compounds are often incorporated in the oil before or after the
addition of the thinner. Such a varnish is apt to be too dark and too insoluble for
use with pictures. Spirit varnishes, being readily soluble in alcohol, are easily
removed from the surface of a painting. Oil varnishes require special methods,
depending on the nature of the resin and on the properties of the ingredients.

Vegetable Oils are oils occurring in the vegetable kingdom (see Oils and Fats).

Vegetable Waxes (see also Waxes and Animal Waxes). These make up a large
class, in which the typical member is carnauba wax. The only other commercially
important member is candelilla. Vegetable waxes are generally found in or upon
the outer skin of leaves, stems, flowers, and fruit, and, also, to some extent, in the
tissues. They contain, as a rule, hydrocarbons of the paraffin series, C„H2n+2}
where ‘ n ’ ranges frequently from about 30 to 60, alcohols of the phytosterol
series, either free or combined with fatty acids, and the higher aliphatic alcohols
of the type of ceryl alcohol, or those of higher carbon content, again either free or
in combination with fatty acids (see Hilditch, p. 127).

Vehicle is a traditional term used interchangeably with ‘ medium ’ as a name
for the film-forming or binding material of paint (see also Meditim).

Venice Turpentine (see also Balsam) . This balsam is produced by the Euro¬
pean larch, Larix decidua^ and is collected chiefly in the Tyrol. It is a viscous,
sticky substance with a characteristic pinaceous odor. It consists of about 63
per cent of resinous acids, 20 per cent of terpenes, and 14 per cent of resins; it is
free from crystals of abietic acid which discolor the common turpentines. Unlike

74

Painting Materials

that of pines, the larch balsam is secreted in the heart of the tree. Therefore, for
tapping, a hole must be bored deep into the tree near the base. There are many
old recipes naming Venice turpentine as a varnish ingredient. De Mayerne, XVII
century (Berger, IV, 185, 2.59, and 269), gives several which call for Venice tur¬
pentine. In one, admixture with sandarac resin is mentioned. Doerner (p. 125)
recommends it as a . . non-yellowing painting medium where an enamel-like
effect of the colors is desirable/ It must be sparingly used, however, especially with
dark, slow-drying colors, or it will dry poorly and give an unpleasant gloss. It is
not recommended as a finishing varnish.

Vinyl Acetate (see Vinyl Resins).

Vinyl Ester (see Vinyl Resins).

Vinyl Resins (see also Synthetic Resins). The so-called vinyl resins may be
considered as the polymerized derivatives of vinyl alcohol, CH2 = CHOH. Al¬
though polymerized vinyl alcohol has been produced commercially and is a
water-soluble, film-forming substance, it is not so important as the polyvinyl
chlorides or acetates, particularly the latter. Although the vinyl halides have
been known for about one hundred years, it is said that the acetate was first dis¬
covered by Klatte in 1912 (German patent, no. 281,687). Since about 1930 the
vinyl resins have acquired a considerable industrial importance because certain
ones can produce protective coatings which are colorless when applied and which
are free from after-yellowing. It is claimed (Ellis, Chemistry of Synthetic Resins^
p. 1025) that coatings of the acetates and chloracetates are transparent to ultra¬
violet light. Their acid number is usually rated as zero. They are used in the
preparation of thermoplastic molding compounds as well as in coatings. Vinyl
acetate is synthesized from acetylene and acetic acid; some mercury salt (as
mercurous sulphate) is employed as a catalyst. The monomer is a mobile, color¬
less, ethereal-smelling liquid, which boils at 73° C. Polymers of varying viscosity
can be made, depending upon the concentration of the catalyst. Polyvinyl acetate
now comes on the market in coarse, granular form (shavings). Since it has a high
cold-flow it Is difficult, in warm weather, to keep it from caking; hence, it should
be packed in small containers, and stored at as low a temperature as possible.

The vinyl acetate resin is a colorless solid, horny, and a little rubbery, but not
brittle. It is soluble in alcohols, ketones, esters, and chlorinated and aromatic
hydrocarbons, but is insoluble in aliphatic hydrocarbons (petroleum naphthas)
and water. It softens at 30^ to 40° C,; it has outstanding light and heat stability.
According to Curme and Douglas (p, 1124), it has a relatively high water absorp¬
tion 3 to 5 per cent in 16 hours at 60° C. Coatings may be made by dissolving
the solid vinyl acetate resin in various combinations of organic solvents mentioned
above. Since the vinyl polymer resins have a high solubility in some of the sol¬
vents, it is possible to prepare lacquers that have a high solids content. Although
polyvinyl acetate resin is insoluble in water, it does swell when immersed in it
and becomes leathery and pliable when a large amount of water has been ab-

Mediums and Adhesives

75

sorbed. The surface, however, is unaffected after drying. The dried film is quite
permeable to water vapor. The water absorption (A.S.T.M.) is given as 2 per
cent in 144 hours. A noticeable property of polymerized vinyl acetate resin is
its low refractive index, 1.4665. This property makes it valuable as a fixative for
granular surfaces, since it does not cause so much darkening as do films with
higher refractive index. Polymerized vinyl acetate can now be obtained in
different viscosities. Where penetration is desired, a low-viscosity resin should
be used. Since films of vinyl acetate are naturally quite flexible, it is not necessary
to incorporate plasticizers except for special purposes. If necessary, any of the
common plasticizers, like the phthalates, abietates, or glycollates can be used;
in general, much less plasticizer is required to impart a definite degree of flexi¬
bility to these films than is required for cellulose nitrate. Vinyl acetate polymers
have conspicuous adhesive properties when used with a wide variety of materials
—cloth, paper, porcelain, metal, stone, leather, and wood.

Polyvinyl chloride is an odorless, chemically inert, and difficultly thermo¬
plastic, synthetic resin. It has a more limited solubility than do the polyvinyl
acetates. It is partially soluble in acetone, but is insoluble in most ketones, is
resistant to acids, alcohols, and water, and is soluble in dioxan, ethylene dichloride,
and chlorobenzene. The refractive index at 20° C. is 1.544. The heat and light
stability of this resin is poor (see Curme and Douglas, p. 1124), It is brittle;
hence, when used industrially, it is always plasticized.

A synthetic resin that is in many respects superior to straight polyvinyl ace¬
tate or polyvinyl chloride is one that is produced by the co-polymerization of the
two. This is not just a physical mixture but is a product formed by simultaneous
polymerization of the monomers. Those now in commercial production contain
65 to 80 per cent vinyl chloride. The polyvinyl chloride in the co-polymer may be
regarded as being internally plasticized by polyvinyl acetate. Curme and Douglas
(loc. cit.) have described many of the properties and uses of these vinyl co¬
polymers. They are colorless, tasteless, odorless, non-toxic, and permanently
thermoplastic compounds. Their chief advantage over the polyvinyl acetates lies
in the fact that they are not swelled by water. Moreover, they are stable to
alcohol and to petroleum naphthas. They are soluble only in ketones and related
compounds, esters, and chlorinated compounds, but they only swell and dissolve
slightly in aromatic hydrocarbons. Heat and light stability are not quite so good
as in straight polyvin^d acetate, but are much better than in the chloride. The
vinyl acetate-chloride co-polymer darkens on prolonged exposure to heat or
direct sunlight, but may be improved by the use of small amounts, usually i or
2 per cent, of stabilizers such as the commonly used lead pigments or lead stearate,
lead oleate, or slaked lime. The co-polymers are marketed as white, fluffy powders
which may be dissolved in mixtures of the solvents mentioned above, and the
solutions can be diluted with cheap aromatic hydrocarbons like toluene. The
refractive index of the vinyl co-polymer is 1.53.

Painting Materials

A useful synthetic resin can be made by replacing part of the acetate groups
of polyvinyl acetate by acetaldehyde. It is known as polyvinyl acetal Elliot
{loc. cit.) describes it and says that protective films made from it have greater
toughness, hardness, and adherence than polyvinyl acetate, but that resistance
to weathering is not so good. Woodbury has described the use of this resin in an
anthropological museum for impregnating decayed and brittle bone specimens.
The polyvinyl acetals so far produced have a pale straw color in solution.

Polyvinyl alcohol is of some interest because it is soluble in water to the
extent of forming a ao per cent solution, but it is insoluble in most organic sol¬
vents. In water the polyvinyl alcohol behaves as a reversible colloid. Solutions
are similar to those of dextrin, albumen, and gum arabic. Its film-forming proper¬
ties seem to be as good as those of the polyvinyl esters. It can be used as a water-
color medium, and, in aqueous solutions, as a sizing agent for fabrics (see Ellis,
Chemistry of Synthetic Resins^ p. 1058).

Because of their recent development, the vinyl resins have had a small place
in painting. Herbert E. Ives and W. J. Clarke, however (‘ The Use of Polymerized
Vinyl Acetate as an Artist's Medium,’ Technical Studies^ IV [1935], pp. 36-41),
have suggested in detail the way to use polyvinyl acetate as a medium; Stout
and Gettens have described its use, also, for the treatment and transfer of Oriental
wall paintings.

Viscose (see also Cellulose Coatings). Viscose is a regenerated cellulose now
widely used in the making of rayon and transparent wrapping films (cellophane).
It is made by treating sulphite wood-pulp, first with sodium hydroxide and then
with carbon disulphide. The product, cellulose xanthate (a yellow solid), is a
definite though unstable compound, is soluble In caustic soda (viscose proper),
and from this caustic solution cellulose can be precipitated rapidly by an acid
in a setting bath. By forcing the viscose through spinnerets directly into the acid
bath, viscose rayon threads are formed. Transparent films (cellophane) are made
by extruding a viscose solution into a long, narrow slit in a setting bath. After
passing through baths to neutralize the acid, it passes through a glycerine solu¬
tion from which it takes up 17 per cent. The process is continuous and a sheet
of any length can be produced. Riegel says (p. 347) that cellophane is made
moisture-proof and waterproof by passing the sheet, after the glycerine bath,
through a dilute lacquer solution with rather volatile solvents such as ethyl
acetate; after passing through driers, the lacquer, with the proper amount of
plasticizer, remains as a coating.

This regenerated cellulose Is not soluble in organic solvents. Wire netting
and fabrics, however, can be impregnated by precipitating the viscose directly
upon them. Heaton (p. 426) says that it has been suggested that the glue sizing
of artists canvas be replaced by a viscose sizing since a canvas treated this
way would be less likely to mold. For application, the canvas is first impregnated

Mediums and Adhesives

11

with a solution of viscose, then immersed in an acid bath, when the fabric becomes
cemented together with regenerated cellulose.

Walnut Oil is expressed from the nuts of the walnut tree {Juglans regia) which
contain about 65 per cent. The best is from cold expression which yields a pale-
colored oil with an agreeable taste and the odor of walnuts. The hot-pressed oil
is greenish in color and has an acrid taste. It may also be separated by boiling
the kernels in water, a process recommended by Leonardo da Vinci. Nut oil has
an iodine number of 140 to 150 and dries more slowly than linseed oil but does
not turn yellow so readily. Laurie {The Painter's Methods and Materials^ p. 133)
says that it dries very slowly, but, if exposed to light or air over water, it can be
obtained very pale in color and will then dry quite as quickly as the linseed oil
prepared for painters’ use. It is said to have less tendency to crack than linseed
oil. As early as the V century it was recommended by Aetius for varnishing wax
pictures and gilt surfaces. Its use was formerly more widespread than it is today.
It is recommended for all light pigments in the treatises on painting by Leonardo
da Vinci, Vasari, Borghini, Lornazzo, ArmeninI, Bisagno, Volpato, and others.
It is said to have fallen into disuse because it easily becomes rancid, and manu¬
factured tube colors ground in it are difficult to keep in storage for any length of
time. (See also Oils and Fats.)

Water Color. The modern term is used to describe a standard preparation of
pigment ground in water-soluble gums, usually from Acacia Senegal or from Acacia
arahica Willd. (see Gums). The typical water color painting Is executed with this
paint thinly on a support of paper.

Water-Glass is a thick, syrupy, clear liquid, an aqueous solution of potassium
or sodium silicate. It differs from ordinary insoluble glass in that it contains no
calcium, barium, or aluminum. Water-glass serves as an inorganic binding medium
because, on evaporation of water, it leaves an amorphous, coherent film. By
virtue of its strong alkalinity, water-glass can be used only with a limited number
of pigments. Church (p. 85) says that as early as 1648 a silicate of potash, called
' fluid silica,’ was made but that the commercial production of these salts dates
from 1825. Water-glass Is used for mineral painting or so-called stereochromy (see
Wolfgangmuller, ‘ Erfahrungen mit Keim’scher Mineralfarbe “ A,” ’ Technische
Mitteilungen fur Malerei^ XLVIII [1932], pp. 124-126, 134-135). In the practice
of painting, water-glass is ordinarily used as a fixative for colors that are put
on merely with water. Potassium silicate appears to be preferable to sodium
silicate for this purpose. It is only useful on plaster supports, since a chemical
combination of the alkali silicate with lime of the plaster is essential to the for¬
mation of a durable film. Laurie {The Painter's Methods and Materials^ 215)
says that certain murals in the House of Lords which were executed in this medium
have suffered from disintegration. Water-glass is used extensively as an adhesive.

Painting Materials

Waxes. The composition of these substances is more varied than is that of
the!ih and, at present, has been less investigated. Like most natural products
they are mixtures of several components, and t^he isolation of these is a di cu
and lengthy task which has been attempted by comparatively few. The chief
plLic exponents arc: (.) esters of the fatty acids «.th a^coho s cont.m.ng
fh eh number of carbon atoms; W free fatty ac.ds; (3) free alcohols eonta.mng
a high number of carbon atoms; and (4) hydrocarbons They can be d.y.ded as
animal, vegetable, bitumen, and mineral waxes. The following table gives the
chief analytical characteristics of the principal animal and vegetable waxes and
of montan wax. (For a discussion of the meaning of these characteristics, see

UliS anu x'ai.o.; , , -i a. r ^ •

The waxes are esters of monohydric alcohols whereas oils are esters of a tri«

hydric alcohol. The protective strength of most waxes against permeation of
moisture has given them an extensive use in the modern treatment of ^paintings,
thoucrh they are little used as a medium. Gettens and Bigelow, in a series of tests
of the moisture permeability of coatings, gave beeswax and ceresin a rating of 4
(average permeability in milligrams lost per day). Commercial mastic had a
rating of 35, dammar had 24, and the ratings for oils ran from 250 to 5 ^ 7 * ^
application over a normal and firm paint film with a thin film of varnish—thor~
ouglily dry—Rosen (p. 115) recommends beeswax, i part, carnauba, 2 parts,
ceresin, 2 parts, as a surface coating for paintings. Wax adhesives have been
found to possess certain very decided advantages over the traditional glue mix¬
ture as well as over synthetic materials for the purpose of lining canvases. 1 lender-
leith and Cursiter subjected a wax-lined painting, in which a well known recipe
was employed (equal parts of beeswax and resin with a small quantity of Venice
turpentine) as the cementing material, to a series of extreme tests of endurance,
and found that the wax proved remarkably unaffected by such drastic treatment.
Fink The Care and Treatment of Outdoor Bronze Statues, Technical Studies^
11 [1933], p. 34) has found paraffin to be the best protective coating which can
be applied to outdoor bronze statues. Wax has also proved itself suitable for
impregnating objects made of wood.

Waxes, history in painting. The art of using colors prepared with wax began in
antiquity. The term, ^ encaustic,’ which has long been applied to this method,
strictly means,' burning in/ an expression which is scarcely applicable to the mere
melting of wax colors. According to Pliny, the process was not originally restricted
to painting but included other processes in which melted wax was used. Lucas
{Ancient Egyptian Materials and Industries^ pp. 280-281) says that the only wax
known to have been used in ancient Egypt was beeswax. It figured in mummifica¬
tion, served as an adhesive, as a luting material on the covers of alabaster vases, as
a paint vehicle, as a fixing substance in the curls and plaits of wigs, and had a place
in the making of magical figures, in ship-building, and in other crafts. Eleven
specimens of beeswax from mummies examined by Lucas had melting points

Mediums and Adhesives

79

cn

W

X

§

8

ta

O

CO

W

i—t

a<

§

(1^

Per cent of
alcohols and
hydrocarbons

\o 0 0

»-rv

1 1 1 1 Vi

ON Tf-

WT, xh VO ^

Acid

number

M ^ 0 Q 0

c^ M Cl !>.

1 1 I 1 1

VO M <s 0

M M ^ CO

Iodine

number

1

to

M 0 CO 0

w (S <S t-i

1 1 1 1 1

00 M to CO MD

Saponi¬

fication

number

!>•

CO ^ CO c l

ov C^VO 00 t-t

oi, i i V

00 00 to !>.

Melting

point

°C.

0

0000 0

0 CO 0 ^ CO

!>. 00 CO M

1 1 1 1 1

00000

CO 0 CO VO

VO CO VO 00 !>.

Setting

point

°C.

0000

CO M 00

VO 00 VO 00 ,

1 1 1 i

0000 1

0 0 CO 0

VO CO VO CO

Refractive

index

0 0 0

to M 0

'o’ 1 ^ cl 1

f i* f

Specific

gravity

15715° c.

VO 0 cv

VO OV C3V

Gv Crs Cn

Hn 1

VO 0 to cv

CV OV OV

d do

Source

Apis mellifica

Coccus ceriferus
Pedilanthus pavanis
Corypha cerifera
Lignite and peat

Wax

Beeswax

Chinese insect wax
Candelilla wax
Carnauba wax
Montan wax

8o

Painting Materials

ranging from 6 f to 70° C. and five specimens from wigs varied from 60° to 63° C.
The melting point of modern beeswax is from about 60.5 to 64.25 ^ hese

specimens, although light-colored and somewhat friable on the surface, had ap¬
parently not undergone any considerable change. Partington (p. 140) has this to
say about Egyptian practice:

The process of enc3.iistic painting (with. a. wax mediuni) was not in use in ancient
Egypt but appeared in the Ptolemaic Period for painting on wood, although
Herodotus says Amasis (559-525) sent a portrait of himself to Cyrene. The encaus¬
tic paintings on wood on mummy cases are Greek and Roman. The encaustic
technique may have originated in Egypt; preparations of wax for preserving paint¬
ings are said to go back to the XVIII dyn., and the names of most of the encaus¬
tic painters of antiquity appear to be Alexandrian or Egyptian. The first literary
mention is after Alexander: a reference in a supposed ode of Anacreon (c. 550 B.C.)
is of doubtful date. Eusebius (264-340) calls the process kvpoxvtos ypa^i] C drawing
in liquid wax ”): it continued in use till the Middle Ages, but had declined after the
9th century A.D. The pigments (now in the British Museum) found at Hawara by
Petrie are really water colors, but it is probable that they would be similar to the
pigments used by the encaustic painter. The process, according to Petrie, was as
follows. The colours were ground in the wax, previously bleached by heating it
to its boiling-point, and fused in the sun in hot weather or in a hot-water bath,
which is mentioned by Theophrastos. The portrait was made on a wood panel,
previously primed with distemper, the wax colour being put on from a pot with a
lancet-shaped spatula or (more probably) with a brush, pressed out at the end of
the stroke.

This makes a description that fits with the appearance of the small mummy
portraits which, as a group, have taken their name from the Fayum district of
Egypt. In Greece, it is said, pictures in wax commanded large prices in ancient
times: 60 talents (about ^85,000) was offered for one and 7,000,000 sesterses
(about 1400,000) was paid for another (Schmid, * La Reconstitution du Procede
a FEncaustique,’ p. 37).

Since the IX century A.D., wax has not been much used as a painting medium.
Eastlake (I, 156) speaks of its prevalence in the first centuries of the Christian
era when it appears to have superseded all other processes, except mosaic. The
Lucca MS. (VIII century) has more about mosaic than about wax painting, but
says that colors mixed with wax were used on walls and on wood. It is scarcely
alluded to in the treatises of the XII, XIII, and XIV centuries.

There has been argument about the type of wax used in ancient times as a
medium for painting. Some reports have attempted to show that the wax was
applied in an emulsified or a saponified state with water, and Berger has taken
the recipe for Punic wax as given by Pliny (XXI, 49) and by Dioscorides ( 11 , 105)
to be proof that the medium was an emulsion. Studies by Eibner, Laurie, Schmid,
and others have, however, made this supposition very doubtful. It is barely

Mediums and Adhesives

8i

possiblcj also, that oils and resins were added to the wax. One such combination,
called Zopissa^ a mixture of wax and balsam, was familiar in ancient industry.
Laurie {Greek and Roman Methods of Paintings Cambridge [1910], p. 65^) argues,
however, that latty acids detected by analysis in some ancient wax could have
been caused by oxidation, and considers the medium of encaustic painting to be
wax alone. This is probably the general opinion. The melting point has sometimes
been found to be high in specimens from Fayum portraits and there remains a
possibility that some other medium was added.

Although wax serves very little now as a painting medium, it finds many
other applications in the arts. Because of its easy solubility in weak solvents and
because of its protective strength and its permanence, its use as a surface coating
for pictures has increased in recent years and it is much used in restoration
(see Waxes).

Wood Oil (see Tung Oil).

Wool Wax (see also Waxes) is the natural grease from the fleece of sheep.
It is a pale yellow, translucent substance with a distinctive odor and an unctuous
consistency. Purified, it forms, together with about 25 per cent of water, the
lanolin of commerce. Its chemical composition is not fully known. It consists of
a mixture of neutral esters and free alcohols, among which occur cholesterol and
isocholesterol (C27H45OH). Although insoluble in water, it emulsifies it (see
Emulsions), and it can easily be made to take up 80 per cent of its weight of
water. It is used extensively in the treatment of leather, as a rust preventative,
and for medicinal purposes.

Zanzibar Copal (see also Copal and Resins) occurs either as a resin from a
living tree, Trachylobium verrucosum Oliv. (of the Papilionaceae family), as a semi¬
fossil in the ground beneath these trees, or as a true fossil deposited by a tree no
longer standing. The fossil resin has a brown crust which, when scraped away,
exposes a transparent, yellow mass on which are small, round elevations called
^ goose skin.’ The exudation from the living tree is not so hard as the fossil resin,
but has a smooth, glossy surface. Very little of this resin is now actually collected
on the Island of Zanzibar; it is sent directly from the mainland. It is sometimes
also called ‘ anime.’ Zanzibar copal is the hardest of the copal resins, has a very
high melting point (240° to 360° C.), and makes a rather dark, oil varnish, used
chiefly for industrial purposes.

Zapon, a lacquer or varnish containing highly viscous nitrocellulose (see G.
Zeidler and F. Wilborn, ‘ Application of Zapon Lacquers to Metallic Surfaces,’
Paint and Varnish Production Manager^ XIX [December 1939], pp. 358-363, 373).
It is sometimes mentioned in works of Continental origin that deal with restora¬
tion of museum materials.

BIBLIOGRAPHY

Terome Alexander, Glue and Gelatin (New York: The Chemical Catalogue Co 1923).
Rev Allen Jr V. E. Meharg, and John H. Schmidt, ‘Chemistry of Synthetic Varnish
"Resins,’ Industrial and Emineerin, Chemistry, XXVI (1934), PP- 663-669
Anonymous, Cleaning and Restoration oj Museum Exhibits, Third Report (London.

H. M. Stationery Office, 1926). , „ . . m • , wvttt

Anonymous, ‘Methacrylate Resins,’ Industrial and Engineering Chemistry, XXVIII

(1936), PP- 1160-1163. _ /T j Tj i\/r Ct

Anonymous, Second Report of the Adhesives Research Committee (London: H. M. Sta-

WilderD""B^ncroft? 3 p//Vi Colloid Chemistry (New York: McGraw-Hill Book Co.,

T Hedley Barry Natural Varnish Resins (London: Ernest Benn, Ltd, 1932).

G. F. Beal, H. V. Anderson, and J. S. Long, ‘X-ray Study of Some Natural and Syn¬

thetic Varnish Resins,’ Industrial and Engineering Chemistry, XXIV (1932), pp.
1068 ff

H. L. Bender, A. F. Wakefield, and H. A. Hoffman, ‘Colloidal Developments in Syn¬

thetic Resins,’ Chemical Reviews, XV (1934), pp. 123-137.

Ernst Berger, Beitrage zur Entwicklungsgeschichte der Maltechniky 4 vols (Munich: G,

D. W. Callwey, 1901-1912). ^ , r v • 7 j

K. G. Blaikie and R. N. Crozier, 'Polymerization of Vinyl Acetate, Industnal and

Engineering Chemistry^ XXVIII (i 936 )> PP*

R. H. Bogue, The Chemistry and Technology of Gelatin and Glue (New York: McGraw-

Hill Book Co., 1922). ^ ^ y . 7

* Properties and Constitution of Glues and Gelatins, Chemical and Metallurgical
Engineering, XXIII (1920), pp. 5-1^7 61-66, 105-109, 154-158, 197-201.

T. F. Bradley, 'Drying Oils and Resins; Influence of Molecular Structure upon Oxygen
and Heat Convertibility,’ Industrial and Engineering Chemistry, XXIX (1937)7

pp. 3 /v 304*

'Drying Oils and Resins; Mechanism of the "Drying” Phenomenon,’ Industrial
and Engineering Chemistry, XXIX (i937)> PP* 440-'445*

Wallace H. Carothers, 'Polymerization,’ Chemical Reviews, VIII (193^:), PP* 353-426.
‘Polymers and Polyfunctionality,’ Transactions of the Faraday Society, XXXII

(1936), pp. 39-53-

A. H. Church, The Chemistry of Paints and Painting, 3d ed. (London: Seeley and Co,,
Ltd, 1901).

D. H* Clewell, 'Drying of Linseed Oil: Electron Diffraction Study,’ Industrial and
Engineering Chemistry, XXIX (1937)? PP*

C. Goflignier, Varnishes, Their Chemistry and Manufacture (London: Scott, Greenwood
and Son, 1923).

James B. Conant, The Chemistry of Organic Compounds (New York: The Macmillan

Co., 1933)*

82

Mediums and Adhesives 83

G. O. Curme Jr and S. D. Douglas, ‘Resinous Derivatives of Vinyl Alcohol/ Industrial

and Engineering Chemistry^ XXVIII (1936), pp. ii 23-1129.

J. O. Cutter, ‘The Polymerisation of Drying Oils,* Journal of the Oil and Colour Chemists^
Association^ XIII (1930), pp. 66-83.

H. B. Devore, A. H. Pfund, and V. Cofman, ‘A Study of the Action of Light of Different

Wave-Lengths on Nitrocellulose,* Journal of Physical Chemistry^ XXXIII (1929),
pp. 1836-1842.

A. De Waele, ‘A Consideration of Some Factors Affecting the Oxygen Absorption of
Linseed Oil,* Journal of the Society of Chemical Industry^ XXXIX (1920), pp.
48T-50T.

Karl Dieterich, The Analysis of Resins, Balsams and Gum Resins, trans, (London:
Scott, Greenwood and Son, 1920).

Max Doerner, The Materials of the Artist and Their Use in Painting, trans. (New York:
Harcourt, Brace and Co., 1934).

Thomas H. Durrans, Solvents, 3d ed. (New York: D. Van Nostrand Co., 1933).

Charles L. Eastlake, Materials for a History of Oil Painting, 2 vols (London, 1847-
1869).

A. Eibner, Entwicklung und Werkstoffe der Tafelmalerei (Munich: B. Heller, 1928).
Entwicklung und Werkstojfe der Wandmalerei (Munich: B. Heller, 1926).
Malmaterialienkunde als Grundlage der Maltechnik (Berlin: J. Springer, 1909).

‘Das Punische Wachs des Dioskurides und seine neuzeitliche maltechnische Be-
deutung,* Technische Mitteilungen fur Malerei, L (1934), pp. 95-“97, 104-107,
111-113.

Cher Fette Ole (Munich; B. Heller, 1922).

Roy Elliot, ‘Vinyl Acetate Resins,* Canadian Chemistry and Metallurgy, XVIII (1934),
pp. 173-176.

Carleton Ellis, The Chemistry of Synthetic Resins, 2 vols (New York: Reinhold Pub¬
lishing Corp., 1935)-

‘The Newer Chemistry of Coatings,* Industrial and Engineering Chemistry, XXV
(1933)5 PP- 125-132.

Synthetic Resins and Their Plastics (New York: The Chemical Catalogue Co.,

1923)-

‘Tailoring the Long Molecule,* Industrial and Engineering Chemistry, XXVIII
(1936), pp. 1130-1144.

A. C. Elm, ‘The Drying and Yellowing of Trilinolenic Glyceride,* Industrial and Engi¬
neering Chemistry, XXIII (1931), pp. 881-896.

‘Fundamental Studies of Paints,* Industrial and Engineering Chemistry, XXVI
(1934)5 pp. 1245-1250,

Encyclopaedia Britannica, iit\i Rnd i4.th

• Lewis Eynon and J. Henry Lane, Starch: Its Chemistry, Technology and Uses (Cam¬
bridge: W. Heffer and Sons, Ltd, 1928).

H. Freundlich, ‘Der Trocknungsprozess des Ldnoles," Kolloidchemische Untersuchungen,
no. 45 (1930), pp.

Percival J. Fryer and Frank E, Weston, Technical Handbook of Oils, Fats and Waxes,

I, Chemical and General (Cambridge: University Press, 1920),

Painting Materials

84

D L Gamble and G. F. A. Stntz, ‘Ultra-Violet Light Transmission Chai^cteristics of
Some Synthetic Resins,’ Industrial and Engineering Chemistry, XXI (192.9),

o fF

H. A^Gfrd°ner and S. A. Levy, ‘Pigment and Color Index,’ Circular no. 352, American

Paint and Varnish Manufacturers Association

W. H. Gardner and W. F. Whitmore, ‘The Nature and Constitution of Shellac, /w 4 ar-
trial and Engineering Chemistry,'XKl {1919), W- ,
Rutherford J. Gettens, ‘Polymerized Vinyl Acetate and Related Compounds m the
Restoration of Objects of Art,’ Technical Studies, IV (i 935 )> PP-
‘Preliminary Report on the Measurement of the Moisture Permeability of Pro¬
tective Coatings,’ Technical Studies, I (1932), PP*

Rutherford J. Gettens and Elizabeth Bigelow, ‘The Moisture Permeability of Pro¬
tective Coatings,’ Technical Studies, II (i 933 ), PP- I 5 “ 25 *

W. E. Gloor, ‘Effect of Heat and Light on Nitrocellulose Films,’ Industrial and Engi¬
neering Chemistry, XXIII (1931), PP- 980-982.

Noel Heaton, ‘The Permanence of Artists’ Materials,’ Journal of the Royal Society of
Arts (London), LXXX (1932), PP- 4 II- 43 S-
W. O. Hermann and W. Haehnal, ‘fiber den Poly-vinylalkohol,’ Berichte des Deutschen
Chemischen Gesellschaft, LX (1927), PP- 1658-1663.

Christiana J. Herringham, The Book of the Art of Cennino Cennini (London: G. Allen,

1899).

T. P. Hilditch, The Industrial Chemistry of the Fats and Waxes (New York: D. Van

Nostrand Co,, 192.7).

J. R. Hill and C. G. Weber, ‘Stability of Motion-Picture Films as Determined by
Accelerated Aging,’ JouTtial of R€se(ZTch of ths Nationul Ruveau of Standavds^ XVII

(1936), PP. 871-881. , r -7 • 7 ^ 17 •

H. E. Hofmann and E. W. Reid, ‘Cellulose Acetate Lacquers, Industrial and Engu

neering Chemistryy XXI (1929), pp. <)SS~ 9 ^S^

‘Formulation of Nitrocellulose Lacquers,’ Industrial and Engineering Chemistryy
XX (1928), pp. 687-693.

R. Houwink, ‘Synthetic Resins, their Formation, their Elastic and Plastic Properties,
and their Possibilities,’ Journal of the Society of Chemical Industryy LV (1936),
pp. 247-259.

George S. Jamieson, Vegetable Fats and Oils (New York: The Chemical Catalogue
Co., 193^^).

J. F. John, D/V Malerei der Alten (Berlin, 1836).

R. H. Kienle, ‘Observations as to the Formation of Synthetic Resins,’ Industrial and
Engineering Chemistryy XXII (1930), pp. 590-594.

G. M. Kline, ‘Permeability to Moisture of Synthetic Resin Finishes for Aircraft,’
Journal of Research of the National Bureau of Standardsy XVIII (i937)i PP- 235-249.
G. M. Kline and B. M. Axilrod, ‘ Method of Testing Plastics,’ Industrial and Engineering
OmfVry, XXVIII (1936), pp. 1170-1173.

A. P. Laurie, Greek and Roman Methods of Painting (Cambridge: University Press,

1910).

The Materials of the Painter*s Craft (Philadelphia: J. B, Lippincott Co., 1911).

Ww 0 /^/(London; The Sheldon Press, 1935).

Mediums and Adhesives

85

* Notes on the Medium of Flemish Painters/ Technical Studies, II (1934), pp,
124-128.

The Painter's Methods and Materials (London: Seeley, Service and Co., 1926).
'The Yellowing of Linseed Oil/ Technical Studies, IV (1936), p. 145.

J. Lewkowitsch, Chemical Technology and Analysis of Oils, Fats and Wa^es (London:
Macmillan and Co., Ltd, 1913).

J. S. Long, 'Drying Oils,' Paint and Varnish Lecture Course (American Paint Journal
Co., St Louis), (1933), PP* 3^-4^-

J. S. Long and W. S. W. McCarter, 'Studies in the Drying Oils, XV: Some Aspects
of the Oxidation of Linseed Oil up to Gelation,' Industrial and Engineering Chem^
istry, XXIII (1931), pp. 786-791.

J. S. Long, A. E. Rheineck, and G. L. Bali, 'Studies in the Drying Oils, XVII: Influence
of Several Factors on the Mechanism of Drying of Oil Films,' Industrial and Engu
neering Chemistry, XXV (1933), PP* 1086-1091.

A. Lucas, Ancient Egyptian Materials and Industries, 2d ed. (London: Edward Arnold
and Co,, 1934).

Antiques: Their Restoration and Preservation, 2d ed. (London: Edward Arnold and
Co., 1932).

E. W. J. Mardles, 'The Dissolution of Substances in Mixed Liquids with Special Refer¬
ence to Colloids,' Journal of the Chemical Society, CXXV (1924), pp. 2224-2259.
'Solvents of Some Cellulose Esters,' Journal of the Society of Chemical Industry,
XLII (1923), PP* 127-136.

'The Swelling and Dispersion of Some Colloidal Substances in Ether-Alcohol Mix¬
tures,’ Journal of the Chemical Society, CXXVII (1925), pp, 2940-2945.

Jacques Maroger, 'Essai de Reconstitution de la Mati^re Picturale de Jean Van Eyck/
Mouseion, XIX (1932), pp. 39-46.

J. G. McIntosh, The Manufacture of Varnishes and Kindred Industries (London: Scott,
Greenwood and Son, 1911).

M. P. Merrifield, Original Treatises on the Arts of Painting, 2 vols (London: John
Murray, 1849).

R. S. Morrell and W. R. Davis, ‘Studies in the Oxidation of Drying Oils and Cognate
Subjects,' Journal of the Society of Chemical Industry, LV (1936), pp. 237T-246T,
261T-265T, 265T-267T.

R. S. Morrell and S. Marks, 'The Polymerisation of Drying Oils,' Journal of the Oil
and Colour Chemists' Association, XIII (1930), pp. 84-90.

R. S. Morrell and H. R. Wood, The Chemistry of Drying Oils (London: Ernest Benn,
Lta, 1925).

H. T. Neher, 'Acrylic Resins,' Industrial and Engineering Chemistry, XXVIII (1936),
pp. 267-271.

E. J. Parry, Gums and Resins (London: Sir Isaac Pitman and Sons, Ltd, 1918).

J. R. Partington, Origins and Development of Applied Chemistry (London: Longmans,
Green and Co., 1935).

H. J. Plenderleith, The Preservation of Antiquities (hondoni The Museums Association,

1934)*

86

Painting Materials

H. J. Pienderleith and Stanley Cursiter, ‘The Problem of Lining Adhesives for Paintings
—Wax Adhesives,’ Technical Studies, III (1934), PP- 90 -ii 3 -
M. Ranagaswami, ‘A Note on the Determination of Melting Point of Resins,’ Journal
of the Oil and Colour Chemists' Association, XIII (193°), PP- 287 ff.

E. W. Reid and H. E. Hofmann, ‘Cellosolve and its Derivatives in Nitrocellulose Lac¬

quers,’ Industrial and Engineering Chemistry, XX (1928), pp. 497-504.

F. H. Rhodes and T. T. Ling, ‘The Oxidation of Chinese-Wood Oil,’ Industrial and

Engineering Chemistry, XVII (1925)) PP- 5 o^“S^ 2 .

Samuel Rideal, Glue and Glue Testing (London: Scott, Greenwood and Son, 1926).

E. R. Riegel, Industrial Chemistry (New York: The Chemical Catalogue Co., 1933 )-
David Rosen, ‘A Wax Formula,’ Technical Studies, III (1934), PP- 1 14-115-
A. H. Sabin, The Industrial and Artistic Technology of Paint and Varnish (New York:
John Wiley and Sons, 1927)-

J. Scheiber and K. Sandig, Artificial Resins (London: Sir Isaac Pitman and Sons, Ltd,

1931).

Hans Schmid, Enkaustik und Fresko auf antiker Grundlage (Munich: G. D. W. Callwey,
1926).

‘La Reconstitution du Proc 6 d 6 h. TEncaustique/ Mouseion^ XXIII-XXIV (1933),

pp. 30-49.

Julius Schmidt, ‘Zur Kenntnis der Kiinstlerolfarbe/ Technische Mittetiungen fur Maleret,
LI (1935). PP’ 186-188, 192-194, 199-203; LII (1936), pp. 7 -S> 16--18, 24-26, 33-3^,
39-41, 49-5L 7^-74, 79-80, 87-88, 96-98, 103-104, 111-112.

H. Schonfeld, Chemle und Technologie der Fette und Fettprodukte^ I, Chemie und Ge-
winnung der Fette (Vienna: J. Springer, 1936).

A. Schwarcman, ‘Linseed Oil and the Chemical and Colloidal Nature of Films,’ Journal
of the Society of Chemical Industry^ LIV (1935), PP- ^07 ff.

W. F. Seyer and Kuramitsu lonouye, ‘Paraffin Wax: Tensile Strength and Density at
Various Temperatures,’ Industrial and Engineering Chemistry^ XXVII (1935),
PP* S^l~Slo>

A. J. Stamm and R. M. Seborg, ‘Minimizing Wood Shrinkage and Swelling,’ Industrial
and Engineering Chemistry^ XXVIII (1936), pp. 1164-1169.

H. Staudinger, ‘The Formation of High Polymers of Unsaturated Substances,’ Trans^
actions of the Faraday Society^ XXXII (1936), pp. 97-121.

H. Staudinger, K. Frey, and W. Starck, ‘Hochmolekulare Verbindungen, 9 Mit-
teilungen: Uber Poly-vinylacetat und Poly-vinylalkohol,’ Berichte der Deutschen
Chemischen Gesellschaff LX (1927), pp. 1782-1792.

J, Vernon Steinle, ‘Carnauba Wax, an Expedition to its Source,’ Industrial and Engi¬
neering Chemistry^ XXVIII (1936), pp, 1004-1008.

George L. Stout and Harold F. Cross, ‘Properties of Surface Films,’ Technical Studiesy
V (1937), pp. 241-249.

George L. Stout and Rutherford J. Gettens, ‘Transport des Fresques Orientales sur de
Nouveaux Supports,* Mouseiony XVII-XVIII (1932), pp. 107-112.

George L. Stout and Minna H. Horwitz, ‘Experiments with Adhesives for Paper/
Technical Studiesy 111

Mediums and Adhesives

87

D. E. Strain, ‘Viscosity Variations in Methacrylic Ester Polymer Solutions,’ Industrial
and Engineering Chemistry, XXXII (1940), pp. 540-551.

D. E. Strain, R. G. Kennelly and H. R. Dittmar, ‘Methacrylate ^tsins,’ Industrial and
Engineering Chemistry, XXXI (1939), pp. 382-387.

Edwin Sutermeister, Casein and Its Industrial Applications (New York: The Chemical
Catalogue Co., 1927).

R. S. Taylor and J. G. Smull, ‘Studies in the Drying Oils, XIX: Oxidation of Linseed
Oil,’ Industrial and Engineering Chemistry^ XXVIII (1936), pp. 193-195.

Daniel V. Thompson Jr, II Lihro del! Arte: The Craftsman's Handbook of Cennino
d'Andrea Cennini (New Haven: Yale University Press, 1933).

The Materials of Medieval Tainting (New Haven: Yale University Press, 1936).

The Practice of Tempera Painting (New Haven: Yale University Press, 1936).

Daniel V. Thompson Jr and George Heard Hamilton, Be Arte Iliuminandi (New Haven:
Yale University Press, 1933).

Edward Thorpe, A Dictionary of Applied Chemistry (London: Longmans, Green and
Co., 1922).

J. F. Thorpe and M. A. Whitely, Supplement to Thorpe's Dictionary of Applied Chemistry
(London: Longmans, Green and Co., 1934).

Maximilian Toch, Paint, Paintings and Restoration (New York: D. Van Nostrand Co.,
1931).

T. R. Truax, The Gluing of Wood (Washington, D. C.: Government Printing Office,
1930).

A. Tschirch and Erich Stock, Die Harze^ 4 vols (Berlin; Gebriider Borntraeger,

1936).

Hermann Vollman, Zur Kolloidchemie des Leinois,’ Zeitschriftfilr Angewandte Chemie
XXXVIII (1925), pp. 337-339.

Hans Wagner and Georg Fischer, ‘Filmbildung aus Emulsionen,” Kolloid Zeitschrift,

LXXVII (1935), pp. 12-30.

R. Walton, A Comprehensive Survey of Starch Chemistry (New York: The Chemical

Catalogue Co., 1928).

J. F. Watin, L'Art du Peintre, Doreur, Vernisseur, 2d ed. (Liege, 1778).

S. Werthan, A. C. Elm, and R. H. Wien, ‘Yellowing of Interior Gloss Paints and

Enamels,’ Industrial and Engineering Chemistry, XXII (1930), pp. 772-784.

R. V. Williamson, ‘Relation of Oil Absorption to Consistency of Pigment-Oil Pastes,’

Industrial and Engineering Chemistry, XXI (1929), pp. 1196-1198.

S. P. Wilson, Pyroxylin Enamels and Lacquers, 2d ed. (New York; D* Van Nostrand

Co., 1927).

Hans Wolff, ‘The Critical Oil Content and the Properties of Paint Films/ Paint and
Varnish Production Manager, VIII (October, 1932), pp. 5-6; (November, 1932),
pp. 8-10.

George Woodbury, ‘Note on the Use of Polymerized Vinyl Acetate and Related Com¬
pounds in the Preservation and Hardening of Bone/ American Journal of Physical
Anthropology, XXI (1936), pp. 449-450.

88

Painting Materials

R L Wray and A. R. Van Vorst, ^Permeability of Lacquer Films to Moisture, Indus¬
trial and Engineering Chemistry, XXVIII ( 193 ^)? PP- 12.68-1269.

H. YosMda, ‘The Chemistry of Lacquer/ Chemical Society Transactions, XLlil (1883)5

pp. 472 if.

C« D. Young, ‘Lacquer,’ Industrial Furnishing, XII (1928), pp- 30 ff*

Fritz Zimmer, Nitrocellulose Ester Lacquers, trans. (New York: D. Van Nostrand Co.,

1934)-

PIGMENTS AND INERT MATERIALS

Alizarin (alizarin crimson) (see also Madder) Is the coloring principle of the
madder root and it was first isolated from that source in i8a6 by Colin and
Robiquet ('Recherches sur la Matiere colorante de la Garance/ Annales de
Chimie et de Physique^ 2 d series^ XXXIV [1827], pp, 225-253). It Is 1^2 dihy-
droxyanthraquinoiie, and was first synthesized by two German chemists, C.
Graebe and C. Lieberman, who reported their discovery in 1868 (‘Ueber Alizarin
und Anthracen,^ BePichte der Deutschen Chemischen Gesellschaft^ I [1868], pp. 49-
51; see also English patent 3850, December, 1868). This is important in the
history of organic chemistry, for alizarin was the first of the natural dyestuffs
to be made synthetically. Its discovery caused the rapid decline and the almost
complete disappearance of the large madder-growing industry in France. The
‘alizarin crimson' lake used so extensively In artists' paints is nearly all from this
source. It is made with aluminum hydrate which gives a transparent lake;
different shades of red can be made with different bases. It is more light-fast than
natural madder lake because it contains none of the fugitive purpurin associated
with alizarin from that source (see Eibner, Malmaterialienkunde^ p. 202), and is
among the most light-fast of the organic red pigments. Some painters have said,
however, that synthetic alizarin does not give the pleasing, saturated, and fiery
tone that madder alizarin gives. In ultra-violet light, synthetic alizarin does not
give any of the strong fluorescence that is characteristic of madder lake. It may
not be permanent when mixed with earth colors like ochre, sienna, and umber
(see Toch, Paints^ Paintings and Restoration^ p. 97). Microscopically, alizarin lake
is not readily distinguished. Merwin says (p. 517) that the isotropic base with
the coloring matter has a variable low refractive Index, about 1.70 for red. The
color by transmitted light is purplish red. It is soluble and turns purple in dilute
sodium hydroxide, but this behavior is hardly characteristic.

Alizarin Crimson (see Alizarin).

Aluminum Hydrate (transparent white), or aluminum hydroxide, AhOH)^, Is
a light, white material which is prepared by treating a solution of aluminum
sulphate with an alkali such as soda ash or potash. The gelatinous aluminum
hydroxide, because it adsorbs dyestuffs easily, may be used directly in pulp form
as a base in the preparation of transparent lake pigments or it can be dried to
a very light, white powder. This is used in paint manufacture, largely as a filler.
It is the most common cheapening agent for artists’ oil colors and many of them
now on the market contain It. Because of its low density and low refractive index,
it lacks covering power and lends transparency to colors. It has high oil absorp¬
tion and, for this reason, tends to increase yellowing in paints. Excess of it some¬
times causes a rubbery consistency in prepared paste paints. Aluminum hydrate
is difficult to detect in paint films by microscopic methods because of its lack of
form and its lack of characteristic optical properties. It appears simply in clots
of fine grains showing no birefringence. The hydrate is soluble in acids and
alkalis, but is otherwise a stable material. When heated to a high temperature,

9S

Painting Materials

92

it loses its combined water and is changed to aluminum oxide, AI2O3, which is of
no value for pigment purposes. Alum and similar substances were used as early
as classical times as a source of substrates for dye colors (see Bailey II,

233-238). , . .

Aluminum Leaf and Aluminum Bronze Powder are made from sheet aiumi-

num by a beating and a stamping process, respectively. The name, ‘bronze,’ is
still retained, no doubt from its association with metal powders made from copper
alloys (see Bronze Powders). Although aluminum powder was probably available
as early as the middle XIX century, it was not until a decade or so after 1886,
when aluminum began to be produced in large commercial quantities by the Hall
process (see Aluminum, section on supports, pp. 221 and 222) that the powder be¬
came readily available. It was first used for coating picture frames and radiators.
Aluminum powder did not become important as a pigment for commercial paints
until after 1920 (see J. D. Edwards, Aluminum Bronze Powder and Aluminum
Paint [New York: The Chemical Catalogue Co., 1927], pp. 26-29). Its develop¬
ment for outside and for protective painting followed experiments and field tests
carried on in the Forest Products Laboratory, Madison, Wisconsin, and by the
H. A. Gardner Laboratory in Washington.

When aluminum bronze powder is stirred into a suitable vehicle like oil or
varnish, the flakes swirl and some come quickly to the surface layer where they
spread out to form an almost continuous film of flat particles. This phenomenon,
which is called ‘leafing,’ is caused by surface tension and is shown only to a
marked degree by the polished powder and not by the unpolished powder (see
J. D. Edwards, F. C. Frary, and Z. Jeffries, The Aluminum Industry^ II [New
York: McGraw-Hill Book Co., 1930], p. 803). No grinding of the powder and
vehicle is necessary. In pyroxylin medium (nitrocellulose) aluminum powder has
no leafing properties and does not form a durable film. Because of its leafing
properties, it is now finding wide use for moisture- and waterproof paints. For
exterior use, long oil spar varnishes are the best vehicle. Experiments at the
Forest Products Laboratory show that this coating has outstanding moisture
resistance and maintains its moisture-proofing efficiency over relatively long
periods of time.

Aluminum bronze leaf in a vehicle has a reflectivity of 60 to 75 per cent for
light, but it has low emissivity or radiating power for heat. At 40° C. the emis-
sivity of aluminum paint is only about 20 per cent of that of a ‘black body,’
which is the theoretically perfect radiating surface (see Edwards, of. cit., pp. 47
and 51).

Microscopically, the particles of aluminum bronze powder are irregular in
shape; in reflected light the individual flakes are lined with irregular, dark mark¬
ings which are the result of having been stamped in contact with other flakes.
Although the flakes are very thin (in the order of i micron), they are opaque to
strong transmitted light.

Pigments

93

Aluminum Stearates Al(CisH35 02)3, is a soap made by the saponification of
tallow and treatment with alum. It is a white powder which forms colloidal
solutions or gels with linseed or other oils, turpentine, or mineral spirits. For this
reason. It is often used in artists’ oil pastes and prepared paints to prevent sepa¬
ration of the oil from the pigment. Small quantities only are desirable because
too much of it hinders drying and develops a 'cheesy’ film. It is used also as a
flatting agent in varnishes and lacquers (Gardner, p. 788). Because of its colloid¬
forming properties, aluminum stearate is not easily recognized in paints; it has
no outstanding optical properties.

Anhydrite is the mineralogical name for native anhydrous calcium sulphate,
CaS045 which is often associated in nature with calcium sulphate dihydrate or
gypsum (see G3rpsum). Although it has no useful setting properties, it occurs
occasionally as an impurity in gypsum and plaster of Paris. Sometimes it is
observed as a component of the gesso in Italian paintings. Anhydrite is a color¬
less inert like gypsum, but It differs from that material in the nature of its crys¬
tallinity and in its optical properties. It crystallizes in the orthorhombic system,
has higher refractive index (/3 = 1.575) ^^an gypsum, and is strongly birefracting.
Particles of it appear as small, square tablets in gypsum gesso; it Is characterized
by cleavage in three rectangular directions (Dana, p. 630). The chemical proper¬
ties are about the same as those of gypsum.

Aniline Pigments (see Coal-Tar Colors).

Antimony Oxide, Sb203, was introduced to the paint trade as a pigment under
the trade name, 'Timonox,’ in 1920 by the Cookson Lead and Antimony Co.,
Ltd, of England. It has good hiding power; the refractive index is about 2.20,
nearly that of a reduced titanium oxide. Some commercial samples that have
been examined (Merwin) were found to contain crystals which correspond to the
two known mineral forms of antimony oxide: senarmonite, which is isotropic,
and valenitinite, which is orthorhombic. They also contain some octahedral
arsenic oxide as impurity. Antimony oxide is an inert substance to vehicles and
its oil absorption is low (i 1.2 grams oil to 100 grams pigment [Gardner-Coleman]).
Since it is darkened by hydrogen sulphide, it is usually mixed with zinc oxide,
which has preferential absorption for that gas. Antimony oxide has not been
mentioned specifically as an artist’s pigment, and it has no advantage over other
white pigments.

Antimony Vermilion is antimony sulphide, Sb2S3; it may be prepared by pre¬
cipitating antimony chloride with sodium thiosulphate or with hydrogen sul¬
phide, and it may be had in hues varying from orange to deep red. It precipitates
in minute isotropic red globules. It was first made by C. Himly in Kiel in 1842
(see Rose, p. 15). Although antimony sulphide figures as a pigment in the rubber
industry, it is little used in paint because it is fugitive and not very stable chem¬
ically. It is said (Weber, p. 120) to have been used as an adulterant for real

94

Painting Materials

mercury vermilion. It is soluble in alkalis and in strong acids^ and turns black
on heating (Rose, p. i6).

Antimony Yellow (see Naples YeEow).

Antwerp Blue (see Prussian Blue).

Armenian Bole (see Bole).

Artificial Pigments (see Synthetic Pigments).

Arzica (see Weld).

Asphaltum (bitumen) is a brownish black, native mixture of hydrocarbons
with oxygen, sulphur, and nitrogen, and often occurs as an amorphous, solid or
semi-solid liquid in regions of natural oil deposits. It is thought to be formed from
the evaporation of the lighter components of the petroleum and from polymeriza¬
tion and partial oxidation of the residue. It is found widely, but that used in
European paintings came, perhaps, from the region of the Caucasus or the borders
of the Dead Sea. In Mesopotamia and Egypt in very early times it was known
and used for various purposes (see Partington, index). Asphaltum has little use
now, but is still listed by artists' supply dealers. Not much is known about its
preparation, but Church says (p. 235) that the crude asphaltum is usually heated
to a fairly high temperature to drive off moisture and volatile materials before it
is ground in oil or other mediums. The pigment is partially soluble In oil, like a
stain, and gives a semi-transparent, reddish brown film. In the film, it may be
occasionally observed microscopically as tiny brown flakes without structure.
Only thin grains are transparent brown. It is soluble in turpentine, naphtha, and
other organic solvents.

Asphaltum and other similar tarry compounds are among the least desirable
pigments known because they never become permanently dry. In thick oil films,
they have a tendency to run and to crawl, but, if they are properly prepared,
such difficulties may be partially overcome (see Church, p. 236). Doerner says
(p. 189) that Rembrandt used asphaltum as a glaze with no harmful effect. It Is
unaffected by acids and is unsaponifiable; it requires about 150 per cent of oil to
grind. Under ordinary circumstances, it is unaffected by light but Is faded by
strong exposure. It was much favored by the XVIII century English school,
with unfortunate consequences; those paintings which contained it have become
disfigured because of shrinkage of the paint films and 'alligatoring.' Harder paint
films put over it sometimes crack and curl. Neuhaus says (see footnote in his
translation of Doerner's The Materials of the Artist^ p. 89): 'Under high summer
temperatures in museums without thermostatic control whole areas of the pic¬
ture surface have moved and become permanently dislocated. Thus in several
warm climatic belts of America it has caused the destruction of many paintings
of the Munich school which at one time was passionately fond of asphaltum as
a frottie.’

Asphaltum is also sold to the artists' trade under the name 'bitumen.' Both
mummy (see Mummy) and bistre (see Bistre) are similar in color and composition

Pigments

95

to asphaltum in that they are tarry, organic substances, but their origin is quite
different.

Aureolin (see Cobalt Yellow).

Azurite (mountain blue) is a natural blue pigment which is derived from the
mineral, azurite, a basic copper carbonate, aCuCOs- Cu(OH)2. The mineral occurs
in various parts of the world in secondary copper ore deposits where it is fre¬
quently associated with malachite, a green basic carbonate of copper (see Mala¬
chite). Like other mineral pigments, this has been prepared from carefully selected
material by grinding, washing, levigation, and flotation (see Thompson, The
Materials of Medieval Painting, pp. 131-132). It has long since ceased to be of
importance in Western painting, and is rarely used today, except perhaps to a
limited extent in the East.

Azurite is crystalline and is fairly highly refracting and birefracting. For use
as a pigment, it is ground rather coarsely because fine grinding causes it to be¬
come pale and weak in tinting strength. Ninety-mesh azurite, however, is deep
violet-blue in color. Areas of dark azurite on paintings can often be recognized by
their sandy texture and by their thickness. Traditionally, it appears to have
been most used in a tempera medium because in oil it would be dark and muddy
and would not have the sparkle that it has in tempera. The characteristics of
azurite blues in old paintings are well described by Thompson {Joe. cit., pp. 132-
135)- The penetration into azurite paint in European panel paintings of successive
layers of oil and varnish films has often caused such areas to become nearly
black. If cleaned, the particles are usually revealed unchanged. Although there
may be instances where the pigment has turned green (to malachite) by hydra¬
tion, the more usual cause of the change in color is the optical effect of super¬
imposed layers of discolored varnish. Azurite is blackened by heat and by warm
alkalis, and it is soluble in acids, even in acetic acid; but, under ordinary condi¬
tions, it appears to be a remarkably stable pigment.

This natural copper carbonate was no doubt the most important blue pigment
in European painting from the XV to the middle of the XVII century and in
paintings of that period it is found more frequently than ultramarine. De Wild
(p. 23) lists nineteen early Dutch and Flemish paintings on which he identified
azurite. Europe had various sources of the mineral. There is evidence (see Laurie,
New Light on Old Masters, p. 42) that Hungary was the principal source in the
XVI century, but the pigment disappeared from the painter’s palette in the
middle XVII century when Hungary was overrun by the Turks. One of the early
names for azurite was azure d’Alemagna, indicating that it came from Germany.
It is the azurro della magna of Cennino Cennini, and was known by numerous
other names in mediaeval times (see Thompson, ‘Trial Index for Mediaeval
Craftsmanship,’ p. 415, f.n. 4 and 5).

Azurite was the most important blue pigment in the wall paintings of the
East. It was employed in the cave temples at Tun Huang in Western China and

96

Painting Materials

was used lavishly in wall paintings of the Sung and Ming dynasties in Central
China. With difference in the fineness of grinding, different shades were produced
(see Gettens, ‘Pigments in a Wall Painting from Central China’). The source of
azurite in China is not known, but there are extensive copper deposits in the
provinces of Kwei-chou and Yunnan. Uyeinura lists azurite among the ancient
pigments of Japan. It was known and used in ancient Egypt. Lucas says that it
occurs both in Sinai and in the Eastern Desert, and he cites (p. 283) examples
of its use in very early dynasties. Strangely enough, it seems not to have been
reported among pigments identified in Roman paintings.

Barium Wliite (barytes, blanc fixe, permanent white) is barium sulphate
(BaS04), which may be obtained naturally from the mineral known as barite,
barytes, or heavy spar, or it can be made artificially. Barytes is found widely in
Europe and in the United States. It can be prepared for use as a filler or extender
in paints by the simple process of grinding and settling. Frequently it serves as
a base for lake pigments. An extremely inert material, it .is quite unaffected by
strong chemicals, by heat, and by light. In judicious quantities, it may improve
the wearing and weathering qualities of lead and zinc white paints (see Toch,
The Chemistry and Technology of Paints, pp. 110-114). Barytes is a heavy inert
(sp. gr. = 4.3 to 4.6), but it does not have enough hiding power for a pigment
because of its transparency and medium refractive index = 1.637 [Larsen and
Berman]). Barytes has low oil absorption; some colors, which alone have high oil
absorption, need much less oil when ground with it {Colour Index, p. 303).

Blanc fixe is the name given to the artificial barium sulphate made by pre¬
cipitation from barium chloride solution with sodium sulphate. It is identical
with barytes, except that it is a finely divided powder and has much greater hid¬
ing power than the natural material. When co-precipitated with zinc sulphide,
lithopone (see Lithopone) is formed and, with titanium oxide, titanium barium
pigment is formed. Like natural barytes, it is an important lake base. Blanc fixe
is not opaque enough to be ground alone with oil for a white paint. As an extender
it is sometimes put into artists’ flake white and other artists’ oil paints. Several
grades of both barytes and blanc fixe are available now, but most of them contain
98 per cent or over of BaS04 (see Gardner, p. 1241). Barium sulphate has been
used in connection with paints since about the beginning of the XIX century
(see Trillich, II, 45-46).

Barium Yellow Oeinon yellow) (see also Strontium Yellow) is barium chro-
rnate (BaCr04), which is a pale green-yellow pigment made by mixing solutions
of neutral potassium chromate and barium chloride. The pigment formed is
deficient in brightness and hiding power. Microscopically, it may sometimes be
observed in nearly colorless, birefracting, rhombic plates. Other varieties are so
fine that crystal character and optical properties can not readily be observed.

^ urc says (p. 151): Of all the chromates which have been used in painting,
banum chromate is the most stable. It is nearly insoluble in water but soluble

Pigments

97

in dilute alkalis and in dilute mineral acids. It is decomposed by beat at high
temperatures but is little affected by light.’ It may become slightly greener on
exposure to light because of formation of chromic oxide. Although Vauquelin,
the discoverer of the metal, chromium (1797), described the preparation of barium
chromate as early as 1809, little seems to have been recorded concerning its first
use as a pigment. J. D. Smith compared the solubilities of barium and strontium
(mromates as early as 1836 (see ‘On the Separation of Barytes and Strontia,’
Philosophical Magazine, 3d series, VIII [1936], pp. 259-261). Barium chromates
and strontium chromates, which are quite similar, are both frequently sold as
lemon yellow.’

Barytes (see Barium White).

Bentonite is a colloidal clay which consists chiefly of the mineral, mont-
morillonite, a hydrous aluminum, iron and magnesium silicate. It is of volcanic
OTigin. and occurs rather widely. Large quantities are now mined near the Black
Hills in Wyoming and South Dakota and, also, in California. The outstanding
cha,racteristic of bentonite is its colloidal behavior when mixed with water, in
which it swells as much as 22 times its absolute volume and forms a stiff, jelly-like
paste which is smooth and soft like soft soap. Best dispersion is obtained by
adding the bentonite to the water. It does not swell and form gels if water is
added to^ it. Bentonite is unaffected by acids and alkalis. When heated above
205° C., It begins to lose water and, also, its colloidal properties. Only special
forms of bentonite are pure white; it is generally a warm gray because of its iron
content. Microscopically, it is characterized only by its very fine grain size.
(This information is taken from the data sheets furnished by the American
Colloid Company.)

Berlin Blue (see Prussian Blue).

Bistre (see also Asphaltum) is a brown water color pigment which is derived
from the tarry soot of burned, resinous wood and beechwood. It is similar to
asphaltum in color and composition. The color varies from saffron yellow to
brown-black, depending upon the source and treatment of the raw material. It
was sometimes mixed with red ochre to give it a warmer tone (see Meder, p. 68).
Church says (p. 234) that the ground raw product is washed with hot water
before it is mixed with gum and glycerine in the preparation of water color. The
tarry nature of bistre (as with asphaltum) makes it an unsuitable pigment, except
perhaps in very thin washes. He also says (p. 234) that exposure to strong sun-
light oxidizes the tarry matter of bistre and the residual pigment becomes cooler
in hue^ and paler. Tarry materials of wood origin have probably been used for
centuries. Meder says (p. 66) that first literary mention of bistre (caligo) was by
Jehan le Begue in 1431 ; it had been used extensively, however, in Italian book
illustrations in the XIV century. It was used by Rembrandt for wash drawings.

It is still listed by artists’ supply dealers, but is little used since it is admitted
by them not to be permanent.

Painting Materials

98

Bitumen (see Asphaltum).

Black Chalk, a bluish black clay containing carbon. (See also p. 285.)

B l ack Lead, the material used for ‘lead’ pencils, has no relation to lead metal,
nor is it a lead compound; it is a common term for the mineral, graphite (see
Graphite), or a mixture of clay and graphite which is more useful for writing
purposes. The confusion in terminology arose from the color similarity of graphite
and lead for marking purposes.

Blanc Fixe (see Barium White).

Blue Bice (see Blue Verditer).

Blue Verditer (blue bice) is a name now given to an artificial basic copper
carbonate, aCuCOs-Cu(OH)2, which is similar in chemical composition to the
mineral, azurite. This pale greenish blue pigment is little used today, but can
still be obtained from some artists’ colormen. Recipes for making artificial copper
blues or ‘azures’ are very old (see Thompson, ‘Trial Index for Mediaeval Crafts¬
manship,’ p. 415, f.n. 7). The more practical of these call for the addition of lime
or potash and sal ammoniac to a soluble copper salt like blue vitriol (copper
sulphate). Microscopically, blue verditer may be seen as tiny, rounded, fibrous
aggregates, even in size, highly birefracting, and blue by transmitted light. It is
similar in color to finely ground azurite. The artificial copper blues have not
been credited with great permanence, and Thompson says {The Materials of
Medieval Painting, p. 151) that they had a tendency to revert to green through
the loss of their ammonia content (see also Bearn, p. 93). According to Laurie
{The Pigments and Mediums of the Old Masters, p. 43), the manufacture of blue
verditer seems to have been carried on in England in large quantities at one time.
Thompson states {loc. cit.) that ‘the artificial blues from copper are probably
more significant in medieval painting than all the rest (of the blue pigments) put
together.’ They were the best cheap substitutes for the more expensive azurite
and ultramarine. Laurie {op. cit., p. 122) identified this pigment in various English
illuminated manuscripts {Coram Rege Rolls) of the early XVII century. He re¬
cords, in another place {New Light on Old Masters, p. 42) that it was used through¬
out the XVIII century and that the ‘Madame de Pompadour’ by Boucher, in the
National Gallery, Edinburgh, is painted with it.

Bole (Armenian bole, red bole) is the name frequently given in the arts to
clay, either white or colored. White bole is about identical with kaolin (see Cidna
Clay). Red bole is a natural, ferruginous aluminum silicate which was originally
found in Armenia but now elsewhere in Europe. It is similar to ochre in compo¬
sition but is softer and more unctuous, and because It is capable of receiving a
high polish, it has served since early mediaeval times as a ground for gilding.
It Is obtainable today under various names such as ‘gilders red clay’ or ‘red
burnish gold size.’

Bone Ash (see Bone White).

Pigments

99

Bone Black (animal black, drop black) (see also Carbon Black and Ivory
Black) is made by charring animal bones in closed retorts; usually bones from
glue stock, boiled to remove fat and glue, are used. Bone black is blue-black in
color and is fairly smooth in texture (see Bearn, p. 131). It is denser than carbon
or lamp black. It contains about 10 per cent carbon, 84 per cent calcium phos¬
phate, and 6 per cent calcium carbonate. Although the calcium compounds (ash)
in it have no color value, they improve the working (Quality and give a superior
black. Microscopically, it is also quite different from lamp black; the particles are
coarser and more irregular in shape and size, many of them being transparent
and some brown. Merwin, who has described some optical properties of ivory
and bone black,^ says (p. 514): ‘In charred bones a small amount of dark carbo¬
naceous matter is held in the most minute subdivision throughout a large amount
of calcium phosphate. Grains 5 iU in diameter transmit an appreciable brownish
color and appear practically homogeneous optically. Xheir apparent refractive
index, when their pores are filled with oil, ranges from about 1.65 to 1.70; thus
the desirable characteristics of slight diffusing power and effective absorbing
power are combined. Bone black sold under the name, ‘ivory black,’ is a favorite
among artists today. It works well in water color.

Bone White (bone ash), which is made by calcining animal bones, is composed
chiefly of tricalcium phosphate, Ca3(P04)2 (85 to 90 per cent); calcium carbo¬
nate and minor constituents make up the rest (see Thorpe). Ash from the bones
of different animals varies little in composition. Bone white is a grayish white
and slightly gritty powder. In mediaeval times, it was used on paper or parch¬
ment to give it tooth or abrasive quality to receive the streak of silver point
(Thompson, The Materials of Medieval Painting, pp. 94—95).

Brazil-Wood is a natural red dye from the wood of Caesalpinia braziliensis.
That used in the Middle Ages came from Ceylon. It was known and called
‘brazil’ many centuries before the discovery of the country, Brazil (Thompson,
The Materials of Medieval Painting, p. 116). Pernambuco wood, Caesalpinia
crista, from Jamaica and also from Brazil has about twice as much coloring
matter, and various other trees of the family, Leguminosae, yield brilliant dye¬
stuffs. The interior of the live wood is light yellow, but changes rapidly to deep
red on exposure. Brazil-wood extract is made by boiling the finely chipped wood
with water and by concentration of the liquor in vacuo. The leuco compound is
brazilin, CieHuOs, which forms, on exposure to air, brazilein, C16H12O6, which is
deep red to brown in color [Colour Index, p. 297).

Brazil-wood lakes, which are prepared with different mordants, vary in color
from bright cherry to deep red and are sold under a variety of names. One method
formerly used was to extract the chips with hot alum solution, followed by pre¬
cipitation with lye (Thompson, The Materials of Medieval Painting, p. 118; see
also Perkin and Everest, pp. 627-628). These lakes are insoluble in water and in
alcohol but are partially soluble in alkalis, giving them a brownish red color.

lOO

Painting Materials

Mineral acids decompose them with a yellow to orange-red solution. They are
not stable in strong light. Brazil-wood dyes are said to have been used in great
quantities in mediaeval times in dyeing, in painting, and in inks, peihaps more
than madder at an early date (Thompson, loc. cit, pp. 120-121), but were later
replaced by more brilliant colors.

Bronze Powders are metal flake pigments made commonly from copper-zinc
alloys (brass), but for some powders copper-tin alloys (bronze) are also used.
The copper-zinc powders go into imitation gold paint. Formerly all metal bronze
powders were made by the same method: first rolling or beating into foil or leaf,
and then powdering. This method was expensive. Since about i860 to 1865, they
have been made directly from sheet metal, up to one eighth inch thick, in special
stamping machines (see Otto Von-Schlenk, ‘The Manufacture of Bronze Powder,’
The Metal Industry [New York], XIV [1917], pp- 77~78, 161-163, 200-203).
Numerous shades and colors, ranging from citron yellow to orange, can be made,
depending upon alloy composition. The alloy, Cu, 95 : Zn, 5, gives a powder the
color of nine-carat gold; 90 : 10 is pale gold; 85 : 15 is yellow gold; 70 ; 30 is
greenish gold, etc. (see Oliver Smalley, ‘The Manufacture of High-Grade Alumi¬
num and Bronze Powders,’ The Metal Industry [London], XXVII [1925], pp. 1—2).

The flakes of these powders are very thin: in the order of 1/50,000 to 1/100,000
inch (Von-Schlenk, he. cit., p. 77). In spite of thinness, the flakes are quite opaque
when viewed under the microscope; they have fairly regular and uniform dimen¬
sions and contain little impurity (see three photomicrographs shown by Smalley,
op. cit., XXVII [1925], p. 186).

Bronzing liquids are generally solutions of nitrocellulose in amyl acetate
(banana oil) or other organic solvents.

Brown Madder is made by the gentle charring of madder lake or alizarin to
give a dull, brownish red color. Although it is not permanent and seems to be an
unnecessary color in these days, it is still listed by a few colormen. Paints now
sold under this name may be mixtures of pure alizarin with burnt sienna or
another similar earth color.

Brown Ochre (see Ochre).

Brown Pink (see Pink).

Buckthorn Berries (see Persian Berries Lake).

Burnt Sienna (see Sienna).

Burnt Umber (see Umber).

Cadmium Green is not a color produced by any pure cadmium compound
but is the name given to a warm green made from a mechanical mixture of trans¬
parent oxide of chromium (hydrated green or viridian) with cadmium yellow.
That produced by one manufacturer is said to be 93 to 94 per cent hydrated
green plus cadmium yellow (Gardner, p. 1349).

Cadmium Orange (see also Cadmium Yellow) is one of the color modifications
of cadmium sulphide (CdS). Toch {The Chemistry and Technology of Paints,

Pigments

lOI

P-_ 73 )) in dealing with the preparation says: ‘If the solution is made slightly
acid a yellow shade is produced and by changing the proportions of acid and
adding ammonium sulphide deeper shades may be made up to the deepest orange.’
The history of cadmium orange is about the same as that of cadmium yellow.

Cadmium Red (see also Cadmium YeUow) is a cadmium sulpho-selenide,
CdS(Se), which is prepared by precipitating cadmium sulphate with sodium
sulphide and selenium. By adjusting the proportion of sulphur to selenium and
by regulating the conditions of precipitation, shades varying from vermilion to
deep maroon may be obtained. Cadmium red is now a popular and favorite
pigment, and today it has, to a great extent, replaced vermilion on the artist’s
palette. Microscopically, it may be seen as tiny red globules less than i in
diameter, without appearance of crystallinity, and with very high refractive
index. The particles are strongly colored, deep red by transmitted light, and have
a characteristic appearance. The various cadmium sulpho-selenides are stable
and light-resistant under ordinary conditions. Their history is more recent than
that of the straight cadmium sulphides. Although a red-orange cadmium pig¬
ment containing selenium was mentioned in a German patent (no. 63558; see
Rose, p. 107) in 1892, it seems that the commercial production of cadmium reds
did not begin until about 1910 (see Toch, Paints, Painting and Restoration,
p. 105; also Rose, loc, cit,),

Cadmum Red Lithopone (see also Cadmium Yellow Lithopone) is a mixture
of cadmium sulpho-selenide co-precipitated with barium sulphate. It is made in
a way similar to lithopone; in one method the metal, selenium, is dissolved in
barium sulphide solution, the two co-precipitated with cadmium sulphate, and
the residue, after washing, is calcined (see U. S. patent no. 1,894,931 [1933], to
W. J. O’Brien). One manufacturer reports (see Gardner, p. 1274) that the barium
sulphate content in a variety of shades ranges from 55.0 to 58.5 per cent. In
microscopic appearance, it is very similar to pure cadmium red, except that at
high magnification irregular prismatic grains of barium sulphate can also be seen.

It IS stable under ordinary conditions and is light-fast. It is a strictly modern
pigment, having been in use only since 1926 (see H. W. D. Ward, under Cadmium
Yellow Lithopone).

Cadmium Yellow is cadmium sulphide (CdS) which is prepared by precipita¬
tion from an acid solution of a soluble cadmium salt (chloride or sulphate) with
hydrogen sulphide gas or an alkali sulphide. The color of the pure cadmium sul-
phide ranges in hue from lemon yellow to deep orange, depending upon the con¬
ditions of precipitation. Cadmium sulphide is found in nature as the mineral,
greenockite, but the use of the mineral as a pigment has not been mentioned.

E. T« Allen and J. L. Crenshaw (^The Sulphides of Zinc, Cadmium and Mer¬
cury; their Crystalline Forms and Genetic Conditions’ [Microscopic Study by
H, E. Merwin], American Journal of Science Jourth series, XXXIV [1912], pp.
341—396), from an extensive study of the crystalline nature of cadmium sulphides

102

Painting Materials

made in different ways have concluded (p. 365)3 basis of microscopic

examination, that differences in color are primarily dependent upon the amor¬
phous or crystalline nature of the sulphide and on its state of division Tney say
further (p. 366) that particles of the orange amorphous cadmium sulphide^are
in the order of 50 times as great in diameter as the yellow particles (see Cadimum
Orange) Many of the light or lemon shades of cadmium yellow are really cad¬
mium lithopone (see Cadmium Yellow Lithopone) which are co-precipitates with
barium sulphate. All of these cadmium yellows are very finely divided and even
in particle size and at high magnifications can be observed in tiny globules.
Cadmium sulphide has a high refractive index and, hence, good hiding power.
It is permanent and fast to light. The modern product, because of freedom from
excess free sulphur, is compatible with most other pigments. When stiongly
heated, cadmium sulphide burns to yellowish brown cadmium oxide. It is in¬
soluble in cold dilute acids and alkalis but readily soluble in concentrated mineral
acids with evolution of hydrogen sulphide gas. Although cadmium sulphide was
first observed by Stromeyer as early as 1817 and was introduced into oil painting
by Melandri in 1829 (Eibner, MalmateriaUenkunde, p. 128), it did not becoine
commercially available as a pigment until about 1846 (see Weber, p. 29). Laurie
(Jhe Pigments and Mediums of the Old Masters, p. 16) says that the cadmium
yellows were first shown in the 1851 Exhibition. It is now perhaps the most
important yellow pigment on the artist’s palette and is widely available in numer¬
ous shades. It appears that, until recently, the cadmium lithopones (see Cadmium
Yellow Lithopone) were the only cadmium yellows manufactured in this country,
the straight commercial cadmium sulphides being chiefly a German pioduct.
Now, however, the latter are beginning to be manufactured here.

Cadmium Yellow Lithopone (see also Cadmium Yellow) is a co-precipitated
mixture of cadmium sulphide and barium sulphate (from cadmium sulphate and
barium sulphide), made in a way similar to zinc lithopone (see Lithopone). The
precipitate is washed and calcined; it contains about 38 per cent cadmium sul¬
phide and can be produced in a variety of shades ranging from lemon yellow to
orange and at a cost considerably less than that for pure cadmium sulphide. It
is a very finely divided pigment and its properties are similar to the straight
sulphide. When this pigment was first introduced, H. W. D. Ward (‘New Cad¬
mium Pigments,’ The Chemical Trade fournal and Chemical Engineer , LXXX
[1927], pp. 59—60) reported that cadmium yellow lithopone had all the fastness
to light and heat of the pure sulphide but did not quite equal it in covering power.
On a cost basis, however, the ratio was in favor of the ‘ cadmopone,’ as he called
it. At the time of his report (January, 1927), he said that this pigment had been
on the market three or four months. Until very recently the cadmium lithopones
were the only cadmium yellows manufactured in the United States.

Carbon Black (see also Lamp Black, Ivory Black, Charcoal Black, Vine Black,
Graphite) includes various pigments that are derived from the partial burning

Pigments

lo;

or carbonizing of natural gas^ oil, wood^ and other organic materials. Almost none
of these products is pure carbon^ but all contain mineral impurities and hydro¬
carbons that are tarry in nature. Carbon makes a very stable pigment; it is un¬
affected by light and air and by hot concentrated acids and alkalis; it can only be
destroyed by burning at very high temperatures. As a pigment, it has excellent
hiding power in all its forms. Oil paints made from carbon black are sometimes
slow-drying; the freer they are from tarry matter, the better they dry (see Bearn,
p. 125). The organic black pigments, although they all contain carbon as their
essential constituent, vary considerably in shade and strength according to the
amount and particle size of the amorphous carbon in them.

Specifically, 'carbon black' is used to designate that produced in America by
allowing the smoky flame from natural gas to impinge against cooled, revolving
metal drums from which the black is automatically removed by scrapers (see
Cabot, p. 13, and Bearn, p. 130). This product is deep brownish black in color,
strong tmctonally, and is more granular and harder than lamp black and, unlike
the latter, wets well in water.

Carthame (see Safflower).

Cassel Earth (see Van Dyke Brown).

Celite (see Diatomaceous Earth).

Cerulean Bhie^ which is essentially cobaltous stannate, CoO-nSnOz (see
Church, p. a I a), is made by precipitating cobaltous chloride with potassium
stannate, thoroughly washing, mixing with pure silica and calcium sulphate, and
heating. It is a stable and inert pigment and is not affected by light or by strong
chemical agents. Physically, it is finely divided and consists of homogeneous,
rounded particles which are isotropic, high in refractive index, and green-blue
by transmitted light. It has limited tinting strength, but is the only cobalt blue
pigment without violet tint. It was known at the beginning of the XIX century
as a blue compound that could be made by heating tin oxide with a cobalt solu¬
tion, but not until the year i860 was it introduced under the name, ‘coeruleum,’
by Messrs G. Rowney and Co., who suggested its use for aquarelle and for oil
painting (Rose, p. 289). (The word, caeruleum, was used in classical times rather
loosely to indicate various blue pigments [see Bailey, I, 234].)

Chalk (whiting, lime white) is one of the many natural forms of calcium car¬
bonate (CaCOs). It occurs widely distributed over the world (see Ladoo, pp.

123-130). The deposits on the English coast and those in northern France,’Bel¬
gium, Denmark, and other European countries are well known. It is also found
in the United States, but not in quality good enough for whiting manufacture.
Natural chalk is a soft, white, grayish white, or yellowish (iron oxide) white rock
which is largely composed of the remains of minute sea organisms {FoTaminifera.).
The crude lump from the quarries is prepared by grinding with water and by
levigation to separate the coarser material. A very fine variety prepared in this
way is known as ‘gilder’s whiting.’ Chalk is quite homogeneous microscopically;

104

Painting Materials

at high magnifications (400 to 500 X) fossiliferous remains in the shape of tiny,
hollow shells can be seen. The material is highly birefracting, the shells being
made of tiny calcite crystals with their vertical axes in nearly radial directions;
its refractive index, however, is low, and this, in part, explains its poor covering
power and its discoloration in oil, although it covers well when used in water paints
and in distempers. Mixed with white lead and linseed oil, it makes glazier’s
putty. As a filler and adulterant, It is put into cheap paints and it serves as a
base for lake colors. In northern Europe, particularly in England, France, and
the Low Countries, chalk was the inert commonly mixed with glue in the prepa¬
ration of grounds for painting just as gypsum (gesso) was used in Italy and in
the south. De Wild (p. 44) found it in the grounds of thirty-six Dutch and Flem¬
ish paintings. Under ordinary circumstances, chalk is stable, but when heated
strongly it changes to calcium oxide (lime), and it is decomposed by acids, with
effervescence of carbon dioxide gas. Made artificially, it is known as 'precipitated
chalk,’ and this is whiter and even more homogeneous than the natural material.

There are many other natural forms of calcium carbonate, some of which are
useful in painting. One of them, marble, is a familiar crystalline variety of calcium
carbonate or limestone. Marble dust has been mixed with lime for the plaster
ground of fresco painting and of lime-wax painting. Another, oyster shell white,
can be made from the shells of almost any mollusk. It was perhaps usual to burn
the shells before powdering them. This white was a pigment in Chinese and
Japanese painting (see Uyeihura, p. 47), and Thompson says that it appeared
also in mediaeval England, chiefly mixed with orpiment. Even calcined egg shells
were used as the source of a fine lime white. Coral, the calcareous remains of
various marine animals, yields, when ground up, a pale pink powder that the
Chinese and Japanese made into paint for certain purposes. Uyeinura says it
was used as early as the Tempyo period (VIII century) in Japan. Lime white,
derived from lime putty, Ca(0H)2, or water-slaked lime, went into Italian fresco
painting under the name, bianco sangiomnni (Thompson, The Materials of Medu
eml Tainting^ p. 97). On exposure to air, this was slowly reconverted to calcium
carbonate or chalk.

Charcoal Black (see also Carbon Black), the residue from the dry distillation
of woods, is made by heating the wood in closed chambers or kilns. That which is
produced from the willow, bass, beech, maple, or such other even-textured wood
is the best. For pigment purposes, the charcoal is ground and well washed to
remove potash. It may be used in stick form for sketching purposes and for the
preparation of cartoons. Charcoal is light and porous; in part, it retains the fine
structure of the wood from which it was made and, for this reason, it is quite
characteristic in appearance when viewed microscopically. It may be seen as
small, opaque, elongated, and splintery particles. This form of carbon has been
used as a pigment since very earliest times. It is gray-black and is weak tinc-
torially. It is found on the wall paintings at Bamlyan in Afghanistan (see Gettens).

' 3/7

Pigments

Laurie says that the cool grays of Frans Hals are a mixture of white lead ahSj®
charcoal black (New Light on Old. Masters, p. 127).

China Clay (pipe clay, kaolin, white bole), the natural hydrated silicate of
aluminum, Al203-2Si02-2H20 (kaolinite), is found in vast beds in many parts
of the world and is the essential raw material of the ceramic arts (see Ladoo,
pp. 138-161). The term is usually reserved, however, for the nearly pure (iron
oxide free) white clay with satin lustre that is used in the manufacture of fine
porcelain. Its plastic qualities, when it is mixed and worked with water, are of
great importance for ceramic purposes. In European paintings, China clay was,
on rare occasions, mixed with glue for a ground or priming material on canvas or
panel. Among painters it has been known as ‘white bole,’ which is closely related

.^ to the red bole (see Bole) used so commonly as a ground for gilding. The Chinese

^ jX to have used it rather extensively in the priming for clay wall paintings

ii (see Gettens, ‘Pigments in a Wall Painting from Central China’). The term,

■1 kaolin, is Chinese in origin and is said to be a corruption of Kauling, meaning
‘Y- ‘high ridge,’ the name of a hill near Jauchau Fu, where the material is obtained
(see Dana, p. 57 ^)• Kaolin is not very characteristic microscopically, although
with suitable magnification vermicular crystals can be seen. It is semi-transparent,
finely divided, and homogeneous. The refractive index is low (/3 = 1.565), and
it is only weakly birefracting. It is inert chemically. When heated, it loses water
and becomes harder, as in the firing of pottery. It is unaffected by strong acids
or alkalis.

Chinese Blue (see Prussian Blue).

Chinese Ink (India ink) has been the favorite writing and painting material
of the East for centuries. It is lamp black (see Lamp Black and Carbon Black)
which is prepared by the imperfect burning of pine-wood or oil in earthenware
lamps. Bearn says (p. 129): ‘The soot formed is mixed with fish glue size, scented
with musk or camphor and moulded into sticks and dried.’ For use, the stick is
rubbed with water on a slate-like slab. The modern India ink, waterproof, pre¬
pared for draughting, is a proprietary material with a resin in the binding medium.
Chinese Vermilion (see Vermilion).

I Chinese White (see Zinc White).

^ Chrome Green (cinnabar green) is a name that has come into very common
^use for a green pigment that is made by mixing Prussian blue (see Prussian Blue)
and lead chromate (see Chrome Yellow). In the ‘wet’ method of preparation, a
slurry of Prussian blue is added to a pulp of barytes, China clay, and chrome
yellow, and the whole is stirred until thoroughly mixed (see Bearn, p. 96). The
product is a very homogeneous mixture and usually the components can not be
distinguished microscopically. In the light and medium varieties, the Prussian
blue seems to be smeared thinly and evenly over the yellow grains, but in the
darker varieties, separate particles of blue can be seen. Because this green has
excellent hiding power and has body and can be produced at low cost, it is the

io6

Painting Materials

most important commercial green pigment. Chrome green is not very suitabl
for an artist’s pigment^ however, because it is not light-fast. It has a tendency
to become blue in strong light because of the darkening of the chrome yello’v
component. It is sensitive to acids (turns blue) which dissolve the lead chromate
and to alkalis which cause it to turn dark orange because they effect the decom
position of the Prussian blue; it is, therefore, unsuitable for fresco. No date fo,
the introduction of this mixed green can be given. It must have been availabf
and in use shortly after the introduction of chrome yellow in the first quarter o
the XIX century.

Chrome Redj a brick-red, crystalline powder, is basic lead chromate
PbCr04-Pb(0H)2. It is made by boiling a strong solution of potassium dichrO'
mate with white lead and a small amount of caustic soda (see Bearn, p. 74)
Many of the particles of the deeper shades may be seen as perfect rectangular
tabular crystals that are highly birefracting. It is stable under ordinary condi¬
tions, but is not widely used as an artist’s pigment because it lacks brillianc]
and is readily affected by sulphur gases. Chrome red was first mentioned b)
L. N. Vauquelin, the discoverer of the metal, chromium, in 1809 {Annales
Chimky LXX [1809], p. 91). Little is known about its history as a pigment, bui
it probably came into use in the early part of the XIX century.

Chrome Yellow, the most important of the commercial yellow pigments, if
lead chromate (PbCr04). It is made by adding a solution of a soluble lead sail
(acetate or nitrate) to a solution of an alkali chromate or dichromate. (See Bearn
pp. 65-76, for full details.) Lead chromate is a crystalline material which car
vary in shade from lemon yellow to orange, depending upon the particle siz^
which, in turn, depends upon the conditions of precipitation. Lighter shades
usually contain lead sulphate, or other insoluble lead salts. The middle hues are
neutral lead chromate, and the orange leads are basic lead chromate. The pig¬
ment is finely divided, dense, and opaque. At high magnification, its crystalline
character can be observed; it consists of small, highly birefracting, monoclinic
prisms.

When chemically pure, chrome yellow is fairly permanent to light, but it is
frequently observed to darken and become brown on aging (see Weber, p. 40,
and Doerner, p. 63). Sometimes, especially when mixed with colors of organic
origin, it takes on a green tone (by reduction to chromic oxide). It is most satis¬
factory when used in oil. In fresco painting, only a basic lead chromate (chrome
orange or red) can be used, for yellow chromes are turned by alkali. Much chrome
yellow is used with Prussian blue to make chrome green (see Chrome Green),
As a pigment, it dates from the beginning of the XIX century. L, N. Vauquelin.
the discoverer of chromium (1797), described the preparation and properties oi
lead chromate in his 1809/Memoir’ {Annales de CA/mzV, LXX, pp. 90-91). He
mentioned that it could be prepared in different shades, depending on the con¬
ditions of precipitation. Chrome yellow did not come into commercial production.

Pigments

107

however^ before 1818 (De Wild, p. 69). One finds It occasionally on XIX century
paintings. Laurie says {New Light on Old Masters^ p. 44) that Turner used chrome
yellow and chrome orange. It is not much used now in painting because more
permanent yellows are available.

Ckronduni Oxide Green, opa«|tie. This is the most stable of the green pig¬
ments; it is the anhydrous oxide of chromium (Cr203), and Is made in various
ways, usually by calcining a mixture of potassium bichromate with boric acid
or sulphur. The product is a dull, opaque green which is irregular and fairly
coarse in particle size. This oxide is unaifected by heat, strong acids, and alkalis,
and is not faded by light. It is permanent in all painting techniques. The opaque
oxide is not so much in use by artists as the transparent oxide (see VMdian)
because it is dull. Vauquelin, the discoverer of chromium (i797)> suggested its
use for coloring ceramic glazes in 1809, but it evidently did not appear as an
artist’s pigment until about 1862 (see Laurie, New Light on Old Masters^

p. 44)*

Chromium Oxide Green, transparent (see Viridlan).

Chrysocolla was a classical name to indicate various compounds that were
useful in the hard soldering of gold (Greek: = gold; /coXXa = glue), and

among these were certain green copper minerals, the basic carbonate, the silicate,
etc. Pliny may have meant malachite by it (see Bailey, I, 105-111, and note
on p. 205). The name is now used by mineralogists, specifically, for natural copper
silicate (CuSi03*:?2H20), a mineral fairly common in secondary copper ore de¬
posits. In the natural state, its appearance is similar to malachite, except that
the color is somewhat more blue. When ground to a fine powder, it retains its
green color quite satisfactorily and may serve for a pigment in a water-soluble
medium. When seen microscopically, it is nearly amorphous or cryptocrystalline,
and is practically colorless or, at most, only a pale green by transmitted light;
particles with a crinkled surface are birefracting. The pigment is stable to light
and to ordinary environments but is decomposed by acids and is turned black
by heat and warm alkalis. This mineral has had little mention as a painting
material. Gettens identified it on wall paintings at Kizil in Chinese Turkestan
and described some of its properties. It occurs in Egypt and the Sinai peninsula
and has been identified by Spurred as a pigment on certain Twelfth Dynasty
tombs at ELBersha (see Lucas, p. 288), and at Kahun (see Spurred, p. 227).

Cinnabar (see Vermilion).

Cinnabar Green (see Chrome Green).

Clay is any plastic, variously colored earth consisting, essentially, of hydrous
aluminum silicate, H4Al2Si2093 formed by the decomposition of feldspar or other
aluminum silicates (see also Kaolin aW Bole). It may contain, also, undecom¬
posed feldspar and quartz and may be colored by iron oxides and other minerals.
Clay may be used as a filler in paints or it may be present as a necessary compo¬
nent of earth colors like the ochres, umbers, and green earths.

io8

Painting Materials

Coal-Tar Colors are made from the distillation products of coal tar, a by¬
product of coke and coal gas manufacture, and are compounds which contain
chiefly carbon, hydrogen, nitrogen, and sometimes sulphur. Benzene, toluene,
anthracene, naphthalene, phenol, and pyridine are all direct coal-tar distillation
products. By processes of synthetic organic chemistry, these distillation products
may be changed to dye intermediates like aniline, phthalic acid, etc., which, in
turn, may be synthesized to color products which are dyes. Since the discovery
of the first aniline dyestuff, mauve (see Mauve), by William Perkin in England
in 18563 many thousands of coal-tar dyes have been prepared. Some have become
important in the preparation of lake pigments, being valued for their richness
and brilliance in color. Many coal-tar lakes lack permanence and have rightly
caused the whole range of lake pigments to be looked upon with suspicion by the
artist. In recent decades, however, there has been a very decided improvement
in the permanence of coal-tar dyestuffs; like the dyes of natural origin, those in
the red region of the spectrum are the more permanent, but there has been a
great improvement in the stability of lake pigments for other regions of the
spectrum, examples of which are the Hansa yellows and the phthalocyanine
blues. For the future, there may be developed organic colors which will rival the
inorganic colors in light stability and general permanence.

Cobalt Blue (Thenard’s blue) is now the most important of the cobalt pig¬
ments. The simplest form is made by calcining a mixture of cobalt oxide and
aluminum hydrate to form, in part, cobalt aluminate (CoO-AhOs). One modern
manufacturer gives the composition as C03O4 = 32 per cent and AI2O3 = 68 per
cent (see Gardner, p. 1359). ^^7 be made in other ways: the original Thenard’s

blue was said to be cobalt phosphate on an aluminum base (see Church, p. 211).
The color varies slightly with different methods of manufacture and with the
amount of impurities present, but it is usually a pure shade of blue, especially
in natural light. Microscopically, the particles are characteristic; they are mod¬
erately fine, irregular in size, and rounded; the surface of some of the larger
particles has a crusty texture; they are bright blue by transmitted light, and are
isotropicthe refractive index is medium, about 1.74 in blue light (Merwin).

Chemically, cobalt blue is very stable; it is insoluble in strong acids and
alkalis and is unaffected by sunlight; it can be used in all painting techniques,
evp for the blue coloring of ceramic glazes, in much the same way as cobalt
oxide is used.

_ Cobalt blue was discovered by Thenard in 1802. De Wild gives a brief account
o Its history and says (p. 28): ‘Since the new pigment satisfied a recognized
demand, it was employed everywhere relatively soon after its discovery especially
m France, as was natural.’ The earliest picture painted in Holland on which it
was identified by De Wild (see p. 30) was 1840 and he adds, ‘Hence its use did
not penetrate into Holland directly after its discovery.’ It has been identified
on a water color painting by R. P. Bonington, 1801-1828. Since it is one of the

Pigments

109

most costly of artists^ colors, it is liable to adulteration and to substitution by
ultramarine and even blue lakes.

Cobalt Green (Rinmann’s green, zinc green) is similar to cobalt blue, except
that zinc oxide replaces wholly or partly the aluminum oxide in the latter. In
one of the ways of making it, a solution of a cobalt salt is added to a paste of
zinc oxide and water; the mass is then dried, calcined, and prepared for pigment
purposes by usual methods (see Church, p. 196). In the final product, there is
only a small proportion of CoO to ZnO, but the color, which is a bluish green,
remains much the same with widely varying proportions of cobalt. This indicates
that the two oxides, zinc and cobalt, form a solid solution and not a definite
compound like CoO-AI2O3 (see De Wild, p. 82). Cobalt green is semi-transparent
and does not have great hiding power. It is fine and regular in particle size; the
grains are rounded and transparent, bright green in transmitted light, and they
are highly refracting and birefracting. It is a stable and inert pigment and can
be used in mixtures and in different techniques. Although it Is soluble in concen¬
trated acids, it is unaffected by alkalis and by moderately high temperatures.
Church says (p. 196): Cobalt green is, in fact, one of the too-rare pigments which
is at once chemically and artistically perfect.' It has not had, however, great
favor with artists because in oil it covers only moderately well, is costly, and
because its color can so easily be imitated by mixtures. Although it was dis¬
covered by Rinmann in 1780 (see Rose, p. 290), it was not until after the middle
XIX century, when zinc oxide became available in large quantities, that cobalt
green in turn became commercially possible. Laurie {The Pigments and Mediums
of the Old Masters, p. 16) gives 1835 the date of the first literarv mention of
cobalt green as a pigment.

Cobalt Violet has been made in various ways, but the violet cobalt pigment
on the market today appears to be either anhydrous cobalt phosphate, Co3(P04)2,
or arsenate, Co3(As04)25 or a mixture of the two. The darker variety is the phos¬
phate. It Is made by precipitating a soluble cobalt salt with dIsodium phosphate,
washing, and then strongly heating the precipitate. The color is reddish violet;
it is transparent and weak in tinting strength, and this fact, in addition to its
high cost, seems to be the reason why it is not more generally used as a pigment.
It is stable and unaffected by most chemical reagents and can be used in all
techniques. Microscopically, it can be seen to consist of irregular particles and
particle clusters which are red-violet in transmitted light, highly refracting, and
brilliantly birefracting. Samples from different sources differ quite a little in
microscopic character. They are still listed by artists' colormen. The preparation
of cobalt phosphate as a pigment was first described by Salvetat (‘Matieres
minerales colorantes vertes et violettes,' Comptes Rendus des Seances de FAca¬
demic des Sciences, XLVIII [1859], pp. 295-297).

Cobalt YeEow (aureolin) is a complex, chemical compound, potassium co-
baltinitrite, CoEi3(N02)6*H20. It is made by precipitating a cobalt salt in acid

no

Painting Materials

solution with a concentrated solution of potassium nitrite (see Bearn, p. 79).
The precipitate must be thoroughly washed; otherwise it is not stable. The pig¬
ment has a very pure yellow color and a fair hiding power. It is fast to light and
air and is stable with other inorganic pigments, but it may accelerate the fading
of some organic colors and itself turn brown. It is decomposed by heat, by strong
acids and alkalis, and is slightly soluble in cold water. At fairly high magnifica¬
tion, it can be observed to be made up of tiny crystals and crystal clusters which
are yellow by transmitted light and appear isotropic in polarized light. The
pigment has been used perhaps more in water color than In oil. The compound,
potassium cobaltinitrite, was discovered by N. W. Fischer in Breslau in 1848
(see Rose, p. 296). It was first introduced as an artist's pigment in 1861 (Laurie,
New Light on Old Masters, p, 44). Messrs Winsor and Newton, Ltd, say (1930
catalogue, p. 14, and in a private communication) that it was first introduced by
them and was popularized by Aaron Penley, a celebrated water color painter.
They also say that they introduced primrose aureolin in 1889. Although avail¬
able today in water color medium, cobalt yellow does not appear to be widely
used as an artist's color, one reason being that it is expensive.

Cochineal (carmine lake, crimson lake) is a natural organic dyestuff that is
made from the dried bodies of the female insect. Coccus cacti, which lives on
various cactus plants in Mexico and in Central and South America. It was first
brought to Europe shortly after the discovery of those countries (see Beckmann,
I, 396-404). Eibner says {Entwicklung und Werkstofe der Wandmalerei, table,
p. 51) that it came in after the conquest of Mexico in 1523 and was first described
by Mathioli in 1549. The coloring principle of cochineal extract is carminic acid,
C22H20O13.

Carmine is an aluminum and calcium salt of carminic acid, and carmine lake
is an aluminum or aluminum-tin lake of cochineal extract. Carminic acid, the
pure extract of cochineal, gives a scarlet-red solution with water and alcohol, and
a violet solution with sodium hydroxide {Colour Index, p. 295). Crimson lake is
prepared by striking down an infusion of cochineal with a 5 per cent solution of
alum and cream of tartar. Purple lake is prepared like carmine lake, with the
addition of lime to produce the deep purple tone. Perkin and Everest (pp. 625-
627) describe methods for making the various cochineal-carmine lakes.

The cochineal lakes are not permanent to light. They turn brownish (Church,
p. 186) and then fade rapidly in strong sunlight, particularly when used in water
color. In oil, however, they are fairly stable and were used formerly in the prepa¬
ration of fine coach colors.

Cologne Earth (see Van Dyke Brown).

Color is a term used not only to indicate a certain region of the visible spec¬
trum but also to indicate the substances of pigments and dyes; or, frequently, as
a synonym for pigment or paint.

Copper Resinate is a green compound formed by dissolving copper acetate,
verdigris, or other copper salt in Venice turpentine, balsam, or similar resinous

Pigments

III

solution. It has been suggested by Laurie {The Pigments and Mediums of the Old
Masters, pp. 35-38) that the transparent green colors of early illuminated manu¬
scripts and of the 'oiF paintings of the early Flemish masters were of this nature.
(For more complete discussion^ see Verdigris.) Paint films colored with copper
resinate appear green-stained and do not owe their color to discrete green par¬
ticles of any crystalline copper mineral or salt.

Coral (see Chalk).

Cremnitz White (see White Lead).

Biatomaceous Earth (infusorial earthy diatomite^ celite) is a hydrous or
opalescent form of silica which is composed of the skeletal remains of very minute
aquatic organisms known as diatoms or radiolaria (see Ladoo, pp. 190-197).
Under the microscopCj with fairly high magnification^ many varieties of the tiny
diatom fossils can be seen. Deposits of this earth are found all over the world.
The largest and most extensively worked are in California, but they occur, also,
in Germany and in other parts of Europe. It is a light colored, light weight, finely
granular, and porous aggregate, insoluble in acids but soluble in alkalis. Diatoma-
ceous earth is widely used as a filter medium and as a bleaching agent for oils,
fats, and waxes; also as an inert and as an anti-settling agent in paints. Since it
adsorbs dyes, it serves as a base for certain lake colors.

Diatomite (see Biatomaceous Earth).

Bragohs Blood is a dark red, resinous exudation from the fruit of the rattan
palm, Calamus draco, which is indigenous to eastern Asia. The resin which is
collected from wounds in the bark, as well as from the fruit, is heated and molded
into short sticks which are sent to the market wrapped in palm leaves. The resin
is odorless. It is soluble in alcohol and other organic solvents, giving a red solu¬
tion. Although dragon’s blood has been used in the arts as a stain for coloring
varnishes, principally varnishes to be used over gold and other metals, it seldom
has been used directly as an artist’s color. It is fugitive unless locked in a resin
film. Little is known about it in painting, but probably it has been used to a
slight extent In all periods up to the present. It was known very early in the
Near East, having been called 'cinnabaris' by Pliny (see Bailey, I, 121), who
admits confusion with the Greek word, ‘cinnabar,’ which meant minium or
mercuric sulphide. It is he who established the myth that it was a product of the
mingling of the blood of those traditional enemies, the dragon and the elephant,
in a furious death struggle. Cennino Cennini mentions its use in illuminations
but he discredits it (Thompson, The Craftsman^s Handbook, p. 2.6). Dragon’s
blood is no longer listed by artists’ colormen.

Butcli Pink (see Pink).

Bye is a coloring matter which is used in solution as a stain. It is different
from a pigment which is used suspended in a medium for painting (see Hackh).
Most of the dyes are complex, organic, chemical compounds and may be derived
from natural sources (like madder), but the great bulk is now made synthetically

II2

Painting Materials

(see Coal-Tar Colors). There are many classes of dyes; classification can be based
upon the method of application (acid dyes, basic dyes, mordant dyes, etc.) or
upon their structure (azo, triphenylmethane, etc.). They consist in structure of
a chromophore (coloring) group and a salt-forming (anchoring) group. Some may
be used directly in vehicles for staining but, for pigment purposes, most of them
are precipitated or struck on inerts to form lake colors (see Lake). Although the
pure, solid dyes are frequently crystalline when dissolved in a solvent or paint
medium, they are quite without structure and, even under the microscope at high
magnification, reveal no discrete particles.

Earth PigmentSj broadly speaking, are those which are derived from minerals,
ores, and sedimentary deposits of the earth's crust. More specifically, they are
those complex mixtures of minerals that comprise the clays, ochres, siennas, and
umbers. Carbonaceous pigments, like Van Dyke brown, also belong in this group.
Earth pigments were among the earliest employed, and they include many of the
highest stability.

Egyptian Blue (blue frit, Pompeian blue). The inorganic blue color most
commonly found on wall paintings of Egyptian, Mesopotamian, and Roman times
is an artificially made pigment which contains as its essential constituents copper,
calcium, and silica. Lucas, who gives a very good summary (pp. 284-285) of the
history and occurrence of this blue, says it was made by heating a mixture con¬
taining silica, a copper compound (probably generally malachite), calcium car¬
bonate, and natron (natural sodium sesquicarbonate). A. P. Laurie, W. F. P.
McLintock, and F. D. Miles ('Egyptian Blue,' Proceedings of the Royal Society
of London^ Series zf, LXXXIX [1914], pp. 418—429), who carried out an investi¬
gation of methods of preparing it, found that the blue crystalline compound is
formed only in the rather narrow temperature range of 800 to 900° C., probably
about 830°. Chaptal appears to have been the first to call this material a 'frit'
but, although it does contain some glass as impurity, the blue is definitely a
crystalline compound. Laurie and co-workers {loc. cit,) point out that the Egyp¬
tian blue pigment is closely related to the well known blue glaze of Egyptian
ceramics; that glaze was applied to a base of carved sandstone at a temperature
somewhat lower than that required to form the crystalline blue. There is con¬
temporary mention of this artificial blue which includes descriptions of its method
of preparation. It is no doubt the Egyptian caeruleum of Pliny (see Bailey, I,
145 and 234). Vitruvius (VII, Chap. XI) describes its manufacture but errone¬
ously statp that the method for making it was first discovered in Alexandria.

blue which is coarsely crystalline and pure blue in color is similar,

appearance, to finely ground azurite. Unlike azurite, however, it is insoluble
in acids, is not affected by light or heat (except at very high temperatures), and
by alkalis only on fusion. Many specimens, well over 3000 years old, appear to
be little changed by time or environment. The blue is characteristic microscopi¬
cally; It IS birefractmg (o) =1.635), and it is moderately pleochroic, the crystals

Pigments

113

varying in color from deep blue to faint lavender. This ancient blue invariably
contains some calcite and quartz as impurities. Raehlman (pp. 67-68) has well
described, with the aid of color plates, its appearance in a paint film.

The history of Egyptian blue is largely ancient. Spurred states (p. 227) that
it was found as early as the IV Dynasty in Egypt. Laurie (‘The Identification of
Pigments . . . / p. 166) observed it on paintings from the palace at Knossos.
Raehlman (loc. ciL)y Chaptal {loc, ciL)^ and others have found it on Pompeian
and other Roman wall paintings. It has further been identified as the dark blue
material of a mace bead from Nuzi, Iraq (c 1500 B.C.); as the material of the
blue inlay in ivories from Samaria; and as a blue pigment on Roman wall paint¬
ings from Dura-Europos in Syria. Partington has reviewed (pp. 117—119) the
history and occurrence of Egyptian blue and he says (p. 118): ‘No ancient Euro¬
pean people could successfully imitate Egyptian blue and the secret of Its manu¬
facture was lost between A.D. 200 and 700.’ Lump specimens of Egyptian blue
can be seen at the Naples Museum, at the Fogg Art Museum, Cambridge, Mass.,
and at other places. A modern blue pigment called ‘Pompeian blue,’ which is
entirely similar in chemical composition and in optical properties to the ancient
copper-lime-silicate blue but which is purer and finer, is now available from a
French source.

Emerald Green (Schweinfurt green, Paris green) is an artificial pigment which
was first made at Schweinfurt, Germany, in 1814 (see Rose, p. 140). It is copper
aceto-arsenite (Cu[C2H302]2*3Cu[As02]2) and can be prepared in several ways,
in all of which the important raw materials are copper, acetic acid (or verdigris),
white arsenic, and sodium carbonate. These are mixed in hot solution and the
precipitate is thoroughly washed and dried (Bearn, pp. 102-104 gives all the
details). Emerald green, as the pigment is now called, is bright blue-green in
color, is one of the most brilliant of the inorganic colors, and is quite unlike any
other green pigment. It has fair hiding power. Some specimens of emerald green
are quite characteristic; in these the particles consist of small, rounded grains,
uniform in size and, at high magnifications, are seen to be radial in structure.
Many particles appear to have a pit or dark spot in the center. The grains are
strongly birefracting. The particles in other samples of emerald green, however,
are not so characteristic in shape. It does not enjoy popularity as an artist’s
pigment chiefly because it is blackened by sulphurous air and pigments, and also
because it is poisonous and dangerous to handle. (As Paris green, it has long
been used as an insecticide.) It is readily decomposed by acids and by warm
alkalis, and it is blackened by heat. It is fairly permanent, however, in an oil or
varnish medium.

Emerald green has not been identified frequently on paintings. De Wild found
it on only one (dating i860). Occasionally it is seen as the green pigment used for
making an imitation patina over repairs on ancient Chinese bronzes.

Painting Materials

114

EngHsli Red is a name sometimes used for a light red iron oxide (see Iron
Oxide Red) formerly natural in origin but now^ as a rule^ made chemically by
heating iron vitriol (ferrous sulphate) with chalk. Artificial English red usually
contains gypsum.

English Yermilion (see Vermilion).

Eosine is the potassium salt of tetrabromofluorescein, C2oH606Br4K25 and was
first made by Caro in 1871 {Colour Index^ p. 194). It was formerly used for pre¬
paring red inks of a very fine scarlet hue, but is not a fast color; it fades rapidly
in sunlight. ‘Geranium lake' is the name sometimes given to a brilliant bluish .red
lake made by precipitating eosine on an aluminum hydrate base.

Extender is an inert (see also Inert), colorless or white, and usually trans¬
parent body used to diffuse or to dilute colored pigments. Extenders up to certain
proportions may increase and improve the wearing qualities of paints. The
barium sulphate which is used up to 75 per cent with titanium dioxide may be
regarded as an extender. The cost of the mixture is materially less than that of
pure titanium dioxide, but there is not a proportionate lessening in hiding and
covering power. The same is true of the calcium sulphate which is often present
in considerable quantities in artificial iron oxide reds. Some of the insoluble dye
pigments or toners have such high tinctorial power that it is more economical
and practical to use them with carriers and extenders. When an extender is added
to a paint or pigment in such quantity that it lowers the tinting strength, it
becomes an adulterant.

Filler is a white, inert, transparent material, low in refractive index, which is
used In paste form to fill imperfections in a surface that is being prepared for
finishing. Wood filler is a paste made with crystalline silica (silex). It is used to
fill the pores or grain of the wood with hard, non-shrinking, transparent material
so that varnish coats will go on smoothly and take a fine polish. The word,
‘filler,' Is sometimes used synonymously with ‘extender' (see Extender).

Flake White (see White Lead).

Flavine Lake (see Quercitron Lake).

French Ultramarine (see Ultramarine Blue, artificial).

Fuchsin (see Magenta).

Fuller’s Earth is a hydrous aluminum silicate of variable composition belong¬
ing to the clay group of minerals (see Ladoo, pp. 231-240). It occurs in sedi¬
mentary deposits in many parts of the -world but particularly in Florida and in
England. It is white, buff, gray, or olive in tint, and has physical properties about
the same as clay, except that it is characterized by a marked ability to absorb

vegetable oils. It gets its name from its original use

which was for fulling or removing grease from cloth.

Gamboge is a yellow gum resin which for centuries has been used as a pig¬
ment in the Far East. It is produced by several species of trees of the genus
indigenous chiefly to India, Ceylon, and Siam. It came to Europe quite

Pigments

115

early as an article of commerce, and Church says (p. 153) that it was used by
the early Flemish oil painters. It has been principally a water color or a color for
spirit varnishes and gold lacquer. Mixed with Prussian blue or indigo, it makes a
rich green that was formerly a water color paint (see Hooker's Green). It is
drawn from trees by means of artificial incisions from which it runs as a yellowish
brown, milky juice that hardens in the air (Bearn, p. 172). It is marketed in the
form of yellow cakes and lumps which are rather brittle and are often covered
with yellow dust. Gamboge, w^hen powdered and ground in oil, has a rich golden
hue and in this medium it is fairly permanent. As a water color, it is less per¬
manent and fades rapidly in sunlight; but in manuscript painting where it is well
protected it has, in some cases, lasted for centuries.

Gamboge burns with an odor of burning resin. It first turns deep orange or
red in dilute caustic soda and then dissolves; since it is a resin, it is partially
soluble in alcohol and in some other organic solvents. Microscopically, it is not
particularly characteristic; the particles are deep yellow or orange and are fairly
transparent by transmitted light.

Gesso (see also Gypsum), in its broadest meaning, is any aqueous, white
priming or ground material that is used to prepare wooden panels or other sup¬
ports for painting or gilding. The word is Italian for gypsum. The white ground
for an Italian panel painting was usually a mixture of glue and burned gypsum
(plaster of Paris). Among the Italian painters, two distinct kinds of gesso were
recognized. As described by Cennino Cennini (see Thompson, The Craftsman's
Handbook^ p. 70), gesso grosso is the coarser and thicker undercoating for a panel;
it was made directly from sifted plaster of Paris and applied with size. Gesso
sottile^ however, was the fine, crystalline gypsum made by soaking plaster of
Paris for some weeks in excess water so that it did not set. The residue, after the
water was poured oflF, was made up into small loaves, was dried, and was kept
for future use. Mixed with glue size, it served for the final coatings over the
foundation of gesso grosso (Thompson, op, cit.^ pp. 71-72). Today the word,
gesso, has taken on even wider meaning and may include grounds made from
chalk (whiting), zinc oxide, or any other inert white. Among modern sculptors,
the word is used for plaster of Paris alone, without glue binder (see Thompson,
op. cit.^ p. xvi).

Gold Leaf and Gold Powder have been valued in painting for making back¬
grounds and details. Because gold is the most malleable of the metals and may
be beaten into leaves of extreme thinness, it can be used quite economically even
to cover large surfaces. Gold beating is a special craft; the foil is placed between
sheets of parchment called ‘gold-beater's skin,' and these are stacked, one upon
the other, and beaten until the metal seems almost to lose its third dimension.
In mediaeval times, gold coins were the direct source of most of the leaf, and its
thinness was reckoned in terms of the number of leaves that could be made from
a ducat (Thompson, The Materials of Medieval Paintings pp. 190-229) . The color

Painting Materials

ii6

depends to a great extent upon purity. When the surface of pure gold leaf is
highly burnished or polished^ it becomes a very good reflecting surface^ less yellow3
more dark and metallic.

Gold powder can be made in various ways^ but not by direct stamping or
rubbing because it is too plastic. One method during the Middle Ages was to
make it into an amalgam with mercury and then to drive off the mercury by
heatj leaving the gold in powder form. Another was to grind gold leaf in a mortar
with honey and then wash away the honey with water. Today there are various
electrolytic and reduction methods that serve the purpose equally well. There is
evidence that river gold or gold dust was used on certain English manuscripts
(Thompson, he. cit.^ p. 198). Powdered gold leaf was used in mediaeval times for
a writing ink; it was applied mixed with egg white or gum and, when dry, was
burnished so that the letters looked as though they were cut from gold leaf.
Unburnished, powdered gold was used quite freely in panel paintings where
brilliance and luminosity were demanded; it was even mixed with transparent
colors; painted hair was sometimes streaked with gold to increase its luminosity.

Chiefly, however, gold was laid as leaf. There were various ways of making
the leaf adhere to the surface, but for large areas a bole or fine earth with size
was usual. For initial letters and illuminations on manuscripts, an aqueous me¬
dium (glair, size, honey, plant juices) was brushed onto the part to be gilded.
The film was allowed to dry and was moistened by breathing to make it sticky
just before the leaf was laid on. For panel and wall painting, however, oil mor¬
dants were more common. These were really thin coatings of an oil varnish on
which the gold was laid while they were still tacky. (See also Bole and Mordant.)

Golden Ochre (see Ochre).

Grain Lake (see Kermes).

^ GrapMte (see also Carbon Black) is a crystalline form of carbon which is
widely distributed naturally as a mineral in different parts of the world. The
most important modern source is Ceylon; European sources have been Cumber¬
land, Bavaria, and Bohemia, where deposits have been worked for centuries. It
has also been made artificially by a furnace process (Acheson process) since about
1891. Graphite has long been used as a writing material and it gets its name
appropriately from the Greek, ypa<l>eLv (to write). It was early confused with
lead which was also used for writing purposes, and hence the names ‘black lead’
and plumbago are also used for it. For use in lead pencils, it is compressed with
very fine clay. Graphite has a greasy texture and is dull gray. Merwin in de-
scribing it says (p. 514): ‘Microscopic flakes thin enough to transmit grayish
ight have been prepared. Its refractive index is about 3 and its reflective power
high (about 37 per cent).’ Graphite is one of the most stable and refractory of
materials and would be permanent in any technique. It has been used chiefly as
a^ikawing material, however, and rarely as a pigment (Thompson, The Materials
of Meateval Pmntmgyp. 2^).

Pigments

117

Green Earth, (terre-verte) has been used in European paintings since before
classical times. It occurs rather widely but that which is suitable for a pigment
is found only in restricted areas. A good quality (celadonite) is found north of
Monte Baldo, near Verona {terre de Verone; see Church, p. 190), and also in
Germany, France, Cyprus, and Cornwall (see Rose, pp. 205—206). Most of the
green earths seem to have originated as marine clays. They are complicated in
composition but are made up chiefly of two indefinite but closely related min¬
erals, glauconite and celadonite, which are essentially hydrous iron, magnesium,
and aluminum potassium silicates. Green earth varies in composition like so
many of the complex silicates (see Clarke, pp. 519-523). Although the color may
be caused in part by a small content of iron in the ferrous state, the greater part
of the iron is ferric. The shade ranges from a neutral yellow-green to pale greenish
gray. The bpt quality is a neutral sage green. Green earth has a low hiding
power, especially in oil, but it works well in tempera. Microscopically, it is char¬
acteristic ; it consists of coarse, rounded, smoky green particles with many trans¬
parent, clear, and angular silica and silicate particles. Quite frequently, scattered
bright blue particles may be seen. De Wild states (p. 74) that these blue particles
are like cobalt blue; but there is no cobalt present and, unlike cobalt blue, the
particles are birefracting. The green is turned red-brown on strong heating; other¬
wise, it is a very stable pigment, unaffected by light or air or by chemical agencies
such as dilute acids or alkalis. Church (p. 192) says, however, that some samples
of terre-verte are liable to become rusty when brought into contact with lime
hydrate in true fresco painting. Although good grades of green earth are still
obtainable, it is subject to substitution by mixtures of transparent oxide of
chromium (viridian) and red earth pigment. The true green earths, which are
supplied by different dealers, usually vary in character and shade because of their
many different natural sources.

Green earth was used as a pigment on Roman wall paintings at Pompeii and
at Dura-Europos. It was widely used by Italian painters as a foundation for flesh
tones, and is the pigment that gives the greenish tone to so many of the abraded
Italian panel paintings. De Wild has reported it (p. 75) on three Dutch paintings
of the XVII century and on one of the XIX century. The green from the ceiling
of Cave I at Ajanta (India) was identified as green earth.

Green Lake is no particular compound, but a name to indicate various green
organic colors of natural or synthetic origin. Mixtures of Prussian blue with zinc
yellow or yellow lakes may be sold under this name.

Guignet’s Green (see Viridian).

Gypsum (terra alba) (see also Gesso) is important among the'raw materials
that have been used in works of art. It is calcium sulphate dihydrate, CaS04- 2H2O,
and, often associated with salt deposits, occurs widely over the world; important
workings are found in most of the countries of Europe, in the United States, and
in Canada. There are several varieties: selinite is crystalline, transparent, and

ii8

Painting Materials

foliated; satin spar, a fibrous form with silky lustre; alabaster is fine-grained,
massive, and may be nearly pure white or delicately shaded (to be distinguished
from Egyptian alabaster, which is a compact, crystalline form of calcium car¬
bonate [see Lucas, pp. 56-57]); ordinary, dull-colored rock gypsum, which is a
compact granular form, coarser grained than alabaster, often contains impurities
of calcium carbonate, clay, and silica (see Ladoo, pp. 281-299). Gypsum gets
its name from the Greek, which means, more specifically, the calcined

mineral (Dana, p. 634).

The raw, unburned gypsum finds little use in the arts. It was perhaps occa¬
sionally ground and used directly with glue in the preparation of gesso grounds
for mediaeval and Renaissance panel paintings. Lately it has been recommended
by Doerner (p, 14) to be used with zinc white and glue for such purposes. Since
raw gypsum itself has no setting properties, it acts solely as an inert, and depends
upon the glue for binding. A fine grade of ground native gypsum or alabaster,
which may be used for this purpose, is sold today under the name, "terra alba’;
it is ground to 200-mesh screen or finer, is bolted or sized by air separation, and
is used principally as a filler in paper and paint. The most important use of
gypsum is in the preparation of setting cements and plaster of Paris. Gypsum has
some utility as a base for lake pigments; it is a component, also, of certain arti¬
ficial iron oxide reds, like modern Venetian red (see Venetian Red), which are
made by calcining green vitriol (ferrous sulphate) with calcium carbonate

Gypsum is a stable material. The only effect of heat, as mentioned above, is
to drive off combined water. It is slightly soluble in water (2.41 grams per liter
at 0° C.); for this reason, gypsum plasters sometimes effloresce in damp places.
It is fairly soluble in dilute hydrochloric acid. From a saturated aqueous or weak
acid solution, it precipitates in characteristic needle-like crystals (monoclinic)
which group themselves in sheaf-like bundles about the edge of the test drop.
The refractive index of gypsum is low {p = 1.523); hence, it is not useful in oil.
It is weakly birefracting.

Haematite (see also Iron Oxide Red) is a hard, compact, and nearly pure
natural variety of anhydrous ferric oxide. The hard, specular kind is found in
columnar (pencil haematite) and reniform (kidney ore) shapes. Although prob¬
ably this compact form of haematite was sometimes ground and used for a dark,
purple-red pigment, ordinarily it was for the preparation of burnishers for gold
leaf (see Thompson, The Materials of Medieval Paintings pp. 213-214).

Hansa Yellow, which is among the most permanent of the modern synthetic
yellow dyes suitable for making yellow lake pigments, is formed by the coupling
of diazotized aromatic amines containing nitro or halogen groups or both, with
acetoacetanilide or its simple derivatives. There is a range of these Hansa yellows
all of which belong to the class of diazo coloring matters. Those with the desig¬
nation 5G and loG are used for the preparation of greenish yellow lakes good in

Pigments

119

hiding power^ fast to light (see Doerner^ pp. 66 and 92), and suitable for artists’
purposes.

Harrison Redj which Is said to get its name from the artist, Birge Harrison
(Weber, p. 62), is a brilliant red lake color or toner which is similar to, if not
identical with, Toluidine Red. Some other lake colors, however, may be sold under
that name.

Hooker’s Green is not a single pigment but is a mixture of Prussian blue (see
Prussian Blue) and gamboge (see Gamboge), used for water color, and is called
Hooker’s green after an artist who is said (Weber, p. 63) to have introduced it.
Different hues of green are obtained by varying the proportions of the blue and
yellow components. The mixture has the chemical properties of its components;
in strong light it is likely to turn blue because of the fading of the gamboge.
Microscopically, it may be seen as a distinct physical mixture of blue and yellow
particles. For oil colors, other yellows like cadmium yellow or yellow lakes are
used with Prussian blue to produce similar hues.

India Ink (see Chinese Ink).

Indian Lake (see Lac).

Indian Red was formerly a variety of natural iron oxide red (see Iron Oxide
Red) imported from India. It varied in color from light to deep purple-red and
contained, generally, over 90 per cent iron oxide. Although the term still indicates
a dark red oxide, it is now used for a pigment artificially made by calcining
copperas (ferrous sulphate) which is a waste material in certain industries. The
product must be carefully washed to get rid of soluble iron salts. Artificial Indian
red is pure, homogeneous, and dense, and has great hiding power. Other names
for it are ‘rouge,’ ‘colcothar,’ and 'caput mortuum'

Indian Yellow {purree) is a yellow organic extract formerly prepared at
Monghyr in Bengal from the urine of cows that were fed on the leaves of the
mango. Its manufacture is now mercifully prohibited by law. The coloring matter
is principally the magnesium or calcium salt of euxanthic acid, CigHieOnMg* 5H2O
(see C. Graebe, ‘Ueber die Euxanthongruppe,’ Annalen der Chemte^ CCLIV
[1889], pp. 265-303). The dried extract formerly came on the market in round
lumps, brown or dirty green outside and brilliant yellow-green inside. The crude
material must be powdered and washed and, when thus purified, it has a deep,
rich, translucent, orange color. Microscopically, it may be observed as a yellow
crystalline material with weak birefringence. This pigment was used in India in
the manufacture of paint and also as an artist’s oil and water color because of
its fastness to light. Church (pp. 154-156) found that even direct sunlight only
bleached it slowly.

Indian yellow is slightly soluble In water, is decomposed by hydrochloric acid
with precipitation of white, flaky euxanthic acid. The color which is sold today
under this name, however, either in oil or in water color, is a synthetic substitute
that may be just as permanent or more permanent than the original Indian yellow.

120

Painting Materials

Indigo (see also Woad) is a blue vegetable coloring matter which seems to
have been used in the Far East very early for dyeing cloth and for painting.
The dye is yielded by different plants of the genus Indigojera^ among which
J. tinctoria^ probably of Indian origin, was the .chief source of the indigo of com¬
merce until the time of the discovery of the process for making synthetic indigo
by Baeyer in 1880 (see A. Baeyer, ‘Synthese des Isatins und des Indigblaus/
Berichte der Deutschen Chemischen Gesellschqft^ XI [1878], pp. 1228-1229; 'Ueber
die Beziehungen der Zimmtsaiire zu des Indigogruppe,’ ibtd.^ XIII [1880], pp.
2254-2263; English patent no. 1177). Indigo was formerly grown all over the
world, particularly in India and China, but since 1900 the synthetic product has
almost entirely replaced the natural. Bengal indigo was one of the best grades
and was used widely in dyeing textiles. In the plant indigo exists as a colorless
glucoside called ‘indican.’ For the preparation of the dye, the freshly cut plants
are macerated, packed into large vats, and allowed to ferment; in this process
the glucoside is hydrolyzed to indigo and sugar. The dark precipitate is strained,
pressed, and dried into cakes (see Perkin and Everest, pp. 475-524). For use as
a paint pigment, it is not precipitated with a mordant but is ground directly to
a fine powder suitable for mixture with artists’ mediums. (For details, see Church,
pp. 217-223.) It is still used to a small extent for artists’ water color. Cake indigo
is a deep violet-blue with a bronze-like lustre. The coloring matter is indigotin,
C16H10N2O2 {Colour Index^ pp. 279 and 299).

Indigo has fair tinting strength; in thin films it is green and blue by trans¬
mitted light. Physically, it is much like Prussian blue but, chemically, it is quite
different. In an oil medium, no distinct particles can be seen at ordinary mag¬
nification; the dye appears to dissolve in the film and to stain it. De Wild says,
however (p. 30), that at a magnification of 1500X intensely blue discrete par¬
ticles appear. Though it can be worked in oil, it is better in tempera or in water
color. Indigo may fade rapidly when thin and exposed to strong sunlight, yet
specimens of it are frequently seen where it has lasted for many centuries without
apparent change. Locked in tempera beneath varnish films, it is very stable. It is
also stable chemically, being insoluble in water, ether, alcohol, lyes, or hydro¬
chloric acid. Nitric acid, however, decomposes it, with the formation of a yellow
compound called ‘isatin.’ It sublimes when heated at 300° C. Indigo is reduced
by several reducing agents to soluble indigo white, called ‘leuco indigo.’ This
reduction is an important operation in dyeing. The dye is taken up by the
fibres in this reduced soluble form and is then oxidized by the air to the insoluble
indigo blue. It is bleached by hypochlorite solutions.

Indigo was known and used as a dye in early Egypt (Lucas, p. 313). It was
mentioned by Pliny, who called it 'indicum^ \stt Bailey, I, 145, and II, 87 and
89)- It has been identified as one of the pigments used for decorating Roman
parade shields of r 200 A.D. found at Dura-Europos in Syria, was mentioned as
early as the XIII century in European commercial transactions (Church, p. 217),

Pigments

I 2 I

and was used in Italian painting certainly as early as the XV century and prob¬
ably even much earlier.

A coloring material much like indigo has been observed in a blue layer be¬
neath an azurite film in a Sienese painting. (See R. J. GettenSj 'Microscopic
Examination of Specimens from an Italian Painting/ Technical Studies^ III [1935],
pp. 165-173.) De Wild lists (p. 31) four paintings^ three by Frans Hals and all
of the XVII century^ in which he found indigo. Perkin and Everest say (p. 475):

Its employment in Europe was very limited until in 1516 when it began to be imported from India
by way of the Cape of Good Hope, but its introduction in large quantity did not occur until about
1602. Owing chiefly to the opposition of the growers of woad, its European rival as a dyeware, it
met with much opposition, and various laws were enacted both on the Continent and in England
prohibiting its use. It was called a ‘devilish drug’ and was said to be injurious to fabrics. In 1737
its employment was legally permitted in France, and from this period its valuable properties appear
to have become gradually recognized throughout Europe.

In the Far East, indigo was used as a pigment as well as for dyeing cloth.
It was identified in a blue layer beneath azurite on a Chinese painted clay statue
of the T'ang Dynasty from Tun Huang in Central Asia, and also on a wall
painting of later date from Kara Khoto in Central Asia.

Inert is the name given to any inactive white pigment which has little or no
hiding power or tinting strength when it is used in a paint vehicle. Examples are
gypsum, barium sulphate, chalk, etc. These generally have a refractive index
below 1.70. They may have a considerable whiteness when used with tempera
medium, but In oil they are nearly transparent and give only dull yellow films.
Inerts are employed for ground and priming materials (see Gesso). They may
be used as extenders (see Extender) for pigments with high tinctorial power, and
they may also be used for the bases, carriers, or substrates of lake pigments
(see Lake).

Infusorial Earth, (see Diatomaceous Earth).

Ink is a liquid or viscous material used for writing, printing, lithographing,
stamping, and staining. Inks are made from dyes and from pigment suspensions
like carbon black. Those used for printing and lithographing are made by grind¬
ing pigments in oils and varnishes. Ordinary writing inks are iron gall inks in
which the color and stain are formed by the combination of gallotannic acid from
oak galls and green vitriol (ferrous sulphate) in the presence of air (see Iron Gall
Ink and Chinese Ink). Both vegetable and aniline dyes are used for special,
colored writing inks (see Mitchell).

Inorganic Pigments are those natural pigments prepared from minerals and
ores (see Earth Pigments) or those synthetically made which are chemically pre¬
pared from the metals. The most stable and inert pigments are in this class.

Iris Green is an organic dyestuff of natural origin from the juice of iris flowers.
Thompson says {The Materials of Medieval Paintings p. 171) that it was used
extensively in the XIV and XV centuries, particularly in manuscript painting.
The beautiful green color was best developed by mixing with alum.

122

Painting Materials

Iron Gall Ink Is made from tannin or gallotannic acid which is derived from
oak galls. When this is combined with ferrous sulphate, a colorless compound,
ferrous gallotannate, is formed which develops a black color on exposure to air
because of oxidation to ferric gallotannate. Because 7 to 10 days are required for
complete oxidation, a dye or other provisional coloring matter is added to the
ink to give it immediate color. The ink ingredients are suspended in a solution
of gum and water (see Thorpe’s Dictionary^ Ink). It Is uncertain just when iron
gall inks came into use, but apparently it was some time in the Dark Ages or in
very early mediaeval times (see Mitchell, p. ii).

Iron Oxide Red (see also HaematitCj Indian Red, Light Red, Mars Colors,
Tuscan Red, Venetian Red). Ferric oxide in both its anhydrous (Fe203) and
hydrous (Fe203*?2H20) forms has been used as a coloring material since pre¬
historic times. It was formerly all derived from natural sources, but at present
much that is used is artificial in origin. The extensive deposits of iron oxide which
occur all over the earth vary widely in hue, depending upon the degree of hydra¬
tion and subdivision. The anhydrous oxide is dark purple-red or maroon while
the hydrated varieties range from warm red to dull yellow, as in yellow ochre.
Iron oxide is a very stable compound; it is unaffected by light and by alkalis;
it is soluble only in hot concentrated acids; and the only effect of heat Is to darken
the lighter colored varieties. Microscopically, it is moderately characteristic. The
natural forms are heterogeneous in composition and in particle size; In the darker
varieties, elongated and splintery, dark brown, lustrous particles of haematite
can be seen. In some varieties, the smaller particles are ruby-red by transmitted
light, similar to vermilion, but usually they are opaque and dense. The artificial
varieties are finely divided and homogeneous and have no very characteristic
optical properties. Distinction is difficult, even microscopically, between the finer
grades of natural iron oxide and the artificial varieties.

The iron oxide pigments have had such continuous use in all periods of paint¬
ing and in all parts of the world that it is unnecessary to go into details concerning
their history and occurrence in paintings. Even today they are commercially
among the most Important pigments. Both the natural and artificial varieties of
iron oxide are known by numerous names. Some names show the source; some
originally were applied to natural products but are now used for artificial ones;
others indicate some very special kind of preparation.

An excellent natural red oxide comes from Ormuz In the Persian Gulf and is
sold in large quantities under the name, ‘Persian Gulf Oxide.’ It contains about
70 per cent of Fe203 and 25 per cent of silica. The well known Spanish red oxide
contains, usually, more than 85 per cent of Fe203. These crude natural oxides
require only grinding and sieving to convert them into pigments. Finer products
are obtained by washing and levigation.

ItaHan Pink (see Pink).

Ivory Black, strictly speaking, is made by charring waste cuttings of ivory in

closed vessels and then grinding, washing, and drying the black residue. It is the

Pigments

123

most intense of all the black pigments {Colour Index j p. 314). The term is now
commonly used for the black from animal bones (see Bone Black).

Kaolin (see China Clay).

Kermes (kermes lake, grain lake) is one of the most ancient of the natural
dyestuffs. It was derived from the dried bodies of the female insect. Coccus ilicis^
found on the kermes oak, which was indigenous to many parts of southern Europe
(see Beckmann, I, 385-404). It is similar to the New World ‘cochineaF in origin,
color, and chemical composition. It contains the coloring matter, kermesic acid,
C18H12O9 {Colour Index j p. 295). Lucas suggests (p. 37) that it was used very
early in Egypt for dyeing leather, and, he thinks, with an alum mordant. It was
well known to the Romans; Pliny called it Coccum granum and praised it highly
(see Bailey, I, 33 and ai8). Thompson {The Materials of Medieval Paintings
p. 112) says that the English word for it, "grain,’ comes from the Latin, grana^
the equivalent of the Greek, kokkosj which means "berry.’ The ancients mis¬
took the dried red clusters of the dead insect, Coccus ilicis^ for berries. He goes
on to say, however {loc. ciL^ p. 113), that grain and kermes dyestuffs are not the
same, though similar in origin. According to him, the English word, vermilion,
comes indirectly from the Latin, vermiculum^ or "little worm,’ which was de¬
scriptive locally of the clusters of dead insects and the berries they caused to
grow around them. The word, "kermes,’ is Arabic in origin and is the source of
the English word, "crimson.’ Kermes and grain dyes were precipitated with alum
to form crimson lakes. Sometimes they were prepared with clippings of cloth
that had been dyed with grain. The dye was extracted with alkali and then was
precipitated with alum (see Thompson, loc. cit.j p. 115). Kermes lakes are not
brilliant, and in the Middle Ages they were displaced by the lac lakes from India
and, still later, by cochineal lakes from Mexico. They are perhaps of no importance
in modern painting.

King’s Yellow (see Orpiment).

Kremnitz White (see White Lead).

Lac, Lac Lake (Indian lake), is a natural organic red dyestuff prepared from
the resin-like secretion of the larvae of the lac insect. Coccus lacca^ which lives on
certain trees of the species, Croton jicus^ in India, Burma, and the Far East. From
the lac secretion which also produces the "seed lac’ or "shellac’ of commerce, the
red dye is extracted with hot dilute sodium carbonate solution when the shellac
is purified. The dye infusion is evaporated and the residue is made into cakes and
dried. The lake may be prepared by extracting the dry residue with sodium car¬
bonate, after purification in turpentine or benzene, and by precipitating it with
alum. The coloring principle is laccaic acid, C20H14O11, or its salts (see A. Dim-
roth and S. Goldschmidt, "Uber den Farbstoff des Stocklacks,’ Annalen der
Chemie^ CCCXCIX [1913], pp- 62-90). It is similar in composition and color to
the carmine dyes from cochineal, but the lac dye is somewhat faster if duller in
shade. Perkin and Everest (pp. 9i“94) say that lac dye is a very ancient dyestuff

124

Painting Materials

and that it was employed in the East many centuries before it was known in
Europe. R. Pfister ('Materiaux pour server au classement des Textiles Egyptiens
posterieurs a la Conquete Arabe/ Revue des Arts Asiatiques^ X [1936], pp. 1-16)
says that lac dye was brought from India and introduced into the West by the
Arabs after their invasion of Egypt and the fall of Alexandria in the VII
century.

Lakes as a term^ may be applied to any pigment which is made by precipi¬
tating an organic coloring matter or dye upon a base or substrate which is usually
an insoluble, finely divided, semi-transparent, inorganic inert, such as aluminum
hydrate or calcium sulphate. The word is derived from the Italian, lacca^ which,
in turn, seems to be associated with dac’ from India, the source of a red dye (see
Lac Lake), as well as ordinary shellac (see Thompson, The Materials of Medieval
Paintings p. 109). The earliest lakes were made from such natural dyes (see
Laurie, Materials of the Painter s Craft, pp. 253-278), but now they are prepared
in enormous quantities from synthetic dyestuffs. The true lake is a transparent
color precipitated on a transparent base like aluminum hydrate, but the word
has been extended to include those colors struck on barytes, tin oxide, zinc oxide,
and a number of other materials which produce pigments with body and hiding
power (see Bearn, p. 133). The same dye often produces different shades and
different hues with different bases (see also Coal-Tar Colors and Mordant).

Lamp Black (see also Carbon Black) is nearly pure (over 99 per cent), amor¬
phous carbon which is collected in brick chambers from the condensed smoke of
a luminous flame from burning mineral oil, tar, pitch, or resin. It is not quite a
true black but is slightly bluish in color, and makes good neutral grays. Micro¬
scopically, it is very finely divided, uniform, and homogeneous; in mounting
mediums, the particles appear to collect in chains and filaments. It does not wet
well with water on account of the slight amount of unburned oil it contains.
The preparation of lamp black {atramentum) from burning resin and pitch, as
well as the preparation of other blacks, is described by Pliny the Elder (see
Bailey, II, 87 and 216).

Lapis Lazuli (see tritraHiarine Blue, natural).

Lemon Yelow (see Barium Yellow and Strontium Yellow).

Light Red (see Iron Oxide Red) is a term sometimes used to Indicate a red
ochre prepared by calcining yellow ochre; it is a light, warm red, the hue depend¬
ing upon the degree of calcination. Today the term is also used for a processed
blend of ferric oxide and calcium sulphate which is almost identical in compo¬
sition with Venetian red (see Venetian Red).

Lime White (see Chalk).

Litharge (see Massicot).

Lithol Red or Lithol Toner is one of the most important and widely used of
the synthetic red dyestuffs in the modern lake pigment industry. It is i-sulpho-
^-naphthalene-azo-^-naphthol, C20H14N2O4S {Colour Index, p. 43). It is prepared

Pigments

125

as a lake by precipitation on barytes or chalk. LIthol red Is bluish red with a deep
blue-red undertone (see Bearn, p. 144). It does not bleed in oil and has good
stability to light and heat. It has not been offered to the artists’ trade under this
name, but no doubt is found in some cheap red paints as a substitute. It was first
made by Julius in 1899.

Lithopone is a co-precipitated pigment which is made by adding zinc sulphate
to barium sulphide in solution. The press cake, which is a mixture of zinc sul¬
phide and barium sulphate (ZnS + BaS04)5 is dried, calcined at red heat, and
quenched, a process necessary to give it useful pigment properties (see Bearn,
p. 53). The mixture of the two components, zinc sulphide and barium sulphate
(28 : 72)3 is so intimate that they can hardly be distinguished microscopically.
It is very finely divided, opaque, and without distinguishing optical character.
It has about the same whiteness but has greater hiding power than zinc white.
Lithopone is partially soluble in dilute mineral acids, with the release of hydrogen
sulphide from the zinc sulphide component; it is unaffected by alkalis and by
heat. This pigment, in the early days of its manufacture, had one serious defect,
a tendency to darken (gray or blacken) in strong light but to turn white again
in the dark (see Bearn, p. 54). The trouble was traced to various causes, among
which were the presence of foreign metallic impurities, but, after years of re¬
search, a lithopone is now produced which does not suffer change in light. The
so-called Titanated lithopones,’ which contain about 15 per cent titanium oxide
(see Gardner, pp. 1230-1232), have hiding power superior to that of straight
lithopone.

Lithopone was apparently first produced and patented by John Orr in Eng¬
land about 1874. It is now industrially important and widely used in interior
paints, lacquers, and enamels, for it has a combination of exceptional whiteness,
brightness, and low cost. It has not been much used as an artist’s pigment be¬
cause, perhaps, of its unfortunate early history. It is used for poster colors and
for cheap water colors. One may expect to find it in the priming coats of modern,
prepared artists’ canvas.

Litmus (archil) is a natural organic red coloring matter that is procured from
such lichens as Lecanora tartarea or Roccella tinctoria. It is extracted from the
dry plants by potassium carbonate solution in the presence of air. It is soluble
in water and in alcohol, giving a carmine red solution, and in alkalis, giving a
bluish violet color. Archil is quite similar to litmus, although obtained from other
species of Roccella and Lecanora (see Colour Index^ p. 297),

Logwood is the name of a red dye that is extracted from the wood of Haema-
toxylon campechianum which is indigenous to Mexico, Central America, and the
West Indies. The interior of the live wood is yellow but changes rapidly to dark
brown on exposure to air. The leuco compound, haematoxylin (CieHwOe), changes
to red-brown haematein (C16H12O6) on exposure to air. It is extracted by boiling
logwood chips in water over steam under pressure. The red-brown haematein
crystals are sparingly soluble in water. With sodium hydroxide, there is formed

126

Painting Materials

a purplish blue solution which changes to brown on exposure to air. Brown^
reddish brown^ blacky and blue-black lakes can be prepared from logwood ex¬
tracts with various mordants. All are insoluble in water and alcohol, are turned
bluish violet by alkalis, and are decomposed by mineral acids with the formation
of a blood-red solution. Logwood extract (as haematoxylin) is widely used as a
biological stain. When treated with bichromate, it was formerly used in the
manufacture of writing inks. Extracts were used, also, in dyeing and in the lakes
for water colors, but, because they are fugitive in strong light, they have been
discarded.

Madder^ Madder Lake (see also Alizarin), is a natural dyestuff from the root
of the herbaceous perennial, Rubia tinctorium^ which formerly was cultivated
extensively in Europe and in Asia Minor. Roots from plants i8 to 28 months old
grown in a calcareous soil are best. The coloring matter, which is chiefly alizarin,
or 1,2 dihydroxyanthraquinone (C14H8O4), is extracted from the ground root by
fermentation and hydrolysis with dilute sulphuric acid. The madder plant is
native to Greece and was used as a source of dye perhaps as early as classical
times (Church, p. 171). It is understood to be the rubia of Pliny (see Bailey, I,
37) and other classical writers. It has been identified as the source of a pink color
on a gypsum base from an Egyptian tomb painting of the Graeco-Roman period.
There are specimens of it in the Naples Museum (Lucas, p. 287). Perkin and
Everest say (p. 23):

About the time of the Crusades the cultivation of madder was introduced into Italy and probably
also into France. The Moors cultivated it in Spain, and during the sixteenth century it was brought
to Holland. Colbert introduced it into Avignon in 1666, Frantzen into Alsace in 1729, but only
toward 1760-1790 did it become important. During the wars of the Republic, its cultivation was
largely abandoned, and only after 1815 did this again become regular.

Madder lake and rose madder for artists’ pigments are prepared from the
madder extract by adding alum and precipitating with an alkali {Colour Index^
p. 296; see also Perkin and Everest, pp. 623-625). Thompson thinks that the
madder lakes were less employed in mediaeval painting than were the brazil
lakes. He says {The Materials of Medieval Paintings pp. 123-1 24) that pure
madders, as they are known now, came into use in the XVII and XVIII centuries
and that they were not important in the Middle Ages.

Madder was the source of the dye, Turkey red, formerly used in large quan¬
tities in textiles andjs still the color for French military cloth {Colour Index,
p. 296). The cultivation of the madder root and its employment for dyeing and
pigment purposes almost ceased shortly after a synthetic method for making
alizarin was discovered by the German chemists, Graebe and Lieberman, in 1868
(see Alizarin).

^ The extract from the madder root also contains another natural dye called
purpurin. This is closely related chemically to alizarin and is 1,2,4 trihydroxy-
anthraquinone, CuHgOs (Co/(?z^r Index, p. 251). The presence of purpurin dis-
tinguishes natural alizarin from the synthetic product. Pure purpurin gives lakes

Pigments

127

which are more orange and red than those of alizarin (Church, p. 173)’ This
accounts for the warm tone of madder lakes as compared with the pure alizarin
lakes. Eibner says {Malmatericiltenkunde^ p. 20) that purpurin is not so light-fast
as alizarin. He gives chemical methods by which the two may be distinguished
{op, cit.^ p. 203). He says, also ('Les Rayons Ultra-Violet Appliques a FExamen
des Couleurs et des Agglutinants,’ Mouseion^ XXI~XXII [1933], pp. 32-68), that
the presence of purpurin causes madder lake to fluoresce a fiery yellow-red in
ultra-violet light whereas synthetic alizarin lakes show only a feeble violet lumi¬
nescence.

Paint films colored with madder lake are nearly transparent and appear bright
red, with a definite violet hue by transmitted light. The base on which the dye is
prepared, particularly if it is aluminum hydrate, can seldom be distinguished,
even at high magnifications, because it is amorphous and transparent. Madder
lake is among the most stable of the natural organic coloring matters. The color
is turned purple by dilute sodium hydroxide but is only destroyed by much
stronger reagents. Harrison says (p. 231) that natural madder is still used in
France on a small scale for the production of fine artists^ colors. Their manu¬
facture is carried on by traditional methods. He further says (p. 239) that ali¬
zarin lakes now far surpass lakes from the natural madder in purity, brilliance,
and range of colors.

Magenta (fuchsin) is a brilliant red-purple organic dye, C2oH2oN3Cl, of the
triphenylmethane group of dyestuffs. It was first prepared by Natanson in 1856
{Colour Index^ p. 173). It is soluble in alcohol, acetone, and aqueous solutions.
Although a fugitive dye, it has been used for water colors and is still listed among
them by artists’ colormen.

Malachite (mountain green) is perhaps the oldest known bright green pig¬
ment. It is the natural (mineral) basic copper carbonate, CuCOa* Cu(OH)2, and
is similar in chemical composition to the blue basic copper carbonate, azurite
(see Azurite), except that it contains a greater amount of combined water. Like
azurite, it occurs in various parts of the world associated with secondary copper
ore deposits. It is prepared as a pigment by careful selection, grinding, and sieving,
but today it is seldom used, except perhaps to a small extent in the East.

Malachite is crystalline (monoclinic) and is fairly characteristic microscopi¬
cally. Particles of some varieties have a clear, faint, bottle-green color by trans¬
mitted light, and show high relief, strong birefringence, and pleochroism. Prisms
with longitudinal striations are common. Since it is a carbonate, it is decomposed
by acids, even acetic acid. It is unaffected by cold dilute sodium hydroxide but
blackens when warmed with that reagent and, also, when it is heated alone. In
spite of its ready decomposition, it has remained unchanged in many paintings
for centuries, just as it has in the earth. It is unaffected by light.

The history of malachite in painting runs closely parallel to that of azurite.
It occurs on Sinai and in the eastern desert of Egypt, and was used there for

128

Painting Materials

eye paint as early as predynastic times (see Lucas, p. 287). It was found side by
side with azurite in Chinese painting at Tun Huang and other temple sites, and
it has perhaps been used in the East continuously to the present day. This copper
green is found in all periods of European painting up to about 1800, but at that
time it was nearly supplanted by artificial green pigments. It was used much in
trees and foliage. Like azurite, it worked better in tempera than in oil. Thompson
remarks {The Materials of Medieval Paintings pp. 160-162) that malachite, al¬
though widely used in the Middle Ages, is mentioned but little in contemporary
literature on painting materials whereas azurite is spoken of repeatedly. This
pigment is no doubt the verde azzurro of Cennino Cennini (see Thompson, The
Craftsman's Handbook^ p. 31).

Manganese Blue is a comparatively new pigment which seems to have been
first mentioned in the patent literature about 1935. This green-blue pigment Is
essentially barium manganate fixed on a barium sulphate base. It is made by
calcining mixtures of sodium sulphate, potassium permanganate, and barium
nitrate, or their equivalents, to a temperature of 750-800° C. in the presence of
air. The blue pigment formed is very inert chemically; it Is unchanged by heat
and is insoluble in strong acids and alkalis. The pigment is fairly coarse and
somewhat irregular in particle size. Many rectangular particles with rounded

corners can be observed; they are moderately birefracting. Although weak in
pnctorial and in hiding power, this pigment may have special uses because of
its chemical stability. So far, it has been used almost exclusively for coloring
cement; it should be of interest to fresco painters. (See French patent 778,290,
March 13, 1935, to 1 . G. Farbenindustrle A. G. {Chemical Abstracts, XXIX, 1935,'
4610^]; French patent 802,687, September 10, 1936, to Wolfgang Muhlberg
{Chemical Abstracts, XXXI, 1937, 2029®]; British patent 465,912, May 19, 1937
to Wolfgang Muhlberg {Chemical Abstracts, XXXI, 1937, 8230^]).

Manganese Violet (permanent violet) is said to be manganese ammonium
phosphate, (NBDMn 2 {P 20 ,), (Rose, p. 255). In the method of preparation de¬
scribed by Weber (p. 88), manganese dioxide and ammonium phosphate are
melted together, with the evolution of ammonia, and the fused violet mass is
digested with phosphoric acid and heated until a correct color is produced; the
product must be thoroughly washed free from phosphoric acid. Church (p. 225)
says that it has a truer violet hue than cobalt violet (cobalt phosphate) which is
redder as well as brighter. The pigment is permanent to light and is unaffected
by heat but it is decomposed by strong acids and by alkalis, which makes it
unsuited for fresco. It is not much used by artists because it is dull in tone and
has poor^hiding power (see Doerner, p. 81). The manganese violet described
by Merwin (p, 521) was birefracting and belonged, probably, to the orthorhom¬
bic system. Little is known apparently about the history of this pigment, except
or a statement by Messrs Winsor and Newton in their catalogue (1930 ed., p. 18)
that It was first introduced by them in 1890. It is understood, however, to have

Pigments

129

been first prepared by E. Leykauf in 1868, and named by him ‘Nurnberg violet’

(Rose, p. 254).

Mars Colors (Mars yellow, Mars orange. Mars red, Mars violet). The Mars
colors, so-called, are artificial ochres which are made by precipitating a mixture
of a soluWe iron salt (ferrous sulphate) and alum (or aluminum sulphate) with
an alkali like lime or potash. The depth of the yellow color of the primary product
can^be controlled by the proportion of alum used. The product is a mixture of
ferric and aluminum hydroxides with gypsum (if lime is the precipitant) and, if
simply worked and dried, is Mars yellow. When this Mars yellow is heated,
various shades of orange, red, brown, and violet result, depending upon the
degiee and duration of the heat. The product must be thoroughly washed free
from soluble salts to be useful as an artist’s pigment. The preparation of artificial
iron oxide colors of this nature from iron vitriol was described in the middle
XIX century (Rose, p. 222). Although these Mars colors are very homogeneous
and fine, they possess no advantage over the natural iron yellows and reds. They
are sometimes sold for the natural iron oxides.

Massicot (litharge). Both massicot’ and * litharge’ are names which have
long been used for the yellow monoxide of lead (PbO). Some writers have used
them as synonyms but, on better authority, they are separated in meaning to
indicate lead monoxides that are derived from different sources and have slightly
different properties.

Massicot is understood to be the unfused monoxide of lead that Is made by
the gentle roasting of white lead. At a temperature of about 300° C., white lead
gives off carbon dioxide and water, and the oxide is left as a soft, sulphur-yellow
powder. It is not an intense yellow but it has good hiding power and is similar
to white lead in pigment properties. Microscopically, it is not characteristic; it
appears to be nearly amorphous, but Merwin states (p. 519) that natural massi¬
cot occurs in orthorhombic, thin plates or scales. Chemically, it has properties
like white lead; it dissolves in nitric and acetic acids, and may even give off car¬
bon dioxide from undecomposed white lead. It melts in strong heat and is changed
to litharge or red lead, depending upon the temperature. It is unaffected by
strong light but may revert to white lead on long exposure to damp air.

Litharge or ‘flake litharge’ is the fused and crystalline oxide which is formed
from the direct oxidation of molten metallic lead. The molten lead, in rever¬
beratory furnaces, is stirred from time to time to expose fresh surfaces of lead
to the oxidizing action of the hot air above (see Bearn, p. 114). A more modern
way is to atomize molten lead by whirling propellers and allow it to oxidize in
contact with hot air. It has long been a by-product of the refining of silver by the
‘cupellation’ process. Litharge is more orange in color than massicot, caused by
the presence of some red lead (Pb304). It is not used as a pigment but is exten¬
sively employed as a drier in paints and varnishes; it is important as an inter¬
mediate step in the preparation of red lead (see Red Lead).

130

Painting Materials

Yellow lead monoxide was known^ certainly^ as early as metallic lead, which
has teen found in sites that date from predynastic times in Egypt (see Lucas,
p. 200). Laurie {The Pigments and Mediums of the Old Masters^ p. 10) found it
on a scribe's palette dating 400 B.C. Davy (p. 105) identified an orange color on
a piece of stucco in the ruins near the monument of Caius Cestius as a mixture
of massicot and minium. Pliny described the preparation of both litharge (see
Bailey, I, 117-119, and II, 73) and massicot (see Bailey, II, 83-85).

Yellow lead oxide was an important pigment in European painting. De Wild
(pp. 49-50) lists thirty-nine Dutch and Flemish paintings of the XV to the XVII
centuries on which he identified it. In modern times, however, it seems com¬
pletely to have passed out of use, and it is no longer listed by colormen.

Mauve is an artificial organic dyestuff belonging to the azine group of dyes; it
is mainly amino-phenylamino-^-tolyl ditolazonium sulphate, €271125X4(304)1/2
{Colour IndeXy p. 211). This was the first dyestuff ever to be made synthetically.
It was discovered in England in 1856 by Sir William Perkin, who prepared it by
the oxidation of crude aniline with chromic acid. Because aniline was the starting
point for this as well as for several others which followed, the term, 'aniline dyes,'
came to indicate all those made synthetically, particularly those from chemicals
derived from the distillation of coal tar (see also Coal-Tar Colors). The term
has been carelessly applied to dyes not derived from aniline or related to it.
Pure mauve dye comes in the form of reddish violet crystals. When applied, the
color is dull violet. It was early patented in England where it was widely used
for a time in dyeing cloth. Although it is fugitive, it has been used as an artists'
water color to a small extent, and today is still listed by some colormen among
water color paints.

Mayan Blue is a name here provisionally given to a peculiar blue pigment
which is found rather extensively on wall paintings and painted objects from
ancient Mayan sites like Chichen Itza in Yucatan and other localities in Central
America. It is green-blue, an inorganic pigment, probably natural in origin.
Apparently, it owes its color to the iron it contains and not to any copper. In a
microscopic study made for the Fogg Museum of a sample from Chichen Itza,
it was observed to be of grains that are very small and faintly greenish blue by
transmitted light. The mineral is weakly birefracting and has a refractive index
between 1.54 and 1.55. It is pleochroic, being blue in one direction and yellow in
another. The blue is intimately mixed with calcite. A spectrogram shows the
elements, calcium, magnesium, silicon, aluminum, and iron. It is not affected by
alkalis and is only taken into solution by hot concentrated acids. This mineral,
in its optical and chemical behavior, compares quite well with the rare mineral,
aerinite (from Spain), of the chlorite group.

Merwin has described a blue pigment from Chichen Itza (see E, H. Morris,
Jean Chariot, and Axtell Morris, 'Temple of the Warriors,’ Carnegie Institution
of Washington y Puhlication no. 406 y I [1931 Ij pp- 35 5"“3 5 ^) > he says;

Pigments

131

The blue paints as contrasted with the others form fairly coherent films. These
can be isolated by dissolving away the underlying plaster in dilute acid. Micro¬
scopically the blue material consists of indefinite spherulitic aggregates of a bire-
fringent substance resembling the clay mineral beidellite. It stains like beidellite.
X-Ray powder photographs, taken by Dr. E. Posnjak, show the blue pigment,
beidellite, and a blue chromiferous clay, to be similar. Not enough of the blue clay
was available to give decisive tests for chromium, but, like the blue clay, the color
is not discharged by boiling nitric acid nor by heating much below redness. The
conclusion seems justified that this is an inorganic color.

The green paints are a mixture of the blue and yellow ochre.

The occurrence of this blue mineral or clay may have been very restricted
or known only in that single locality, but there its use seems to have been quite
general.

Mica, In mineralogy, various micaceous minerals are recognized, but the name
most commonly applies to muscovite, or hydrous potassium aluminum silicate,
H2K Al3(Si04)35 which is found in nature in thin laminae with perfect cleavage.
It occurs widely in small deposits all over the world. Other names are ‘isinglass’
or ‘Muscovy-glass.’ Its widest present use is as an insulating material in the
electrical industry. Ground mica is used as a lubricating agent and as a re¬
enforcing pigment in paints. In the Far East it had an occasional place in painted
designs where its lustrous surface gave an effect somewhat like that of metal.

Mineral Pigments are those derived from the natural minerals of the earth.
Although, broadly speaking, they may include complex mixtures and aggregates,
like earths and clays, the term is more properly restricted to those minerals which
are, on the whole, definite chemical compounds with characteristic physical form
and constant chemical behavior. Pigments derived from azurite, orpiment, and
lapis lazuli are examples.

Minium (see Red Lead).

Molybdate Orange, a pigment of recent origin, is a mixed crystal compound
of lead chromate, lead sulphate, and lead molybdate in the approximate ratio,
7PbCr04-2PbS04' iPbMo04. This molybdate-modified lead chromate belongs to
the tetragonal system whereas lead chromate alone is usually either rhombic or
monoclinic. The particles are small, rounded, and uniform in size, have high
index of refraction, and are moderately birefracting. As a pigment, it has high
covering power and tinting strength. A. Linz has described (‘ Molybdenum Orange
Pigments,’ Industrial and Engineering Chemistry^ XXXI [1939], pp- 298-306)
the conditions of precipitation necessary to produce a pigment having the most
desirable properties. Molybdate orange can claim only moderate fastness to light.
Mixed crystal pigments of lead chromate and lead molybdate were first described
by E. Lederle in a German patent (no. 22F. 1.52.30) applied for by the 1 . G.
Farbenindustrie A. G. on August 30, 1930 (see Linz, loc, cit,^ p. 298; see also
German patent 574>3795 April 12, 1933, and 574,380, April 13, 1933; also U. S.
patent 1,926,447, September 12, 1933)- Although molybdenum orange went into

132

Painting Materials

production soon after 1935 for use in printing inks and paints, it is not known
that it has been used, as yet, in artists’ paints. Because of its brilliant color and
other desirable properties, however, it may be expected soon to find use for that
purpose.

Monastral Blue (see Phthalocyanine Blue).

Mordant is a general term for a fixing agent. It is derived from the Latin,
mordere^ "to bite.’ In dye processes, the term is used to Indicate certain chemicals
which are used for fixing colors on textiles by adsorption. The more common
mordants are the soluble salts of aluminum, chromium, iron, tin, etc., which are
precipitated on the fibres along with the dye by an alkali. These mordant salts
are particularly necessary for fixing adjective dyestuffs to fibres. Occasionally,
the term applies to the base or substrate employed in the preparation of lake
pigments from organic dyes (see Lake). In the technical language of the fine arts,
the term refers to the adhesive film used to fix gold leaf in one type of gilding
(see also Mordant, p. 35).

Mosaic Gold is artificial tin (stannic) sulphide (SnS2), and it is made by
combining tin with sulphur in various ways (see Colour Index^ p. 306). This
sulphide, which looks like bronzing powder, consists of fine, soft, yellow scales
with a golden glint. It was formerly used for gilding purposes as a sort of gold
substitute. According to Thompson, who has described the preparation and his¬
tory of this material {The Materials of Medieval Tainting^ pp. 182-184), it was
known in Europe as early as the XIII century. There are many recipes for making
it in XIV and XV century texts, and it is found occasionally on mediaeval manu¬
scripts. In the Bolognese MS. (XV century), a whole chapter is given to the
preparation of mosaic gold or 'purpurino' and its use in painting (see Merrifield,
II, 458). Stannic sulphide is a fairly stable compound; It is unaffected by light
and by mineral acids, but is soluble in sodium hydroxide and In aqua regia.

Mountain Blue (see Azurite).

Mountain Green (see Malachite).

Mummy, A brown, bituminous pigment was once actually prepared from the
bones and bodily remains of Egyptian mummies which had been embalmed with
asphaltum (see Church, p. 236, and Eibner, Malmaterialienkunde, p. 213). It was
claimed that, through time, the asphaltum had lost some of its volatile hydro¬
carbons, and the powder from the ground-up, embalmed remains was more solid
than recent asphaltum and was better suited for a pigment. Apparently, it was
once a favorite with some artists. Church says (p. 237) that it was certainly used
as an oil paint at least as early as the close of the XVI century. Little is known
about its history; it has not been mentioned in reports on the identification of
materials in paintings- It is now perhaps unobtainable and is no longer desired
in the^arts. Some oil paints sold under that name are substitutes which contain
bituminous earths like Van Dyke brown. The microscopic character of true
mummy has not been described, but probably its properties and behavior are
much like those of asphaltum (see Asphaltum).

Pigments

13:;

^Naples Yellow (antimony yellow) is essentially lead antimoniate (Pb3[Sb04]2)j
whlcli^may be considered to be chemically combined lead and antimony oxides.
It varies in color from sulphur-yellow to orange-yellow, depending upon the pro¬
portion of the two materials. It is made in various ways: from the prolonged
roasting of the mixed oxides of lead and antimony, or from salts of those metals,
like tartar emetic (potassium antimonyl tartrate) and lead nitrate with sodium
chloride (see De Wild, p. 56; also Rose, pp. 306-310). The pigment is homo¬
geneous ^and finely divided, and it has good hiding power. It resembles massicot
in its microscopic character, and De Wild says (p. 57) that no crystalline form
can be detected, even at high magnification. Chemically, it is quite stable; it is
little affected by alkalis or by dilute and concentrated nitric or hydrochloric acids.
It^fuses only at a high temperature but turns permanently dark brown. Since
it is^a lead pigment, it is darkened by hydrogen sulphide; hence, it is more useful
in oil than in water color medium.

The history of this pigment is rather obscure. A compound of antimony and
lead seems to have been used in Babylonia and Assyria in the production of
yellow ceramic glazes (see Partington, pp. 256, 283, and 29a). It was found
(Partington, p. 292) in cake form among other pigments in Sargon IPs palace
at Khorsabad. Lucas (p. 125) found lead and antimony in a specimen of Egyptian
glass of the XIX Dynasty. Little is known about its early history in Europe.
Naples yellow has been vaguely connected by some with the giallorino of Cennino
Cennini (see Thompson, The s H^tidbook^ p. 28, and Ths MateHcils of

Medieval Painting, pp. 179-180), but the Identity of that yellow is still uncertain.
R. Jacobi has reported ( Uber den in der Malerei verwendeten gelben Farbstoff
der Alten IVIeister, jingewandte Che?nte, LIV [19^-0], pp. 28—29) on specimens of
a yellow pigment used in paintings from the XVIII century, particularly in
northern Europe which, on the basis of spectroscopic studies, were found to be a
cornpound of lead and tin oxides. Recipes for the preparation of lead antimoniate,
as it is now known, first appeared around the middle of the XYIII century (see
Guignet, p. 105, and De Wild, p. 56). Rose says (p, 307) that the first recipes are
given by Passeri in 1758. De Wild (p. 58) does not believe that it was used by the
old masters. Doerner (p. 62) thinks, however, that it was used by Rubens. Only
careful chemical analysis could distinguish it, on paintings, from massicot. Naples
yellow is still used as a ceramic pigment, but not much by artists, although it can
be got from a few colormen. Several substitutes, which are sold under that name,
are mixtures like cadmium yellow, zinc white, Venetian red, etc. The name has
come to indicate a shade of yellow rather than a definite chemical compound.

Natural Pigments are those which are mineral, vegetable, or animal in origin.
They are the pigments most generally used in the early history of painting. In¬
cluded are many of great stability and usefulness, like ochres and umbers. Some
are of fine quality, like azurite, malachite, and ultramarine. A few of the natural
animal and vegetable colors, like indigo and madder lake, are moderately stable,
but many, like saffron and carmine, are fugitive. Natural mineral pigments are

134

Painting Materials

often characterized by their coarseness and irregularity in particle size andj also^
by the presence of impurities. Coarse and impure mineral pigments can impart
certain desirable qualities of texture and of tone to painted surfaces not attainable
with artificial precipitated pigments. The inert pigments^ like gypsum, clay, and
chalk, are of natural origin and have only to be purified to be used as paint
materials.

Ochre (yellow ochre, golden ochre, red ochre, brown ochre). An ochre is 'a
natural earth which consists of silica and clay, and which owes its color to iron
oxide in either the anhydrous or hydrous form. Red ochre is colored by anhy¬
drous iron oxide, Fe203 (see Iron Oxide Red), but in yellow ochre the color is
caused by the presence of various hydrated forms of iron oxide, chief of which
is the mineral, goethite, Fe203* H2O. This is seen at high magnifications in strongly
birefringent spherulites less than i /x in diameter. Brown ochre is nearly pure
limonite. In addition to iron minerals, yellow ochre may contain impurities of
gypsum, magnesium carbonate, etc. There is a wide variation in the iron oxide
content, but French ochre, which is one of the best varieties, contains about
no per cent; it is low in aluminum and high in silica. The best ochre district of
France centers about Apt in the Department of Vaucluse in the south (see Ladoo,
p. 380). Ochre, however, occurs all over the world and ochre pits are worked in
many places. It is prepared for use by selection, grinding, washing, levigation,
and drying (see Bearn, p. 60). Since it is a natural product, it Is found in a num¬
ber of shades which vary from dull, pale yellow to reddish brown. Some ochres
have good hiding power; others, like the siennas from Italy (see Sienna), are es¬
pecially valued for their transparency. Microscopically, the pigment is hetero¬
geneous in particle size and in composition; it is a mixture of colorless silica and
semi-opaque, pale yellow and brown particles which are sensibly isotropic. Like
other forms of iron oxide, yellow ochre is permanent in all techniques; it is not
affected by dilute acids or alkalis. It turns red (to red ochre) on burning from loss
of water of hydration.

Yellow ochre has been universally used as a pigment from earliest history.
It was known and used in ancient Egypt, in Roman times, and in the East. It
was important in the Middle Ages and in all periods of European painting.
De Wild has reported it (p. 53) In twenty specimens from paintings of all periods
of Flemish and Dutch art. Yellow ochre is now listed by all artists' supply dealers
and is a fairly dependable product. In the recent past, it was occasionally bright¬
ened with chrome yellow or by natural or aniline dyes (see Weber, p. 94). Arti¬
ficial iron yellows or ochres are now common (see Mars Colors).

Orange Mineral (see Red Lead).

Organic Pigments are those which belong to the organic division of chemical
compounds. They are compounds of carbon with hydrogen, oxygen, nitrogen,
sulphur, and other elements. They may be derived from vegetable sources or they
may be made synthetically. Vegetable coloring matters are those like madder.

Pigments

iJS

saffron, indigo; synthetic or coal-tar colors are those like magenta, alizarin, and
tolmdine red. Although a few organic pigments are stable and are considered
permanent, in general they are fugitive.

Orpimeni (King s yellow) was once widely used, particularly in the East, but
has now fallen into complete disuse because of its limited supply and because of
its poisonous character. It is the yellow sulphide of arsenic, AS2S3, occurring
naturally in many places but not in large quantities. The principal sources in
ancient times appear to have been in Hungary, Macedonia, Asia Minor, and
perhaps in various parts of Central Asia. There was a large deposit near Jula-
merk in Kurdistan (see Dana, p. 358). It is said that some hundreds of tons of
orpiment are exported annually from Shih-haung-Ch’ang in Yunnan province in
China (see Thorp). The natural product must have been prepared by methods
common for other natural pigments. It can also be made artificially by precipita¬
tion or sublimation. Orpiment is brilliant, when pure, with a rich, lemon-yellow
tone, and fair covering power. Laurie says (‘The Identification of Pigments . . . ,’
p. i 60 : ‘The brilliant color and tint on a MS. are usually unmistakable.’ In old
paintings and illuminations, it was rather coarsely ground to preserve its rich
yellow color. Microscopically, the larger particles glisten by reflected light and
have a waxy-looking surface. It often contains orange-red particles of realgar,
to which it is closely related (see Realgar). Orpiment is crystalline (monoclinic)
and is highly refracting. It sometimes appears to have a fibrous structure. This
natural sulphide is stable to light and air. It is not affected by dilute acids and
alkalis but only by strong acids. When ignited, it burns to arsenic trioxide. Since
it is a sulphide, it is incompatible with copper and with some lead pigments.

Orpiment was known in classical times. It Is mentioned by Pliny (see Bailey,

I, loi) as occurring in Syria, and as a pigment which can not be used in fresh
plaster {op. cit, II, 91). It appears to get its name from a corruption of the Latin,
aunpigmentum (gold color or paint). By Vitruvius it is mentioned (VII, Chap!
VII) among the natural colors. Spurred records it as having occurred in Egyptian
paintings at Tell el Armarna of the XVIII Dynasty. Lucas (p. 292) says that
the mineral does not occur in Egypt and must have been imported, perhaps from
Persia. It has been identified many times in old illuminations. Laurie {The Pig¬
ments and Mediums of the Old Masters, p. 72) reports a peculiar kind on VIII
century Irish manuscripts. He says, also, that it was used on Byzantine and on
early Persian pages (see ‘The Materials in Persian Miniatures,’ pp. 146-147).
It has been found, along with realgar, on mud wall paintings from Kara Khoto
in Central Asia (XI—XIII centuries). Although it was mentioned by Cennino
Cennini (see Thompson, The Craftsman s dlandbook, p. 28), it does not much
appear In easel or monumental painting of the West. De W^ild did not find it on
any of the Dutch and Flemish paintings he examined, perhaps because it was
not necessary when a good grade of massicot was available.

Oyster Shell White (see Chalk).

136

Painting Materials

Para Red or Para Toner was one of the early synthetic diazo dyestuff colors;
it is yj-nitrobeiizene-azo-jS-naphtholj CisHnNsOs^ and is derived from the dye
intermediate, paranitraniline {Colour Index^ p. ii). For pigment purposes, the dye
is usually precipitated on barytes. It is brilliant in hue, but darkens on exposure
to strong light and sometimes becomes brown; it has a tendency to bleed in oil
paints. Although Para red was formerly widely used in the manufacture of red
paints and enamels, it has been replaced in recent years by the more stable Lithoi
Red and Toluidme Red. Para red has perhaps never been offered to the artists’
trade under this name, but it has no doubt occurred in pigments as a toner and
substitute in cheaper lines of paints. It was first made by Messrs Holliday and
Sons in England in 1880 {ColourIndex^ p. ii).

Paris Blue (see Prussian Blue).

Paris Green (see Emerald Green).

Permanent Blue is a name used by some artists’ colormen for ultramarine (see
Ultramarine Blue, artificial). It is occasionally given to a pale variety of that
pigment.

Permanent Green is a name sometimes applied by artists’ colormen to mix¬
tures of viridian with a yellow pigment like cadmium yellow or zinc yellow; it may
also contain zinc oxide.

Permanent Yiolet (see Manganese Violet).

Permanent White (see Barium White).

Persian Berries Lake (yellow berries, buckthorn berries) is a yellow lake made
from the dried, unripe berries of various shrubs of the buckthorn family, Rhamnus,
found in the Near East and now imported from Smyrna and Aleppo ( ColourIndex^
p. 293). The berries are also available from European members of the family.
The coloring principle is rhamnetin, C16H12O7, which is extracted by boiling water.
The lake is made by the addition of alum, followed by soda {op. cit.^ p. 301; see
also Perkin and Everest, p. 628). Other colors are produced with other mordants.
It is insoluble in water and in alcohol but is soluble in alkalis, forming a yellowish
brown solution; it is decomposed and decolorized by mineral acids, but is moder¬
ately stable in light. Colors of this origin were popular in France and in England
in the XVIII century (Thompson, The Materials of Medieval Paintings p. 187),
but are no longer current.

Phosphotuugstic Acid Bases are complex compounds of phosphorous pen-
toxide and tungstic acid combined approximately in the ratio, 1P2O5 : 24WO3.
These bases are now quite extensively employed with organic dyestuffs in making
lake pigments and toners of excellent strength and light-fastness. They were de¬
veloped in Germany, and it appears that first patents were issued to the Badische
Anihn und Soda Fabrik (see British patent iS.gS'^y 1914; French patent 474,706,
1914; also British patent 216,486, 1924).

r C Monastral ’ blue) or copper phthalocyanine,

CjsHieNaCu, is an organic blue dyestuff that was recently developed by chemists

Pigments

137

of the Imperial Chemical Industries, Ltd, and was first introduced to the pigment
trade under the name, ‘ Monastral blue,’ at an exhibition in London, November
1935- American manufacture and trade introduction under other names followed
early in 1936 (see first notice in Industrial and Engineering Chemistry, News
Edition [(January 10, I 93 ^II)- The claim was made that it was the most important
blue discovery since those of Prussian blue in 1704 and artificial ultramarine blue
in 1824, and that in many respects it was superior as a pigment to either of these.

Copper phthalocyanine is prepared by a complex organic synthesis. Phthalic
anhydride and urea (or phthalonitrile) are fused together with copper chloride and
the product is first washed in dilute caustic soda and then in dilute hydrochloric
acid. At this stage it is copper phthalocyanine, but it is not in physical form suit¬
able for pigment. It is conditioned by being dissolved in concentrated sulphuric
acid and precipitated again in excess water. After careful washing and filtering,
the resulting paste can be used directly in the preparation of lakes by adsorption
on aluminum hydrate or it can be dried for incorporation directly into non-
aqueous mediums.

M. A. Dahlen of Messrs E. I. duPont de Nemours and Co. (in an article en¬
titled, ‘The Phthalocyanines: A New Class of Synthetic Pigments and Dyes,’
Industrial and Engineering Chemistry, XXXI C1939II) PP- 839-847), who has
described the properties and uses as well as the method of synthesis of this new
class of pigments, says that pure copper phthalocyanine in crystalline form is
deep blue with a strong bronze reflection, but the dry, disperse form is bright
blue with little or no bronziness. It is insoluble in organic solvents, even at
high temperatures, in alkalis and in acids, except concentrated sulphuric and
phosphoric acids, and is highly resistant to oxidizing and reducing agencies.
Dahlen says that when tested in pigment applications the phthalocyanines as a
class have good fastness to light and certain members show outstanding fastness.
It is very close to the ideal pure blue, for it absorbs light almost completely in the
red and yellow, and reflects only green and blue bands. This makes it the true
‘ minus red ’ so much needed in three-color printing. The color is very strong
tinctorially, having about twice the strength of Prussian blue and twenty to forty
times that of ultramarine. It was proposed and offered for sale under the trade
name, ‘ Monastral blue,’ and other trade names, as an artists’ color very shortly
after its first commercial introduction in 1936. Other phthalocyanine pigments
were shortly introduced and more can be expected because of the current interest
in this new chromophore. Among the first of these was chlorinated copper phthalo¬
cyanine which yields a green dye and pigment having properties quite similar
to those of the blue pigment, including light-fastness. It first became available
commercially at about the beginning of 1938. (See Dahlen for extended bibliog¬
raphy on this class of pigments to 1938.)

Pigment is a finely divided coloring material which is suspended in discrete
particles in the vehicle in which it is used as a paint (thus being opposed to a dye

Painting Materials

138

[see Bye], which is soluble in the vehicle). Pigments are derived from a wide
variety of substances, organic and inorganic, natural and artificial They may be
classified according to color, chemical composition, or source.

PigmentSj chemical properties. Pigments comprise a wide variety of chemical
compounds; hence, they differ greatly in respect to their chemical properties.
Among the inorganic coloring materials are the oxides, sulphides, carbonates,
chromates, sulphates, phosphates, and silicates of the heavy metals. A very few
like Prussian blue and emerald green are complex metallo-organic compounds.
Carbon in the form of lamp black or charcoal and the metal pigments like gold
and aluminum are the only elements that serve in a relatively pure state. Dye¬
stuffs are complex organic compounds.

For certain special purposes, a pigment should be as nearly chemically Inert
as possible and be unaffected even by strong acids, alkalis, and heat. Only very
few of the paint pigments, however—carbon black, oxide of chromium, and cobalt
blue—are so resistant. A few compounds like the oxides of cobalt, chromium, tin,
and iron are so stable at high temperatures that they can serve for coloring
ceramic glazes. So far as the demands of ordinary painting are concerned, how¬
ever, a pigment need only be stable and chemically inert enough to withstand
light, air, and moisture or environments in which all three agencies are combined.

Light, especially strong sunlight, is the promoter of certain photochemical
reactions which result in dimming some colors, in browning and darkening some,
and in producing a distinct color change in others. In the case of organic dye¬
stuffs, light causes a definite degradation to colorless products, a change called
‘ fading.f The effect of light is usually accelerated by heat and moisture. The
oxidizing action of chromates, with reduction to chromic oxide, is accelerated by
light. Red lead in a glue medium has been observed to turn to brown lead dioxide
through the combined action of light and moisture. Air is the carrier of moisture
and certain obnoxious industrial gases like sulphur dioxide and hydrogen sulphide,
and the oxygen which is an important component of it may itself take part in
reactions which cause fading or discoloration. Church and others (pp. 334 and 339)
showed years ago that fugitive pigments exposed in thin washes to light faded
less when moisture is excluded and still less rapidly when both air and moisture
{in vacuo) were excluded. Many chemical reactions require a certain amount of
moisture before they can take place. The exact role of moisture is not entirely
understood, but it may be considered to act as a catalyst.

Stability and inertness enough to insure complete compatibility with others
are among the first requirements of artists’ pigments because they are intermixed
or intimately juxtaposed much more than are those in house paints. Under certain
conditions, it is possible for sulphides to interact with copper and lead pigments
and produce black or brown copper and lead sulphides, with resulting discolora¬
tion. Actual happenings of this phenomenon are rare, particularly when the
pigments are used in oil medium, because the oil encloses each particle in an

Pigments

139

envelope that protects it from moisture and contact with other particles. Some
oxfgen-bearing pigments^ especially the chromates, seem to have an oxidizing
action on certain organic pigments with reduction of themselves to a lower state
of oxidation. Yellow lead chromate, for example, is reduced to green chromic
oxide. Pale tints made with zinc oxide and lake pigments are known to fade more
rapidly in direct sunlight than those made with other white pigments.

The simple oxide pigments, as a rule, are regarded as the most stable, particu¬
larly to light, air, and moisture. Stable in this respect are also the sulphates,
phosphates, and carbonates. Although some of the most important pigments are
sulphides, they may not, as already mentioned, be stable with certain lead and
copper compounds. Vermilion, which is mercuric sulphide, is so insoluble, how¬
ever, that specimens are frequently seen where it has been intimately mixed with
white lead for centuries without change or darkening of the white lead. The same
is true of ultramarine which is partially a sulphide. It is not safe, however, to mix
cadmium sulphide yellow with verdigris or with emerald green.

Chemical properties of pigments may be looked at from the point of view of
their behavior to strong chemical reagents. Carbonates, ultramarine, and some
oxides and sulphides (zinc and lead oxides and cadmium sulphide, for example)
are readily decomposed by acids. Prussian blue is sensitive to alkalis and, hence,
can not be used for true fresco.

Pigments themselves may have either acid or alkaline properties. The oxides
of the heavy metals, in general, are basic (alkaline)—so basic that they can react
with free fatty acids of drying oils to form metallic soaps. Zinc oxide has this
tendency and so have some of the lead pigments, and it appears that this is one
of the reasons why white lead in oil forms such a compact, elastic, and durable
paint film. Titanium oxide, on the other hand, is perfectly inert, does not tend to
form a titanium soap, or to react in any way with paint and varnish vehicles.

Pigments, history. Coloring materials from animal, vegetable, and mineral
sources to be used for personal adornment, for decorating tools, weapons, and
utensils, and for making pictures were sought by man as early as remote pre¬
historic times. Most easily procurable were vegetable colors, flowers, seeds, berries,
nuts, bark, wood, and roots of plants. Most of these were fugitive and they soon
faded when exposed to sunlight. There were notable exceptions, however, like the
materials obtained from the madder root, the woad plant, or from the lac insect
which, under conditions not too unfavorable, sometimes lasted for centuries. Only
slightly less available were the colored earths which included the yellow, red, and
brown ochres and clays that abound on the earth’s surface in sedementary de¬
posits. Carbon black in the form of soot, charcoal, and even charred bones,
could have been found about the most primitive hearth. Such coloring materials
as these were known and used as early as there are archaeological criteria. Some¬
what less readily available than the earths were the colored rninerals of the heavy
metals, but, even so, such brightly colored minerals as cinnabar, orpiment, realgar.

140

Painting Materials

azuritej malachite, and lapis lazuli were known to the ancients and were used for
pigments in very early historic times. Since these minerals were not widely
distributed but were almost in the class of semi-precious stones, their earliest use
was restricted to the particular regions in which they were found. Long before
classical times, how^ever, such minerals became articles of commerce and were
transported to regions far beyond their origin. There is archaeological evidence
of the use of cinnabar (see Vermilion) as a pigment in China as early as the third
millenium B.C.; azurite (see Lucas, p. 283) was used in Egypt fully that early.

Artificial pigments came to be made almost with the beginning of written
history. There is evidence that the blue artificial pigment, copper calcium silicate,
more commonly known as Egyptian blue, was manufactured by 2000 B.C. and
perhaps much earlier (see Egyptian Blue). The artificial yellow and red oxides of
lead, as well as basic lead carbonate, were known in classical times and perhaps
much before that. And so was verdigris. There is knowledge of the use of these
from archaeological as well as from literary sources. It seems that artificial
vermilion was not known in the West that early but it is mentioned in the Arabic
alchemical writings of the VIII and IX centuries. It may have been made by the
Chinese centuries before.

The archaeological remains of ancient Egypt are rich in information about
pigments used for decorative and architectural purposes (see Lucas, pp. 282-292).
So are those of classical times, particularly of the later and more far-reaching
Roman period. Raehlmann has carefully described his findings made from studies
of Pompeian wall paintings, and Laurie {Greek and Roman Methods of Painting)
has commented generally on literary and archaeological information about
painting materials of classical times. The materials employed for pigments re¬
mained much the same through the Dark Ages, and information about them
comes largely from direct observations on parchment illuminations. On the pages
of books, where light and moisture have been excluded, conditions have been
nearly ideal for the preservation of painting materials.

Pigments in mediaeval as in earlier times were still important articles of trade
and were carried for long distances. In value, some of them were in a class with
precious metals and stone, and, since they were light and not bulky, they were
easily transported. In Byzantine times, ultramarine (see Ultramarine Blue,
natural) began to be brought to Europe from the region of the headwaters of
the Oxus in modern Afghanistan, and for centuries remained the most precious
of all artists' colors. Ultramarine was carried north to Chinese Turkestan at
the same time, but it was probably only the bright, exotic pigment materials
that were carried such distances. The more sombre colors, apparently, were usually
obtained nearer at hand and for such colors there are perhaps greater geographical
limitations of use. The green earths, the siennas, and the umbers were first used
in Italy and adjaceni: regions because best supplies were found there and still are.
In later periods smalt seems to have been favored in the Low Countries because

Pigments

141

it was made principally in Germany. The Mayans in Central America used a
native earth blue in their paintings, a color that seems not to have been known
in any other place in the world.

The mineral pigments on the palette of the European painters of the XV and
XVI centuries differed little from those of the classical painters, with the possible
exception of the blue glass pigment, smalt, which came into use in Europe in
this period. Perhaps, also, some new vegetable colors were added about then.
Knowledge of the painting materials of this time begins to be handed down in
numerous treatises and manuscript writings. From this period there has come,
also, a wealth of objective evidence in the form of actual paintings. Examination
of these paintings and the identification of materials in them yields technical
information that is often more important than that to be had from literary
sources. During these centuries of the late Renaissance, many vegetable pig¬
ments continued to be used, particularly for book illumination, and they included
coloring materials from safflower, brazil-wood, turnsole, and woad. Later sepia and
bistre came into use for water color work on paper.

During all these centuries the pigments employed for painting in the Far East
were similar to those of the West. The Chinese had basically the same things to
start with as the Europeans. Vermilion was used in China as a pigment as early
as the Shang period (1766—1122 B.C.). It is not known when it came to be made
artificially, but it was probably the Chinese alchemists who first learned how to
make mercuric sulphide by the dry process. The minerals, malachite and azurite,
were important in Chinese painting and so were the plain earth colors. The lead
pigments, red and white, were used in the T^ang period and perhaps much earlier.
Vegetable pigments like indigo, safflower, gamboge, were also known and used.
The ways in which pigments were employed differed from practices of the West
more than the pigments themselves.

The first years of the XVIII century mark the beginning of modern synthetic
pigments. It was in 1704 that Diesbach in Germany discovered how to make the
pigment that is still made in large quantities under the name, Prussian blue. It is
the first pigment about which there is fairly definite knowledge and written
contemporary record of the circumstances surrounding the discovery. From then
on, equally definite knowledge of the date of discovery of new pigments is avail¬
able from published records in scientific journals. Unfortunately, often much
less is known about the approximate date of introduction of a new color as an
artists' material than about its date of discovery. The middle XVIII century was
not productive of new coloring materials, but in the last quarter, beginning with
the discovery of copper arsenite by Scheele (see Scheele’s Green) in 1778, new
pigments began to appear in fairly rapid succession. These were the direct out¬
come of the discovery of several new chemical elements about that time, prin¬
cipally zinc, cobalt, and chromium. Cobalt green first appeared about 1780, zinc
oxide in 1782, and cobalt blue in 1802. In 1797 the French chemist, L. N. Van-

142

Painting Materials

quelin^ first announced the discovery of the new metal, chromium. This was an
important event in pigment history because from that element, which so ap¬
propriately gets its name from the Greek word, xpS/xa, meaning color, are derived
more pigments and a greater color range than from any other single element. In
his memoir of 1809 (see Chrome Yellow), he first described some of these new
chromium compounds which later were to become useful as pigments. In the XIX
century, nearly every decade was marked by the discovery of some such com¬
pound that later became a pigment. Some of them remained useless, scientific
curiosities for years before they were finally put into production. Conspicuous
developments were the discovery of cadmium yellow by Stromeyer in 1817, of
artificial ultramarine by Guimet in 1824, and of viridian in 1838. These increased
the color range of the artists' palette and, in some cases, added to its permanence.
The artist was now independent of the costly and uncertainly available mineral
pigments like genuine ultramarine and azurite.

A new epoch in the history of pigments began in 1856 when William Perkin in
England announced the preparation of the first synthetic dyestuff, mauve.
Although many of the dyes or so-called ' coal tar ' colors that soon followed were
taken up with enthusiasm by artists, they soon received a bad name for lack of
permanence. It is only in recent years that synthetic dyestuff lake colors have
been prepared which vie with mineral colors in stability and permanence, and
even today the number is very limited. The last half of the XIX century was not
notable for development of new inorganic pigments. Lithopone was first produced
in the 1870's, but did not become general as an artists' pigment. Since the begin¬
ning of the XX century, however, there have been some important additions. The
first of these, the cadmium reds, that now somewhat displace time-honored ver¬
milion, came along about 1910. They were followed by the titanium oxide pig¬
ments about 1920. Most recent additions to the organic pigments are the blue and
green copper phthalocyanines, and in the inorganic class are molybdate orange
and manganese blue.

Since about 1850 first dissemination of knowledge of new pigments and dye¬
stuffs has come from the patent literature. Dozens of patents on pigments and
dyestuffs are now taken out every year, but most of them are concerned with
improvements and variations in methods for manufacturing long-established
materials. Although artists are still conservative in accepting new painting
materials, the interval between discovery of a new color and acceptance for
artists' purposes is much shorter than it was formerly. Today the artist has a
greater range and variety of durable pigment materials to choose from than at any
time in history.

Several attempts have been made to show by graphic methods or in tabular
form the history and periods of uses of the principal pigments. Laurie, the first
to do this, presented the results of the Identification of pigments of illuminated
manuscripts of various countries in tabular form so that the data could be readily

Pigments

143

compared and referred to (see The Pig?nents aj'id Medhms of the Old Masters^
insert after p. 112). Another type of chart {op. cit.^ p. 136, and The Ai^alyst, p. 176)
shows by vertical lines the occurrence of pigments in Western paintings from 800
A.D. to 1800. De Wild (see inserted table) made up a chronological chart of
pigments based on his investigations of Dutch and Flemish paintings. Although
his researches covered paintings of limited geographical origin, his data serve to
indicate some of the history of pigments in Europe from the XV to the XX cen¬
tury. Eibner {Wandmalerei, pp. 549-554) has prepared'perhaps the most com¬
prehensive table on the history of pigments. He uses information from several
sources, mainly literary evidence, both classical and contemporary, but, also,
to some extent, his own objective findings. Noel Heaton (' The Permanence of
Artists' Materials, Journal of the Royal Society of yfr/j [London], LXXX [1932],
pp. 415-416) has published a table which lists in chronological order the dates of
introduction of the important artists’ pigm.ents.

Pigments, physical properties. Pigments are materials which are useful for
painting mainly because they have some outstanding physical properties, even
when they exist in a fairly minute state of subdivision. Physical properties are
those properties inherent in a material itself and which do not involve its relation¬
ship or combination with other materials.

Color is the most important physical property of a material in determining
its immediate usefulness as a pigment. A material has color because of its selective
absorption for the component colors of white light. The color of painting materials
can be treated from many points of view. One of these has for its end the com¬
prehension of all possible visual tones within a single system. Morton C. Bradley,
Jr (‘ Systems of Color Classification,’ Technical Studies^ VI [1938], pp. 240-275)
has made a brief critical review of that phase of color study. It has to do not with
pigments specifically but rather with color as visual tone. In isolated instances a
pure material itself may serve as a standard of reference in a system of visual
tones. True color characteristics are best established analytically by spectro-
photometrlc measurements. Barnes has recently worked out the descriptions of
some fifty artists’ pigments and has given curves for them on the basis of light
reflectance from the surface for different wave-lengths of incident visible light.
Maerz and Paul treat the language of color, its origin, growth, and usage, and by
means of color plates showing graduations of hue, purity, and value, they offer
a quick and practical method for relating colors with the names by which they are
commonly identified. Standards of color for many of the pigments described in
these data appear on their plates.

Merwin, in treating optical properties and theory of color of pigments, has
shown that the color characteristics, the hue, purity, and brightness of the light
diffused, depend upon the color absorption, size, shape, and texture of pigment
grains. He describes the optical properties of individual pigment substances
in detail. For instance, he says of cobalt blue (p. 520) that grains vary in depth of

H 4

Painting Materials

color; those most deeply colored have the highest refractive index, which for blue
is 1.745 for red a little greater than 1.78, and for green less than 1.73, Grains 10^
in diameter are practically opaque to wave-lengths between 560 and 6iom/x
whereas wave-lengths in the red longer than dcomju and in the blue-violet are
transmitted freely. Most pigment grains are minute crystals and, since many of
these crystals are anisotropic, color absorption and transmission of light are
different along different axes.

The colors of different classes of pigments cover unequally well the different
regions of the visible spectrum. There are no colors of the short wave-length region
among the common earth pigments. There is also a deficiency of bright mineral
and inorganic pigment colors in that region. The organic dyestuff colors, however,
cover all portions of the spectrum almost equally well.

The refractive index of a pigment, which is the measure of light-bending power
of particles as light passes through them, is important because the hiding power
of a transparent pigment is proportional to the refractive index of its grains.
Titanium dioxide with a refractive index of 2.55 has the greatest whiteness and
hiding power of any white pigment. Both white lead and zinc white, with refrac¬
tive index approximating 2.00, have lower covering power. Merwin says (p, 497)
that the amount of light reflected from a unit area of surface of a pigment grain
increases with the refractive index. Pigment grains reflect most light when sur¬
rounded with air, less light when surrounded with vehicle, and paint reflects in
proportion to the difference between the refractive indices of the pigment and the
surrounding medium. The higher the refractive index of the pigment and the lower
that of the vehicle, the greater the light reflection, and, with white pigments, the
greater is the resulting whiteness and hiding power. There is also a close relation¬
ship between refractive index and color. Merwin says (p. 501):

To be most effective as a pigment when used alone, a substance should have a high refractive index
for the color which it most freely transmits. In general there are large variations of refractive
index near and through a region of color absorption. Refractive index is higher on the long-wave
side of such a region than on the short-wave side. For this reason red, orange, and yellow pigments
usually have much higher refractive indices than blue and violet pigments. The refractive index of
lakes is largely determined by the base, and is always comparatively low. Some pigments so nearly
match vehicles in refractive index that they diffuse very little light. They become effective only
when mixed with a pigment of high refractive index which will diffuse their color, or when painted
in thin films over a surface covered with a strongly diffusing paint. For example, Prussian blue,
verdigris, and alizarine lakes.

The refractive indices for many pigments are given in the table of physical prop¬
erties. Pigments may belong to the isotropic, uniaxial, or biaxial groups of
crystals and the n index, the e and co indices, and the a, jS, and 7 indices are given for
members of each group respectively. The indices for many of the mineral pigments
and inerts are known accurately to the third decimal place. Indices for many of
the precipitated chemical pigments are not known with such accuracy because
they are too fine to be measured or do not take definitely crystal forms when pre-

Pigments

145

pared. Many of the pigments are not uniform in particle composition; hence, the
refractive indices he over a range of values. The degree of hydration and water
inclusion are other factors that determine the light-bending properties. The purely
opaque^pigments, of course^ have no measurable refractive index.

Hiding power is the property of a pigment, when made into a paint, to obscure
the surface over which it is applied. In the case of white pigments, the ability
to reflect light and to obscure black is the measure of hiding power; in the case
ot black pigments the opposite is true. As a general rule, hiding power of a pig¬
ment IS proportional to its refractive index, to fineness of particle size (down
to a certain limit) and to depth of color. Usually pigments which are compounds
of the heavy metals have the greatest hiding power, but there are exceptions like
carbon bkck and ultramarine. Lake pigments, especially when prepared on an
alumina base, are transparent and have very little hiding power.

Size and shape of pigment grains are important for various reasons. Pigments
are ordinarily very fine substances. To be useful, they must produce a paint that
can be applied evenly and smoothly in a uniform film. This requires fine and uni¬
form particle size. In the production of pigments, that is attained in various ways.
Those pigments produced from minerals are simply broken crystal fragments.
Their particle size is governed by the ease and kind of fracture and by the amount
of grinding, but ordinarily mineral pigments are not very finely ground. Small
particles produced m this way have the appearance of broken fragments, the
edges are sharp and irregular and usually angular; the shape is, in fact, governed
by the cleavage properties of the mineral. Azurite and cinnabar vermilion are
examples. Many of the earth pigments of sedimentary origin consist naturally of
small, discrete particles which, however, are usually very uneven in size. In pre-
paration for use as pigrnent, the raw earth must be stirred up in large tanks of
water and let stand (levigation) to allow coarser particles to settle away from the
finer particles which are held in suspension. The supernatant liquid bearing the
finer particles is drawn off, is passed from tank to tank, remaining in each longer
than It did in the preceding one, and producing in each successive tank a finer and
finer deposit. Particles from this kind of source are usually irregular in shape but
are rounded or have rounded edges. They are often quite heterogeneous in com¬
position and color. Examples are green earth and raw sienna. Many of the modern
pigments are produced as chemical precipitates by the interaction of salt solutions
which make an insoluble substance. Manysuch precipitates are crystalline in nature
and each particle is more or less a small, perfect crystal. Pigments that are pyro-
genetic in origin, like ultramarine blue and oxide of chromium, have variable
particle characteristics because conditions of formation differ greatly. They are
produced by complex chemical reactions that may take place between several
substances at high temperatures. Pigments made by the corroding action of
chemicals upon metals, like white lead and verdigris, are usually fairly coarsely
crystalline. Several important pigments are fume and smoke products and, hence.

146

Painting Materials

are finely divided and uniform in particle size. Zinc oxide and lamp black are
examples; particles of the former may be perfectly crystalline^ existing as stout
prisms. Vermilion^ prepared in the dry way^ is a sublimation product and each
particle is more or less a perfect crystal. Pure dyestuffs and toners often appear as
stains without any discontinuity in the film, particularly if they are soluble in the
film-forming substance. Lake colors are variable in character^ depending upon the
base on which they are precipitated. The size of grains is usually expressed in
microns (i micron^ /x = 0.001 mm.). Merwin (p. 499^ f.n. i) calls grains very small
that are less than 0.8/x in diameter; small, those that are between 0.8 and 2^;
medium, 2 to 5/x; large, 5 to lo/x; and very large, over 10^. The most effective black
and white pigments are those which have an average particle size in the order of
i/x in diameter or slightly less. White pigments, however, with average grain size
much below do not have such good hiding power as larger grains, they tend
to diffuse blue light more than red, and, when mixed with black, give blue-grays
(see Merwin, p. 494). Most colored pigments have grains ranging from 0.5 to
lo/x in mean diameter. Prussian blue and indigo are extremely fine-grained, but
pigments like emerald green, verdigris, and cobalt blue are comparatively coarse
(see Merwin, p. 499, f.n. 2). Pigments in older paintings, in general, are coarse,'
particularly the mineral pigments. Azurite and smalt had to be used coarsely
ground because, when very finely ground, so much white light is reflected from the
surfaces of their particles that they become pale and unsuitable as coloring ma¬
terials. Large particle size and graininess are characteristic of the pigments used in
early Chinese paintings. Granular, crystalline pigments give a certain pleasing
quality to paint films that can not be had from fine, well-dispersed pigments such
as are produced for the modern paint industry. Fine and uniform particle size in
modern pigments is also partly the result of modern mechanical methods for
grinding dry pigments. Control of particle size of pigments is carried out in pre¬
paration or in dry grinding. The grinding of a pigment in a vehicle ordinarily does
not reduce particle size but merely effects wetting and dispersion of each individual
pigment particle.

Individual pigments vary greatly in density or specific gravity and this varia¬
tion has to be taken into consideration, both in the preparation and in the prac¬
tical use of paints. Some pigments, particularly the organic lakes and toners,
are light and bulky, and so are a few of the inert materials like aluminum hy¬
drate (sp. gr. == 2.45). Lamp black is one of these very light materials (sp. gr.
= 1.77). Many of the pigments, however, are compounds of the heavy metals and,
hence, have a high specific gravity. Examples are vermilion (sp. gr. = 8.09) and
red lead (sp. gr. == 8.73). Pigments with high specific gravity settle rapidly in
liquid paints. In paints that contain mixtures of light and heavy pigments, there
are sometimes indications of a slight separation of the light and heavy components
when the paint is spread thickly on a horizontal surface. Specific gravity has an
important bearing in centrifugal methods for the separation and analysis of paint

PHYSICAL PROPERTIES OF PIGMENTS

Pigment Name and
Chemical Composition ^
Aluminom hydrates Al(OH )3
Aluminum stearate, x 41 (Ci 8 H 2502)3
Anhydrite, CaS 04

Antimony oxide, Sb 203

Antimony vermilion, Sb 2 S 3
Asphaltum (bitumen), carbonaceous
Azurite, aCuCOs* Cu(OH )2
Barytes (barite, nat.), BaS 04

(blanc fixe, art.), BaS 04
Barium yellow, BaCr 04
Blue verditer, zCuCOa-Cu(OH )2
Bone black, C + Cas(P 04)2
Cadmium red, CdS(Se)

Cadmium red lithopone, CdS(Se) 4- BaSOi

Cadmium yellow, CdS

Cadmium yellow lithopone, CdS + BaS 04

Cerulean blue, CoO*wSn 02

Chalk (whiting), CaCOa

Charcoal black, C

China clay (kaolinite), Al 203 * 2 Si 02 * 2 H 20

Chrome green (med.), Fe 4 [Fe(CN) 6]3
+ PbCr 04

Chrome red, PbCr 04 -Pb( 0 H )2
Chrome yellow (med.), PbCr 04
Chromium oxide green, opaque, Cr 203
Chrysocolla, CuSiOs • WH 2 O
Cobalt blue, CoO-AbOs
Cobalt green, CoO-wZnO
Cobalt violet, Co 3 (P 04)‘2

Cobalt yellow, CoK 3 (N 02 ) 6 *H 20
Diatomaceous earth, Si 02
Egyptian blue, CaO-CuO *48102
Emerald green, Cu(C 2 H 302 ) 2 ' 3 Cu(As 02)2
Gamboge, organic resin
Graphite, C

Green earth (celadonite and glauconite),
Fe, Mg, Al, K, hydrosilicate
Gypsum, CaS 04-21120
Indian yellow, Ci 9 Hi 60 iiMg* 5 H 20
Iron oxide red (haematite), Fe 203
Lamp black, C

Lithopone (regular), ZnS (28-30%), BaS 04
(72-70%)

Malachite, CuC 03 - Cu( 0 H )2

Manganese blue, BaMn 04 BaS 04

Manganese violet, (NFl 4 ) 2 Mn 2 (P 207)2

Massicot (litharge), PbO

Mayan blue, Fe, Mg, Ca, Al, silicate (?)

Mica (muscovite), H 2 KAl 3 (Si 04)3

Molybdate orange, Pb(Mo, S, Cr, P )04

Naples yellow, Pb 3 (Sb 04)2

Ochre, yellow (goethite), Fe 203 -H 20 , clay.

Specific Particle

Gravity 2 Characteristics ®

2.45

V, fine amorph. part.

0.99

agg. of spher. gr.

2.93

cryst. frag.

5-75

V. fine cryst.

—

V. fine red glob.

—

irr. amorph. part.

C4

00

0

cryst. frag.

4.45

cryst. frag.

4-36

V. fine cryst. agg.

4.49

V. fine cryst. gr.

—

fibrous agg.

2.29

irr. coarse lumps

4-5

min. round, gr.

4.30

min. round, gr.

4.35

min. round, gr.

4.25

fine comp. gr.

—

round, gr.

2.70

hollow spheruiites

—

irr. splintery part.

2.60

fine, vermicular cryst.

4.06

fine green agg.

6.7

tabular cryst.

5.96

fine prism, gr.

5.10

fine cryst. agg.

2.4

crypt, agg.

3-83

round, gr.

—

spher. gr.

—

round, gr.

—

fine dendritic cryst.

2.31

min. fossil forms

—

cryst. frag.

3-^7

spheruiites and disks

—

irr. amorph. part.

2.36

irr. plates

2.5-2.7

round, irr. gr.

2.36

fine cryst. gr.

—

prisms, plates

5-2

min. cryst.

1.77

min. round, part.

4.30

fine comp. gr.

4.0

cryst. frag.

—

gr. and stubby prisms

—

fine cryst. gr.

9.40

min. flakes

—

porous irr. agg.

2.89

platy frag.

—

min. round, gr.

—

round, gr.

2.q-4.0

irn spheruiites

Refractive Index ^
1.50-1.56 [M*]

1.49 (w. bi.) [W]

0:1.570,7 1.614,^1.575 [LB]

Ivalentinite, o: 2.18,7 and 2.35 ^
(senarmonite, 2.09 (isot.) ^
'/iLi 2.65 (isot.) [M*]

1.64- i.66[M*]

o: 1.730,7 1.838,^3 1.758 [LB]
o: 1,636, 7 1.648, ^ 1.637 [LB]
1.62-1.64 [M*]

1.94-1.98 (bi.) [M]

1.72, 72 slightly > 1.74 [M*]

1.65- 1.70 (for larger translucent gr.)
2.64 (bright red)-2.77 (deep red) (

[M*]

2.50-2.76 (for CdS(Se) part) (isot.) |
2.35-2.48 (isot.) [M*]

2.39-2.40 (for CdS part) [M*]

1.84 (isot.) [M="]

ej c I.510, 0)2 ^ ^•645 [M*]

(opaque)

0:1.558, 7I.565> 1.564 (all

[LB,

c 2.4 (cf. Prussian blue and chrome ye]
[M*]

o: 2.42, 7 2.7 -F, P 2.7 [M*]

amQmn < T650mj[i 2*49 [M]

?2Li2.5[M]

a 1.575, 7 1.598, ^1.597 [LB]
n var.; max. c i.74biue (isot.) [M]
1.94-2.0 (w. bi.) [M*l
€1.65-1,79 (dull violet), CO 1.68-
(salmon) (s. bi.) [M*]

1.72-1.76 (isot.) [W]
n mostly 1.435, some 1.40 [M*]

€ 1.605, 1.635 (pleo.) [APL]

^*7^ 7s ^-78 (w. pleo.) [M*]
1.582-1.586 [Wl
(opaque) [M]

n var. c 1.62, (porous) [M*]

a 1.520,71.530.^^-5^3 [LB]

1.67 (w. bi.) [M*]

€Li 2.78, OJU 3.01 [M]

(opaque)

2.3 (ZnS)^i.64 (BaS04) [M]

a 1.655,7 1.909. ^ 1-875 [LB]
c: 1.65 [W]

a 1.67, 7 1.75, j8 1.72 (for violet) [M]
au 2.51, yu 2.71, fe 2.61 [M]

2.54 (irr.; bi. and pleo.) [M*J
o: 1.563, 7 1.604, ^ 1.599 [LB|

Bu 2.55 (s. bi.) [M*] ;

2.01-2.28 (isot.) [M*]

(isot. nart^: (a 8 )^ 2.0<-2.2i

PHYSICAL PROPERTIES OF PIGMENTS

Pigment Name and
Chemical Composition ^
Aluminum hydrate, Al(OH)3
Aluminum stearate, xA.l(Ci8H3602)3
Anhydrite, CaS04

Antimony oxide, Sb203

Antimony vermilion, Sb2S3
Asphaltum (bitumen), carbonaceous
Azurite, aCuC03-Cu(0H)2
Barytes (barite, nat.), BaS04

(blanc fixe, art.), BaS04
Barium yellow, BaCr04
Blue verditer, zCuCOs* Cu(OH)2
Bone black, C + Ca3(P04)2
Cadmium red, CdS(Se)

Cadmium red lithopone, CdS(Se) -j- BaS04

Cadmium yellow, CdS

Cadmium yellow lithopone, CdS + BaS04

Cerulean blue, Co 0 *? 2 Sn 02

Chalk (whiting), CaCOs

Charcoal black, C

China clay (kaolinite), Al203*2Si02*2H20

Chrome green (med.), Fe4[Fe(CN)6]3
+ PbCr04

Chrome red, PbCr04-Pb(0H)2
Chrome yellow (med.), PbCr04
Chromium oxide green, opaque, Cr203
Chrysocolia, CuSiOs • WH2O
Cobalt blue, CoO-ALOs
Cobalt green, CoO-wZnO
Cobalt violet, Co3(P04)2

Cobalt yellow, CoK 3 (N 02 ) 6 - H2O
Diatomaceous earth, Si02
Egyptian blue, CaO-CuO *45102
Emerald green, Cu(C2H302)2-3Cu(As02)2
Gamboge, organic resin
Graphite, C

Green earth (celadonite and glauconite),
Fe, Mg, Al, K, hydrosilicate
Gypsum, CaS04-2H20
Indian yellow, CigHieOnMg* 5H2O
Iron oxide red (haematite), Fe203
Lamp black, C

Lithopone (regular), ZnS (28-30%), BaS04
(72-70%)

Malachite, CuC03* Cu(0H)2

Manganese blue, BaMn04 -h BaS04

Manganese violet, (NH4)2Mn2(P207)2

Massicot (litharge), PbO

Mayan blue, Fe, Mg, Ca, Al, silicate (?)

Mica (muscovite), H2KAl3(Si04)3

Molybdate orange, Pb(Mo, S, Cr, P)04

Naples yellow, Pb3(Sb04)2

Ochre, yellow (goethite), Fe203‘H20, clay.

Specific Particle

Gravity ^ Characteristics ^

2.45

V. fine amorph. part.

ON

ON

d

agg. of spher. gr.

2.93

cryst, frag.

5*75

V. fine cryst.

—

V. fine red glob.

—

irr. amorph. part.

3.80

cryst. frag.

4.45

cryst. frag.

4-36

V. fine cryst. agg.

4-49

V. fine cryst. gr.

—

fibrous agg.

2.29

irr. coarse lumps

4.5

min. round, gr.

4.30

min. round, gr.

4-35

min. round, gr.

4.25

fine comp. gr.

—

round, gr.

2.70

hollow spherulites

—

irr. splintery part.

2.60

fine, vermicular cryst.

b

On

fine green agg.

6.7

tabular cryst.

5.96

fine prism, gr.

5.10

fine cryst. agg.

2.4

crypt, agg.

3-83

round, gr.

—

spher. gr.

—

round, gr.

fine dendritic cryst.

2.31

min. fossil forms

—

cryst. frag.

3*^7

spherulites and disks

—

irr. amorph. part.

2.36

irr. plates

2 . 5 - 2.7

round, irr. gr.

2.36

fine cryst. gr.

—

prisms, plates

5.2

min. cryst.

1.77

min. round, part.

4.30

fine comp. gr.

4.0

cryst. frag.

—

gr. and stubby prisms

—

fine cryst. gr.

9.40

min. flakes

—

porous irr. agg.

p

bo

NO

platy frag.

—

min. round, gr.

—

round, gr.

2 ,Q- 4.0

irn soherulites

Refractive Index ^
wstf 1.50-1.56 [M*]

1.49 (w. bi.) [W|
0:1.570,71.614,181.575 [LB]
fvalentinite, a 2.18,7 and ^ 2.35 _

|senarmonite, 2.09 (isot.) ^
nu 2.65 (isot.) [M*]

1.64- i. 66 [M*]

q: 1.730,7 1.838, iS 1.758 [LB]
a 1.636,7 1.648,^3 1.637 [LB]
1.62-1.64 [M*]

1.94-1.98 (bi.) [M]

0:2 1.72, 72 slightly > 1.74 [M*]

1.65- 1.70 (for larger translucent gr.)
2.64 (bright red)-2.77 (deep red) (

[M*]

2.50-2.76 (for CdS(Se) part) (isot.) j
2.35-2.48 (isot.) [M*]

2.39-2.40 (for CdS part) [M*]

1.84 (isot.) [M*]

€2 r 1.510, 1.645 [M*]

(opaque)

0:1,558, 71.565. 1^1.564 (all ± ,
[LB, M*]

c 2.4 (cf. Prussian blue and chrome ye!
[M*]

o: 2.42, 7 2,7 +5 0 2.7 [M*]

(xrnmn < 2 - 3 L Tesom/i 2.49 [M]
nu 2.5 [M]

« 1*575.71.598.^ 1.597 [LB]
n var.; max. c i. 74 biue (isot.) [M]
1.94-2.0 (w. bi.) [M*]

€1.65-1.79 (dull violet), cu 1.68-
(salmon) (s. bi.) [M*]

1.72-1.76 (isot.) [W]
n mostly 1.435, some 1.40 [M*]

€ 1.605, ^ ^*635 (pleo.) [APL]

0:2 1*7^> 7 s 2.78 (w. pleo.) [M""]
1.582-1.586 [W]

(opaque) [M]

n var. c 1.62, (porous) [M*]

0:1.520,7 1.530, ^1.523 [LB] ;
1.67 (w. bi.) [M*]

€Li 2.78, OiLi 3.01 [M]

(opaque)

2.3 (ZnS)^i.64 (BaS04) [M]

a 1.655, 7 1.909. ^ ^•875 [LB]
oi.65[W]

a 1.67,71.75, iS 1.72 (for violet) [M] I
au 2.51, yu 2.71, &Li 2.61 [M]
i32 1.54 (irr.; bi. and pleo.) [M*]
a 1.563, 7 1.604,181.599 [LB]
to2.55(s.bi.)[M*]

2.01-2.28 (isot.) [M*]

n^ 2 .o (isot. Dart'): 2.0C-2.2I

PHYSICAL PROPERTIES OF PlGMLiNlb

Pigment Name and
Chemical Composition ^

Orpimentj AsaSs

Prussian blue^ Fe4[Fe(CN)6]3

Pumice (volcanic glass)^ Na^ Kj Aij silicate

RealgaPj AS2S2

Red lead, Pb304 {c 95%)

Sepia, organic

Sienna, burnt, Fe203, clay, etc.

Sienna, raw (goethite), FesOs•H2O, clay, etc.
Silica (quartz), Si 02

(chalcedony), Si02
Smalt, K, Co(Al), silicate (glass)

Strontium yellow, SrCrOi

Talc, 3 MgO* 4 Si 02 “H 2 O

Titanium barium white, TiOa (2.5%)

+ BaS04(75%)

Titanium calcium white, Ti02 (25%)

-P CaS04 (75%)

Titanium dioxide (anatase), Ti02
(rutile), Ti02

Ultramarine blue (art.), Nas-.ioAl6Si6024S2-4
(nat.j iazurite), 3Na20*-
3 AI2O3 • 6Si02 • 2N a2S
Ultramarine violet

Umber, burnt, Fe203 + Mn02, clay, etc.
Umber, raw, Fe2034-Mn02+H20, clay, etc.
Van Dyke brown, bituminous earth
Verdigris (copper basic acetate),
Cu(C2H302)2*aCu(0H)2
Vermilion (art.), HgS

(nat,, cinnabar), HgS
Viridian (chromium oxide, transparent),
Cr203 • 2H2O

White lead (basic carbonate),
2 PbC 03 ‘Pb( 0 H )2
Zinc white (ordinary), ZnO
(acicular), ZnO
Zinc yellow, ZnCr04

Specific

Particle

Gravity ^

Characteristics ®

3-4

min. flakes

1.83

colloidal agg.

—

vesicular vitr, frag.

3-56

cryst. frag.

8-73

crypt, agg.

—

angular frag.

3-56

uneven, round, part.

3-14

uneven spherulites

2.66

cryst. frag.

2.6

crypt, agg.

—

splintery, vitr. frag.

_

small needles

2.77

platy frag.

4.30

min. round, gr.

3.10

prism, or ragged gr.

3-9

min. round, gr.

4.2

round, or prism, gr.

2.34

uniform small round, g

2.4

angular, broken frag.

—

round, gr. (blue, rose,
and violet)

3.64

uneven, round, gr.

3.20

uneven, round, gr.

1.66

irr. amorph. part.

—

cryst. frag.

8.09

hexagonal gr. and
prisms

8.1

cryst. frag.

3*32

spheruL gr.

6.70

V. fine cryst.

5.65

V. fine cryst. gr.

—

spicules, fourlets

3.46

min. spher. gr.

Refractive Index ^
au 2.4 ±3 yLi 3-02, 2.81 [LB]

I. 56460 mM [M*]
c 1.50 (isot.) [M*]
au 2.46, yu 2.61 2.59 [LB]

2.42Li (w. bi.; pleo.) [M]

(opaque) [M*]
c 1.85 (var.) (isot.) [M]

1.87-2.17 (mostly 2.06) (isot.) [M*]

€ 1.553, 0) 1.544 [LB]

€, 0? 1.54 [LB, M*]

1.49-1.52 [M=^]

O', id (or co) 1.92,7 (or e) 2.01 (I| ext.) [I

a 1.539,7 ^*589 [LB]

n^c 1.7-2.^ [M*]

mostly 1.8-2.0 (irr.) (bi.) [M*]

€ and 6^ 2.5 (w. bi.) [M*]

€ 2.9, w 2.6 [M*]

^ ^*5^green? ^*^3red (IsOt.) [M]

1.50 ± (isot.) [LB]

Cl.^6 (isot.) [M*]

mostly 2.2-2.3 [M*]
mostly 1.87-2.17 [M*]
i.62-i.69[M*]
a 1.53,7 1.56 [M]

€i,i 3.14, wLi 2.81 [M]

€1,13.146, coLi 2.819 [LB]

o:,/325 l.82, 72;2.I2[M*]

€ I-94, OJ 2.09 [M]

€ 2.02, 0) 2.00 [M]
e 2.02, CO 2.00 [M*]

1.84-1.9 (irr.; bi.) [M*]

1 Abbreviations: art. = artificial; med. = medium; nat. = natural. The chemical formulas are those commonly acce]
in chemical and mineralogical literature, but they may not compare exactly with structural formulas based on x-ray difi
tion data or even on critical chemical analysis.

2 The figures for specific gravity of the artificial pigments are mainly from H. A. Gardner, pp, 710-^12, and those on
mineral pigments are chiefly from E. S. Larsen and H. Berman.

^Symmetry terms (monoclinic, orthorhombic, etc.) are omitted because pigments are so finely divided that it is rare v
observations on crystal symmetry can be made. The term, ‘ spherulitic,^ as used here means aggregates that tend tot
radial structure and spherical shape. ‘Amorphous^ describes materials that are microscopically formless but may be t
crystalline on the basis of x-ray diffraction data. Abbreviations: aeg. = aggregate(s); amorph. = amorphous; comp. = <
posite; crypt. = cryptocrystalline; cryst. = crystal(s); frag. = fragment(s); glob. = globule(s); gj. = grain (s); irr. =f
regular; min. = minute; part. = particle(s); prism. = prismatic; round. = rounded; spher. = spheroidal; spheruL = spli
litic; var. = variable; V. = very; vitr. = vitreous.

^ Unless otherwise inicated, all refractive index measurements are by sodium light. S is the symbol used by H. E. Me
to indicate greater or less indefiniteness or irregularity in the case of aggregates, especially in respect to refractive ir
Abbreviations: bi, = birefringent; c = circa; ext. = extinction; isot. = isotropic; ] j = parallel; pleo. = pleochroic;
strongly; w. = weakly. The letters in brackets refer to^the authorities for the refractive index data: M = H. E. Mer
M* » H. E. Merwin, data by private communication, hitherto unpublished; W = C. D. West, data by private commui
tion, hitherto unpublished; LB ~ E. S. Larsen and H. Berman; APL « A. P. Laurie and co-authors.

ririi^iCAi^ rKuriiKiiiib riurivim'N 1

Pigment Name and
Chemical Composition ^

'rpimentj AS2S3

russian blue, Fe4[Fe(CN)6]3

umice (volcanic glass), Na, K, Al, silicate

ealgar, AS2S2

ed lead, Pb304 (c 95%)

jpia, organic

enna, burnt, Fe203, clay, etc.
enna, raw (goethite), Fe203*H20, clay, etc.
lica (quartz), Si02
(chalcedony), SiOa
nalt, K, Co(Al), silicate (glass)
rontium yellow, SrCr04
lie, 3MgO-48102-HaO
tanium barium white, Ti02 (25%)

+ BaS 04 ( 75 %)

tanium calcium white, Ti02 (25%)

4- CaS 04 ( 75 %)

itanium dioxide (anatase), Ti02
(rutile), Ti02

Itramarine blue (art.), Na8_ioAl6Si6024S2_4
(nat., lazurite), 3Na20*-
3AI2O3 * 6Si02 • 2Na2S
'tramarine violet

Tiber, burnt, Fe203 4 * Mn02, clay, etc.
nber, raw, Fe2034Mn024Pl20, clay, etc.
in Dyke brown, bituminous earth
Tdigris (copper basic acetate),
Cu(C2H302)2-2Cu(0H)2
Tmilion (art.), HgS

(nat., cinnabar), HgS
ridian (chromium oxide, transparent),
Cr 203 * 2 H 20

hite lead (basic carbonate),
2PbC03-Pb(0H)2
1C white (ordinary), ZnO
(acicular), ZnO
1C yellow, ZnCrOi

SPECiriG
Gravity ^

Particle
Characteristics ®

Refractive Index ^

3-4

min. flakes

au 2.4 ±, jLi 3.02, 0Li 2.81 [LB]

1.83

colloidal agg.

I* 5 ^ 4 G 0 m;i [Ml*]

—

vesicular vitr. frag.

r 1.50 (isot.) [M*l

3-56

cryst. frag.

au 2.46, JLi 2.61, ^Li 2.59 [LB]

8-73

crypt, agg.

2.42^1 (w. bi.; pleo.) [M]

—

angular frag.

(opaque) [M*]

3-56

uneven, round, part.

r 1.85 (var.) (isot.) [M]

3-14

uneven spherulites

1.87-2.17 (mostly 2.06) (isot.) [M*]

2.66

cryst. frag.

«i-S 53 . <*^ 1-544 [LB]

2.6

crypt, agg.

€, 6? 1.54 [LB, M*]

—

splintery, vitr, frag.

1.49-1.52 [M*]

—

small needles

a, (S (or co) 1.92, y (or e) 2.01 ( 1 | ext.) [M*]

2.77

platy frag.

^i- 539 >ti. 589 j^ 1-539 [LB]

4-30

min. round, gr.

^2 c 1.7-2.5 [M*]

3.10

prism, or ragged gr.

mostly 1.8-2.0 (irr.) (bi.) [M*]

3*9

min. round, gr.

6 and w 2.5 (w. bi.) [M*]

4.2

round, or prism, gr.

e 2.9, o) 2.6 [M*]

2.34

uniform small round, gr.

Tl I* 5 ^greens ^-^ 3 red (isOt.) [M]

2.4

angular, broken frag.

1.50 ± (isot.) [LB]

__

round, gr. (blue, rose,

c 1.56 (isot.) [M*]

3-4

and violet)
uneven, round, gr.

mostly 2.2-2.3 [M*]

3.20

uneven, round, gr.

mostly 1.87-2.17 [M*]

1.66

irr. amorph. part.

1.62-1.69 [M*]

—

cryst. frag.

a 1.53,7 1.56 [M]

8.09

hexagonal gr. and

€Li3-l4j COLi 2.81 [M]

8.1

prisms
cryst. frag.

€Li 3-1465 OOLi 2.819 [LB]

3*32

spherul. gr.

O', iSs 1.82, 73 2.12 [M*]

6.70

V. fine cryst.

€ 1.94, CO 2.09 [M]

5.65

V. fine cryst. gr.

€ 2 . 02 , CO 2.00 [M]

—

spicules, fourlets

€ 2 . 02 , CO 2.00 [M*]

3-46

min. spher. gr.

1.84-1-9 (irr.; bi.) [M*]

abbreviations: art. = artificial; med. = medium; nat. = natural. The chemical formulas are those commonly accepted

at. siSiTO" s "• ^

lymmetry terms (monochnic, orthorhombic, etc.) are omitted because pigments are so finely divided that it is rare when

can be made. The term, ‘spherulitic.-^as used here means aggregates thlt tenTtowaS
retillne on tile describes materials that are microscopically Termless but may be truly

Se- “ypt - crvXr^^aXi? T JS- = ag-egate(s); amofph. i amorphous; comp. = com!

^ . «y«al(s); _frag._= fragment(s); glob. = glofule(s); gr. = grain(s); irr. = ir-

ic;vM. = VariaWeTv! =^v4- v!tr'^= vit^^ ~ = rounded; spher. = spheroidal; spherul. = spheru-

PHYSICAL PROPERTIES OF PIGMENTS

Pigment Name and

Specific

Particle

Chemical Composition ^

Gravity ^

Characteristics ®

Refractive Index ^

Orpiment, AssSs

3-4

min, flakes

au 2.4 dr, yu 3.02, 2.81 [LB]

Prussian blue, Fe4[Fe(CN)6]3

I. S 3

colloidal agg.

I . [M*]

Pumice (volcanic glass), Na, K, Al, silicate

—

vesicular vitr. frag.

c 1.50 (isot.) [M*]

Realgar, AS2S2

3-5^

cryst. frag.

<XL% 2.46, jLi 2.61, (Sii 2.59 [LB]

Red lead, Pb304 {c 95%)

8-73

crypt, agg.

2.42lx (w. bi.; pleo.) [M]

Sepia, organic

—

angular frag.

(opaque) [M*]

Sienna, burnt, Fe203, clay, etc.

3-56

uneven, round, part.

r 1.85 (var.) (isot.) [M]

Sienna, raw (goethite), Fe203‘H205 clay, etc.

3-14

uneven spherulites

1.87-2.17 (mostly 2.06) (isot.) [M*]

Silica (quartz), Si02

2.66

cryst. frag.

e 2 - 553 > ^ 1*544 [LB]

(chalcedony), Si02

2.6

crypt, agg.

£, oj 1.54 [LB, M*]

Smalt, K, Co(Ai), silicate (glass)

__

splintery, vitr. frag.

1.49-1.52 [M*]

Strontium yellow, SrCr04

—

small needles

a, /3 (or co) 1.92, 7 (or €) 2.01 (11 ext.)

Talc, 3Mg0-4Si02-H20

2.77

platy frag.

^i*539>T 1-589.1.589 [LB]

Titanium barium white, TiOa (25%)

4-30

min. round, gr.

n^c 1.7-2.5 [M*]

+ BaS04(7S%)

Titanium calcium white, Ti02 (25%)

3.10

prism, or ragged gr.

mostly 1.8-2.0 (irr.) (bi.) [M*|

+ CaS 04 ( 7 S%)

Titanium dioxide (anatase), Ti02

3*9

min. round, gr.

€ and 6) 2.5 (w. bi.) [M*]

(rutile), Ti02

4.2:

round, or prism, gr.

e 2.9, cx) 2.6 [M*]

Ultramanne blue (art.), Nas_ioAl6Si6024S2-4

2.34

uniform small round, gr.

^ ^• 5 ^greenj ^*^ 3 red (isOt.) [M]

(nat., lazurite), 3Na20*-

2.4

angular, broken frag.

1.50 db (isot.) [LB]

3AI2O3 * 6Si02 • 2Na2S

Ultramarine violet

—

round, gr. (blue, rose,
and violet)

c 1.56 (isot.) [M*]

Umber, burnt, Fe203 + Mn02, clay, etc.

3*64

uneven, round, gr.

mostly 2.2-2,3 [M*]

Umber, raw, Fe203-|-Mn02+H20, clay, etc.

3.20

uneven, round, gr.

mostly 1.87-2.17 [M*]

Van Dyke brown, bituminous earth

1.66

irr. amorph. part.

1.62-1.69 [M*]

Verdigris (copper basic acetate),

—

cryst. frag.

1.53,7 1.56 [M]

Cu(C2H302)r2Cu(0H)3

Vermilion (art.), HgS

8.09

hexagonal gr. and

eLi3.i4, <^Li 2.81 [M]

(nat., cinnabar), HgS

8.1

prisms
cryst. frag.

€1,13.146, 03Li 2.819 [LB]

Viridian (chromium oxide, transparent).

3-32

spheruL gr.

ay jSv 1.82, 7v, 2.12 [M *1

CrsOs-aHaO

White lead (basic carbonate).

6.70

V. fine cryst.

€ I.Q4, 0> 2.00 [M]

2 PbC 03 *Pb( 0 H )2

Zinc white (ordinary), ZnO

5.65

V. fine cryst. gr.

€ 2 . 02 , CO 2.00 [M]

(acicular), ZnO

—

spicules, fourlets

C 2 . 02 , CO 2.00 [M*]

Zinc yellow, ZnCr04

346

min. spher. gr.

1.84-1.9 (irr.; bi.) [M*]

^ art. artificial; med. ~ medium; nat. = natural. The chemical formulas are those commonly acceptc

m chemical and mmeralogical literature, but they may not compare exactly with structural formulas based on x-ray dif&ai
tion data or even on critical chemical analysis. ^

2 The figures for specific gravity of the artificial pigments are mainly from H. A. Gardner, pp. 710-712, and those on tl
mineral pigments are chiefly from E, S. Larsen and H. Berman. ^ / ’

(monoclimc, orthorhombic, etcj are omitted because pigments are so finely divided that it is rare wht
r ations on crystal symmetry can be made. The term, ‘spherulitic,’ as used here means aggregates that tend towat
crvstill^ron^h^ht spherical shape. Amorphous'describes materials that are microscopically^formless but may be trul
crystalline on the basis of x-ray diffraction data. Abbreviations; agg. = aggregate(s); amorph. = amorphous* como = con
posite; crypt._= cryptocrystafline; cryst. = crystal(s); frag. = ligmentfs);%lob. - Tl

pctvarf'L%ari™ ^ prismatic; round. = rounded; spher. = spheroidal; s|heruL » spheri

indicated, all refractive index measurements are by sodium light. S is the symbol used by H. E Merwi
ibbrtSrH - ^ in the case o^f aggregatfs, especially In respect to JefractiVeinTe’

PHYSICAL PROPERTIES OF PIGMENTS

Pigment Name and
Chemical Composition ^

Orpimentj AS2S3

Prussian blucj Fe4[Fe(CN)6|3

Pumice (volcanic glass), Na, K, Al, silicate

Realgar, AS2S2

Red lead, Pb304 (cgsfo)

Sepia, organic

Sienna, burnt, Fe203, clay, etc.

Sienna, raw (goethite), Fe203‘H20, clay, etc.
Silica (quartz), Si02

(chalcedony), Si02
Smalt, K, Co(Al), silicate (glass)

Strontium yellow, SrCrOi

Talc, 3Mg0-4Si02-H20

Titanium barium white, Ti02 (25%)

+ BaS04(75%)

Titanium calcium white, Ti02 (25%)

+ CaS04 (75%)

Titanium dioxide (anatase), Ti02
(rutile), Ti02

Ultramarine blue (art.), Na3_ioAl6Si6024S2-4
(nat.j lazurite), 3Na20*-
3 AI2O3 • 6Si02 • 2N a2S
Ultramarine violet

Umber, burnt, Fe203 + Mn02, clay, etc.
Umber, raw, Fe203+Mn02+H20, clay, etc.
Van Dyke brown, bituminous earth
Verdigris (copper basic acetate),
Cu(C2H302)2*2Cu(0H)2
Vermilion (art.), HgS

(nat., cinnabar), HgS
Viridian (chromium oxide, transparent),
Cr203 • 2.H2O

White lead (basic carbonate),
aPbC03-Pb(0H)2
Zinc white (ordinary), ZnO
(acicular), ZnO
Zinc yellow, ZnCr04

Specific
Gravity ^

Particle

Characteristics ^

Refractive Index

3*4

min. flakes

au 2.4 'YLi 3.02, ^Li 2.81 [LB]

1.83

colloidal agg.

1 . 5 ^ 460111^1 [M*]

—

vesicular vitr. frag.

r 1.50 (isot.) [M*]

3-56

cryst. frag.

au 2.46, jLi 2.61, ^Li '2>S9 [LB]

8.73

crypt, agg.

2.42Li (w. bi.; pleo.) [M]

—

angular frag.

(opaque) [M*]

3*56

uneven, round, part.

1.85 (var.) (isot.) [M]

3 -H

uneven spheruiites

1.87-2.17 (mostly 2.06) (isot.) [M*]

2.66

cryst. frag.

€ 1.553, «1.544 [LB]

2.6

crypt, agg.

£, ct) 1.54 [LB, M*]

__

splintery, vitr. frag.

1.49-1.52 [M*]

small needles

O', (or 00) 1.92, y (or e) 2.01 ( 1 | ext.) [M*]

2.77

platy frag.

a 1.539, TI.589J/5 1.589 [LB]

4.30

min. round, gr.

n-^c 1.7-2.5 [M*]

3.10

prism, or ragged gr.

mostly 1.8-2.0 (irr.) (bi.) [M*|

3-9

min. round, gr.

€ and to 2.5 (w. bi.) [M*]

4.2

round, or prism, gr.

€ 2.9, 0) 2.6 [M*]

2.34

uniform small round, gr.

^ ^* 5 ^greens ^*^ 3 red (isOt.) [M]

2.4

angular, broken frag.

1.50 =b (isot.) [LB]

—

round, gr. (blue, rose,

iT 1.56 (isot.) [M*]

3-^4

and violet)
uneven, round, gr.

mostly 2.2-2.3 [M*]

3.20

uneven, round, gr.

mostly 1.87-2.17 [M*]

1.66

irr. amorph. part.

1.62-1.69 [M*]

—

cryst. frag.

a 1.53, T 1.56 [M]

o^

q

00

hexagonal gr. and

€Li 3 -Hy [M]

8.1

prisms
cryst. frag.

€Li 3-146, COLi 2.819 [LB]

3 - 3 ^

SpheruL gr.

a, / 3 s 1.82,732.12 [M*]

6-70

V. fine cryst.

€ 1.94, to 2.09 [M]

5-65

V, fine cryst. gr.

€ 2.02, to 2.00 [M]

—

spicules, fourlets

e 2.02, to 2.00 [M*]

3.46

min. spher. gr.

1.84-1.9 (irr.; bi.) [M*]

1 Abbreviations: art. = artificial; med. = medium; nat, == natural. The chemical formulas are those commonly accepted
in chemical and mineralogical literature, but they may not compare exactly with structural formulas based on x-ray diifrac-
tion data or even on critical chemical analysis.

®The figures for specific gravity of the artificial pigments are mainly from H. A. Gardner, pp. 710-712, and those on the
mineral pigments are chiefly from E. S. Larsen and H. Berman.

® Symmetry terms (monoclinic, orthorhombic, etc.) are omitted because pigments are so finely divided that it is rare when
observations on crystal symmetry can be made. The term, ‘ spherulitic,^ as used here means aggregates that tend toward
radial structure and spherical shape. ‘Amorphous^ describes materials that are microscopically formless but may be truly
crystalline on the basis of x-ray diffraction data. Abbreviations: agg. ~ aggregate(s); amorph. = amorphous; comp. = com¬
posite; crypt. = cryptocrystalline; cryst. = crystal(s); frag. = fragment(s); glob. = globule(s); gr. “ grain (s); irr. = ir¬
regular; min. = minute; part. — partide(s); prism. = prismatic; round. == rounded; spher. s= spheroidal; spheruL = spheru-
litic; var. — variabje; y. = very; vitr, = vitreous.

^ Unless otherwise indicated, all refractive index measurements are by sodium light. S is the symbol used by H. E. Merwin
greater or less indefiniteness or irregularity in the case of aggregates, especially in respect to refractive index.
Abbreviations: bi. — birefringent; c = circa; ext. = extinction; isot. = isotropic; jl = parallel; pleo. — pleochroic; s. =
s^ongly^ w. = weakly. The letters in brackets refer to the authorities for the refractive index data; M = H. E. Merwin;
- t' Merwin, data by private communication, hitherto unpublished; W = C. D. West, data by private communica¬
tion, hitherto unpublished; LB = E. S. Larsen and H. Berman; APL « A. P. Laurie and co-authors.

Pigments

149

materials. Specific gravities of the more important pigments and inerts are listed
in the table of physical properties.

The oil absorption of a pigment is the amount of oil that is just required to wet
each of the pigment particles and to convert the mass into a mobile paste. Pig¬
ments differ greatly in the amount of oil required for this purpose. It is often ex¬
pressed as the number of grams of oil required to grind 100 grams of pigment into
a stiff, putty-like paste that does not ‘ break ’ or separate (see Gardner, pp. 539—
560). Oil absorption is no exact physical constant. It varies slightly from lot to
lot of pigment, with the kind and condition of the oil used, and with the degree
and duration of mixing and rubbing. Some pigments, like basic carbonate white
lead, are characterized by low oil absorption, which is generally as low as 9 to 12
per cent by weight of oil, to make it into a workable paste; raw sienna, on the other
hand, takes upwards of 50 per cent oil to grind. Pigments with low oil absorption
are favored, in general, because paints made from them have less tendency to
discolor as a consequence of the yellowing of the oil. Many of the pigments with
high specific gravity have low oil absorption. Gardner says (p. 544) that oil ab-
sorption is dependent essentially upon the total surface of the pigment, the Inter¬
facial tension relations between pigment and vehicle, particle shape, size, and
distribution, and the chemical nature of oil and pigment. All these are Important
factors that have much influence on the plastic and flow properties of oil paints
(see also R. Houwink, Elasticity^ Plasticity and Structure of Matter [Cambridge:
University Press, 1937], pp. 311-327).

Pink (Dutch pink, Italian pink, brown pink), in addition to Its meaning as a
tint of red, is also used for certain yellow lakes prepared from quercitron (see
Quercitron Lake), or from Persian berries (see Persian Berries Lake), or from
similar, natural, yellow coloring matters. (The Shorter Oxford English Eictionary
says that the origin of the word in this connection is obscure.) Brown pink is a
deep variety of quercitron lake (Weber, p. 127),

Pipe Clay (see China Clay).

Plaster of Paris (see Gypsum),

Pompeian Blue (see Egyptian Blue).

Pozzuoli Red is a red iron oxide of volcanic origin from Pozzuoli, near Naples.

Prussian Blue (Berlin blue, Paris blue, Antwerp blue, Chinese blue) is the
earliest of the modern synthetic colors. It is a complex chemical compound which,
technically, is ferric ferrocyanide, Fe4(Fe[CNjl6)35 or a closely similar compound.
It is now commonly made by the action of an oxidizing agent, such as potassium
bichromate and sulphuric acid, upon a mixture of copperas (ferrous sulphate),
sodium ferrocyanide, and ammonium sulphate, giving a blue with the approxi¬
mate formula, Fe(NH4)Fe(CN)6. The pigment which is precipitated from dilute
solutions of those salts is a deep blue, finely divided compound which, after It has
settled and after the mother liquor is drawn off, is washed, filtered, and dried.
(See Bearn, pp. 85-92 for details.) The product is amorphous in colloidal ag-

150

Painting Materials

gregates and so finely divided that it has almost the characteristics of a dye. By
control of conditions of precipitation and oxidation, variations in shades and
physical characteristics of the blue may be had. (Antwerp blue is a light shade of
Prussian blue, made by precipitating zinc along with the iron, or it may contain
inerts like gypsum, barytes, etc.) The dry powder or lump form is dark blue, al¬
most blue-black, but some varieties, especially those prepared with bleaching
powder as the oxidizing agent, have a reddish, coppery lustre. It is a transparent
color but has very high tinting strength. One part of Prussian blue will render 640
parts of white lead perceptibly blue (see Colour Index, p. 309). The color by trans¬
mitted light is green-blue which is also its color in tint. In an oil film, discrete
particles can not be seen, even at high magnification. De Wild has compared its
appearance in oil with that of indigo (p. 32). It seems to form thin smears on the
surfaces of other pigment grains (see Chrome Green).

Prussian blue is fairly permanent to light and air. Laurie mentions {The
Painters Methods and Materials, p. 94) a ceiling painted with it in the first part
of the XIX century in which the color is still good. It sometimes acquires a
peculiar, metallic bronze cast when subject to out-of-door weathering, and some¬
times paint films that contain it turn green because of yellowing of the oil. The
blue is unaffected by dilute mineral acids, but it is very sensitive to alkalis which
cause it to turn brown; for this reason, it can not be used in true fresco. It is
soluble in 10 per cent oxalic acid (Bearn, p. 89). It decomposes rapidly on ignition
and leaves a residue of ferric oxide.

Huge quantities of Prussian blue are now used in the paint and printing ink
trades; the most important commercial green pigment, chrome green (see Chrome
Green), is made by adding it to chrome yellow.

Prussian blue has a conspicuous place in the history of painting materials
because it is the first of the artificial pigments with a known history and an
established date of first preparation. Moreover, it Is a material so complex in
composition and method of manufacture that there is practically no possibility that
it was invented independently in other times or places. It was first made in Berlin
on or about the year 1704 by Diesbach, a dyer or color maker. Kopp (IV, 369) says
that it was first mentioned in an anonymous communication entitled ‘ notitia
coeruki Berolinensis nuper inventi^ in a report of the Miscellanea Berolinensis,
1710. In that notice the beauty of the color was praised and it was claimed that it
was useful as a painting pigment. It was said then to be for sale by the book dealers
of the Berlin Academy. In that account there was nothing about the discoverer
or the method of preparation. Stahl (Georg Ernst, 1660-1734) gave a more ac¬
curate and detailed report of the discovery in his Experimentes, Obseroationibus,
animadversionibus CCC, etc. (1731). According to him, it came from an accident
which resulted when Diesbach wished to prepare Florentine lake by the precipita¬
tion of an extract of cochineal with alum and iron vitriol and a fixed alkali. He
asked the well known alchemist, Johann Konrad Dippel (1673-1734), to let him

Pigments

151

have for the purpose some of the waste potash over which he had distilled in
process of purification some of the animal oil with which he was then working
ippe s 01 a distillacion product of bones and other animal matter which
consists chiefiy of pyridine and pyridine bases). With this alkali Diesbach got,

f ^ blue one. He told Dippel, who realfzed

that the foimation of the blue color was the result of the action of the spent alkali
upon the iron vitriol. Dippel had prepared his animal oil from blood. (The calcina¬
tion of the blood with alkali had formed potassium ferrocyanide, which was the
reagent that had reacted with iron vitriol under oxidizing conditions to form
Berlin blue.) Kopp goes on to say that the preparation of Berlin blue was kept
secret until the Englishman, Woodward, published it in the Philosophical Trans¬
actions for 1724. It was soon demonstrated that the blue could be prepared from
other animal remains (nitrogenous substances) as well as from blood. (In German
the word, Blutlaugensah: is still in use, however, for potassium ferrocyanide.)
Bearn, in his short historical introduction to the preparation of Prussian blue,
sa.ys (p. 85) that Diesbach communicated his discovery to a French pupil De
Pierre, who jater started making this pigment in a small way in Paris; hence’ the
name,_ Pans blue.’ He adds that Wilkinson in London next commenced manu¬
facturing it, and that gradually more and more color firms took up its production
It must have been well known all over Europe by 1750. The earliest painting on
w 1C e lid reports it (p. 33) is one by J. E. La Farque, dated 1770, and it is
quite commonly found on late XVIII century and XIX century works.

Pumce (and pumicite) is a light, porous stone or natural vesicular glass of
vo came origin, and consists of silicates of aluminum, sodium, and potassium (see
Ladoo, pp. 455-464). Ground pumice is a light gray or warm white, gritty powder
Under the microscope, particles appear like broken glass, with the rounded
surfaces of broken bubbles appearing prominently. Much that is produced com¬
mercially comes from the Lipari Islands off the coast of Sicily. It is widely em¬
ployed as an abrasive and polishing agent. It is put in certain types of paint,
particularly that for masonry, where its open cellular structure allows air dif-
usion. Being a volcanic ash, pumicite is a fine powder or dust composed of sharp,
angular grains of volcanic glass of about the same composition as pumice. It is used
extensively in cleaning powders.

Purree (see Indian Yellow).

Quartz (see Silica).

Quercitron Lake (yellow lake, flavine lake) is a yellow coloring matter made

kom the inner bark of a species of oak, Quercus tinctoria, that is indigenous to
T- principle is quercetin or tetrahydroxyflavonal,

'-.16W10U7. Ihe bark IS extracted with water and the lake is made by adding alum
and precipitating with chalk (see Perkin and Everest, pp. 628-629). It is soluble
in water and in alcohol, but forms a yellowish brown solution with alkalis and is
decolorized by mineral acids. Yellow lakes of this nature are rapidly faded by

152

Painting Materials

sunlight but are said to retain their color well in artificial light {Colour Index
p. 301).

Raw Sienna (see Sienna).

Raw Umber (see Umber).

Realgar is the natural orange-red sulphide of arsenic, AS2S2, and it is closely-
related chemically and associated in nature with orpiment, the yellow sulphide of
arsenic (see Orpiment). The two minerals are often found in the same deposits.
Although it occurs perhaps as widely in nature as orpiment, realgar appears not
to have been used so widely. Like orpiment, it was known in ancient times. All
are agreed that the ‘ sandarack ' of Pliny (see Bailey, II, 75-77) 'is identical with
the modern realgar. It was confused by the ancients with red lead because it
resembles it in color (see Bailey, II, note on p. 205). It is said to get its name from
the Arabic, alghdr (powder of the mine) (see Dana, p. 357). Cennino Cennini
mentioned it but did not hold it in favor (Thompson, The Craftsman's Handbook
p. 29).

The chemical and physical properties of realgar are similar to those of orpi¬
ment. It belongs to the same crystal system (monoclinic), but has slightly lower
refractive index. Its color is orange by transmitted light but usually many yellow
particles of orpiment can also be seen. It may be made artificially, but the artificial
product is not used as a pigment in modern times, being too poisonous for that

purpose.

Realgar has not been identified in ancient paintings so frequently as orpiment.
It was observed, however, as the pigment in an orange-red area on a fragment
of wall painting from Kara Khoto in Central Asia (XI-XII centuries).- One may

expect to find it, along with orpiment, in Eastern miniatures and illuminated

manuscripts.

Red Bole (see Bole).

Red Lead (minium, orange mineral). The red tetroxide of lead, Pb304, is made
by heating litharge or white lead for some hours at a temperature of about 480° C.
( ee earn, pp. 114 117, for details.) Care must be taken that the temperature
does not get too high or it will be decomposed again into litharge, because the
reaction is reversible. Most of that now made commercially is from the direct
0X1 ation o ead through the monoxide stage and the product may contain 'X
per cent or more of free litharge. ‘ Orange mineral,’ which is made by the roasting
of white lead, has the same composition as red lead, except that it contains less
ree litharge. There is still some question about the structure of red lead but, for

analytical purposes, it is treated as PbO^-aPbO, in which the Pb02 functions as an

oxidizing agent.

The pigment is bright scarlet, has good hiding power, and excellent texture.

be either crystalline or amorphous, depending upon
SoL Af r P- 5^8). MicroscopicaUy, it is not very characteristic,

the particles are transparent and orange-red by transmitted light. The

Pigments

^53

refractive Index is high but it is only slightly birefracting. Chemically, red lead is
fairly active. It is turned brown rapidly by nitric or acetic acid, which is the result
of the formation of brown lead dioxide; hydrochloric acid turns it white (lead
chloride); sulphides and hydrogen sulphide blacken it; it is not affected by dilute
alkalis. When exposed to light and air, it is not a particularly stable pigment
and has had, for centuries, a poor reputation in that respect. Red lead will turn
chocolate brown in color when exposed to either strong or diffuse light over a
period of centuries, particularly when it has been applied in a water color or
tempera medium. This darkening has been particularly noticeable on the wall
paintings of China and Central Asia. Gettens observed the phenomenon on the
wall paintings of Kizil in Chinese Turkestan (V-VIII centuries, A.D.), and the
strange, chocolate-colored faces on wall paintings at Tun Huang appear to add
further evidence. In films prepared from a glue medium, the darkening can be
produced artificially by exposure to light and high humidity, and seems to come
from the formation of brown lead dioxide. This fault of red lead was mentioned by
Cennino Cennini (see Thompson, The Craftsman's Handbook^ p. 25), who says
that it is good for painting on panel, but on the wall ‘ it soon turns black, on
exposure to air and loses its color ’ (see Thompson, loc. cit.). Red lead in oil, when
strongly exposed out-of-doors, may eventually turn pink or white because of the
formation of lead carbonate (white lead).

Red lead was a pigment of antiquity and was probably known as early as lead
itself. Lucas holds, however (pp. 290 and 308), that it was not used in Egypt until
Graeco-Roman times. By early classical writers, it seems to be confused with
other reds, particularly cinnabar or mercuric sulphide. Pliny describes it under the
name, secondarium minium (see Bailey, I, 119, 123, 217, 220-221), but appears to
have recognized it as distinct from minium, which was the name he used for
cinnabar or mercuric sulphide. The name, minium, came to be applied to red lead,
rather than to cinnabar, at some time in the Middle Ages.

Red lead is often found as a pigment on objects that date from antiquity.
Davy (p. loi) identified It as the orange-red on a vase at the Baths of Titus.
Laurie (' The Materials in Persian Miniatures’) says it was a favorite with the
Byzantine and Persian Illuminators. Thompson {The Materials of Medieval
Paintings p. 101) records that orange lead was common all through the Middle
Ages in manuscript embellishments and in painting, but that It was not used on
walls and not much on panels. This is in agreement with the negative findings for
red lead by De Wild on the Dutch and Flemish paintings he examined. It has been
identified, however, as a pigment on one of the panels of the Monte Oliveto altar-
piece, by Spinello Aretino (XIV century) in the Fogg Art Museum. It was widely
used on wall paintings in China and Central Asia, as has already been indicated.
In spite of its bright color and good covering power, artists do not much use it now,
although it is still obtainable in water color form. Commercially, however, it is

154

Painting Materials

important, being used extensively as an anti-corrosive pigment for iron, and
familiar in the priming coats used for structural steel and for iron fences.

Red Ochre (see also Iron Oxide Red) is an earthy variety of iron oxide which
contains more or less clay and silica and is one of the most important of the natural
pigments. The best contains as high as 95 per cent of ferric oxide. It has been
widely used. It was the red paint of the American Indian and the sinopis or
sinopta of classical antiquity (see Thompson, TAe Materials of Medieval Painting,
p. 98).

Rhodamine (Rhodamine toner) is one of the stable, synthetic, organic dye¬
stuffs used for making red lake pigments. Rhodamine 6G, discovered by Bernthsen
in 1892, is the ethyl ester of diethyidiamino-o-carboxy-phenyl-xanthenyl
chloride, C26H26N2O3CI {ColourIndex, p. 190).

Rinnman’s Green (see Cobalt Green).

Safflower (carthame) is a natural organic red coloring matter which is pre¬
pared from^ the dried petals of the Dyer’s Thistle, Carthamus tinctorius, which is
cultivated in the East, in Egypt, and in southern Europe. The red coloring matter
is carthamin, or carthaminic acid, C25H24O12. This is extracted by steeping the dry
petals in cold dilute sodium carbonate solution. Safflower extract is only slightly
soluble in water and alcohol. The dye is an orange solution with alkalis and is
turned dull red by dilute sulphuric acid. It was once used widely in the East as a
dye and for the manufacture of pigments and cosmetics.

Saffron is the golden yellow coloring matter that is extracted from the dried
stigmas of the crocus flower, particularly Crocus sativus. It has long been a flavor¬
ing for food and was formerly used in painting, especially, for illuminating and
embellishing the pages of books. The color was put directly into a medium without
a mordant. Jehan le Begue (Merrifield, 1 , 128) and other writers of his period speak
of adding it to green, particularly copper green, to make a warmer tone. Saffron
IS mentioned by Theophilus (p. 41) and by Le Begue {op. cit., p. 158) for aiiri-
petrum, a transparent yellow coating over tin foil to make it look like gold. Beck¬
man has given a whole chapter to saffron (I, 175-180), and believes it originally
was brought from the Levant into Spain and Europe by the Arabs.

Sap Green is a natural organic dyestuff derived from the ripened berries of
several varieties of the buckthorn, Rhamnus. The juice of the berries may be used
directly or may be precipitated with alum. In early times it was not dried but was
sold in bladders as a dense syrup (Thompson, The Materials of Medieval Painting,
rP' u ^ most fugitive substance, it is still used in water color.

^ manuscripts, it has often survived because of ideal condi¬

tions. Oil paints now sold under this name usually contain coal tar lakes.

Scheele’s Green, an acid copper arsenite, CuHAsO^, was the first artificial
green pigment in which copper and arsenic were the essential constituents. It
was first prepared by the eminent Swedish chemist, Carl Wilhelm Scheele, in 1778.
Methods of preparation varied in details, but it was usually made by dissolving

Pigments

155

white arsenic (AS2O3) in a solution of soda ash or potash and adding the hot
arsenite solution to a solution of copper sulphate. The precipitate needed only
washing and drying. De Wild says (p. 79) that the pigment consists of small and
large/irregular-shaped, green flakes which are only slightly transparent. Since
Scheele’s green is inferior as a pigment, it w^as soon displaced by the superior
copper aceto-arsenite (see Emerald Green) when that was introduced in 1814. It
is blackened by lead and is decomposed by acids. Yellowish green when made, it
fades rapidly and is blackened by sulphur-bearing air and sulphide pigments. It
is very poisonous. Although it has not been reported in the examination of paint¬
ings, one may expect to find it in works of the late XVIII and early XIX centuries.

Schweinfurt Green (see Emerald Green).

Sepia is the black or dark brown secretion from the ' ink bag ’ of the common
cuttle-fish or squid {Sepia officinalis)^ and also from other species of the Cephalo¬
poda. Although it has come from cuttle-fish of the English Channel and the
Mediterranean, most modern sepia is from Ceylon. The dark ink has very high
tinctorial power, the secretion from one cuttle-fish being able to turn a thousand
gallons of water opaque in a few seconds (see Mitchell, p. 19). For sepia prepara¬
tion the ink sacs are removed from the cuttle-fish, dried, pulverized, and boiled
with lye solution. The extract, which is soluble in the lye, is precipitated out with
hydrochloric acid, is washed, and is dried at a low temperature; it is ground very
finely with gum arabic and is made into cakes or prepared in tubes for use in water
color.

Sepia is a complex nitrogenous organic compound with characteristic fishy
odor. It is in the nature of an organic acid (see Mitchell, pp. 22-25); it is soluble
in alkalis and is reprecipitated by acids. It is insoluble in water, alcohol, ether,
and similar organic solvents; it is decolorized by nitric acid and by chlorine
water. Under ordinary conditions, sepia is fairly permanent, but in strong sunlight,
especially in thin washes, it is quite fugitive. It is relatively opaque to infra-red
rays. The color of sepia, when recently applied, is warm black but it gradually be¬
comes reddish brown, the color commonly associated with the name. Under the
microscope, its appearance is similar to bone black and it may be observed in
irregular, fairly coarse particles, most of which are opaque, although there are
many that are semi-transparent yellow-brown. Oil paints sold under the name,
‘sepia,’ are mixtures of such pigments as burnt umber, Van Dyke brown, and
lamp black.

Although there is reason to believe that sepia was known and used as early as
classical times, particularly as an ink for writing purposes (see Mitchell, p. 8), it
was not until the latter part of the XVIII century that it became popular in Eu¬
rope for ink drawings and for water color painting. Meder (pp. 69-70) refers to
Hebenstreit {Encyklopddie der Aesthetik) as authority that sepia was introduced
as a color by Professor Seydelmann of Dresden some time after 1778. Until about

Painting Materials

156

the end of the XVIII century^ only bistre and India ink were used for washes
(see Meder^ p. 70). The comparatively late use of sepia makes possible a distinc-
ticn between late XVIII century sepia additions to earlier bistre drawings.

Sienna (raw sienna^ burnt sienna). Raw sienna is a special kind of yellow ochre
which gets its name from the well known Tuscan city near which one of the best
grades of it has long been found; it is still produced there and is shipped from
Leghorn. Good commercial grades are also found in the Hartz Mountains, Ger¬
many, and in America. Like ochre, sienna is hydrated ferric oxide (goethite,
Fe203*H20) with alumina and silica, but it has a little deeper tint than yellow
ochre, is a little warmer, and is considerably more transparent. A good sienna
should contain at least 50 per cent of iron oxide (Fe203); some of the best contain
70 per cent or over. It generally has a small amount of manganese dioxide (0.6
to 1.5 per cent). Raw sienna is prepared for commerce by processes similar to
those used with the ochres, and the physical and chemical properties are like those
of the other hydrous iron oxides. Microscopically, the pigment is quite hetero¬
geneous; it is a mixture of transparent, colorless, yellow and brown-red particles,
along with opaque brown particles and a few scattered pink ones. The transparent
grains are highly birefracting, but the brownish material is quite isotropic. This
latter material is a darkened variety of goethite (see Ochre), occurring in fairly
large spherules.

Burnt sienna is prepared by calcining raw sienna; in the process, the raw sienna
undergoes a considerable change in hue and depth of color. In going from the ferric
hydrate of the raw earth to ferric oxide, it turns to a warm, reddish brown.
Microscopically, it becomes more even in color and the grains are reddish brown
by transmitted light. Merwin says (p. 578) that it shows no evidence of crystal¬
linity, is not birefracting, and the grains have variable, moderate refractive index.

By artists, sienna has been used as a glaze because of its transparency. For
the same reason, both raw and burnt sienna are used in wood finishing for stains
and for graining work. In the microscopic and chemical examination of paintings
the siennas are usually not reported under that name but are grouped under the
ochres or native iron oxide pigments. Often distinction among earth colors is
difficult, because the differences are of degree and not of kind. The siennas have
been available in all periods of European painting and have been used in all
processes.

Silex (see Silica).

Silica (quartz, silex) is silicon dioxide which occurs in clear, crystalline form
as quartz or rock crystal. It is common in other less pure forms as quartzite,
sandstone, sand, and in crystalline grains or masses in granite (see Ladoo, pp. 521-
526). It is widely distributed, being one of the most abundant constituents of the
earth’s crust. Finely ground and sieved quartz (silex), with oil or varnish, serves as
a primer for filling the grain of wood before staining and varnishing. Tripoli (not
to be confused with tripolite which Is the same as diatomaceous earth) is an amor-

Pigments

157

phous or cryptocrystalline (chalcedonic) form of silica also nsed for wood fillers
and in paints (see Ladoo^ pp. 641—651). Diatomaceous earth (see Diatomaceoiis
Earth) Is a fossil form of silica. Silicic acid or precipitated silica, Si02*;f2H20, pre¬
pared by the action of an acid on an alkali silicate, is a pure white, amorphous
powder which has special uses as a filler and extender. Silica in all its forms is
extremely inert. It is unaffected by heat and is insoluble in strong acids (except
hydrofluoric) 5 but it is slowly attacked by strong alkalis. Quartz, or crystalline
silica, can be recognized by its optical properties. It has medium refractive index
(o) = 1.544), and is only moderately birefracting. Particles of quartz are often seen
as an impurity in mineral pigments and other natural products. Sand particles are
usually rounded and frosty in appearance as a result of the wearing action of wind
and wave.

saver Leaf and Sfiver Powder were used occasionally in mediaeval paintings,
but their very great tendency to tarnish and to blacken limited their effectiveness'
This fault was known very early and Cennino Ceniiini warns against silver for
that reason (see Thompson, The CTciJts 77 ici?f s HciTidboohy p. 60). Laurie speaks of
Byzantine manuscripts {The Pigments and Mediums of the Old Masters ^ pp. 78-79)
where not only the silver but the mordant also has become black and appears
to have stained through the manuscript page. There seems to be no connection
between the discolorations of the two. When protected with a good varnish coat¬
ing, however, this metal may retain its lustre for years. Silver leaf was used for
rendering armor in battle scenes and pageants (Thompson, The Materials of
Medieval Paintingy p. 190). In some early paintings, it was used for a background
like gold leaf. Methods of application were much the same as those for gold.

Smalt was the earliest of the cobalt pigments. It is artificial, in the nature of
glass, a potash silicate strongly colored with cobalt oxide and reduced to a powder.
The origin is obscure- For years there has been much debate concerning whether
or not cobalt was used by the Egyptians and by other peoples of classical times
to color glass. Marie Farnsworth and P. D. Ritchie have shown recently (" Spec¬
troscopic Studies on Ancient Glass,* Technical Studiesy VI Ci938]> pp. I55”*i73)
that cobalt was definitely present along with copper in much Egyptian blue glass,
but only in amounts of the order of o.i to 0.2 per cent. They assume that the
cobalt may have been used intentionally with full knowledge of its properties
for that purpose. There is no evidence as yet, however, that any powdered cobalt
glass was ever used as a painter’s pigment in ancient times. When cobalt was first
employed in Europe for glass making is not known, but probably the Venetian
glass makers knew of its properties. B. Neuman C Antike Glazer,* Zeitschrift
fur Angewandte Chemicy XXXVIII [1925], p. 863) remarks that it was first used
by them in 1443, but he does not give his source of information. According to
Laurie {The Pigments and Mediums of the Old Masters, pp. 12,-16), the word,
smalto, was used as early as 149^5 a glass pigment under the name, azzurro
di smalto, was described in 1584. About the middle of the XV century, certain

158

Painting Materials

cobalt minerals such as cobaltite (CoAsS) and smaltite (C0AS2) were discovered
on the borders of Saxony and Bohemia. At the time, the nature of these minerals
was not known and, since they gave the miners a certain amount of trouble,
they were called ^ cobalt" for spirits or ' kobolds ’ which were thought to haunt
the mines. The Bohemian glass makers soon found, however, that these strange
minerals had the property of coloring glass blue. Beckman, who treats the early
history of smalt (I, 478-487) relates (p. 483) that information about the early
discovery has come through a certain Christian Lehmann (d. 1688) who wrote an
historical work on the mines in Misnia. Lehmann said that the color mills, at the
time when he wrote, were about a hundred years old, and Beckman, therefore,
places the invention at about 1540-1560. Lehmann added that smalt was dis¬
covered by Christian Schurer (or Christoph Schiirer [see Rose, p» 277]), a glass
maker of Flatten in Bohemia, who found that when cobalt mineral was added to
a molten, vitreous mass, he obtained a fine blue glass. At first, he prepared it only
for the use of potters, but in the course of time it was carried as an article of
merchandise to Nuremberg and thence to Holland. Later it came to be manu¬
factured in Holland and in better quality than that previously prepared by the
Saxons.

As has been said, smalt was first prepared by roasting native cobalt minerals
like cobaltite and smaltite to form cobaltous oxide, CoO. ‘ Zafer^ it is called by
Beckman. Rose mentions (p. 278) that Biringuccio, in his Pyrotechnia (1540),
calls it by that name. The oxide was then added to a mass of molten glass and,
when thoroughly combined, the molten mass was poured into cold water to break
it into fine particles. It was further ground and then washed and allowed to settle.
The finer particles, which settled last, gave only a pale blue pigment, but the
coarser particles gave a deep, rich, purple-blue. Smalt may contain in 100 parts,
65 to 71 parts of silica (Si02), 16 to 21 of potash (K2O), and 6 to 7 parts of cobalt
oxide (CoO), as well as some alumina (see Church, p. 224). In an analysis given by
Bearn (p. 92), the cobalt oxide content is much lower, only 0.8 per cent. Farns¬
worth and Ritchie say (Joe. cit.y p. 160) that in a modern cobalt glass, a strong
blue is imparted by 0.15 per cent of cobalt oxide and a quite perceptible blue by
0.006 per cent. Blue glasses, however, with such low cobalt content, when ground
to a powder, have little or no color.

Smalt, since it is a glass and is transparent, has very poor covering power and,
for this reason, it has had to be used very coarsely ground. This has led to certain
difficulties: a tendency to settle in the paint film; poor spreading qualities, and a
tendency to streak down the surface of a painting. Various tricks were employed
by early painters to overcome this fault (see De Wild, p. 25).

Smalt can usually be recognized easily by its microscopic appearance. The
glassy character and conchoidal fracture are seen even at low magnifications.
Quite characteristic are particles with square and angular corners and others wit§i
thin, flat edges; some are shaped like sharp splinters; tiny air bubbles in the

Pigments

159

particles are common. Like all true glasses, the pigment is isotropic; the refractive
index is low. Large particles are purple-blue by transmitted light, but small
particles are clear blue. Smalt, like other glasses, is ordinarily considered to be
quite stable. It is commonly seen on pictures two or three centuries old, without
sign of alteration. ’W'hile most samples are insoluble, even in strong acids, a few
have been seen which are quite readily soluble, even in dilute hydrochloric acid.
Church says (p. 224) that it is gradually altered by moisture and by carbonic acid
of the air, becoming paler and grayer, and Doerner also speaks (p. 80) of this
tendency. These unstable pigments may be high in potash and not true glasses.

In Europe smalt seems to have had its beginnings as a pigment certainly as
early as the late XIT century. In the early XVII centiiiy it came into general use
m oil painting. There is evidence, however, that smalt was known in Asia even
earlier than in Europe. It has been identified on Chinese wall paintings: one from
Kara Klioto in Central Asia and dated perhaps as early as tlieXI-XIII centuries;
anotliei, a Seated Buddha of the Ming Dynasty (Fogg Museum, 110. 1933.190),
Farnsworth and Ritchie say?- (loc. ctt.^ p. 160): ^ Some of the finest blue underglazes
in Chinese blue-and-white pottery, occurring about the middle of the Ming
Dynasty, vrere made by means of cobalt ores imported from some Islamic area
west of China, and are alway’-s described as ^ Mohammedan blue.’

The use of smalt as an artist’s pigment was discontinued around the beginning
of the XIX century. This was because of its shortcomings as mentioned above,
and also because its place w?as taken by the more satisfactory cobalt aluminate
(cobalt blue) and by artificial ultramarine. It is still available, however, and is
used to some extent by sign-painters for strewing on a background for gold letter¬
ing. Smalt is quite frequently found on old paintings. It was identified on a
portrait by Hans Holbein the Younger (i 497 ~i 543 ) Sir William Butts, and this
indicates that it was known before the time which Beckman suggests for its
discovery (1540-1560). Sixteen specimens of blue pigment were identified as smalt
by De Wild on XVII and XVIII century paintings, one by Rubens, dated 1620,
being the earliest. Laurie records (New Light on Old Masters 126) that Teniers
used smalt in his skies and that Velasquez (op. cit.y p. 129) used a mixture of smalt
and azurite in the green drapery of the Rokeby ' Venus,’

Soapstone (see Talc).

Steatite (see Talc).

Strontium Yellow (lemon yellow) (see also Barium Yellow) Is strontium chro¬
mate, SrCr04. It is prepared much like barium yellow except that strontium
chloride replaces barium chloride. The finely divided, crystalline precipitate that
is formed must be thoroughly washed to be useful for pigment purposes. It is a
little deeper and brighter in lemon hue and has greater hiding power than barium
chromate. It takes the form of blades or needles which are pale yellow by trans¬
mitted light and are strongly birefracting. Strontium chromate is slightly soluble
in water and soluble in alkalis, in dilute mineral acids and in acetic acid; it some-

i6o

Painting Materials

times takes on a greenish tone (reduction to chromic oxide) when exposed to
strong sunlight (see Eibner, Malmaierialienkunde, p. 165). Like barium chromate,

this alkaline earth chromate is also sold as‘lemon yellow. , . r

Syntiietic Pigments are those made by processes of chemical synthesis from

chemical elements or compounds. They may be inorganic, compounds of the
metals, or they may be organic, complex compounds of carbon, like the dyestuffs

of coal-tar origin. Some synthetic or artificial pigments, like Egyptian blue, white
lead and verdigris, have been known since classical times or earlier.

Talc (soapstone, steatite) is a natural hydrous magnesium silicate, sMgO-
4Si02-H20, and is found as a soft stone. It is smooth and unctuous to the touch
because the cleavage is highly perfect and it powders to form thin, laminar
particles. It has properties like those of China clay, and in the arts is used for
similar purposes. Talc is white to grayish white in color. It is very inert and is used
commonly as a filler in paints and paper, in colored crayons, and for dusting.
Steatite or soapstone, which is a massive variety of talc, serves as a sculptor’s
medium and for certain ornamental purposes. A fibrous form of talc from New
York State called ‘asbestine’ is widely used in outside paint films to increase
strength and weathering properties; it contains about 92 per cent magnesium
silicate (Gardner, p. 1250).

Terra Alba (see Gypsum).

Terre-Verte (see Green Earth).

Th6nard’s Blue (see Cobalt Blue).

Tin Leaf and, to a smaller extent. Tin Powder were very early used to embellish
paintings. The metal was used in its own right or for imitating silver, or it was
lacquered yellow to imitate gold. Tin, which was one of the metals known to the
ancients, is soft and malleable; hence, it is easily beaten into leaf or foil. It is
superior to silver in that it does not tarnish and blacken with time. Mediaeval
recipes for the use of tin in imitation of gold are numerous. With a yellow varnish
to give it a golden glint, it was named ‘ auripetrum.’ Many of the recipes for this
call for saffron (see Saffron) or other transparent yellow or red vegetable coloring
matters. Theophilus (pp. 31-33) tells how to beat out the tin foil on an anvil,
how to polish it, how to ornament letters and pictures in books with it (p. 41), and
how to imitate gold by coating it with glair mixed with saffron. Jehan Le Begue
(see Merrifield, I, 304) mentions that tin was used in powder form as well as foil.
Laurie says {Materials of the Painter’s Craft, p._203) that the famous Spanish
leather hangings were decorated with tin foil to give both the silver and the gold
effects. Much of the brassy gilding observed so frequently on Russian icons is
probably tin foil coated with yellow varnish. Thick tin foil has occasionally been
put on the backs of panel paintings and varnished over to make the wood im¬
pervious to moisture and to prevent warping.

Titanium Dioxide (titanium white, ‘ titanox ’), TiOz, is the whitest and has the
greatest hiding power of any of the white pigments. The principal titanium ore.

Pigments

i6i

ilmenite (originally menachanite), was first described by an Englishman, the Rev.
William Gregor, as early as 17915 but the element was named ^ titanium ' by the
German chemist, Klaproth, in 1795 (see Weeks, pp. 142-146). Attempts were
made to develop it for pigment purposes early in this century (Rose, pp. 357-359)5
but not until about 1916-1919, however, were certain companies in Norway and
America (see Toch, Chemistry and Technology of Paintsy p. 48; also Trillich, III,
52) able to overcome difficulties in the purification and manufacture of the oxide
and to put it on the market in regular production. The native oxide of titanium,
rutile, occurs in nature, but the titanated iron ore or ilmenite (FeTiOs), found in
large deposits on the west coast of Norway, today supplies the titanium of
commerce.

For the preparation of this pigment, the ilmenite ore is digested with con¬
centrated sulphuric acid, and the coagulated mass of iron and titanium sulphate
which is formed is dissolved in water and then heated to boiling to precipitate the
titanium as metatitanic acid and separate it from the iron. The precipitate is
neutralized with barium carbonate and is then calcined. Commercially, only a
small amount of titanium dioxide is used, pure, as a pigment for white paints.
Most of it is sold as a composite in which it is precipitated on a base of barium or
calcium sulphate. Barium base titanium oxide is usually about 30 per cent titan¬
ium oxide and 70 per cent barium sulphate. In the preparation of this composite,
the titanium sulphate is mixed with blanc fixe (artificial barium sulphate), and the
mass is boiled to precipitate titanium hydrate (metatitanic acid) upon the blanc
fixe.

Both the pure titanium and the barium base titanium oxides are micro-
crystalline and fine In texture. The high refractive index (a? = 2.5-2.6) and, hence,
the great hiding power, is the outstanding characteristic of titanium dioxide.
Bearn (p. 58) says that, bulk for bulk, paints made with pure titanium white
have nearly twice the opacity or obscuring power of paint made with pure white
lead. The pigment is used extensively In inside white enamels and also as a
ceramic white.

Titanium dioxide Is a very stable substance; it is unaffected by heat, by dilute
acids and alkalis, and by light and air. As a pigment, it Is non-reactive with drying
oils and is a poor drier; hence, it gives soft paint films unless much zinc oxide or
drier is added. The oil absorption of pure titanium dioxide is fairly high, 23 to
25 per cent, but that of the barium base pigment is lower, 17 to 18 per cent (see
Gardner, pp. 1228-1229). Titanium oxide was suggested as an artist’s pigment
very soon after it came into commercial production, and, for some years, titanium
barium pigments have been supplied under special names by various artists’
supply houses. One can not expect to find it used, however, on paintings that were
done much earlier than 1920.

Titanium White (see Titanium Dioxide).

Titanox (see Titanium Dioxide).

62

Painting Materials

ToMdine Red (toluidlne toner) is a synthetic, yellowish red, organic dyestuff,
-nltro-^-toluene-azo-l^-naphthol, C17H13N3O3 {Colour Index^ p. 16). It is one of
he most permanent of its kind and, hence, is now used widely for outside pur¬
poses where a permanent, bright red paint is demanded; it will stand strong
unlight for some months without fading. It is unaltered by heat up to 150 C.
.nd by alkalis, is insoluble in water but soluble in boiling alcohol. Toluldine red
ras first made by the Badische Company in Germany, the patents being dated
905. Although it has not been oifered to the artists’ trade under this name, it
lay occasionally be found in cheaper artists’ colors as a toner or as a substitute.

Toner is a term used in the heavy paint industry to indicate pure or nearly
ure synthetic, organic dyestuff's or lakes of high color concentration. Toners are
ometimes used to bring color mixtures of pigments up to standards of tint (see
Iso Para Red, ToMdine Red, etc.).

Transparent White (see Altuninuna Hydrate).

Turnsole, a blue from seeds of the plant, Crozophora tmctoria, indigenous to
outhern Europe, is an indicator dye like litmus. When freshly squeezed from the
eeds, it is red; but when made alkaline, it turns blue. In mediaeval times, small
.nen cloths were dyed directly with the juice of the seeds and, when dipped in a
um solution, served as a convenient source of the color for manuscript painting,
f violet was desired, the cloths were first limed to neutralize the acidity of the
eed juice. Turnsole violet was highly esteemed in XIV century Italy (Thompson,
^he Materials of Medieval Paintings pp. 143-144), and was used occasionally to
one azurlte.

Tuscan Red is a red iron oxide brightened with one of the more permanent
rganic pigments like alizarin red.

Tyrian Purple, one of the most important and most costly of the organic color-
ng matters of the ancients, was prepared from several mollusks (whelks) including
VLurex brandaris and Purpura haemostoma^ found on the shores of the Medlter-
anean and the x 4 tlantic coast, including the British Isles. Huge quantities of these
nollusks were used for dyeing fabrics in classical times, and on certain shores of
he Mediterranean there still remain heaps of the shells about the sites of ancient
lye works. The color-producing secretion of the whelk is contained in a little vein
)r cyst, and when this is broken and squeezed by hand, it issues as a white fluid.
Dloths to be dyed are dipped in this fluid and are exposed to strong sunlight
vhicli causes the development of the color finally to purplish red or crimson. The
lue obtained depends somewhat on the particular species of mollusk and on the
extraction process. P. Friedlander, in his experiments on the extraction of purple
collected at Trieste in 1908, obtained only 1.4 grams of the
pure dye from 12,000 mollusks. It was he who established the identity of the
coloring principle as 6 : 6' dibromoindigotin (‘Uber den Farbstoff des antiken
Purpurs aus murex brandaris,’ Berichte der Deutschen Chemischen Gesellschaft^
XLII [1909"], pp. 765-770). The purple color is remarkably stable, resisting alka-

Pigments

163

lis, soap3 and most acids^ and Is only destroyed by hot nitric acid or chlorine. It is
insoluble in most ordinary organic solvents^ except hot aniline and nitrobenzene
(Friedlanderj loc. cit.^ p. 768).

Pliny (see Bailey, I, 25-33) describes in some detail the source and method of
extraction, and says that the best quality was made at Tyre, probably the reason
why it is still called ' Tyrian purple ' (Laurie, The Pigments and Mediums of the
Old Masters^ pp. 47-62). This was the color in the togas of Roman emperors and
gave rise to the expression, ' born to the purpled It was used in the preparation of
a purple ink and in dyeing parchments upon which the codices of Byzantium were
written. Whelks that yield a purple dye are also found in waters of the British
Isles, and they furnished the purple color for some of the early English, Irish, and
French manuscripts (see Thompson, The Materials of Medieval Paintings p. 156).
The color w^ent out of general use about the VIII century, though it may have been
used occasionally up into the XII century (Thompson, loc. cit,^ p. 157).

Ultrainarme Blue, artificial (French ultramarine, permanent blue). All the
ultramarine used in commerce is made artificially by a furnace process. In chemical
composition and structure it is identical with the natural ultramarine which is
made from the rare mineral, lapis lazuli (see Ultramarine Blue, natural). The
chemical composition of the mineral was first established by Desormes and
Clement in 1806 (see ' Memoir sur FOutremer/ Annales de Chimie [first series],
LVII [1806], pp. 317-326). They showed that it was essentially a compound of
soda, silica, alumina, and sulphur, and they predicted, on the basis of the analysis,
that artificial production should follow. Already blue masses had been observed
in ovens where the Leblanc soda process was being carried out (see Rose, p. 175)
and, in 1814, L. N. Vauquelln (' Note sur une couleur bleue artificielle analogue
a Foutremer,' Annales de Chimie [first series], LXXXIX [1814], pp. 88-91) de¬
scribed a blue substance taken from the hearth of a demolished soda furnace by
M. Tessaert at the glass works of St Gobain, and showed that it was quite similar
in composition and properties to ultramarine from lapis lazuli. In November, 1824,
the Societe d^ Encou 7 'agement pour ITndustrie National offered a prize of 6000 francs
for a method for making artificial ultramarine at a cost not to exceed 300 francs
per kilogram. The prize was awarded four years later to J. B. Guimet in Toulouse
for his method of preparing artificial ultramarine which he asserted he had
developed in 1826 and had not published, but had kept a secret. Almost simul¬
taneously and quite independently, Christian Gmelin of Tubingen, and F. A.
Kottig of Meissen, perfected processes for the same purpose. Very soon after 1830,
factories were established in France and Germany where it came principally to be
made, although later it was also manufactured in England, Belgium, and the
United States.

Two distinct kinds of ultramarine are made. ' Soda ultramarine ’ is made by
heating in closed fire clay crucibles in a muffle furnace, a finely ground mixture of
China clay, soda ash, coal or wood charcoal, silica, and sulphur. After maintaining

164

Painting Materials

a bright red heat from 12 to 18 hours^ the product is cooled, ground, and lixiviated
to remove soluble salts, dried and again ground until the proper color and degree
of fineness are obtained. With a small amount of sulphur the color is dark blue,
but with a high percentage of sulphur it is dark blue with a reddish tinge. Soda
ultramarine also contains a high percentage of silica and is sometimes called acid-
resisting because it is stable in the presence of alum solutions (important where
ultramarine is used in the paper trades). ' Sulphate ultramarine,’ which has a
greenish tinge (see Ultramarine Green) and little covering power, is made by-
using sodium sulphate (Glauber’s salt) in place of soda ash. By washing and roast¬
ing with additional sulphur, it may be changed to blue ultramarine (see Bearn,
pp. 80-8 5, and Rose, pp. 173-202). Variations in the process give blue, red, and
violet ultramarines in various shades and hues. The chemical synthetic product
does not differ from natural ultramarine in composition or chemical properties
and it is much purer. Ultramarine is essentially a sodium aluminum silicate which
also contains a certain amount of sulphur. F. M. Jaeger says {Optical Activity and
High Temperature Measurements^ Part ZZI, Constitution and Structure of Ultra-
marine [New York: McGraw-Hill Book Co., 1930], pp. 403-441) that it has no
fixed formula and the ratios of the various constituents can change within limits.
There appears, however, to be a fixed component in ultramarine with the formula,
Na8Al6Si6024, which may take on sodium and sulphur atoms to give ultramarines
with formulas ranging from Na8Al6Sl6022S4 to NaioAl6Si6024S2. The cause of the
color and differences of color in ultramarines is still more or less a mystery. It ap¬
pears to be associated with the sulphur present or a combination of sodium and
sulphur. When the pigment is decomposed by acids, sulphur and hydrogen sul¬
phide are released and the color is immediately discharged.

Artificial ultramarine, in contrast with natural ultramarine, is finely divided
and homogeneous. The particles, which are rounded, are about the same size as
Dutch process white lead. The small, individual particles are quite opaque to
transmitted light; they are isotropic, and the refractive index is low {n = 1.50).
The color by reflected light is claimed (see De Wild, p. 20) not to be the pure blue
of natural ultramarine but usually to have a purplish tinge which makes it less
desirable from the artist s point of view. This blue is stable under all conditions,
except in the presence of acids. It is readily decomposed, even by dilute acetic
acid, with decoloration of the pigment and evolution of hydrogen sulphide. It is
permanent to light and is unaffected by high temperatures. Since it is unaffected
by alkalis, it is stable in fresco. Impure ultramarines may contain free sulphur
and, hence, cause darkening when mixed with lead and copper pigments. Artificial
ultramarine in rich oil films occasionally appears to decolorize and to become gray
with age. This phenomenon is sometimes called ‘ ultramarine sickness ’ and seems
to be caused by an acid condition in the film. Weber points out (p. 113) that it
never occurs when white pigments are mixed with the film; this is probably be¬
cause some whites, like those of lead and zinc, can readily neutralize acid.

Pigments

165

Ultramarine is today quite widely used as an artist’s pigment, and is known
to many as^ French ultramarine,’ presumably because of its discovery and long
production m France. Just when artificial ultramarine was first used for pictures
has not been definitely established, but the date must have been about 1810 or
soon after. Laurie says Light on Old Masters, p. 44 ) that Turner used it.

In color, quality, and brilliance, it was superior to Prussian blue and indigo, which
were the only other readily available blue pigments in the first part of the XIX
century.

UHxamarine Blue, natural (lapis lazuli). Genuine ultramarine blue pigment is
from the semi-precious stone, lapis lazuli, which is a mixture of the blue mineral
lazurite, with calcspar, and iron pyrites. (For a complete mineralogical and cry-
stallographic treatment of lapis lazuli, see W. C. Broger and H. Backstrom, ‘ Die
Granatgruppe,’ Zeitschrift fur Krystallographie und Mineralogie,
A.V111 L1890J, pp. 209-276.) Various ancient sources have been ascribed to this
stone, including Persia, Tibet, and China, but the most reliable information
indicates that the lapis lazuli which was brought to Europe in mediaeval times
originated in mines which were located in Badakshan, now a province of north-
east Afghanistan The Badakshan mines, lying in a most inaccessible region at
the headwaters of the Oxus, on the north side of the Hindu Kush near Firgamu
appear to have been worked very early and possibly they were the source of the
lapis lazu 1 used in Mesopotamia and in classical times. They were visited by
Marco Polo in 1271, and were described by Capt. John Wood of the Indian Navy
{A Journey to the Source of the River Oxus [London, i87a[l, pp. 169-172) who
saw them in 1838. Lapis lazuli was probably an important article of trade over the

Mediterranean and thence to Europe, during the
Middle Ages. It was probably imported into Italy through Venice, a terminal for
Oriental commerce. Its present name, ultramarine, derives from azurrum ultra-
mannum or azurro oltramarino which formerly served to distinguish it from azurite
which was variously called azurrum citramarinum, azurro della magnia, or azutro
dell Alemagna (see Merrifield, p. ccxi, and Beckmann, I, 474 ). Significant, also, is
me fact that the blue pigment made from this stone was at one time known in
^ain as atzur dlAcre (see J. Gudiol, La Pintura Mig Eval Catalana- II Els
Trescenttsies [Barcelona: S. Babra, 1924], p. 89). ’

Although lapis lazuli was used throughout the East in remote antiquity and
classical times for lapidary purposes, there is no evidence, as yet, that it was used •
r centuries after the beginning of the Christian Era. Lucas

(p. 286) finds no evidence for it as a pigment among the ancient Egyptians, al¬
though the stone was imported into Egypt as early as predynastic times. It first
became a pigment, apparently, in the region of its origin, in Afghanistan, and
adjacent countries. Gettens has reported its occurrence in VI and VII century
wall paintings in the cave temples at Bamiyan in Afghanistan. It was also in con¬
temporary wall paintings at Kizil in Chinese Turkestan. Laurie says (‘ Materials

166

Painting Materials

in Persian Miniatures/ p. 146) that blue from lapis lazuli was used in Byzantine
illuminated manuscripts from the VII century on but that the quality of the early
blue was poor. Laurie adds {he, ctL, p. 148) that this dull blue was used in Persian
miniatures in the XIII and XIV centuries, but that in the XV century it was re¬
placed by fine ultramarine which may have had its source ui the East but was
refined in Europe and then returned to the East again. In China, however, azurite,

not ultramarine, w^as the chief mineral blue.

Methods for purifying and concentrating ultramarine from the crude lapis
lazuli appear to have been developed in the West in the XII and XIII centuries,
although the raw material still came from the East. These meAods have been
handed down in numerous recipes which are quite similai in principle and vary
only in details. One of the best is given by Ceiinino Cennini (see Thompson, The
Craftsmanh Handbook, 36 “ 39 ). who directs that the powdered mineral be
kneaded in a weak lye solution with a paste or dough of wax, pine rosm, linseed
oil, and gum mastic. The dough retains the foreign pai tides (silica, calcite, pyrite,
etc.), but the fine particles of blue color settle out in the alkaline water. The first
extraction gives the finest and purest colors each successive exti action gives a
product less pure until, finally, there is a pale gray-blue called ultiamaiine ash.
The reason for the separation is the preferential wetting and retention of the
impurities by the dough mixture. Microscopically, natural ultramarine is charac¬
teristic in appearance, and one can quite easily distinguish it from modern
artificial ultramarine (see Ultramarine Blue, artificial). The particles are clear
blue and translucent; their fracture is conchoidal and, when not too finely divided,
certain ones with squared corners and others shaped like sharp splinters can be
seen. The blue particles are isotropic and have a very low refractive index,
n = 1.50, lower than dried linseed oil or Canada balsam. Unless it is very highly
purified, ultramarine contains colorless, birefracting particles of calcite; occa¬
sionally small, golden particles of iron pyrites can be seen by reflected light. Since
the refractive index of ultramarine is so low, it served better and was far brighter
in tempera than in oil. It has apparently discolored (turned green) in many old
paintings because of the yellowing of oil and varnish films that are applied over it.

Natural ultramarine is unaffected by red heat or by alkalis but is decomposed
by dilute acids, even acetic acid, with complete loss of color and the discharge of
hydrogen sulphide gas (see Ultramarine Blue, artificial). This sensitivity to acids
may be the cause of the so-called ‘ ultramarine sickness,* a phenomenon that is
occasionally met with in old pictures where areas painted with ultramarine have
turned gray-blue. As explained by De Wild (pp. I4“i6), this may perhaps be
caused by either an acid medium or varnish or by an acid atmosphere. The blue
is stable, however, in strong light and many specimens which are several hundred
years old show no apparent change in color.

In mediaeval times, natural ultramarine was a costly material; it was in a
class with gold as a symbol of luxury and it was frequently specified by the rich

Pigments

167

in contracts and commissions for paintings. It was used from Byzantine times
through the XVIII century in European paintings and, though it had more fre¬
quent literary mention than azurite, it is found less often in old paintings than
this latter pigment. Diirer used ultramarine, and in his letters to his patron,
Jacob Heller (see W. M. Conway, Literary Refnains of Albrecht Durer [Cam¬
bridge: University Press, 1889], PP- 66-69), he complains of its cost. De Wild
lists (p. 18) several Dutch and Flemish paintings, beginning with the Van Eyck
St Baibara (14375 sit x4ntwerp), on which he identified it. He also found it on a
Dutch painting dated as late as 1810. It disappeared from the painter’s palette
soon aftei that but can still be bought from certain English colormen, and it is
claimed that the ultramarine is purified by methods similar to those described in
the mediaeval recipes.

Ultramarine Green is nearly identical in composition with ultramarine blue
(see Ultramarine Blue, artificial) and is produced in a similar way. It is the palest
in color of the ulti amarines and the most sensitive to acidsj otherwise, it is quite
similar, physically and chemically, to ultramarine blue. Microscopically, it is
heterogeneous; the bright, transparent particles of ultramarine green are mixed
with a large proportion of ultramarine blue, as well as colorless particles. Though
still listed by a few colormen, it apparently has never been extensively used.
It was first prepared by Kottig in Meissen in 1828, but was not manufactured on a
commercial scale until 1854-1856 (see Trillich, III, 78).

Ultramarine Violet (ultramarine red) is made by mixing soda ultramarine blue
with sal ammoniac and heating for some hours at a temperature of about 150"’ C.
Ultramarine red is prepared in a similar way, except that dry hydrochloric acid
gas replaces sal ammoniac. These special ultramarines were developed in Germany
mainly in the years between 1870 and 1880 (see Rose, pp. 193-195). Chemically
and physically, ultramarine violet is similar to ultramarine blue and usually con¬
tains much of it. Not being affected by alkalis, it is one of the very few violets
that can be used in fresco painting. In oil technique, it is pale and has poor covering
power. It is readily available today in dry powder and in various mediums, in¬
cluding oil. ^

Ultramarine Yellow, a name sometimes misapplied to Strontiuin Yellow.

Umber (raw umber, burnt umber). Raw umber is a brown earth pigment simi¬
lar to the ochres and siennas but contains manganese dioxide as well as hydrous
ferric oxide. The general run of raw umbers has 45 to 55 per cent iron oxide, 8 to
16 per cent manganese dioxide, silica, alumina, etc. Raw umber is rather widely
distributed in nature. One of the best, found on the island of Cyprus, has long
been known as Turkey umber. Others are found in England, France, Germany,
and America. From the crude lump umber, it is prepared as a pigment by the usual
process^of grinding and levigation. The best earths have a warm, reddish brown
color with a greenish undertone. Microscopically, the pigment is heterogeneous
in composition and particle size. It contains much goethite, but the grains of that
mineral are finer and darker yellow-brown or more nearly opaque than the

i 68

Painting Materials

goethite of raw sienna and yellow ochre; there are many orange, yellow, and color¬
less ones and there is also a small proportion of birefractmg material. Umber is
durable; it is compatible with other pigments, and is adaptable to all mediums.
Poor grades, which contain humus matter, are not so stable and are liable to fade

in strong sunlight. , ,t j i _ ..-i ..

Burnt umber is made by roasting the raw earth at a dull red heat until the

desired shade is obtained. The heating changes the ferric hydrate to ferric oxide
and the product is redder and warmer than raw umber. Microscopically, the
burnt differs little from the raw, except that it is a little redder and more

transparent. . . , • i r

The umbers are unaffected by alkalis and by dilute mineral acids. Because of

their manganese content, they dry well in oil, and have been used as driers for
varnishes. They have high oil absorption, requiring up to 8o per cent to grind, and
for this reason oil films pigmented with them have a tendency to become darker

The umbers have been available since earliest times, but Thompson says [The

Materials oj Medieval Painting, pp. 88-89) that they were not found on the early
mediaeval palette and they did not come into general use in Europe before the
close of the XV century.

Van Dyke Brown (Cassel earth, Cologne earth) is a name commonly used to
designate a brown organic pigment which is derived from earthy substances simi¬
lar to lignite or brown coal. It contains usually over 90 per cent of organic matter,
with a small amount of iron, alumina, silica, etc. Most of the raw Van Dyke
brown has come from Germany, from places near Cassel and Cologne. Harrison
(p. 242) says that the best grades of this pigment are prepared from good, clean
peat deposits which have been well carbonized by slow formation and long weath¬
ering. It is said (Weber, p. 115) that it got its name from ‘ the famous artist who
was partial to the use of brown in his pictures.’ It is prepared first by heating to
drive off excess moisture, and then by the usual processes for earth pigments. It
has a warm, reddish brown shade and, since it is partially transparent in oil, it is
used for staining of woods and for glazing in pictures. When seen microscopically,
it is heterogeneous in particle size and composition, and the particles seem more
opaque and less crystalline than .ochres and umbers. V^hen ignited, it burns and
leaves a gray ash, and when heated in an ignition tube, tarry vapors are given off.
It dissolves in dilute sodium hydroxide to give a deep brown solution. Because of
its tarry and bituminous nature, it is a fugitive pigment. It fades on exposure to
strong light and develops a cold, gray tone. In oil, however, it is more permanent
i-ban in water color. The pigment appears to dissolve in oil or varnish and to stain
it; for this reason, it is difficult to isolate and to identify in such mediums.

Little is known about the early use of the lignite colors. Probably they came
into use in the late XVII and early XVIII centuries when brown shadows and
backgrounds became popular.

Pigments

169

Van Eyck Green (see Verdigris).

Venetian Red is an iron oxide (see also Iron Oxide Red) with a brick-red color.
Formerly a natural oxide^ partially hydrated (see Church, p, 180), today it is
obtained by calcining a mixture of copperas (ferrous sulphate) and whiting
(calcium carbonate). The product, a finely divided mixture of ferric oxide and
calcium sulphate, does not require washing but is simply ground and sieved. Al¬
though the best Venetian red may contain as high as 50 per cent ferric oxide, the
greater part has 10 to 30 per cent with the rest a mixture of calcium carbonate
and calcium sulphate.

Verdigris is specifically the normal acetate or one of the basic acetates of
copper but, on occasion, the term is also used to indicate copper carbonate or any
of the other blue or green corrosion products which form on copper, brass, or
bronze. The verdigris {vert de Grece) of commerce is usually the dibasic acetate,
Cu(C2H302)2-2Cu( 0H)2. It is a greenish blue, crystalline powder with acetic
odor. Its preparation was known in ancient times. Theophrastus (p. 225) and,
later, Pliny (see Bailey, II, 41-43) tell how to prepare it by exposing copper to the
vapors of fermenting grape skins or in closed casks over vinegar. There are
numerous mediaeval recipes for its preparation. Production of this material has
long centered about Montpellier, France {verdet de Montpellier), and the methods
used there differ little from those of ancient times (see Beckman, I, lyi—iy^). The
crude verdigris produced by the action of acetic vapors on strips of metallic copper
can be lixiviated and the product recrystallized from acetic acid, after which It can
bemused as a pigment. Well crystallized verdigris particles have the shape of
pointed needles. De Wild says (p. 7^)^ ^ They are often united in bundles, while
the larger pieces show fibrous structure. If, however, the pigment is not recrystal¬
lized but ground directly, it is seen in transparent grains/ Verdigris is strongly
birefracting {a = 1.53; = 1.56 [Merwin]), and it is pleochroic, changing from

light blue-green to deep green-blue.

This green is the most reactive and unstable of the copper pigments. It is
slightly soluble in water and readily soluble in acids. When heated, it decomposes
with the escape of acetic acid and water, and leaves a black residue (CuO). Unless
locked up with protective coatings, it is a fugitive color; it blackens readily with
sulphur-bearing pigments. Under very favorable circumstances, as, for example,
where used in the illumination of books and manuscripts which have been kept
closed, it has sometimes endured well. Laurie says {New Light on Old Masters^
P* 45) ^kat Watteau frequently used a mixture of verdigris and ultramarine for his
skies and, in spite of the theoretical incompatibility of these two pigments, there
has been no apparent reaction; his skies are as luminous as ever. De Wild (p. 78)
found it on several paintings of the Flemish School. Thompson says {The Materials
of Medieval Painting, pp. 163-169) that it was a favorite pigment in the early
days of oil painting in Italy, particularly in landscape painting, but it was a
fugitive color and there are many cases where it has turned dark brown. He says

Painting Materials

170

it WS.S much used by scribes End illuminators of books5 in many of these it is we,
preserved but in others it has eaten into the parchment so that parts painted wit
it drop out and leave gaps in the page. Laurie states {The Pigments and Medium
of the Old Masters, p. 100) that real crystalline verdigris is not found on painting
or manuscripts until the beginning of the XV century but from that time its us
continued up to the XIX century. It is now seldom listed by colormen.

A pigment allied with it is transparent copper green. Laurie, in several of hi
published w^orks (particularly in The Pigments and Mediums of the Old Master.
pp. 35-39 and 99-103) has described a peculiar grass-green paint which frequent!
is found on illuminated manuscripts that date from the VIII through the XF
century. When examined microscopically, it exhibits no discrete crystallin
particles of verdigris or other copper salt, yet it tests positively for copper. It i
green-stained, pellicular paint. The resinous character of the medium is quit
evident from its fracture and brittleness. In dilute hydrochloric acid the color i
discharged and it is soluble enough to test for copper. The color is destroyed b
dilute alkali and by heat. Laurie believes that this color was produced by com
bining copper acetate or verdigris with some balsam like Venice turpentine. It i
well known that copper and copper salts react readily with resin solutions to forr
copper resinates, and these solutions become green-stained. Laurie suggests {0].
cit., p. 37) that this transparent copper green could have been applied by dilutio
with turpentine or it could have been dried, ground to a powder, and mixed wit
gum, or with white of egg, or even emulsified with egg. He says there appear to b
no early recipes for the preparation of this green, and it is first mentioned, so fa
as he knows, by De Mayerne (Md*. Shane, 105a) in the XVII century. Further (|
164), just as it disappeared from illuminated manuscripts in the late XV century
it began to appear in the ‘ oil ’ paintings of the Van Eycks and their followers, an
its use continued until about the middle XVI century in Germany and norther
Italy. He sometimes terms it ' Van Eyck green ’ (see p. 128) because it is foun
in so many of the paintings of those masters. The color obtained by the direc
action of copper salts on pure balsams is blue-green, and Laurie suggests that th
warmer hues of the copper green were made by admixture with organic yello^
pigment like yellow lake, saffron, or gamboge (see p. loi). In many paintings th:
color appears to be unaltered and in much its original condition—the result, part
ally, of the protective influence of the resinous medium,

Verditer (see Blue Verditer).

Vermilion (cinnabar, English vermilion, Chinese vermilion) is red mercur:
sulphide (HgS). It is found in natm-e as the mineral, cinnabar, which is tl
principal ore of the metal, mercury. Although the crushed and ground ore serve
directly as a pigment for centuries, yet in very early times men learned how to r<
combine the elements, mercury and sulphur, to form artificial cinnabar or vermi
ion. Cinnabar was known by the Greeks and Romans, and was mentioned t
Pliny, who called it ^minium’ (see Bailey, II, 119-127), The name, minium, lat(

Pigments

171

became fixed to the artificial pigmentj red lead. Pliny says that almost the entire
Roman supply came from Sisapo in Spain. This source was probably the famous
x 41 maaen mines which today are still the world’s most important source of mer¬
cury. He speaks of its use as a pigment and says that it was costly and its price
was fixed by the government. The pigment, vermilion, has been identified numer¬
ous times on Pompeian and Roman wall paintings. Lucas makes no mention of it
as a pigment in ancient Egypt, and there is some question as to whether or not it
was used in Mesopotamia and the Near East. The pigment has been known in
China since prehistoric times and it has long been held in high esteem there. It
was identified as the red in the fossae of the incisions of the famous Chinese oracle
bones (see A. A. Benedetti-Pichler, ‘ Microchemical Analysis of Pigments,’
Indtish'ial and Engineering Chemishy^ Analytical Editiony IX C1937], pp. 149—
152) which date from the Shang epoch in the second milleiiium B.C. It was used
by the Chinese, probably as early as Han times, for making the red ink which is
so ^often seen on cartouches and stamp seals of early Chinese silk and scroll
paintings.

Cinnabar is fairly widely distributed in nature and sources are known in Eng¬
land, Europe, China, Japan, California, Mexico, Peru, as well as in Spain. Soon
after classical times, artificial cinnabar is noticed. Geber (or Jabir), the VIII-
IX century Arabic alchemist, speaks about a red compound formed by the union
of sulphur and mercury (see Kopp, IV, 184—18 8). Recipes for its preparation are
common in the Middle Ages. From writings of Cennino Cennini, the vermilion
of the Italian painters of the XV century is supposed to have been artificial. He
says (see Thompson, The Craftsmards Handhooky p. 24): ^ A color known as
vermilion is red and this color is made by alchemy prepared in a retort.’ Even in
China they knew very early how to make vermilion by the dry method. They
may have been the first to make it artificially, and their knowledge of the process
could have been carried to the West by the Moors.

The dry method of preparation was the one used by the ancient alchemists
and is used, probably, by the Chinese at the present time. In the Dutch modifica¬
tion of the Chinese method, 100 parts (by weight) of mercury are combined in an
iron pan with 20 parts of molten sulphur to form black amorphous mercuric sul¬
phide (ethiops mineral). The black mass is charged into retorts where it is heated,
and by sublimation and condensation on earthenware pots or iron cylinders is
changed into the red crystalline modification of mercuric sulphide. The product
has only to be treated with a strong alkali solution to remove free sulphur, and to
be washed and ground under water to prepare it as a pigment. The change from
black mercuric sulphide to vermilion is entirely physical. This dry-process ver¬
milion, particularly that produced by the Chinese (Chinese vermilion), is rather
coarsely crystalline and slightly violet-red in color.

The wet method has found favor with English, German, and American pro¬
ducers. It was known as early as the XVII century that the red modification of

Pigments

171

became fixed to the artificial pigment, red lead. Pliny says that almost the entire
Roman supply came from Sisapo in Spain. This source was probably the famous
Almaden mines which today are still the world's most important source of mer-
cur}', tie speaks of its use as a pigment and says that it,was costly and its price
was fixed by the government. The pigment, vermilion, has been identified numer¬
ous times on Pompeian and Roman wall paintings. Lucas makes no mention of it
as a pigment in ancient Egypt, and there is some question as to whether or not it
was used in Mesopotamia and the Near East. The pigment has been known in
China since prehistoric times and it has long been held in high esteem there. It
was identified as the red in the fossae of the incisions of the famous Chinese oracle
bones (see A. A. Benedetti-Pichler, ‘ Microchemical Analysis of Pigments/
Industrial and Engineering Chemistry, Aftalytical Edition, IX [1937], pp. 149-
152) which date from the Shang epoch in the second millenium B.C. It was used
by the Chinese, probably as early as Han times, for making the red ink which is
so ^ often seen on cartouches and stamp seals of early Chinese silk and scroll
paintings.

Cinnabar is fairly widely distributed In nature and sources are known in Eng¬
land, Europe, China, Japan, California, Mexico, Peru, as well as in Spain. Soon
after classical times, artificial cinnabar is noticed. Geber (or Jabir), the VIII-
IX century Arabic alchemist, speaks about a red compound formed by the union
of sulphur and mercury (see Kopp, IV, 184—188). Recipes for its preparation are
common in the Middle Ages. From writings of Cennino Cennini, the vermilion
of the Italian painters of the XV century is supposed to have been artificial. He
says (see Thompson, The Craftsmans Handbook, p. 24): ^A color known as
vermilion is red and this color is made by alchemy prepared in a retort.' Even in
China they knew very early how to make vermilion by the dry method. They
may have been the first to make it artificially, and their knowdedge of the process
could have been carried to the West by the Moors.

The dry method of preparation was the one used by the ancient alchemists
and is used, probably, by the Chinese at the present time. In the Dutch modifica¬
tion of the Chinese method, 100 parts (by weight) of mercury are combined in an
iron pan with 20 parts of molten sulphur to form black amorphous mercuric sul¬
phide (ethiops mineral). The black mass is charged into retorts where it is heated,
and by sublimation and condensation on earthenware pots or iron cylinders is
changed into the red crystalline modification of mercuric sulphide. The product
has only to be treated with a strong alkali solution to remove free sulphur, and to
be washed and ground under water to prepare it as a pigment. The change from
black mercuric sulphide to vermilion is entirely physical. This dry-process ver¬
milion, particularly that produced by the Chinese (Chinese vermilion), is rather
coarsely crystalline and slightly violet-red in color.

The wet method has found favor with English, German, and American pro¬
ducers. It was known as early as the XVII century that the red modification of

172,

Painting Materials

mercuric sulphide could be made by treating the black sulphide with alkali
sulphides (see Kopp, p. 187). Production of vermilion by this method began in the
late XVIII century in Germany (see Rose^ p. iii). The mercury and sulphur are
ground together in the presence of water and^ toward the end of the grinding
operation, a warm solution of caustic potash is added to complete the transforma¬
tion. After being stirred for some time, the black mercuric sulphide develops the
desired vermilion color. In an improved method, potassium pentasulphide is used
in place of caustic potash (see Bearn, p. 119). Vermilion prepared in this way must
be washed and dried to be rid of the soluble sulphur compounds.

Chemically and physically, artificial cinnabar does not differ from the natural.
There are no optical differences between them, and it is often quite impossible to
tell the origin of the vermilion in a paint film. If it is coarse and if the particles
appear to be broken fragments rather than single small crystals and if there are
inclusions of impurities in the broken fragments, then it may be natural in origin.
Impurities in vermilion are no satisfactory criterion of origin, however, since
very pure, natural cinnabar frequently occurs in nature. Artificial vermilion,
particularly that made by the wet process, is very finely divided and homo¬
geneous; differences in the color of different samples are caused mainly by
differences in particle size. The sublimed product from the dry process (^.^.,
Chinese vermilion) is usually more coarsely crystalline and It has a bluish, carmine-
red color. When it is finely ground, the color approaches the orange of wet-process
vermilion.

Vermilion is one of the heaviest pigments (sp. gr. = 8.2). It has excellent body
and hiding power. It Is highly refracting and birefracting {eLi == 3.14, = 2.81).

Under the microscope, the particles are translucent, deep orange-red by trans¬
mitted light. By reflected light at high magnification, the red particles have a
certain waxy lustre which seems to be quite characteristic. Many fragments show
perfect cleavage.

Vermilion, on the whole, has been a permanent pigment. It is frequently seen
on Roman wall paintings, quite unchanged. On many Flemish paintings of the XV
century it appears to have retained all of its original brilliance. It is not permanent
under all conditions, however, and one peculiar property is that specimens of it
are frequently observed to darken when exposed to direct sunlight. This occurs
more often where the pigment is used with tempera or water color mediums than
with oil. Darkening appears to be a purely physical change, and is thought to be
caused by the formation of a metastable black modification of mercuric sulphide
(see De Wild, p. 67, and Church, p. 169), Wet-process vermilion is more often
observed to darken than the dry-process or the natural vermilion; impurities seem
to have some part in the change. The tendency to discolor has caused vermilion
to be replaced on the modern artist’s palette by cadmium red. When heated,
vermilion sublimes at about 580® C. (see Mellor, IV, 944). At higher temperatures
it burns with a bluish flame and leaves no appreciable residue. It is insoluble in

Pigments

173

alkalis and in concentrated acids but is soluble in aqua regia. Although it is a
sulphide^ it is so inert that it does not darken white lead when the two are mixed,
unless it contains free sulphur or soluble sulphide as an impurity. It has often
been used with 'white lead for flesh tints.

Vermilion has been found on numerous paintings of nearly all periods and
countries in the West since classical times. It is a rich color, and Thompson has
remarked {The Materials of Medieval Tainting^ p. 106) how its brilliance increased
the color intensity of the palette of mediaeval painters. It demanded bright blues,
greens, and yellows to go with it and to complement it. In Far Eastern wall paint¬
ings these blues and greens were supplied by azurite and malachite.

Vert Emeraude (see Viridian).

Vine Black (see also Charcoal Black and Carbon Black), which is similar to
charcoal, is prepared by carbonizing vine twigs or vine wood. (One kind, so-called,
is made from wine lees.) The pigment has a blue hue, and gray tints made with
white have a bluish tinge (see Weber, p. 24). Other similar vegetable blacks are
made from peach stones, cocoanut shells, cork, etc. Such sources of black are
mentioned by Cennino Cennini (see Thompson, The Craftsman^sHandbooky^. 22).

Viridian {vert emeraude^ Guignet’s green, transparent oxide of chromium) is the
transparent, bright green, hydrous oxide of chromium and is usually given the
formula, Cr203 *21120, The anhydrous or dull green, opaque oxide of chromium
is also used as a pigment (see Chromium Oxide Green, opaque). Viridian is prepared
today much as it has been for years, by heating to dull red heat a mixture of an
alkali chromate with excess boric acid. After completion of the reduction, the
charge is raked into vats containing cold water and let stand to hydrate; it is then
washed by decantation, ground wet, washed with hot water to free it from
soluble salts, and dried (see Bearn, p. loi). The finished product usually contains
boric acid, some of which may be chemically combined with the chromium oxide.
The color of the hydrous oxide is a deep, cool green of great purity and
transparency. It is a desirable pigment because of its excellent tinting
strength and its stability in all mediums. It is unaffected by dilute acids and al¬
kalis and by light; strong heat only causes it to change to the opaque, anhydrous
oxide. Viridian is characteristic microscopically; the bright green, transparent
particles are quite unmistakable and unlike any other pigment. Usually they are
fairly large, irregular In size, slightly rounded, and appear to be fairly strongly
birefracting in polarized light. H. Wagner and A. Rene (‘ Chromoxydhydrat-
griin, Farhen-ZeitungfKlA PP* 821—823) explain the apparent anisotropy

to strain in the particles caused by cooling. The refractive index is moderate,

Viridian is classed as a modern pigment. Although the element, chromium, was
discovered by Vauquelin in 1797 and the anhydrous green oxide was then known,
it was many years later, in 1838 (Church, p, 194, gives the date), that Pannetier, a
color maker in Paris, began to make a beautiful transparent chromium green.
He and Binet, who took over the process when Pannetier died, manufactured the

174

Painting Materials

product by a secret method for years. Finally, in 1859 Guignet patented a method
for making the hydrous oxide by a process which he described (see Guignet, pp.
149-153) as new and unique and which is the reduction of potassium bichromate
with boric acid, outlined above. This new green immediately replaced Schweinfurt
green (see Emerald Green) for printing and other industrial coloring purposes.
It must have been shortly after that time that the transparent oxide of chromium
was introduced as an artistes pigment. Laurie says ( New Light on Old Mastersy
p. 44) that the date is i86a. Messrs Winsor and Newton, Ltd, state (in their
catalogue, 1930, p. 20) that it was originally introduced by their house and they
say (in a private communication) that this pigment, as well as aureolin, was
popularized by Aaron Penley, a water color painter. Church says (p. 195) that the
pigment came to be best known In England by the name, virldian, and that it was
unfortunate that it came to be called Vert emeraude ’ In France and, hence,
confused with the poisonous copper aceto-arsenite or Schweinfurt green, known in
England as ‘ emerald green.^

Weld (arzica) is a natural yellow dyestuif, obtained as a liquid or as a dry
extract of the herbaceous plant, Dyer’s Rocket, Reseda luteohy formerly culti¬
vated in central Europe. The coloring matter is luteolin or tetrahydroxyflavone,
(CisHioOe). It is extracted with aqueous solutions and may be made into lakes of
various shades of yellow with different mordants. Although weld extract has lower
tinctorial power than some other natural yellow dyes like quercitron (see Querci¬
tron Lake), it yields the purest and the fastest shades of all ( Colour IndeXy p. 294).
Sparingly soluble in hot water and moderately soluble in alcohol, it gives a deep
yellow solution with alkalis. Weld has had a long history as a dye and lake pigment,
and Thompson says {The Materials of Medieval Paintingy p. 187) that it is still
cultivated in small quantities in Normandy for dyeing silk.

White Bole (see Bole and China Clay).

White Lead (flake white, Cremnitz white) is the most important of all the lead
pigments; It Is the basic carbonate of lead, 2PbC03*Pb(0H)2, and ordinarily
contains about 70 per cent of lead carbonate and 30 per cent of lead hydrate.
Although normal lead carbonate occurs In nature as the mineral, cerussite, it has
never been important as a source of white pigment. White lead was known in
early times and was one of the first artificially prepared pigments. Theophrastus
(pp. 223-225), Pliny (see Bailey, II, 75), and Vitruvius (VII, 12) all describe its
preparation from metallic lead and vinegar. There are also numerous mediaeval
recipes for making it. A large part of the white lead used today is made by the
‘ Dutch ’ or " stack ’ process, which differs little in principle from the method used
in classical and mediaeval periods. Metallic lead in the form of strips, ‘ buckles,’
or other shapes is exposed for about three months in clay pots which have a sepa¬
rate compartment in the bottom containing a weak solution of acetic acid. The pots
are placed in tiers in a shed with spent tan bark or manure separating them. When
the building is closed, the combined action of the acetic vapors, heat, and carbon

Pigments

175

dioxide from the fermenting tan bark, oxygen of the air, and water vapor slowly
transform the lead to basic lead carbonate. After being washed, dried, and
screened, the product is ground in linseed oil. Various other processes, including the
chamber, electrolytic, and precipitation processes, most of which are more rapid
than the Dutch, are also used for preparing white lead.

Although white lead can be purchased in a dry powder form, the greater part
of it comes on the market ground to a thick paste with 9 to 10 per cent of linseed
oil. Since whi ue lead is a poisonous compound if inhaled as a dust or if ingested,
grinding and manufacture into paint was long regarded as a hazardous industry
and in several countries it was curbed by legislative action. Now, because of
improved factory methods, such dangers are no longer attendant. Painters, how¬
ever, still suffer from ‘ painters’ colic ’ or ‘ plumbism ’ if they are careless with it.

White lead is^a very finely divided yet a definitely crystalline compound. At
400X magnification, it can be observed to be highly birefracting. Merwin says (p.
514): ‘Individual grains seen in several samples were tabular (perhaps twice as
broad as thick) and hexagonal in outline.’ The refractive index is high, w = a.09.
It is commonly understood (see Bearn, p. 45) that the lead hydroxide, Pb(OH)2,
part of the white lead molecule, is able, partially, to saponify linseed oil and to
form with it a lead soap (lead linoleate). This fact has been used to explain why
white lead in oil forms such a homogeneous, durable, hard, and non-porous paint
film.^ (White lead films are conspicuously strong, and their strength extends to all
mediums.) It is also given as the reason for the apparent increase in transparency
of old white lead films (see Eibner, Malmaterialienkunde, p. 121) with the striking
through of darker underpainting, sometimes called, ‘ pentimento.’

The siccative or drying action of white lead upon oils is another reason for its
being so widely used. Pure white lead in oil is favored as an outside white paint
because it chalks on weathering (does not check or crack) and leaves a satisfactory
surface for repainting. On indoor exposure, however, it has a tendency to yellow,
particularly in the dark. It is darkened, even blackened, by contact with sulphide
pigments and hydrogen sulphide in the air because of the formation of black lead
sulphide. When protected by oil or varnish films, this reaction is very slow and the
effect may be negligible. In fact, white lead is commonly seen in paintings where
it has been mixed with vermilion (HgS), ultramarine, and other sulphur-bearing
pigments for centuries without any sign of incompatibility. With Impure pigments
that bear free sulphur, however, a darkening effect may be quickly noticed. In
water color films, it is often seriously blackened. When heated at a moderate
temperature, white lead turns bright yellow because of the formation of massicot
(lead monoxide, PbO); higher temperatures melt the massicot and change it to
litharge and even further oxidize it to red lead. White lead is readily soluble in
dilute mineral acids and in acetic acid with effervescence (CO2). There is perhaps
no question that, so far as general use is concerned, white lead is the most im¬
portant pigment in the history of Western painting. It is practically the only

176

Painting Materials

white used on easel paintings from remote antiquity to the XIX century. Radi¬
ography of old paintings rests to a great extent upon the generous use of white
lead in the past: since lead has a high atomic weighty it has a high mass ab¬
sorption coefficient for Roentgen rays (see De Wild, pp. 92-98), Its first use as
a paint pigment must have been very early. It is not mentioned extensively,
however, by either Partington or Lucas in their accounts of the materials of the
Egyptian and other early civilizations. It must have been used in classical times
for painting pictures as well as for cosmetic purposes. It has been identified as
a pigment on Fayum portraits (see George L. Stout, ‘ The Restoration of a
Fayum Portrait,' Technical Studies^ I [1932.], p. 86), and was probably known and
used as a pigment in the Orient quite as early as in the West. It lies thickly on
painted sculpture of Tang times from Tun Huang in Western China. In Europe,
hardly an important painting, before the XIX century, is without it. De Wild
has listed (pp. 34-39) over 80 paintings in which it occuis.

Although white lead has been used in tempera and in water color, it is not so
satisfactory in these mediums as in oil. Today its place in water color has been
largely taken by zinc white (Chinese white), and in oil it is meeting serious com¬
petition from the titanium pigments. The pigment is sold to the artist under the
name, ' flake white.' Cheaper grades are sometimes ‘ cut' or adulterated with
barite or blanc fixe.

Cremnitz (Kremnitz) white is a special kind of white lead which is prepared
by the action of acetic acid and carbon dioxide on litharge. It is now greatly
favored by artists because it is considered to be whiter, denser, and more crystal¬
line than ordinary, Dutch process white lead.

Whiting (see Chalk).

Woad is a blue dye very similar to Indigo (see Indigo) which is obtained from
the leaves of the woad plant, Isatis tinctona^ a herbaceous biennial indigenous to
southern Europe. Before the importation of indigo in the XVII century, it was
widely cultivated in England and on the Continent for its dye (see J. B. Hurry,
The Woad Plant and Us Dye [[London: Oxford University Press, 1930]!), which
is extracted by a fermentation process similar to that used with indigo. Although
the coloring principle of woad was formerly thought to be the same as that of
indigo, it is now known to be a distinct substance (see Perkin and Everest, pp.
524-^5). Woad blue was apparently used in mediaeval times for a pigment as
was indigo (see Thompson, The Materials of Medieval Paintings pp. 135—140).
It is quite impossible to distinguish between the two when they occur as pigments
in old paintings.

Yellow Berries (see Persian Berries Lake).

Yellow Lake (see Quercitron Lake).

YeEow Ochre (see Ochre),

Zinc Green (see Cobalt Green).

Zinc White (Chinese white), or zinc oxide (ZnO), has now almost the impor-

Pigments

177

tance of white lead as an artist’s pigment. Neither zinc oxide- nor metallic zinc
seem to have been known as individual substances in the ancient worlds though
certain zinc ores were used in making brass. Zinc was first described as an element
by Margraaf, a German chemist, in 1746. Although the use of the oxide, as a
substitute for white lead, was first suggested by Courtois of Dijon in 1782 (see
Rose, p. 84), more than 50 years passed before it became commercially available.
According to Church (p. 134), as early as 1834 a peculiarly dense form of zinc
oxide was introduced as a water color pigment by Messrs Winsor and Newton,
Ltd, of London, under the name,' Chinese white ’ (see also Winsor and Newton’s
catalogue, 1930 ed., p. 15). The chief difficulty in the way of its commercial use
at that time was its poor drying qualities in linseed oil In the years 1835-1844,
Leclaire in France showed that this difficulty could be overcome by using with the
zinc oxide an oil that had been rendered siccative by boiling with pyrolusite
(Mn02), and in 1845 he began, near Paris, to produce zinc oxide on an industrial
scale. By 1850, it was regularly made as an oil paint. De Wild says (p. 40) that
the first trial orders of such paint, from the firm of Hafkenscheid in Amsterdam,
were in 1854.

In the French process of manufacture zinc vapor, from molten metallic zinc,
is burned in an oxidizing atmosphere at a temperature of about 950° C., and the
fumes of white oxide are collected in a series of chambers. In the American or
direct process, zinc ores, principally sphalerite (zinc blend, ZnS), are mixed with
coal coke and burned, and the white smoke of zinc oxide is collected in suitable
chambers. In either process the occurrence of such impurities as metallic zinc,
soot, and other metallic oxides can seriously impair the general quality and
whiteness of the product. So-called ' leaded zinc oxides,’ which are made by the
direct oxidation of lead-bearing zinc ores, contain several per cent of lead sulphate.
Dry zinc white comes on the market in various qualities and degrees of whiteness.

‘ White seal ’ and ‘ green seal ’ zinc white contain over 99 per cent zinc oxide; the
latter has the better hiding power. ' Red seal ’ and ^ gold seal ’ are understood
to be slightly less pure, and ‘ gray seal ’ zinc white contains metallic zinc.

Zinc oxide is a pure, cold white. In the dry state it is lighter and more bulky
than white lead. It is non-poisonous but is a mild antiseptic. It requires more oil
(18 to 20 per cent) to form a paste than white lead. It has a tendency eventually
to dry brittle and to crack. Mixtures of zinc oxide and white lead combine the ad¬
vantages of both pigments.

As would be expected, since zinc oxide originates as a smoke, it is very finely
divided and separate grains are difficult to observe except at high magnifications,
Merwin says (p. 506) that the pigment from zinc vapor (French process) has a
grain size much less than in diameter. The refractive index (oj = 2.Q0 [jMer-
win]) is about the same as white lead but, unlike the latter, is little birefracting.
In ultra-violet light, the oxide appears bright yellow. It is not affected by strong
sunlight- It is readily soluble in dilute alkalis. Although it can react with hydro-

178

Painting Materials

pn sulphide to form zinc sulphide, it is not darkened, because zinc sulphide itself
is white (see Lithopone) and, for this reason, the oxide has been an important
pigment for use where industrial atmospheres are prevalent. It is claimed to be a
mildew inhibitor for outside paints (see H. A. Gardner, L. P. Hart and G. G
Sward, ‘ Mildew Prevention; Fourth Report on Investigation with Conclusions
and Recommendations, Circular no, 475 ^ National Painty Varnish and Lacquer
Association [January, 1935]). Zinc white, more than some other whites, seems
to accelerate the fading of certain coal tar colors that are mixed with it in tints
and exposed to strong sunlight. It has been widely used in paintings since the
middle XIX century. It_continues to be popular for water color under the name
Chinese white,’ but it is also sold to artists in an oil paste. '

Acicular zinc oxide is^ a special form in which the particles are needle-shaped
an^d crossed and joined in pairs to form X’s. It has greater hiding power and
whitening strength than ordinary zinc oxide which contains mostly rounded
particles. ’

Zinc YeUow is zinc chromate, ZnCr04, which is made artificially by adding a
hot soluton of potassium dichromate to a solution of zinc sulphate. The pigment
has a bright, clean, lemon-yellow shade, much like strontium chromate. lUacks
the body and strength of lead chromate yellow. Since, however, it is not poisonous
and IS not darkened by hydrogen sulphide gas, it has found favor for special uses
It IS partially soluble in water and this behavior has somewhat limited its use
It IS also readily soluble in dilute mineral acids and in acetic acid, but is not
affected by dilute alkalis. It is not very permanent to light, having a tendency to
turn gray-green caused by the formation of chromic oxide. Microscopically it
may be observed to consist of tiny spheroidal grains which have strong bire-
nngence. It has only a moderately high refractive index {n = 1.84-1.90 [Mer-
wm]). Little IS known about the occurrence of zinc yellow in paintings. It was
discovered by Vauquelin in Pans in 1809, but was not produced as a commercial
pigment until after 1850 (Trillich, III, 55). Apparently it has only been used as an

artists color « recent years, and for this purpose only in oil and water color
mediums (see Weber, p. 133).

Chrni^°<?r! synonymous with chrome green (see

^ome Green) which is a processed mixture of chrome yellow and Prussian blue.
More specifically, it is given to mixtures that are olive in hue.

BIBLIOGRAPHY

Anonymous, Merck's Index^ 4th ed. (Rahway, N. J.: Merck and Co., 1930).

K. C. Bailey, The Elder Pliny's Chapters on Chemical Subjects (London: Edward Arnold
and Co., Part I, 1929; Part II, 1932).

Norman F. Barnes, ‘A Spectrophotometric Study of Artists’ Pigments,’ Technical
Studies, VII (1939), pp. 120-138.

J. G. Bearn, The Chemistry of Paints, Pigments, and Varnishes (London: Ernest Benn,

Ltd, 1923).

John Beckmann, A History of Inventions, Discoveries, and Origins, 4th ed., 2 yols
(London, 1846).

Ernst Berger, Beitrdge zur Entwicklungsgeschichte der Maltechnik, 4 vols (Munich: G.
D. W. Callwey, 1901-1912).

Godfrey L. Cabot, ‘Lamp Black and Carbon Black,’ Eighth International Congress of
Applied Chemistry, XII (1912), pp. 13-31.

M. Chaptal, ‘Sur quelques couleurs Annales de Chimie, LXX (1809),

pp. 22-31.

A. H. Church, The Chemistry of Paints and Painting, 3d ed. (London: Seely, Service
and Co., 1901).

F. W. Clarke, ‘The Data of Geochemistry,’ 5th ed., Bulletin no, ygo, U, S, Geological
Sm'vey (Washington D. C.: Government Printing Office, 1924).

Colour Index, edited by F. M. Rowe (Bradford, Yorkshire: Society of Dyers and
Colourists, 1924).

E. J. Dana, A Textbook of Mineralogy, 3d ed. by W. E. Ford (New York: John Wiley and
Sons, 1922).

Sir Humphrey Davy, ‘Some Experiments and Observations on the Colours Used in
Painting by the Ancients,’ Philosophical Transactions, CV (1815), pp. 97-124.

Max Doerner, The Materials of the Artist and Their Use in Painting, trans. (New York:
Plarcourt, Brace and Co., 1934).

A. Eibner, Entwicklung und Werkstofe der Wandmalerei (Munich: B. Heller, 1926).

Malmaterialienkunde als Grundlage der Maltechnik (Berlin: J. Springer, 1909).

H. A. Gardner, Paints, Varnishes, Lacquers and Colors, 8 th ed. (Washington, D. C.:

Institute of Paint and Varnish Research, 1937).

Rutherford J. Gettens, ‘The Materials in the Wall Paintings of Bamiyan, Afghanistan,’
Technical Studies, VI (1938), Pp. 186-193.

‘The Materials in the Wall Paintings from Kizil in Chinese Turkestan,’ Technical
VI (1938), pp. 281-294.

‘Pigments in a Wall Painting from Central China,’ Technical Studies, VII (i 93 ^)>
pp. 99-105.

Ch.-Er. Guignet, Fabrications des Couleurs (Paris, 1888).

Ingo W. D. Hackh, A Chemical Dictionary (Philadelphia: P. Blakiston’s Son and Co.,
1929).

A, W. C. Harrison, The Manufacture of Lake and Precipitated Pigments (London:
Leonard Hill, Ltd, 1930).

179

i8o

Painting Materials

H. Kopp, Geschkhte der Chemie, 4 vols (Braunschweig, 1843-1847).

R. B. Ladoo, Non-Metallic Minerals (New York: McGraw-Hill Book Co., 1925).

E. S. Larsen and H. Berman, ‘The Microscopic Determination of the’Non-Opaque
Bulletin 848, U. S. department of the Interior, Geological Survey (1934).
A. P. Laurie, ‘The Identification of Pigments Used in Painting at Different Periods'
with a Brief Account of Other Methods of Examining Pictures,’ The Analyst I V
(1930), pp. 162-179.

‘Materials in Persian Miniatures,’ Technical Studies, III (1935), pp. 146-156.

The Materials of the Painter's Craft (Philadelphia: J. B. Lippincott Co., 1911).

Hew Light on Old Masters (London: The Sheldon Press, 1935).

The Painter's Methods and Materials (Philadelphia: J. B. Lippincott Co., 1926).
The Pigments and Mediums of the Old Masters (London: Macmillan and Co Ltd
1914).

A. Lucas, Ancient Egyptian Materials and Industries, 2d ed. (London: Edward Arnold
and Co., 1934).

A. Maerz and M. Rea Paul, 0/'Co/or (New York: McGraw-Hill Book Co

1930).

R. C. Martin, Glossary of Paint, Varnish, Lacquer and Allied Terms (St Louis: American

Paint Journal Co., 1937).

Joseph Meder, Die Handzeichnung, ihre Technik und Entwicklung (Vienna: Anton
Schroll and Co., 1919).

J. W. Mellor, A Comprehensive Treatise on Inorganic and Theoretical Chemistry, i6 vols

(London: Longmans, Green and Co., 1922-1937).

M. P. Merrifield, Original Treatises on the Art of Painting, 1 vols (London: John Murray
1849).

H. E. Merwin, ‘Optical Properties and Theory of Color of Pigments and Paints ’
Proceedings of the American Society for Testing Materials, XVII (1917), pp.

C. Ainsworth Mitchell, Inks: Their Composition and Manufacture (Philadelphia- T B*
Lippincott Co., 1937). • J-

J. R. Partington, Origins and Development of Applied Chemistry (London: Longmans
Green and Co., 1935), &

A. G. Perkin and A. E. Everest, The Natural Organic Colouring Matters (London:
Longmans, Green and Co., 1918).

W. M Flinders Petrie, Medum (London, 1892); Chap. VIII by W. J. Russell, ‘Egyptian
Colours,’pp. 44-48. ’

E. Raehlmann, Vher die Maltechnik der Alien (Berlin: Georg Reimer, I910).

Friedrich Rose, Die Mineralfarben (Leipzig: Otto Spamer, 1916).

F. C. J. Spurrell, ‘Notes on Egyptian Colours,’ The Archaeological Journal, LII (i8qO

pp. 222—239. ^

<London"^”847r'''^ Robert Hendrie

Theophrastuds History of Stones, trans. by Sir John HiU (London, 1774).

Craftsman's Handbook of Cennino

d Andrea Cenmnt, 2 vols (New Haven: Yale University Press, 1932-1933)

‘TrifunT 7 “^(London: George Allen and Unwin, Ltd, 1936).
Trial Index for Mediaeval Craftsmanship,’ Speculum, X (1935), pp. 410-431!^

Pigments i 8 i

Edward Thorpe, A 'Dictionary of Applied Chemistry, 7 vols (London: Longmans, Green

and Co.5 1921-1927).

Maximilian Toch^ The Chemistry and Technology of Paints^ 3d ed. (New York: D. Van
Nostrand Co., 1925).

Paints^ Painting and Restoration (New York: D. Van Nostrand Co., 1931).

Heinrich Trillich, Das Deutsche Farhenhuch^ Parts I-III (Munich: B. Heller, 1923).
Rokuro Uyemura, ‘Studies on the Ancient Pigments in Japan/ Eastern Art^ III (1931),
pp. 47-60.

Vitruvius, The Ten Books on Architecture^ trans. (Cambridge: Harvard University
Press, 1926).

F. W. Weber, Artists' Pigments (New York: D. Van Nostrand Co., 1923).

Mary Elvira Weeks, The Discovery of the Elements (Easton, Pa: Mack Printing Co.,
1934). ^

A. M. de Wild, The Scientific Examination of Pictures^ trans. (London: G. Bell and Sons
Ltd, 1929).

SOLVENTS, DILUENTS, AND DETERGENTS

Absolute Alcohol (see Alcohol)*

Acetone (dimethyl ketone [CH3COCH3]). As the most important member of
the ketone group of solvents, this has been known since the early XIX century.
Formerly it was obtained to a large extent from the dry distillation of calcium
acetate which, in its turn, was a by-product of wood distillation. Now acetone
comes as a by-product from the fermentation of molasses or corn mash in the pro¬
duction of n-Butyl Alcohol and is also made synthetically. The liquid is clear and
boils at 56.1 C. The odor is strong and rather sweet. Acetone is completely miscible
with water and with most organic liquids. Industrially, it is perhaps the most
generally used of the low-boiling solvents because of its solvent strength and its
low cost. It dissolves cellulose nitrate, other cellulose derivatives, synthetic resins
of the glyceryl phthalate, thio-urea, and vinyl types, the natural soft resins, and
certain waxes. Fossil resins dissolve in it to a large extent, and shellac to m.ore than
80 per cent. It is miscible in all proportions with linseed, tung, and polymerized
oils.

Since acetone is miscible with water, oils, and most solvents, it makes a good
coupling agent for combining immiscible fluids. It is an important ingredient of
neutral paint removers because of its action on resins and on linoxyn (oxidized
linseed oil). Used in excess with lacquer and varnish films, it may cause them to
blush, especially in humid weather; this is from condensation of moisture within
the film when rapid evaporation of solvent cools the surface. Acetone is only
niildly toxic and is safe to use in well ventilated places, though its vapors are
highly flammable. (See L. C. Cooley, ‘ Acetone,’ Industrial and Engineering
Chemistry^ XXIX [1937], pp. 1399-1407.)

Alcohol (Cologne spirits, grain alcohol, ethyl alcohol, ethanol [C2H5OH]).
The second in the series of monohydric alcohols is inexpensive when obtainable
tax-free, yet is the purest and probably the most useful of the organic solvents.
It acts readily on almost all natural resins, including shellac, and on many of the
synthetic compounds that serve as film materials in painting, but it is compara¬
tively inactive on the drying oils and quite inactive on waxes and aqueous
mediums. The liquid is clear and volatile, boiling at 78.3°C.; vapor pressure at
i2o°C. is 44 mm. Alcohol is miscible in all proportions with water and with most
organic liquids and is a solvent for some inorganic substances like sodium
hydroxide.

The preparation of alcoholic beverages by fermentation from grains and fruits
can be traced to ancient times. A. Lucas {Ancient Egyptian Materials and Indus¬
tries^ [London: Edward Arnold and Co., 1934], p. 23) says that although the
ancient Egyptians made beer and wine, they were not acquainted with the
process of distillation and had no distilled spirits. According to him, also, it is
the general belief that distilled spirits were not known until the Middle Ages,
the first use of them being for the preparation of medicine. E. O. von Lippmann
{Beitrdge zur Geschichte der Naturwissenschajten und der Technik [Berlin: Julius

185

i86

Painting Materials

Springer, 1923], pp. 56-126) corroborates this by concluding that alcohol was not
the discovery of the Alexandrian or Arabian alchemists, but first became known
probably in the XI century and in Italy. He finds the first mention of it in the
Mappae Clavicula, a MS. of the XII century, and a later one in the MSS of
the so-called ‘Marcus Graecus’ which date from 1250 to 1300. Other Italian
writers of the XIII century speak of its medicinal value. Its use spread rapidly
after the Great Plague of 1340 (see G. Sarton, Introduction to the History of
Science [Baltimore: The Williams and Wilkins Co., 1927], I, 533-534; II, 29
130, 408, 1038-1039). It is said (see Samuel Couling, The Encyclopaedia Sinica
[London: Oxford University Press, 1917], under ‘ Wine ’) that distillation of
alcoholic spirits was first introduced into China in the Yuan Dynasty (XIII
century). ^ ^

The name has an Arabic origin^ the particle al and the word kohl^ a term used
for centuries to designate any fine powder, particularly an eye paint from anti¬
mony sulphide. Its application to a volatile liquid, especially the spirit of wine
seems to have been made by Paracelsus in the early XVI century. Before that
the terms aqua ardens in the Middle Ages and aqua vitae in the Renaissance were
commonly given to it. ^

Although much alcohol is made synthetically, most of that used In industry is
still the product of the fermentation of sugar, or of natural carbohydrates which
yield sugar, by enzymic action. Cane sugar and beet sugar molasses are the princi¬
pal sources, but cereals like wheat, barley, and corn, and also potatoes, are like¬
wise important. The Weizmann process for production of butyl alcohol by fer-
mentation of corn starch yields about 10 per cent of ethyl alcohol as a by-product
Alcohol from fermentation has to be concentrated and separated from water and
similar impurities by distillation and rectification. This is done in high columnar
stills where the vapom condense on baffle plates and are redistilled with progressive
ennchment. Industrial alcohol has about 5 per cent of water and can not prac¬
tically be purified further by distillation because alcohol and water form a con-
Ty volume”^ mixture when the distilled vapors contain 97.2 per cent alcohol

Synthetic ethyl alcohol, marketed as ‘ ethanol,’ is now made in quantity from
ethylene gas, obtained from the cracking of petroleum, or is made from acetylene.

Inis is identical in composition and properties with alcohol derived from fer-
mentation.

,, Absolute or anhydrous alcohol is ethyl alcohol free from water. This is 200 proof

50 per cent alcohol; proof spirit [U. S.] has 42.5a per
cent by weight of absolute alcohol in distilled water). The 5 per cent of water left
Z r I that comes from straight distillation can be removed in order to

f ^ ^ of azeotropic distillation with

benzene or by redistfilation over quicklime or over a hydrate-forming salt. Now it
IS made industrially in large quantities and at a fairly low cost. Absolute alcohol is

Solvents and Diluents

187

preferable as a lacquer solvent because it is miscible with a wider range of solvents
than is the usual industrial product and because it is not so likely to cause bloom.
Unlike the ordinary alcohol, it is miscible with turpentine and benzene. It is,
however, strongly hygroscopic and does not remain anhydrous unless carefully
protected from the air.

^ Alcohol probably came first into use as a paint material for the preparation of
spirit varnishes—soft resins in a suitable solvent. The slight literature that exists
in connection with the restoration of pictures mentions it as a solvent to be used
for the removal of old varnish film. The softening and gelation caused by alcohol
vapors on varnish films is the basis of the so-called ^ Pettenkofer process ’ formerly
used for the regeneration of paintings (see Max von Pettenkofer, Uber Olfarbe und
Konsefvierung der Ge^ndlde-Galerlen dutch das Regenerationsverfahren [Braunsch¬
weig, 1870]; also Eibner, pp. 449-452). In this process the painting was put in a
sealed chamber with alcohol vapors. The solvent vapors coalesced the edges of
the cracks of the varnish film, made that film continuous, and gave it added
transparency and the appearance of a revarnished surface. Sometimes copaiba
balsam was first applied and wms driven into the film by the vapors. The assertion
is frequently made that alcohol is a strong solvent for oil paint, but the description
of the oil paint is not given. Possibly action on immature films or films containing
some amount of resin as an admixture have been the basis for this belief, although
it remains to be categorically denied.

Ammonia (NHs) and Ammonitim Hydroxide (NH4OH). The colorless gas is
very soluble in water. Concentrated ammonia water is 28 to 29 per cent NH3 and
has a specific gravity of 0.90. It is also very soluble in alcohol. It goes out of solu¬
tion much more rapidly than the fluid evaporates until, in the case of water, the
concentration of dissolved ammonia falls to 6.1 per cent. There the evaporation
rate is the same for both and the gas can be expelled from the water only by heat¬
ing. In water the solution is alkaline, and takes the name, ammonium hydroxide.
Because of alkalinity, it neutralizes acids and forms ammonium salts. It is a weak
alkali, however, and the weakness seems to come from its low ionization. A dilute
solution has certain uses because it provides a weak alkali which leaves no residue
on evaporation of the water. The presence of the dissolved gas lowers the surface
tension of water and allows it more readily to wet surfaces and to penetrate into
fabrics or other rough textures. The mild alkali properties enable it to cut thin
films of oil and grease. The presence of the dissolved gas in the mixture of water
with organic solvents acts rapidly on fatty or greasy materials and can be used
occasionally with safety in removal of them from surfaces of paintings. As a rule,
however, the water content of such a mixture is dangerous, even if the alkalinity
does not threaten the preservation of the linoxyn film.

As ammonium carbonate, a white solid called spiritus sails urinae^it was known
to the early alchemists. Another name for It was ^ spirit of hartshorn,^ because it
was obtained from the dry distillation of hoofs, bones, and horns. This, in effect,
was the only volatile form known up to about the XVIII century. Ammonium

Painting Materials

chloride (sal ammoniac or sal ammoniacum), a fixed salt of ammonium, was, how¬
ever, known to the ancients. The gas (NH3) was first isolated by Joseph Priestly,
English chemist and discoverer of oxygen, in 1774. Until recently, industrial
ammonia was a by-product of coal through dry distillation in gas works and coke
ovens. Now it is made synthetically from hydrogen and nitrogen.

Amyl Acetate (CH3COOC5H11). At one time this ester of acetic acid and amyl
alcohol was prepared in large quantities from fusel oil, a residue obtained from the
rectification of alcohol. A more uniform product is made synthetically. It is a
clear liquid, with a penetrating odor (giving it the common name, banana oil),
which is overpowering in high concentrations. The boiling point is I48°C. and the
flash point is high, -jfC. In 1882 it was introduced as a solvent for cellulose nitrate
and for many years continued to be the leading solvent for that material. Now it
is largely replaced by butyl acetate. The amyl acetate is a good solvent for many
resins and is miscible with linseed oil. It mixes with water only very slightly.
Much of the synthetic product, composed of five isomeric amyl acetates, is sold
as Pentacetate. There is no record of experimental data on the action of this long¬
standing solvent on resin and linoxyn films.

A min es (see Solvents, classification).

Banana Oil (see Amyl Acetate).

Benzene (benzol) is the simplest of the aromatic series of organic compounds.
This is a hydrocarbon with the formula, CsHe, and is usually represented as having
the ring or cyclic structure shown below. It is a clear, colorless liquid, with a

H

C

HC

Hi

\

CH

i •

CH

C

H

peculiar odor. It boils at 80° to 8i°C., is highly flammable, and, being an un¬
saturated compound, it burns with a sooty flame. It is miscible with most organic
solvents but practically not with water.

The benzene of commerce is largely obtained by the destructive distillation of
coal tar, and. the bulk of it comes over in the first or ‘ light oil ’ fraction. This is
further fractionated to give the industrial product commonly known as benzol, a
mixture of about two thirds benzene with one third toluene and other benzene
omoogu^. Among irnpurities, it contains thiophene, a sulphur compound, and
pyridine. Further fractional distillation of benzol and special chemical treatment
to remove impurities, combined with filtration and crystallization below c t°C

(its solidifying point) yields the pure benzene.

The toxicity of its vapor prevents benzene from being widely used in coating
and cleaning compounds, though it is an effective solvent for a wide variety of

Solvents and Diluents

189

materials^ including some cellulose derivatives. Prolonged exposure to the vapor
affects the bloody producing both anaemia and leucopenia^ the symptoms being
headache and general lassitude, and if exposure is excessive, it leads to delirium
and final death (see Hamilton, pp. 156-165).

Benzine ^(petroleum spirit). This name is now being discarded because of its
confusion with benzene, a coal-tar product. It is still given to a volatile petroleum
distillate similar to gasoline. At times the word is applied indiscriminately to
various petroleum fractions below kerosene.

Benzol is the commercial name for the coal-tar distillate. Benzene, containing
also toluene and other benzene homologues.

n-Butyi Acetate (CH3COOC4H9). One of the recent developments of the solvent
industry, this ester of butyl alcohol and acetic acid is a clear, stable liquid with
strong ethereal odor. When pure, it boils at 126. fC,, but the commercial product
is only about 85 per cent pure. Unlike the lower esters, it is not hygroscopic and,
hence, does not hydrolyze and become acid. It mixes in all proportions with ben¬
zene and many other organic solvents, but is only slightly miscible with w^ater.
The solvent action is strong on cellulose nitrate, other cellulose plastics, and syn¬
thetic resins. The low rate of evaporation prevents blush and gives a good flow
to such mixtures. In the lacquer industry it has largely displaced amyl acetate.
The vapors are not toxic. Secondary butyl acetate and isobutyl acetate, prepared
from isomers of butyl alcohol, are also available commercially, and their properties
are little different from those of the normal acetate.

n-Butyl Alcohol (butanol [C4H9OH]) may be prepared in four isomeric forms,
but the normal alcohol is the most important. The Weizmann process brought it
out on a commercial scale during the war of 1914-1918 when it was derived as a
by-product of acetone in the fermentation of corn mash by a specially developed
bacillus. Somewhat later its value as a lacquer solvent was recognized and its
production became the chief purpose of the Weizmann process. It is separated
from the other solvents, acetone, ethyl alcohol, higher alcohols, and fatty acids,
and is purified by distillation. Little has been done to record experimentally its
action on the film materials of artists’ paint and varnish. Very limited observations
make it seem that the solvent properties of butyl alcohol are somewhat stronger
than those of ethyl alcohol.

It is not miscible with water in all proportions but only to the extent of 7.7
per cent at ao°C. and mixes with most organic solvents. It is able, moreover, to
promote miscibility in other mixtures like that of benzene, petroleum naphtha,
and ethyl alcohol. Commercially, it is said to have a good solvent action on hard
copal and on shellac. Its boiling range is 110° to 118°C. It evaporates at a moderate
rate, is not hygroscopic, and, since it does not incline towards blush, is a favorite
solvent for synthetic lacquers. It is said to be a good solvent for metallo-organic
driers like lead and cobalt linoleates, and to have an effective solvent action on
linoxyn and other oxidized oil films. The vapors of butyl alcohol are distressing

Painting Materials

190

and have a toxic effect if inhaled for any considerable period. They are com¬
bustible but not highly flammable.

Butyl Cellosolve (see Ethylene Glycol Monobutyl Ether and Glycol Ethers)

Butyl Lactate. This solvent (CHs-CHOH-COO-C4H9), an ester of butyl
alcohol and lactic acid, is very slow to evaporate and, for that reason, has certain
special uses. Herbert E. Ives and W. J. Clarke (' The Use of Polymerized Vinyl
Acetate as an Artist’s Medium,’ Technical Studies, IV [1935], PP- 36-41) suggest
it as a solvent for vinyl acetate when that is used as a painting medium. VTen
pure, it is colorless, but commercial varieties have a brown tint. It is only slightly
miscible with water (3.4 per cent at 25°C.) but is miscible with most organic
solvents. Toch says (p. 91) that butyl lactate added in small amounts to tur-
pentine acts as a good cleaning agent for old paintings.

Carbon Disulphide (carbon bisulphide [CSJ). The only important industrial
solvent of this type containing sulphur, this is a volatile liquid which boils at
46.3°C. and is heavy (sp. gr. 1.263). It was discovered in 1796. The commercial
product, made by the direct union of carbon and sulphur at dull red heat, is
slightly yellow in color and has a disagreeable odor caused by impurities of other
organic sulphur compounds. It mixes with most organic solvents but only slightly
with water. The flash point is -2o°C.; it is highly flammable and forms explosive
mixtures with air. It can be spontaneously ignited by contact with objects at
i_5o°C.; even contact with a warm steam pipe or electric lamp bulb may be sufli
cient to cause ignition of the vapor. Because of this, it is about the most dangerous
solvent in commercial use. Its vapors, moreover, are strongly toxic. Although it is
a good solvent for oils, fats, waxes, rubber, and a number of resins, safer materials
are now taking its place.

Carbon Tetrachloride (tetrachlormethane [CCI4]) may be regarded as meth¬
ane, in which all the hydrogen atoms have been replaced by chlorine. Prepared
y the chlorination of carbon disulphide, this is a clear liquid with an odor similar
to that of chloroform, a high specific gravity (1.629), and a boiling point of 77°C.
Because it is non-flammable, it has a unique place among organic solvents and is
miscible with most of them. Solvent action on oils and soft resins is good and it is
frequently mixed with other solvents to cut down the fire hazard. A disadvantage
IS thap at slightly elevated temperatures it reacts slowly with water to form
ydrochloric acid. At moderate temperatures it is stable to water and to light.

. IS a trade name for ethylene glycol monoethyl ether, and the name

is^ used with qualifying adjectives like methyl, butyl, etc., to designate several
othCT closely related derivatives of ethylene glycol (see Glycol Ethers).

Ethera) Acetate (see Ethylene Glycol Monoethyl Ether Acetate and Glycol

Chlorinated Hydrocarbons (see Solvents, classification)

^ Uhloroform^trichlormethane [CHCla]), a clear, colorless liquid, with a charac¬
teristic sweetish odor, and a boiling point of 6i°C., is prepared by the action of

Solvents and Diluents

191

bleaching powder on alcohol or crude acetone. Although rapidly volatile, the
vapors can be ignited only with difficulty. Mixed with alcohol, it is a good solvent
for cellulose acetate and, alone, is active on most resins, but it has little use com¬
mercially. In all proportions it is miscible wdth most of the organic solvents and
with vegetable and mineral oils. When allowed to stand for a long time in the
light, it will slowly react with moisture to form hydrochloric acid and phosgene,
but if I per cent of alcohol is added, its decomposition is hindered. Chloroform
was discovered in 1831 by Liebig and Soubeiran, and its use as an anaesthetic in
1848 by an Englishman, Simpson.

Coal-Tar Hydrocarbons (see Solvents, classification).

Denatured AicohoL Addition of poisonous or distasteful material to ethyl
alcohol so that it can not be safely used in the concoction of beverages, gives it
this name. The Bureau of Internal Revenue, Treasury Department of the United
States, in the Appendix to Regulations, no. 3, Formulae for Completely and Spe--
dally Denatured Alcohol^ 1938, specifies the use of such materials as gasoline,
aliphatic isoalcohols, methyl isobutyl ketone, and organic hydrogenated materials
in three formulas for completely denatured alcohol which can be used without
restriction of sale. There are also given many special denaturing formulas em¬
ploying such materials as methyl alcohol, benzol, ether, pyridine, animal oil,
acetone, and others, but such specially denatured alcohol can only be used under
close government regulation.

Detergent. Broadly this can be taken to mean any cleansing agent or anything
that aids in cleaning. Although soap has long been the common detergent in the
past, a large number of special materials has been developed recently for this
purpose. The most important is that group of compounds known as the sulphated
alcohols. These are sodium sulphuric esters of straight-chain fatty alcohols ranging
from eight to eighteen carbon atoms and higher. The cleansing action of any
detergent is caused by its ability to wet completely particles of dirt and to disperse
or deflocculate them and keep them in suspension so that they can be rinsed away.
(See Surface-Active Agent; also R. A. Duncan, ‘ The New Detergents/ Industrial
and Engineering Chemistry^ XXVI [1934], pp. 24-26.)

Diacetone Alcohol. When pure, this keto-alcohol, (€03)20 (OH) CH2COCH3,
is a colorless liquid having a mint-like odor. It is made by the condensation of
acetone, and the commercial grade contains some uncondensed acetone and some
mesityl oxide. There is no sharp boiling point for diacetone alcohol; it distills
between 130° and i8o^C., about 85 per cent between 150° and 170°. It may
contain a trace of acetic acid, but the acidity should not exceed 0.02 per cent.
It is not miscible with the petroleum hydrocarbons but mixes completely with
water and with most organic solvents. The low vapor pressure (3.3 mm. Hg at
2o°C.) and low evaporation rate make it useful as an ingredient to retard the
drying of varnishes, to make them brush more easily, and to prevent blush.
General experience with it seems to indicate that its solvent action on dried

192

Painting Materials

varnishes is about like that of ethyl alcohol. It is a good solvent for cellulose ace¬
tate. Toch says (p. 91) that diacetone alcohol is one of the most important restor¬
ing agents for paintings because of its miscibility with other solvents, its slow
evaporation, and its ability to coalesce the crackle on old varnish films.

Dibutyl Phthalate (C6H4CCOOC4H9]2) is the most widely used chemical
plasticizer for commercial lacquers. A colorless liquid, it has a boiling point be¬
tween 200 and 216 C. and a flash point of i6o°C. It is made by the direct action
of butyl alcohol on phthalic anhydride and has a high degree of stability, being
only very slightly volatile. Amounts equalling or exceeding the solids content are
used in compounding commercial lacquers. Although the films that contain it
mmain plastic for a long time, they eventually lose it, with a consequent harden¬
ing. Gardner (p. 419) reports experiments in which it was found that a film of 2
parts nitrocellulose and i part dibutyl phthalate lost only 0.98 per cent of the
plasticizer after baking for 20 hours at 5o°C. It is light-stable but yellows very
slightly on long standing. On a number of resins, including mastic, it has some
solvent action. Other alkyl esters of phthalic acid are used as plasticizers—diethyl
phthalate, diamyl phthalate, and dimethyl glycol phthalate.

152-Dicliloretliane (see Ethylene Dichloride).

Diluent In its broad meaning this is any material added to a paint or varnish
to make it thin and easily applied. More specifically with respect to varnishes
and lacquers^ it is an added liquid which by itself would not serve as a solvent for
the film material. As such, it can be put into the mixture only to a limited extent
without causing precipitation of the solid. The amount of diluent that is tolerated
in combinations of film materials and solvents is called ' the dilution ratio/ No
film material can be properly called a ‘ diluent/ Whether or not a completely vola¬
tile fluid is a diluent or a solvent depends entirely on its relation to a particular
solid. If the fluid is capable of dispersing all or part of that solid and holding it as so
much suspended matter, then that fluid is a solvent for that solid. If it can not do
this but is in some degree miscible with a solution formed by another fluid, it is
a diluent. Turpentine, for example, is a solvent for mastic resin but is only a
diluent to a small extent for solutions of certain synthetic resins. Petroleum
naphtha is a solvent for dammar resin but is only a diluent within limited pro-
portions for solutions of mastic.

^ Dioxane (1,4-dioxane [O ; (CH2CH2)2: O]) is a cyclic diether. The boiling
point IS y and the liquid is colorless and stable, with a faint ethereal odor.
It is made by distilling ethylene glycol with sulphuric acid. Completely miscible
with water and with most organic solvents, it has itself a high solvent strength
for natsiral and synthetic resins, and also for oils, fats, and waxes. Like ether, it
IS much used in the extraction of ethereal oils, but since it has a relatively low
volatility. It IS much more useful in the removal of old surface films, especially
those in which a wax or partly dried oil is present. The vapors are explosive, and

Solvents and Diluents

193

there are some indications of toxicity. Oil-soluble dyes dissolve in dioxane, but
those which are usually spirit- or water-soluble are only partially affected by it.

Distillate. This name, given to a liquid that is condensed from its vapors,
covers most of the organic solvents. In distillation a liquid is purified by being
heated to the boiling point, the vapors then being condensed and collected.
Fractional distillation is the slow distillation of a mixture of substances and the
separate collection of distillates at each boiling point or after definite temperature
intervals. The low-boiling fractions are collected first. The distilling range is the
recorded thermometer range at which a liquid is distilled. Destructive distillation
is the heating with exclusion of air of complex organic matter like wood or coal
until it is decomposed or split into a number of liquid and solid substances.
Steam distillation is the passage of steam through a liquid whereby the liquid
vapor is carried off with the steam and, on condensation, the liquid and water
are separated by their immiscibility. Vacuum distillation is done under reduced
pressure; it is usually carried out with high-boiling materials which decompose at
their atmospheric boiling points. Most organic solvents are distilled in processes
of preparation or purification (see Alcohol, Petroleum Thinner, and Turpentine).

Essential Oil, a volatile oil, with characteristic odor, derived from plants.

Esters (see Solvents, classification).

Ether (ethyl ether [[CaHsOCaHs]). Since the XVI century this clear, volatile,
and highly flammable fluid has been made by the dehydration of ethyl alcohol
with sulphuric acid. The solvent action is very wide, including oils, resins, fats,
and waxes. High vapor pressure (442 mm. Hg at 2o°C.), with danger of explosion’
make it hazardous to use as a studio or laboratory solvent. The boiling point is
34.5°C. The vapors are heavy and tend to creep. Because of its rapid evaporation,
it has little use as a commercial solvent for lacquer. It is completely miscible with
alcohol and with most organic solvents, but with water only to the extent of 7.42
per cent at 2o°C. It has some use as an extraction solvent in the analysis of paint
samples, and either alone or in mixtures for the removal of resin and wax.

Ethyl Acetate (CH3COOC2H6). Action of acetic acid on alcohol forms this
ester. It is a colorless liquid, with faint odor, and the pure, anhydrous form boils
at 78°C. Being hygroscopic, it reacts with absorbed water by hydrolysis to form
acetic acid and alcohol. It is miscible with alcohol and with most organic solvents,
but only partially with water. Its acidity and the tendency to cause blush, the
result of rapid evaporation, are factors against its use as a solvent.

Ethylene Dichloride (1,2-dichlorethane [CH2CICH2CI]), a clear, volatile
liquid, has an odor somewhat like that of chloroform and boils at 83.5°C. (It may
be confused with dichlorethylene, C2H2CI2.) Dutch chemists discovered it in 1795,
making it one of the earliest organic compounds to be isolated. Only in recent
years, however, has it become commercially available. Direct chlorination of the
ethylene gas is the method of manufacture. This is an excellent solvent for oils,
fats, and waxes, and for certain resins, but serves only as a diluent for cellulose

194

Painting Materials

derivatives. In its resistance to hydrolysis and oxidation it is outstanding among
chlorinated hydrocarbons^ and is among the most stable of this series (see G. O.
Curme Jrj ' Importance of Olefine Gases and Their Derivatives, III, Ethylene
Dichloride,' Chemical and Metallurgical Engineerings XXV [^1921], pp. 999 ff.).
Being hard to ignite, it is used to lower the flash point of solvent mixtures. The
vapors are not dangerously toxic but are anaesthetic if breathed in high con¬
centrations.

Ethylene Glycol (glycol [CH2OH * 0112011 ]]). This dihydric alcohol is an inter¬
mediate between alcohol and glycerine (glycerol). The liquid is colorless, has a
sweet taste with no odor, and boils at I97.2°C. It will not mix with hydrocarbons
but will mix in all proportions with water, alcohol, and many organic liquids. It is
extremely hygroscopic, absorbing approximately twice its weight in water in an
atmosphere of room temperature, and 100 per cent relative humidity. This unfits
it for the common uses as a softener for mediums and dried films.

Ethylene Glycol Monobutyl Ether (butyl cellosolve [C4H9OCH2 • CH2OH])
has the highest boiling point, lyo.d^C., and the lowest evaporation rate of the
common Glycol Ethers. The solvent action and other properties are similar to
those of cellosolve, but it is less volatile.

Ethylene Glycol Monoethyl Ether (cellosolve [C2H6OCFI2 * 002011 ]]). This is
the most important of the glycol ether series, being a colorless, stable liquid, with
a mild, fruity odor. It is made synthetically from ethylene gas obtained in the
cracking of petroleum. The boiling point is I34.8°C. It is a good solvent for natural
and synthetic resins and Is characterized by its moderate rate of evaporation (see
Glycol Ethers). It is used, in small percentages, in lacquers to improve flow quali¬
ties and to prevent blush. *

Ethylene Glycol Monoethyl Ether Acetate (cellosolve acetate [C2H50CH2*
CH2OOCCH33). This clear liquid is a solvent for most natural and synthetic
resins and is miscible with most of the organic solvents. It is distinguished from
cellosolve, the closely related ethylene glycol monoethyi ether, by its higher
tolerance for petroleum hydrocarbons (see Glycol Ethers).

Ethylene Glycol Monomethyl Ether (methyl cellosolve [CH3OCH2 • CH2OH]).
A member of the series of Glycol Ethers, this is miscible with water and with most
organic liquids. In many respects it is like the other members of the series, but
differs from the rest by being hygroscopic. It is a solvent for essential oils and for
natural and synthetic resins. Its boiling point is I24.5°C. and it is the most volatile
of the commercially available glycol ethers. As a solvent for certain dyes, it is
much used in the preparation of non-aqueous stains for wood and for vegetable
and animal fibres.

Gasoline (petrol). Produced chiefly as a motor fuel, this mixture of volatile,
aliphatic hydrocarbons is obtained from the distillation or cracking of petroleum.
Where rapid evaporation is desired, it is used as a solvent for paint mediums and

Solvents and Diluents

195

is a common cleaning agent. Its volatility and flammability prevent a wide use of
it as a paint thinner. The gasoline from American sources contains several hydro¬
carbons, but chiefly hexane, heptane, and octane. That from Rumanian and
Russian sources may have hydrogenated benzene compounds called naphthenes.
Being a mixture, gasoline has no definite boiling point but, rather, a boiling range
as wide as from 60° to i2o°C.

Glycerol (glycerine [(CHaOH-CHOH-0112011]]), is a trihydric alcohol, clear,
syrupy, and with a sweet taste. It was first prepared in 1779 by Scheele through
the saponification of olive oil with litharge. Now it is obtained chiefly as a by¬
product of soap manufacture. It is miscible in all proportions with water, alcohol,
and many organic solvents. In the practice of painting it is chiefly used as a
moistening and plasticizing agent, particularly for aqueous mediums such as egg
white, gelatin, and gum.

Glycol Ethers. This name is given to a series of solvents rather recently de¬
veloped and sold under the trade name, cellosolve. Besides cellosolve itself, which
is ethylene glycol monoethyl ether, there are: methyl cellosolve (ethylene glycol
monomethyl ether), butyl cellosolve (ethylene glycol monobutyl ether), and
cellosolve acetate (ethylene glycol monoethyl ether acetate). Because of strong
solvent action, medium and low volatility, stability, mild odor, and tolerance for
aromatic hydrocarbons, the series has had a wide commercial use in the manu¬
facture of coatings from synthetic resins. Since they contain no ester group, they
do not develop acidity through hydrolysis. In the treatment of paintings, par¬
ticularly the removal of old and discolored films of varnish, they are used alone or
as ingredients to couple mixtures of hydrocarbons with alcohols or ketones.

Gum Spirits of Turpentine (see Tmrpentine).

Hexane, the colorless, volatile, liquid, aliphatic hydrocarbon (CeHu), is a
normal component of most gasolines, but serves also as a special solvent. It is
prepared by careful fractional redistillation of lower petroleum fractions.

Ketones (see Solvents, classification).

Methyl Acetate (CHsCOOCHs). This ester is derived from methyl alcohol and
acetic acid, and can be prepared by distilling the former over calcium or sodium
acetate in the presence of sulphuric acid. It is also a by-product of wood distilla¬
tion. The odor faintly resembles that of methyl alcohol, the liquid is clear, boils
at 53 to 59°C., and is the most volatile of the ester type of solvents as well as the
most active. In this respect it resembles acetone. It mixes with water but is liable
to decomposition by hydrolysis, with the formation of acetic acid. The vapors are
not strongly toxic but are highly flammable.

Methyl Alcohol (wood alcohol, methanol). This, the simplest member of the
alcohol series, has the formula, CH3OH. Formerly it all came from the dry dis¬
tillation of vegetable waste, particularly wood. Repeated distillation or special
treatment is necessary in order to get a pure product, the chief difficulty being
the removal of acetone which is also the product of this dry distillation. That

196

Painting Materials

method has been largely displaced since about 1920 when a way was found for
synthesizing methyl alcohol from carbon monoxide and hydrogen. The result is
a pure product known as methanol and made more cheaply than that from wood

distillation.

When pure, methyl alcohol has an odor about like that of ethyl alcohol. It is
a clear liquid and boils at 66°C. As a solvent, it acts about as does ethyl alcohol
including solution of shellac. It is the basis of many paint removers and is much
used as a denaturant for ethyl alcohol because it does not affect the properties
of the latter as a solvent, and it can not be easily separated from it by distillation.
The vapors of methyl alcohol are much more poisonous than are those of ethyl
alcohol. Unlike the latter, it is oxidized in the system with the formation of
formaldehyde and formic acid. The vapor is flammable.

Methyl Cellosolve (see Ethylene Glycol Monomethyl Ether and Glycol Ethers).

Methyl Ethyl Ketone (CH3COC2H6). This solvent is like acetone, except ethyl
replaces one methyl group of that compound. Usually made by the dehydro¬
genation of secondary butyl alcohol, it can also be obtained from wood distillates,
though that product contains a considerable amount of acetone. The evaporation
is somewhat slower than that of acetone, but the solvent action is about the same
as is the odor. It acts well on cellulose acetate. It is a colorless, flammable liquid,
boiling at 76.6°C. As a commercial product, it contains methyl acetate and
methyl alcohol.

Methyl Isobuyl Ketone (hexone [(CH3)2CHCH2COCH3:j). As a medium¬
boiling ketone, this solvent is useful in mixtures for lacquers and varnishes. Un¬
like the lower ketones, it is only slightly miscible with water (1.9 per cent). It is
colorless and stable, not highly toxic or flammable. The boiling point is ii6°C.

Methylated Spirit (see Denatured Alcohol).

Methylene Chloride (dichlormethane [CH2CI2]]), a clear liquid resembling
chloroform, is produced by the chlorination of methane. It is the most volatile
of the chlorohydrocarbons and boils at 42°C., but has a very low flammability.
It is a strong solvent for oils, resins, rubber, and some cellulose derivatives.
Because of its action on linoxyn, it is included in many paint removers. Care is
required in handling it because of its volatility, but it is least dangerous physio¬
logically of the chlorohydrocarbons and is stable to light and moisture.

Mineral Spirits^ (white spirits). As a vague synonym of ‘mineral thinner’
j. ., thinner,’ the term designates a rather wide range of petroleum

distillates and has also come to mean a particular kind of thinner suitable for
paint and varnish. The solvent is prepared from petroleum bases which may con¬
tain naph^thenic hydrocarbons and are free from those sulphur and unsaturated
hydrocarbons of the olefin series which give an objectionable odor. It boils be¬
tween 150 and 2io°C., but the distillation range varies according to source and

re _^sry. Some distilleries offer various grades with differences in boiling range
and flash point. & a

Solvents and Diluents

197

Mineral TMnner (see Petroleum Thinner).

Morpholine is a clear liquid with a musty and ammoniacal odor. The chemical
formula is O :^(CH2CH2)2 : NH; the boiling point is I28.9°C. It is miscible in all
proportions tyith water, and dilute solutions boil or evaporate with little change
in composition. A constant alkalinity is maintained in the solution and in the
distillate. Being mildly alkaline, it forms soaps with fatty acids and so may be
used as an emulsifying agent. Since in its structure it is a ring compound, combin¬
ing an ether and an amine, it is a strong solvent for dyes, waxes, shellac, and for
casein. It is one of the few available solvents which has a direct effect on a thor¬
oughly hardened linoxyn film.

Naphtha. In the technical literature on solvents and diluents, there is some
confusion about this term. It comes from the Greek, vaipda^ and, in turn, from the
Acadian, naptu (see R. J. Forbes, ^ Petroleum and Bitumen in Antiquity,' Amhix^
II D93SI p. 76). Originally ^the word was used for crude petroleum or vapor
coming out of the earth in regions of the Near East. Recently it has been applied
to the volatile distilled products of petroleum and coal-tar. Petroleum naphtha
distilled in the range, 100° to 160X., Is from paraffin or naphthenic hydrocarbons
(see V. M. and P. Naphtha). Coal-tar naphtha, a crude distillate, contains chiefly
benzene and its hcmologues. Solvent naphtha is a mixture of toluene, isomeric
xylenes, cumene, and other coal-tar hydrocarbons after the separation of benzene
from coal-tar naphtha by fractional distillation.

Spike Lavender, a product similar to turpentine, is obtained by the
distillation of spike lavender {Lavandula sfica). This is different from the lavender
oils used in perfumes, for that comes from the dried tops of lavender flowers. Oil
of spike is a colorless liquid, boiling between 170° and 2oo°C., and is less volatile
than tarpentine. A solvent for soft resins, it is frequently called for in old recipes
for spirit varnish. It may have been used more for its odor than for its solvent
properties. According to Doerner (p. 122), Rubens used it occasionally, but con¬
sidered It to be inferior to oil of turpentine.

Oil of Tiirpentine (see Turpentine).

Pamt Remover. This name covers a large group of commercial preparations
made for the purpose of softening and dissolving paints and varnishes. Most of
them are mixtures of organic solvents, with a small amount of paraffin wax which
retards evaporation a.nd keeps the solvent itself in contact with the paint for a
long time. Ammonia is a common ingredient also. With it are apt to be acetone,
methyl alcohol, and benzene. To reduce flammability, chlorinated hydrocarbons
like tnchlorethylene are added. Many such mixtures are patented. Speed and
completion of solution in a paint remover, as in any single solvent, are dependent
upon various factors in the paint and on the combination of materials used on it.
Oil films thoroughly dried and containing pigment are very slow to dissolve and
are strongly resistant to the usual organic solvents. For them the type of paint
remover which contains strong alkalis is more effective. Some of these with sodium

198

Painting Materials

Iiydroxidej potassium hydroxide, or ammonia are made up for use in a paste with
sawdust, chalk, starch, or similar bulky materials.

Penetrant is a term used for any one of various sulphated and sulphonated
higher fatty acids, alcohols, and esters. When used even in very small amounts,
these reduce interfacial tension between liquids and solids. Thereby they increase
ease of wetting and penetration (see Surf ace-Active Agent; also B. G. Wilkes and
J. N. Wickert, ' Synthetic Aliphatic Penetrants,' Industrial and Engineering
Chemistryy XXIX [1937], pp. 1234-1239).

Petroleum Ether is a petroleum distillate, chiefly hexane, with a boiling range
between 40° and 6o°C. Although too volatile for a diluent in paints and lacquers,
it is a good extracting agent for soft resins, oils, and waxes.

Petroleum Spiritj a term that may indicate any Petroleum Thinnerj is used
also specifically for a distillate boiling in the range of 60"* to i2o°C. This is similar
to Benzine and Gasoline.

^ Petroleum Thinner (benzine, gasoline, mineral spirits, naphtha, petroleum
spirits). The name is used for those hydrocarbons obtained from the distillation
of crude petroleum and commonly employed with paints and varnishes. They are
produced by most of the large oil companies; Gardner lists many of them (see pp.
570-574) and includes data on evaporation rates, distillation ranges, flash points,
density, and other properties. The solvent properties of the petroleum thinners
vary considerably according to the particular kinds of hydrocarbons they contain,
to their structure, molecular weight, and proportions. They are good thinners
and solvents for mineral and fatty oils, but hard fats (like tristearin) have limited
solubility in them. Resins are poorly dissolved in petroleum thinners containing
high percentages of aliphatics but are more soluble in those containing hydro¬
genated aromatics (naphthenes). Cellulosic compounds and polymerization
products like vinyl acetate are generally insoluble. The higher boiling fractions
are used as thinners for oil paints, varnishes, wax polishes, and emulsions, and
they now displace turpentine for many purposes. They have wide industrial
application because of their cheapness.

Although the Babylonians, the Assyrians, and other peoples of the Near East
were acquainted with crude petroleum and related substances like asphalt and
bitumen obtainable from the regions of the Caspian Sea (Baku), the Dead Sea,
and Mesopotamia, there is no sign that they refined it and put it into any prepara¬
tions of paint and varnish. Even though the art of distillation was perfected in
early mediaeval times (see Alcohol), no definite mention of a distillate as a painter's
material occurs until the XVII century in the MS. of De Mayerne (Berger, III,
108, 188, and 190). Petroleum products did not come into commercial use for
paint thinning until the latter part of the XIX century. Heaton says (p. 17) that
in 1885 Samuel Banner first introduced petroleum distillate as a solvent for the
paint industry under the name, Patent Turpentine (English patent 12,249 of that
year). For some time it was regarded as only a cheap substitute, but within the
last twenty-five years it has found a more respectable place in the paint and

Solvents and Diluents

199

varnish industry. Great care is being taken to prepare petroleum thinners for
specific purposes. Impurities of sulphur compounds, which formerly caused

trouble^ have been largely eliminated.

^ Plasticizer. This term has come into use recently to describe any solid or
liquid substance added to a film material in order to give that material elasticity
and pliancy. More specifically it stands for any one of a series of substances,
largely liquids with high boiling point and low volatility, that are used to reduce
the inherent brittleness of lacquer and synthetic resin coatings. Ordinary solvents
and diluents, even though fairly volatile, may be retained in small quantities as
part of the lacquer or varnish film during some weeks or months. With their final
escape, however, most organic films become brittle and friable, lose their pro¬
tective value, and, through the formation of minute crevices, become dull.
Linoxyn, the dried film of linseed oil, remains plastic for some years, and that is
why it is used as a plasticizer in resin-oil and spirit varnishes, but it, too, eventually
changes into a brittle and crumbling state. Lack of permanent flexibility seems to
be a shortcoming of most organic films, particularly those made from cellulose
derivatives and resin-like polymers. Celluloid is nitrocellulose plasticized with
camphor; castor oil also has long been used to plasticize nitrocellulose. The syn¬
thetic or chemical plasticizers are mainly esters of high molecular weight, but
they include a few amines, ethers, aromatic ketones, and even hydrocarbons.
Most of them boil above 250=0. and have a vapor pressure less than o.i mm.
mercury (see Gardner, pp. 576-578). The various plasticizers of modern industry
have_ been put into two groups: gelatinizing—those which have a solvent or
swelling action on the material to be plasticized—and non-gelatinizing—those
which have no solvent or swelling action but merely fill intermicellar spaces as
water does the pores of a sponge. Probably the best known of these plasticizers are
dibutyl phthalate and tricresyl phosphate.

Reagent Solvents are those which are used to dissolve water-insoluble mate¬
rials by chemical action upon them. Most of them are acids, but some are alkalis.
They react with materials to form water-soluble salts. Hydrochloric and nitric
acids and sodium hydroxide are examples. Almost without exception none of
these reagents can be used directly on artists’ paint. Their activity is apt to be
violent and destructive.

Restrainer. A word restricted almost entirely to the terminology of picture
restoration, it was formerly applied to a solvent which had a low activity on
resinous surface films and which was used in combination with a solvent having a
high activity. In some cases they were mixed together. In others, they were
applied alternately to the film. The theory was that the slow-acting or non-solvent
rnaterial would serve to dilute and make less concentrated the fast-acting one so
that its solvent effect could be controlled. Turpentine and alcohol, the former as
the restrainer, were regularly used in this way before a wider variety of solvent
materials became available.

200

Painting Materials

Saponin is a glucoside obtained from the soapwort and other plants. It is a
white, amorphous powder which forms a solution with water that foams like soap
when shaken. It has found some little use in cleaning paintings, and it is thought
to be less harmful than soap for this purpose (see Mayer, p. 398). A. Lucas
{Antiques^ Their Restoration and Rreservation^ ad ed. [London; Edward Arnold
and Co., 1932], pp. 166-167) recommends a i per cent aqueous solution of saponin
for removing accumulated dirt and smoke from a painting, but cautions that if
used in excess the water may penetrate to the canvas and cause damage.

Soap. Commonly the word is used to define the sodium or potassium salt of
palmitic or stearic acid. It may be given to the mixed salts of several acids ob¬
tained from ordinary fats. Although other metals can form soaps, these are not
soluble in water. Alkaline hydrolysis of fats with strong alkalis, producing a soap,
is called saponification. A soap is hard if made with soda, soft if made with potash.
Sodium carbonate and olive oil make Castile soap.

Occasionally it has been an addition to painting mediums, but the principal
use of soap is as a detergent. It can aid in the removal of grease or dirt from paint
or fabric. In doing so, it acts partly as an emulsifying or surface-active agent
which, by lowering interfacial tension, cuts fatty films and releases and disperses
particles of dirt they contain. In part, also, it appears that dirt particles are
adsorbed to the surface of colloidal soap particles. In its relation to paint, soap
has a number of defects. When put with emulsions, it is said to cause a later
darkening of the paint (Doerner, p. 22,6). Mayer (p. 373) says that when soap is
dissolved in water and used as a cleaning agent for paint, a minute quantity of
alkali may be produced by hydrolysis and may alter the surface color. He also
warns (p. 406) that the oil paint film can be weakened by soap solution, its adhe¬
sion destroyed, and a permanent blush left on it. Undoubtedly part of the ill
eflFects of soap solutions on paintings is from the seepage of water into the ground
and support.

Historically, soap appears to have been known in classical times and small
quantities have been found at Pompeii and other sites dating from the I century
A.D. Early treatises on painting frequently mention mixtures of fat and lye for
use in the refinement of granular blue pigments, and De Mayerne, in the early-
XVII century (Berger, IV, 292), has a brief description of the cleaning of a picture
with soap and fine ashes, followed by washing with water.

Solvent. This name is given to any of the volatile fluid organic compounds
which can, without chemical change, convert a solid or semi-solid organic material
into a technically useful solution. Generally such a solution is a mobile liquid
capable of application in thin films. The organic substances converted are usually
colloidal, oils, waxes, resins, gums, and cellulosic bodies, and are used as varnishes
or mediums for paints.

Solvents, classification. A chemical basis serves best to afford an arrangement
of the various useful solvents in painting and restoration. Most of them are

Solvents and Diluents

201

definite chemical compounds and the larger part, being compounds of carbon,
are organic. Formerly, of course, most of them had a natural origin, but now far
the greater number comes from synthetic manufacture. Industrially, the petro¬
leum hydrocarbons form the most important class and have a definite place in the
practice of painting. These are aliphatic or paraffin hydrocarbons derived from
crude petroleum, a complex mixture containing hydrogen and carbon. Chemically,
they are known as straight-chain hydrocarbons because the carbon atoms of each
molecule appear to be linked in a line by a single bond. This homologous series
of solvents has the general formula, C„H2„+2. They begin with the simple gas
methane, CH4, and extend to compounds that have thirty or more carbon atoms
which are hard waxes. The lower homologues with five to ten carbon atoms are
most used as solvents. Chemically, they are not active, but have a good solvent
action on fluid oils, fats, bitumen, rubber, and wax, as well as on a few resins
such as dammar. They are flammable and form explosive mixtures with air,
but are not highly toxic.

Coal-tar hydrocarbons, another class of solvents, are those derived from the
destructive distillation of coal-tar, a by-product of coke and coal gas. Aromatic
and benzene hydrocarbons are other names given to them. They contain the ben¬
zene ring (see Benzene) and that material is the simplest member of the group.
Toluene and xylene are derivatives. Like the petroleum hydrocarbons, they are
hydrophobic but, chemically, are more active because of the three double bonds
in the benzene ring. Their general solvent action is stronger and they affect most
resins and certain cellulose derivatives, as well as the other materials which the
petroleum hydrocarbons dissolve. The coal-tar group is flammable and generally
toxic.

Chlorinated hydrocarbons make a further class, those obtained by introducing
chlorine into the hydrocarbon molecule. It goes into coal-tar hydrocarbons through
the double bond and into petroleum hydrocarbons by replacement of hydrogen.
Perhaps the most familiar in this class are carbon tetrachloride, chloroform, and
ethylene dichloride. Their solvent power is somewhat stronger than that of the
other hydrocarbons. Like them, they are hydrophobic but, unlike them, are non¬
flammable or of very low flammability. They have, however, injurious physio¬
logical effects when used in large quantities. Under ordinary conditions, most of
them are stable, but a few decompose and liberate hydrochloric acid in the
presence of light or moisture. They have a high specific gravity.

Turpentine hydrocarbons are known almost entirely through the one product
familiar in painting practice. Turpentine is a mixture of various aromatic hydro¬
carbons known as terpenes, and all of them have a ring structure and the em¬
pirical formula, CioHie. They differ accordifig to internal grouping of the atoms.
Composition varies with differences in the species of pine from which the original
balsam comes. Turpentine is fairly active chemically because of its unsaturation,
and has a moderate solvent action, taking effect on fats, soft resins, and liquid

TABLE I. PHYSICAL PROPERTIES OF ORG.

Painting Materials

Oa)<U(l>CD<L)(U<U

flcjcccidcjcj

oooooooo

<0 <U (U <U d

§§§§“§

•z Z 'Z. Z CO U

P , eg CS eg ±J

S ’4J *i3 'Jp -d

^ jax)

o eg eg eg -p

(J P-i Oh Ah CO

S

oococoooEocococo cocococoEoU

OO VO O
J>. P'* CO

9 9 9 ^

rk lloocooo ChOO

vor^l^c)© d d d

d d d

1 >- OO CT\ CS

'-o v-n o^ d ^ 0»

CO CO t{- mD d t>~

1> 4. Pp--v

jU d CO

n ■ eg eg eg 4_j

rj ’PJ ‘jp ‘pj JUi

^ u u u bD

O eg eg eg “p

CJ Oh Oh Oh GO

00000

00 o o

\T r i w I “ “ ”

V 6 6 V «

tlX) b£)

«Kcj
S W A

P C /5 F* o
> « £4^

eg

to CO

4- d o W Vn 00

'o cr\vo On

2 H « o.

3 9 0

Oi VO I

CO q 6

M d

CO P^ M W CO
00 p^ p^ p^vd
P^ w VO P>>. cs

U O 3 s
O U Q U U

2 W D W III

o u u u u u

ffiffi ffi
u u u

gg888

Q 4 u u u

^ ^ £5 K {5

u u u u 0

I .i >§ § ^ "g c d « td ^ ^

; Is gj i2 |o

Solvents and Diluents

203

TABLE II. SOLUBILITY OF FILM SUBSTANCES IN ORGANIC SOLVENTS AND DILUENTS ^

204

Painting Materials

xi3A\ mjjBJUc-];

1

1

1

1

!

i

1

i 1

CO

1

1 s

CO

CO

1

hh

XBAA UUdUf

CO

1

1

CO

1

CO

CO

CO 1

CO

1

1 ^

CO

CO

CO

1

«

XBA\ BqnUUJ'BQ

CO

CO

A

1

CO

CO

1

T

T

1 r

CO

CO

CO

1

CO

CO

CO

CO

1

CO

CO

xBAisaDg

1

1

CO

CO

1

1

1

1 1

CO

CO

CO

1 1

CO

PS

1

CO

CO

joqqn.!

p3;uiiLio|q3

-

1 — 1

-

1 —(

CO

CO

CO 1

-

1 CO

CO

CO

l-H

1—1

isqqn'a

CO

CO

CO

CO

CO

CO

CO

CO CO

CO

i

1 CO

CO

CO

1

1

jamApcI atjuiAio
-uqtjsui p'C:jna;-2i

1

CO

A

T

T

CO

CO

CO 1

CO

1

1 CO

CO

CO

-

PS

jauiAjocI aiEjAJD

1

-

1

1

1

CO

CO

CO 1

-

1

1 CO

CO

CO

t—l

jomAfod 3 :}ej;Ajd
- i3q?3tu lAq^ajAj

1

-

1

1

1

CO

CO

PS

-

1

1 ^

-

CO

t—l

-

95'G:}3DB ptoiAiCpcj

1—(

-

HH

-

CO

CO

CO 1

HH

«

1 1

T

CO

CO

CO

a:}Dj:)iu ssopqDQ

-

-

1 — 1

HH

>-H

HH

-

hh

-

PS

HH

3SO[nipD l-'Cqia

-

HH

-

CO

CO

CO [

CO

T

1 CO

CO

CO

CO

CO

a;-B.!A;nq
-0430B DSOTnqSQ

t—l

HH

-

t—j

-

(-H

CO

Ph

PSI

1 ^

-

CO

CO

aiB:j3D'G asopqp3

-

l-H

1—-<

-

t—(

1—1

h-l

HH

HH

HH HH

t-H

1

DiquiB uin0

i

1

A

A

1

T

1

1 1

1

A

1 1

T

t—l

1

CO

sSuEio ‘DUipqs

1

HH

A

A

1

H-l

A

1 1

t—l

A

[ t—l

CO

Dh

CO

CO

CO

CO

au|;uaclin^

CO

1—]

1

CO

A

A

CO

A

1 1

CO

1

1 CO

A

CO

CO

CO

OqiBOBi'BX/SJ
‘musiEq i3qred[03

1

1

PS

A

A

CO

1

1 1

CO

1

1 CO

A

A

i

CO

(Auoqdopo) UTSo-^

CO

hJ

1

A

A

A

CO

A

1 i

CO

A

1 CO

A

A

CO

CO

uisai luiajTj;

1—1

h—I

H-t

)--<

CO

CO

s 1

LS

1

1

t—1

t—<

CO

CO

CO

Cn

CO

CO

UlSaJ OBiEpUBS

►-H

-

t—t

3

1—1

1—<

l-H

l-H 1—(

t-H

1

t—l

l-H

CO

CO

CO

hP

CO

uisai DpSBl\[

[

1-H

-

h—j

«

CO

CO

S

LS

LS

A

LS

CO

Oi

CO

CO

CO

aiodBSuis
‘UISDJ JBmiUBQ

1

A

A

A

A

T

T

1 1

1

A

1 1

1

CO

1—1

PS

BiAB:}Ba;
‘uisaa JBi’atuBQ

1 , CO

CO

CO

CO

CO

CO

CO

CO CO

CO

A

1 w

CO

CO

PS

PS

OSUOQ

'uisaj xBdoQ

H-H

t—<

H-J

t—»

3

-

hH h—J

-

1 —i

HH HH

hk

CO

CO

CO

CO

(po paasuix |
poup) nAxcuiq i

1

T

A

3

A

-

1 1

-

A

! 1

A

CO

Oh

CO

Ph

PS

(pnrj) po pa3smq[

1

CO

1

1

1

CO

1

1 1

CO

1

1 CO

CO

CO

-

Petroleum ether
Gasoline

Petroleum naphtha

(petroleum spirits)

V, M. and P, naph-

tha

Mineral spirits

Benzene

Toluene

Xylene

Tetralin

Turpentincj gum
spirits

Turpentine, steam

distilled

Dipentene

Chloroform

Carbon tetra-

chloride

Ethylene dichloride

Methyl alcohol

Ethyl alcohol, an¬
hydrous

Solvents and Diluents

I

1

i

1

1

j

SS

j

1

SS

1

1

1

1

1

!

Mi!

1

-

i

-

-

1

SS

1

1

-

j

CP'

X

1-H

i

1

Mil

i

!

1

1

1

i

1

!

SS

-

I

1

SS

SS

1

1

i

1

1

MM

j

1

!

1

1

1

I

!

SS

1

i

CP'

SS

1

j

1

SS

!

1

MM 1

1

-

CP

CP

CP

CP

cn

an

P

CP

'CP

p

Oh

1

an

'X

X

X

X

i

i ,M !

i

1

i

1

i

1

!

«

1

1

i

t

!

!

1

1

!

1

1

1

1 M i

X

!

1

CP

1

i

CP

CP

i

!

I

CP

CP

1

X

1

!

i

1

-Mi

1

1

i

CP

1

i

CP

CP

i

1

!

'CP

CP

!

X

!

1

1

-II!

1

1

1

CP

1

i

CP

CP

I

1

!

1—4

CP

1

1

X

!

!

1

1—! i ! j

!

1

C/2

ff-(

I

CP

CP

i

!

CP

CP

CP'

CP

1—4

!

i

X

X

X

MM

i

1

CP

CP

CP

CP

CP

CP

CP

CP

CP

CP

1

X

X

X

X

!

Mil

i

CP

CP

CP

CP

'CP

CP

CP

PS

CP

-

CP

CP

X

X

X

X

i

1 r M

1

GO

1

CP

-

-

-

■CP

CP

-

CP

Ph

i

CP

-

1—I

X

Oh

1

MM

i

)—4

CP

CP

-

H-)

CP

-

CP

H-4

CP

X

-

t-H

-

1—4

1 - 1 1

i

t—1

1

CP

(-H

CP

1

1

1

1

HH

CP

1

1

X

1

SS

1

MM

i

CP

PS

CP

-

SS

1

CP

Oh

i

-

CP

SS

SS

X

X

X

x

1

MM

1

i

CP

1

1

1

CP

1

1

T

CP

1

T

!

1

T

1

i

MM

X

1

1

CP

Oh

1

1

i

1

1

CP

!

1

T

1

1

T

MM i

X

1

_i_

T

_[

1

I

CP

1

1

1

CP

1

i

1

1

1

MM

X

CP

CP

CP

CP

CP

1

Oh

-

-

SS

CP

CP

X

h4

X X

>-4 1-4

SS

1

MM

X

1-1

CP

t-H

CP

-

CP

1

CP

CP

CP

CP

CP

o<

CP

X

X

X

X

1

MM

H-4

CP

CP

CP

CP

CP

1

cp

1

T

CP

CP

CP

X

X

X

X

j_

MM

SS

CP CP

P-,

PS

LS

1

i

PS

1

1

H-4

CP

LS

PS

!

X

Oh

X

Oh

1

MM

1

PS

CP

04

CP

CP

CP

CP

CP

Ch

CP

CP

Oh

1

1

(--»

CP

CP

(—3

PS

X

Oh

X

Oh

PS

1

MM

X

CP

!d(

CP

Oh

SS

CP

CP

Oh

1

1

CP

CP

04

«

-

«

1—H

1

j *—t >“4 HH

PS

PS

1

1

j_

CP

Oh

1

1

1

1—4

CP

04

i

1—4

1

1

i

1 X X X

i

CP

CP

CP

CP

CP

1

CP

CP

CP

CP

CP

CP

-

X

X

X

SS

Mil

X

w-Butyl alcohol

Methyl acetate
Ethyl acetate

w-Butyl acetate

Amyl acetate

Butyl lactate
Acetone

Methyl ethyl ketone
Methyl isobutyl

ketone

Diacetone alcohol
Ether

Dioxane

Methyl cellosolve
Cellosolve

Butyl cellosolve
Cellosolve acetate

Ethylene glycol
Glycerol

Pyridine

Morpholine

Triethanolamine

Carbon disulphide

^ -a

fcJO

c/2

C/2

o o

-2 ’T 3
O «
OS cs

2 w

yT <ci>

i s

"I ^

- rt

ti3

zi
2

.2
*5

S

^ C! «D
B rt

S:f I

t 3

Rj

I

o S
e a

g 5

T3
Qh CW

OQ

O

o s

CO g

d ’

3 o '

B

■*-' CD ^

d ^
rt 'p! I

1 )

rt

' tsfl

> 3
: o
; ^

1^

-3^-2

Si) 2 '

d d '
S o
11 p ^

(D -r;

S W 3 ttf

« ■ .S *-'
cT >^*3. S

—' cs .t! —

-5 ;.-d

*0 *3

.5 a

S o

1

I'.f 8 !

3 c Ti

*0 B **-

CO cS CO O
'* *5 .2 d

,o " d

c/2

•TP

cS

G

u

"2 ®3 '

2 J

<u

o

<U Jd

H -3

1-4 CO

' JD

, 3

*- O

^ 2 I

d ^
d

C3

^d —

52 S

m.2 B

*« rt 3

b S

.t, J-. TJ

B d F

X rt 8

^ CO

•d ^3

d .2

'o -2

«> cd oj
O a Ja{

8 o

4J

P tH

b <cj

2 s

d

5 <u
I -G d

3 *-?-!

d ^ (U

CO G
<u d

3 OJ

o

X b p
d 2 -G
2

^ d
G «

rS

jd ^

G 33
3 d

2 <

■♦-> dt

’C w

3 «U

?3

-g' g
, 3

d ^

2o6

Paintin-g Materials

oils. It is flammable and, although irritating when breathed, is not particularly
toxic.

Alcohols contain the hydroxyl group, -OH. Because of this group, they are
strongly polar, fairly active chemically, and may be considered as organic bases.
The most common member of the group is ethyl alcohol (ethanol), and its homo-
logues carry names with a similar ending—methanol, propanol, and others. The
latter, synonyms for methyl and propyl alcohol, get their names usually because
they are synthetic products. Lower alcohols are water-miscible in all proportions.
They act as solvents on resins and some synthetic materials but poorly on fats,
waxes, and true gums. Higher alcohols have low miscibility with water. Vapors
of the lower alcohols are flammable and explosive. They irritate the mucous
membrane but, with the exception of methyl alcohol, are not toxic.

Esters are considered to be reaction products of organic bases (alcohols) and
organic acids. An example is Ethyl Acetate derived from ethyl alcohol and acetic
acid. As a group they are noticeable for a sweet and rather fruity odor. They are
weakly polar and only the lower esters are hydrophyllic; these may hydrolyze
back to alcohol and acid. The higher esters, however, are not easily decomposed.
Esters of high molecular weight are much used as plasticizers.

Ketones are distinguished by the presence of the carbonyl group, = CO. The
lower ketones are neutral and highly volatile, and only the lowest are hydrophyllic.
The higher members are solids. The most important of the lower ketones is acetone
or dimethyl ketone, a strong solvent for natural resins, synthetic resins, or cellu¬
lose derivatives, and miscible with most other organic solvents and diluents.

Glycol ethers are derived from the dibasic alcohol, glycol, by replacement of a
hydroxyl group with an ether group. The most important of this group, all of
which are of fairly recent development, is the so-called cellosolve ’—ethylene
glycol monoethyl ether.

Polyhydric aliphatic alcohols differ from common alcohols in having more ‘
than one hydroxyl group. Ethylene glycol or glycol is dihydroxy alcohol and lies
between ethyl alcohol and glycerine. This glycerine or glycerol, in turn, is tri¬
hydroxy alcohol. These are similar to mono-alcohols In their chemical properties
but have higher viscosity and lower volatility. They become more hygroscopic in
proportion to the number of hydroxyl groups they contain.

Ethers are organic compounds in which two hydrocarbon radicals are joined
by an oxygen atom. Being very slightly polar, they have good solvent action on
waxes and fats.

Amines contain the —‘NH2 group and form derivatives with many organic
compounds. A few of these have special uses in connection with organic solvents.
The amino derivatives of the polybasic alcohols, particularly triethanolamine, are
useful emulsifying agents. Pyridine and morpholine have special uses as solvents
for linoxyn, dried linseed oil.

Solvents and Diluents

207

Solvents, evaporation rate. The volatile part of a paint or varnish film is
necessary at the time of application and largely escapes or evaporates after the
film is spread. This escape is a movement of molecules through the surface and

Figure i. Evaporation rate curves of fast to intermediate liquids, adapted from those
of A. K. Doolittle, ‘Lacquer Solvents in Commercial Use/ Industrial and Engineering
Chemistryy XXVII (1935)5 P- fig* 3* According to number, these are as follows:

1. Methyl acetate

2. Acetone

3. Benzene

4. Ethylene dichloride

5. Ethyl acetate

6. Methanol (anhydrous)

13. Amyl

7. Methyl ethyl ketone

8. Ethanol (anhydrous)

9. Petroleum naphtha

10. Toluene

11. ?^-Buty! acetate

12. Xylene
(mixed isomers)

into the surrounding air where they mix with the gaseous components of the air
and become part of them. Escaping molecules exert a pressure called ‘ vapor
pressure.’ The rate of escape differs greatly in different liquids. To some extent.

2 o8

Painting Materials

this is regulated by outside factors^ including temperature, atmospheric pressure
humidity, ventilation, and amount of liquid surface exposed. Internally there are
definite factors also—vapor tension, surface tension, latent heat of evaporation,

Figure 2 . Evaporation rate curves of intermediate to slow liquids, adapted from those
of A. ^K. Doolittle, Lacquer Solvents in Commercial Use,’ Industrial and Engineering
Chemistryy XXVII (1935), P* 117O5 fig* According to number, these are as follows;

I. Methyl cellosolve

7. Cellosolve acetate

a. Steam distilled turpentine

8. V. M. and P. naphtha

3, 72-Butanol

9. Mineral spirits

4. Gum spirits turpentine

10. Di ace tone alcohol

5. Amyl alcohol (mixed isomers)

II. Butyl cellosolve

6. Cellosolve

12. Butyl lactate

specific heat, dissolved impurities, and a few others. Although vapor pressure and
evaporation rate are closely related, they are not the same. Vapor pressure is the
pressure (expressed in millimeters of mercury) when a liquid and its vapor are in
equilibrium in a closed system at a definite temperature. Vapor molecules in air

Solvents and Diluents

209

exert a partial pressure and contribute to the total pressure of the atmosphere they
pervade. When the %^apor pressure in the air in a closed space above a liquid be¬
comes a maximum, the air is saturated in respect to the liquid. When the tem¬
perature of a liquid is raised, its vapor pressure increases and when it equals the
pressure of the atmosphere, the liquid boils. Evaporation rate is the rate at which
vapor evaporates freely and continuously at the surface of liquid in an open system
under a specified set of conditions. These conditions include surface exposed, rate
of air-flow over the surface, temperature, and other factors. Unlike vapor pres¬
sures, evaporation rates are not readily capable of numerical expression but are
best shown by curves like those in figures i and 2. The simplest method of reaching
these determinations is ^gravimetric, the loss of weight of a fixed quantity of
solvent under fixed conditions being measured at regular intervals (see Gardner,
pp. 318-322). Evaporation rates of solvents are not in the same order as their
boiling points. Compounds that contain hydroxyl groups evaporate more slowly
than compounds of the same boiling point not containing hydroxyl; for example,
alcohols evaporate more slowly than their esters of higher boiling point (see
Hofmann, p. 135). Chemical structure and types of molecular aggregates are
influential.

When solvents are mixed together they behave somewhat inconsistently. If
they are members of a homologous series, as would be the case with toluene and
xylene, they^ evaporate at different rates simultaneously and do not affect each
other. Certain liquids, however, may form constant evaporating mixtures. The
solvent with the higher rate evaporates rapidly until, in a binary mixture, a
certain proportion of the two is reached,-and from that point on they evaporate
together at a constant rate. Such a mixture is apt to occur with dissimilar com¬
pounds ^like that of an alcohol and a hydrocarbon. Hofmann gives the relative
proportions of certain constant evaporating mixtures as follows; benzene 47,
ethyl acetate 53> toluene 45, ethyl alcohol 555 there are also a few ternary mixtures
which strike a constant evaporating level. In the application of varnishes and
lacquers, rapidly evaporating solvents and diluents are not usually desired, for
they cause chilling at the surface with possible precipitation of moisture and a
blush in the film.

Solvents, flammability. When mixed with air, the vapors of most organic
solvents, diluents, and thinners are explosive. In general, the degree of flamma¬
bility of hydrocarbons and compounds of carbon, hydrogen, and oxygen is in¬
versely proportional to the boiling points and the vapor pressures. Those con¬
taining sulphur are highly flammable; those with chlorine, much less. Unsaturated
compounds are generally more flammable than saturated compounds. The meas¬
ure of flammability is the flash point—the lowest temperature at which a liquid
gives off enough vapor to be ignited. At this temperature the vapor molecules
are so concentrated that they can unite with oxygen, and combustion becomes
self-propellant. This point is determined by a special test and several types of

210

Painting Materials

apparatus are made for testing. The point is usually designated in degrees
Fahrenheit and a liquid is supposed to be flammable when the flash point is below
I5 o®F. Those which flash below qo^F. are exceedingly flammable and dangerous.
When solvents are mixed, the flash point of the mixture is not necessarily the
average of the points of the components. There may be uneven evaporation or
constant boiling mixtures may be formed. A common addition to the flammable
solvents is that of carbon tetrachloride, and this, in an adequate quantity, seems
to be effective. Ethylene dichloride is used in the same way. The explosive power
of solvent vapor depends on temperature and on the proportion of this vapor to
the air. The lower explosive limit is the smallest amount of solvent vapor according
to volume per cent which will allow the propagation of the flame. An upper
explosive limit is reached when the solvent vapor exceeds the quantity which has
oxygen enough to keep up a fire. The danger of flammability can be much reduced
by careful storage. Frequent inspection of containers to guard against leaks, the
use of tin or lead-lined cans for all organic solvents, and metal cases remote from
any excess heat, from sparks, or from flame will go a long way to prevent accidents.
If film materials conveyed by flammable solvents are sprayed mechanically, there
is an attendant fire hazard unless good ventilation is provided. In particular,
sparks from motors and from electrical connections have to be guarded against.
An open flame should not be used near containers of flammable solvents. It is
particularly dangerous to distill or heat organic solvents in a container, especially
a glass container, over a free flame. If the container breaks, there is nothing to
prevent instantaneous and violent explosion. Such heating, when necessary,
should be done over a steam bath or by properly insulated electrical equipment.

Solvents, miscibility. This is the capacity of a fluid to mix with another.
Interfacial tension between the molecules seems to be the determining factor.
When the difference in tension is small, the two go together easily, but when it is
great, the two are drawn into separate layers. In general, solvents that have a
similar chemical constitution and properties are miscible with each other. Lower
members of a homologous series are more widely miscible than those of higher
molecular weight. The greater part of organic solvents is immiscible with water,
exceptions being the lower alcohols and ketones. Acetic esters and higher alcohols,
butyl and amyl for example, are limited in their ability to mix with water but go
well with the hydrocarbons. Acetone is the only solvent which will mix with water
in all proportions and with all other organic liquids. Frequently two immiscible
fluids can be made to go together by the addition of a small amount of a third.
Butyl alcohol, for instance, will make a mixture out of petroleum spirit and ethyl
alcohol. Solvents with this capacitv are called ' mutual solvents ’ or ' coupling
agents."

Solvents, toxicity. Almost any materials used as solvents or diluents in the art
of painting are poisonous if taken into the human body in large amounts. There is,
however, a great variety among their toxic affects, and the risk of their being
drunk as a fluid is slight. The greatest exposure is to their vapors and, in general.

Solvents and Diluents

2 II

the more volatile among them are the more toxic. Acetone^ alcoholj ether^ and
chloroform are narcotics. They affect the nerve centers causing stupefaction^
intoxication, and ultimately anaesthesia. The coal-tar hydrocarbon, benzene
or benzol, has a direct physiological effect on blood cells, causing leucopenia and
anaemia. Excessive use of this material seems permanently to injure bone marrow
where the cells originate. This poisoning can reach an acute stage resulting in
death. The National Safety Council has set as a safety limit for the concentration
of benzene vapor loo parts of benzene per i million parts of air. Toluene and
xylene are less toxic, but relatively impure grades of toluene may contain benzene.
The vapors of chlorinated aliphatic hydrocarbons—particularly trichlorethylene,
tetrachlorethane, and, to some extent, carbon tetrachloride and ethylene dichlo¬
ride—affect the central nervous system and damage the liver and kidneys.
Petroleum hydrocarbons are far less injurious. Exposure to concentrated vapors
of them may produce dizziness and nausea. Carbon disulphide has an effect on
the central nervous system. The toxicity of methyl or wood alcohol when taken
as a fluid is well known. It injures the central nervous system, but moderate ex¬
posure to^the vapors has not been shown to be seriously harmful. Most operations
m the painting and restoration of pictures do not involve high concentrations of
any of these vapors. Spraying of varnishes, particularly on large areas, is an ex¬
ception, as is the use of a solvent in the removal of such film materials, especially
where the operator is working with magnifying lenses and is close to the evaporat¬
ing surface. In all cases, good ventilation should be provided and work should be
stopped immediately at any sign of physical irritation, headache, or drowsiness.

The National Safety Council supplies the following information with regard
to the toxicity of some of the common solvents. The figures after the solvent are
the maximum allowable concentrations of solvent vapor in parts per million of air.

SOLVENT CONCENTRATION

(p.p.m.)

Amyl acetate 400

Butyl acetate 400

Butyl alcohol 100

Methyl alcohol 100

Benzene 100

Benzol

Spike Oil (see Oil of Spike Lavender).

Spirits. Specifically, this term is used for volatile substances dissolved in
alcohol, but in older usage it was applied to any distilled volatile liquid. It is still
commonly given to ethereal essential oils and distillates—spirits of turpentine,
petroleum spirits, Cologne spirits, and wine spirits. Methyl alcohol is still occa¬
sionally called ‘ wood spirits.’

SOLVENT

Carbon disulphide
Carbon tetrachloride
Ethylene dichloride
Ethyl ether

Turpentine (American)
Xylol

CONCENTRATION

(p.p.m.)

15

100

100

400

200

200

212

Painting Materials

Surface-Active Agent (wetting agents detergent, penetrant) is a compound
which reduces interfacial tension at the boundaries between gases, liquids, and
solids; it especially promotes wetting and penetration of liquids into solids and
serves as a detergent, emulsifying, and dispersing agent. The oldest type of surface-
active agent is a saponified vegetable oil, a soap, but in recent years many special
compounds have been developed. These include principally fatty alcohol sulphates
and sulphonated higher fatty acid esters, ethers, and amides. The latter com¬
pounds have greater adaptability than soaps in respect to the physical conditions
under which they are used. They have high wetting power even in low concen¬
tration, are stable in dry form and in solution, and are soluble both in water and
in organic solutions. They are not precipitated by hardness in water. They are
polar compounds and their activity is dependent upon the ability of their mole¬
cules to become oriented and adsorbed at an interface (see a series of papers on
* Surface-Active Agents,’ Industrial and Engineering Chemistry^ XXXI [1939],
pp. 31-69). Like soap they can be used to remove grime and greasy deposits from
paint, but there is always a risk from the action of water and possibly from
residues of the agents themselves.

Tetralin (tetrahydronaphthalene [C10H12]) is a colorless fluid which boils at
2o5°C. It is derived from the hydrogenation of naphthalene and is a good solvent
for resins, oils, fats, and waxes. Moreover, it is said to have a strong solvent effect
on linoxyn, the dried, oxidized film of linseed oil. In paint and varnish removers
it is a common ingredient. Discoloration from standing is one of its drawbacks.

Thinner, a solvent or diluent or a mixture of both used to reduce a film material
to suitable brushing or spraying consistency. The common thinners for oil paint
are turpentine or petroleum distillates. Strictly, a thinner is a Diluent but, be¬
cause the difference between solvent and diluent exists only in relation to particu¬
lar film-forming solids, this more general term is used for either.

Toluene (toluol, methyl benzene [CeHs-CHs]) is derived from coal-tar by
fractional distillation of commercial benzol. A clear, colorless liquid, it resembles
Benzene but boils at a higher temperature, iio°C., and is less volatile. It is
miscible with most other organic solvents and is said to be the most widely used
hydrocarbon diluent for commercial cellulose lacquers. For resins and for cellulose
ethers, it has a considerable solvent power and a higher dilution ratio than the
petroleum hydrocarbons. Its vapor pressure permits rapid drying without blush.
It is low in cost and, unlike benzene, is not dangerously toxic. The less pure form
of toluene is called toluol.

Toluol is a commercial or industrial grade of Toluene.

Tricresyl Phosphate ([CH3C6H4]3P04) is generally used as a plasticizer in
commercial lacquers, though it is a moderately good solvent for some resins,
including mastic. It has a high boiling point, 275° to 28o°C., and a high flash
point, 23o°C. At a5^C. it is o.a per cent soluble in water.

Triethanolamine ([CH2CH20H][3N). Though extremely active as a solvent for
oils and fats, this is best known to painters and restorers as an emulsifying agent.

Solvents and Diluents

213

It IS a straw-colored, viscous liquid, with a faint odor of ammonia, and in some
respects it combines the properties of that material and glycerine. It is highly
hygroscopic and miscible with water in all proportions. It mixes with alcohol but
not with petroleum or coal-tar hydrocarbons. Like ammonia, it combines with
acids and acidic materials, forming soaps of low alkalinity with fat. It is less
alkaline, however, than ammonia, the pH of a 25 per cent solution being 11.2
at 25°C. Small additions of triethanolamine can produce stable water emulsions
of various oils and waxes. Usually the amount of triethanolamine is 2 to 4 per cent
of the oil or fat to be emulsified. Casein is colloidally dispersed in water by a 5 to
15 per cent solution of triethanolamine.

Turpentine. The name {terebinthos [^Gr.], teribenthine j^Fr.]) was originally
applied to the crude exudation or balsam from various species of pine, and is still
given to certain kinds like Venice turpentine from the European larch, and Stras¬
bourg turpentine from the European white fir. In the United States, however, the
word is commonly used, only for the volatile liquid obtained by the destructive
distillation of an oleoresin. About two thirds of the world’s supply comes from the
long-leaf pine that is grown in Georgia and the Carolinas, though part is from
other closely related species. A fair amount is from the Maritime pine cultivated
along the southwest coast of France. American production began at the close of
the XVI century. The French did not start until the middle of the XIX century,
but, until recently, their production methods were more efficient. Small amounts
of turpentine come from Spain, Greece, and other countries.

In the usual method of collecting the resinous exudate, the tree is chipped
near the base, gutters are attached, and the material is caught in earthenware
cups. (For details, see T. Hedley Barry, Natural Varnish Resins [^London:
Ernest Benn Ltd, 1932], pp. 145-199.) The crude oleoresin (French) yields 18
per cent turpentine, 75 per cent dry resin, 10 per cent water, and 2 per cent solid
impurities.^ It is put into copper stills together with water in order to effect
steam distillation, the water being later separated by gravity. With methods
developed during recent years, the yield from the crude material is 15 to 25 per
cent by weight. The molten residue is rosin or colophony. Most of the distillate is
sold to the paint and varnish trade after a simple purification by redistillation
(rectification) under the name, ‘pure gum spirits of turpentine,’ or ‘oil of tur-
pentine.’

The distilled turpentine is a mixture mainly of various and closely related
aromatic hydrocarbons known as terpenes^ all of which have the empirical formula,

CioHis. The composition includes a-pinene, ^S-pinene, dipentene, terpinene,
borneol, fenchyl alcohol, limonene, and traces of camphor. The American turpen¬
tine is largely a- or dextro-pinene, and the French is j3- or laevo-pinene.

^ When fresh and pure, turpentine is a clear, volatile, flammable liquid which
boils in the range of 150 to i8o°C. Any marked deviation from this range indicates
impurity, and the Federal Specifications Board specifications require that 90

214

Painting Materials

per cent should be distilled over below i7o‘"C. When spread^ it should evaporate
almost completely, and when poured on clean filter paper should leave no appre¬
ciable residue after a half hour. The normally moderate and uniform rate of
evaporation makes turpentine a good thinner for paint and varnish, and it seems
to give paint a particularly acceptable handling quality under the brush.

The components are active chemically. Being unsaturated compounds, they
absorb oxygen and produce non-volatile, resinous substances. Apparently, also,
some polymerization reactions take place. These, of course, occur only in the
presence of air and are particularly noticeable in a light and warm location.
Poorly sealed containers will allow turpentine to thicken into a sticky substance
which can not longer be used as a safe thinner for paints. If the distillate is pure, it
probably has on paint a thinning action and no other. There has been some belief
that through the formation of peroxides it might act as an oxygen carrier and,
hence, promote the drying of oil films, but any siccative action of this kind is
quite uncertain. Heaton (p. 62) says that there is enough oxidizing action and
enough acidity in turpentine to have a destructive effect on fabrics, and he sug¬
gests that its use in connection with artists’ canvases should be avoided where
the maximum durability is essential.

All vegetable and mineral oils in a fluid state are miscible with turpentine,
and it dissolves most resins, except those of a fossil origin, and most waxes. It is
immiscible with water, but mixes in most proportions with nearly all organic
solvents. It is not toxic or narcotic, but prolonged exposure to its vapors may
cause headache and sickness, usually temporary, to some persons.

A variety of turpentine, produced chiefly in America, is made by steam dis¬
tillation of pinewood logs and stumps. After such wood is first reduced to fine
chips or powder, there is an initial distillation to extract turpentine and pine oil.
The wood is then digested with petroleum spirit to extract the resin. It is said that
the yield of turpentine from this source is between four and eight gallons per ton
of the wood. Wood turpentine, as it is called, produced on a large industrial scale,
is more standardized and uniform than gum spirits. It is similar to the oleoresin
product, the main difference being that it contains a greater proportion of dipen-
tene, limonene, and terpinene. In consequence, it has a greater solvent strength.
The odor is distinct from that of ordinary turpentine. Except for this, for slightly
different physical properties, and for a narrower distillation range, there is not
much distinction between it and the turpentine from gum spirits. That made by
destructive (dry) distillation of pine wood is an inferior product.

The history of turpentine is a long one. In crude form, at least, it appears to
have been known in classical times or earlier. Greek writers tell about a material
obtained from coniferous woods and juniper, and Pliny (XV, 7) descrihes pissinumy
made by boiling pitch from trees and catching the vapors in a fleece spread over
the tops of the vessels. A yellow fluid was wrung out. He also mentions (XVI, 22)
a tarry substance from the destructive distillation of coniferous wood, and that

Solvents and Diluents

215

may be the ‘ pitch from trees.’ The distillate, a complex substance containing
terpenes, creosote, acetone, and phenolic bodies, is now sometimes called ‘ tar
spirit.’ It would have good solvent properties but a doubtful value as a thinner or
diluent for resin varnish (see Laurie, Greek and Roman Methods of Painting
[Cambridge, 1910], pp.^27-33). Other references in Pliny, however (XIV, 25*
XVI, 21 and 23c and XXIV, 22), leave little doubt that the distilled volatile
component of coniferous wood or of oleoresins was known and used in ancient
times. (A more complete discussion may be found in A. Lucas, ‘ Cedar-Tree
Products Employed in Mummification,’ Journal of Egyptian Archaeology, XVII
L^9j^ 1^PP* Kenneth C. Bailey, The Rider Plwy^s Chapters on Chemu

cal Subjects [London: Edward Arnold and Co., 1932], II, 238-239.)

In^spite of the evidence about the knowledge of turpentine in classical times,
there is nothing to indicate that it was used with paints. In the early part of the
mediaeval penod, however, when the art of distillation had been perfected and
when turpentine was a more familiar product, it may well have been common as a
punter s material. Leonardo da Vinci in the late renaissance refers to it occa¬
sionally, chiefly in connection with the preparation of ‘ Greek fire.’ That there
may be confusion in his writings between the crude oleoresin and the spirit is
possible, but in one place he definitely speaks about a turpentine of the second
distilling (Edward MacCurdy, The Notebooks of Leonardo da Vinci [New York-
R^eynal and Hitchcock, 1938], II, 196). Berger (I, 228n, and III, i6on) says that
there IS no information about when the distilled product was first used in the art
of painting, though he mentions a description made by Marcus Graecus which
shows that distillation was familiar in the VIII century. By the XVI century
numerous references show that distilled turpentine was a regular ingredient of
varnishes (Charles L. Eastlake, Materials for a History of Oil Painting, 2 vols
[London, 1847-18693, 1 , 291, 301, and 313; Berger, IV, 191, 193).

V. M. and P. Naphtha (Varnish Makers and Painters Naphtha) is a petroleum
distillate with a boiling range, 100° to i6o“C., between those of gasoline and kero¬
sene. It IS a common turpentine substitute and is widely used as a diluent and
thinner for oil paint. (See also Petroleum Thinner.)

Water. This, the common solvent or diluent for aqueous mediums, glues, gums,
egg white, casein, and others, the diluent for emulsions, like yolk of egg, the
solvent for dyes and for inorganic salts, is without doubt the most generally used
of any single material in the arts. Chemically, it is considered to be inert and
stable. Most substances that are dissolved in it can be recovered in their original
^ate after evaporation. To a small extent, however, it does dissociate, forming
H and OH ions, and these ions are capable of entering into a type of chemical
reaction called hydrolysis or hydrolytic dissociation. Many organic materials,
the lower organic esters like ethyl acetate, for example, are readily hydrolyzed
by water with the formation of some acetic acid. Moreover, water serves as a
catalyst for many chemical reactions, and the efi^ects of many gases depend on at

2i6

Painting Materials

least a trace of moisture. Changes in the chemical nature of lime plaster in the
process of fresco painting require water.

Water has a higher heat capacity than any solid or than any other liquid at
ordinary temperatures and pressures. In other words, more heat is required to
raise the temperature of a given mass of water by a given amount than for any
other substance. Water has also the highest latent heat of fusion (8o calories)
and vaporization (536 calories). Heat conduction is less than that of metals but
is higher than that of other liquids and non-metallic solids. Its maximum density
is at 4°C. It expands when cooled beyond this point to zero and further on solidi¬
fication. Surface tension, 73 dynes, is higher than that of any other liquid except
mercury. Because of this many surfaces are hard to wet with water.

In relation to the various film materials and adhesives used in the art of paint-
ing, water may have a solvent action, a swelling action, or practically no action
at all. The waxes and resins are little affected by it. Certain materials like the true
gums, egg white, and fish glue are soluble in cold water; the water causes complete
dispersion (peptization) of the material into particles of colloidal dimensions.
Skin and bone glues and, also, starches are swelled by cold water to form hydro¬
gels. Moderate heat is required to convert these gels to a fluid condition.

Natural impurities common in water may be contributing causes of deteriora¬
tion of paint materials in which water is an original component. All ground waters
contain various gases and salts. The gases are chiefly those of the air—oxygen,
nitrogen, and carbon dioxide. The salts are chiefly sulphates and carbonates of
calcium, magnesium, and iron. These cause what is called ‘ hardness ’ in water. In
addition, there is apt to be organic matter, algae and bacteria. Formerly works on
pairiting techniques frequently called for distilled water in formulas and for
mixing aqueous paints. Today many municipal supplies, particularly in the
eastern seaboard of the United States, are comparatively free from injurious
impurities and distilled water is not necessary. When, however, the local supply is
not good, distilled water, now easily available, had better be used in connection
with important work.

Wetting Agent, any material which lowers the interfacial tension between
liquids and solids, and serves thus to aid in wetting a surface (see Surface-Active
Agent).

White Spirits (see Mineral Spirits).

Xylene (xylol) is dimethyl benzene (C6H4(CH3)2). The commercial product,
xy ol, IS a mixture of three isomeric xylenes, chiefly meta-xylene. Like benzene and
toluene, it is a clear liquid derived from the destructive distillation of coal tar and
fractional distillation of the ‘ light oil.’ Less volatile than toluene, it boils at I39°C.,
and Its lower volatility favors it as a diluent for brushing lacquers. It is not so
dangerous a toxic as benzene.

BIBLIOGRAPHY

AnonymouSj 'Final Report of the Committee on Benzol/ Bulletin of the National Safety
Council (Chicago, 1926).

W, M, Bayliss, Principles of General Physiology^ 3d ed. (New York: Longmans, Green
and C0.5 1924). Chapter VIII, 'Water, Its Properties and Functions/

Ernst Berger, Beitrdge 2ur Entwicklungsgeschichte der Maltechnik^ 4 vols (Munich: G.
D. W. Callwey, 1901—1912).

Ethel Browning, Toxicity of Industrial Organic Solvents (New York: Chemical Publishing
Co., 1937).

Max Doerner, The Materials of the Artist and Their Use in Painting^ trans. (New York:
Harcourt, Brace and Co., 1934).

A. K. Doolittle, 'Lacquer Solvents in Commercial Use,^ Industrial and Engineering
Chemistry^ XXVII (1935), 1169-1179.

T. H. Durrans, Solvents^ 3d ed. (New York: D. Van Nostrand Co., 1933).

A. Eibner, Malmaterialienkunde als Grundlage der Maltechnik (Berlin: Julius Springer,
1909).

H. A. Gardner, Physical and Chemical Examination of Paints^ Varnishes^ Lacquers and
Colors^ 9th ed. (Washington, D. C.: Institute of Paint and Varnish Research, 1939).
Alice Hamilton, Industrial Toxicology (New York: Harper and Brothers, 1934).

Noel Heaton, Volatile Solvents and Thinners (London: Ernest Benn Ltd, 1925).

H. E. Hofmann, 'Evaporation Rates of Organic Solvents,^ Industrial and Engineering
Chemistry^ XXIV (1932), pp. 135-140.

Otto Jordan, The Technology of Solvents^ trans. (New York: Chemical Publishing Co.,
1938).

K. B. Lehmann and F. Flury, Toxikologie und Hygiene der technischen Ldsungsmittel
(Berlin: Julius Springer, 1938).

Ralph Mayer, The Artisfs Handbook of Materials and Techniques (New York: The
Viking Press, 1940).

Ibert Mellan, Industrial Solvents (New York: Reinhold Publishing Corp., 1939).
Maximilian Toch, Painty Paintings and Restoration (New York: D. Van Nostrand Co.,

1931)*

SUPPORTS

Academy Board and Canvas Board. The so-called ^ mill boards/ ^ academy
boards/ and ^ canvas boards ’ for several decades have been supplied for the use
of amateurs and students. The term ‘ mill board ’ is generic for any strong, hard-
pressed, flexible pasteboard made from rope, yarn, or other cheap fibres. C.
Roberson and Company, Ltd, estimate that mill board was developed at the end
of the XVIII century and their records show that it was in existence when the
firm was founded in i8iq. It appears in the Winsor and Newton Company,
Ltd, lists of 1841.

Academy board is simply a mill board which is given a surface in preparation
for painting, primarily oil painting. It is made of paper containing chalk and size
and has a face of pale gray or white ground usually of a lead, oil, and chalk
mixture. In some cases the face is given a rough texture by having a piece of paper
laid on and pulled off again while the ground is still wet. Reeves and Son, Ltd,
and Winsor and Newton Company, Ltd, London, first listed this board in 1850.
The records of George Rowney and Company, Ltd, London, carry it back as far
as 1852. When it reached the continent can not be stated exactly. The old firm of
Lefranc in Paris, founded in 1775, has no records concerning it. It was manu¬
factured in America by E. H. and A. C. Friedrichs Company, New York, in 1868.

Canvas board, a paper board with primed canvas fastened to one face, was
put on the market by George Rowney and Company, Ltd, in 1878. It appears in
the records of Reeves and Son, Ltd, and of Winsor and Newton Company, Ltd,
in 1884. C. Roberson and Company, Ltd, think that it was introduced between
1875 ‘ Russel board,’ a type of canvas board in which the cloth is

turned back and fastened over the edge, has been sold for over fifty years by F.
Weber Company, Philadelphia. In 1887, Rowney introduced what they called
Rushmore boards, a paper board having a surface grained in imitation of canvas.
(This information has been compiled from personal correspondence with several
of the older artists’ supply dealers in England, the Continent, and this country.)

Aluminum. The element aluminum is a white, soft, malleable, and ductile
metal. Its particular property is its lightness; it has only about one third the weight
of the common metals. Although not readily attacked by acids, particularly by
organic acids, it is rapidly corroded by alkalis. It has marked resistance to
atmospheric corrosion. The aluminum metal cap on the Washington Monument,
made in 1884, still reflects sunlight from Its exposed surfaces. Under certain atmos¬
pheric conditions, even brightly polished aluminum becomes frosted with time
because of the formation of a thin oxide coating.

Although compounds of aluminum were known as early as classical times. It
was not until 1825 that the Danish chemist, H. C. Oersted, became the first to
isolate this metal. Aluminum was available in small quantities for the next half
century but at prices comparable to those of the noble metals; it did not come into
commercial use until after 1886 when Charles Martin Hall discovered the process
that led to large-scale production. Although ingot aluminum was produced as

221

222

Painting Materials

early as 18935 the making of sheet metal did not assume large proportions until
' about a decade later.

Aluminum is prepared by the electrolysis of the mineral bauxite (AI203-2H20)
in a fused bath of cryolite. For this reason, the commercial product is of high
purity. Sheet aluminum is obtained by either hot or cold rolling. Modern alloys
of aluminum have a wide range of hardness and working properties.

Because of its very recent commercial historyj few paintings are done on this
metal. Its light weight, however, has suggested its use as a paint support and as a
new support for the transfer of old paintings. Previous use in the automobile body
industry shows that it gives a suitable surface for the application of paint. Highly
polished or ‘ bright-finish ' aluminum is too smooth for satisfactory coating.
‘ Gray plate,’ which is made by putting the aluminum through rolls that are
covered with a coating of aluminum metal particles and aluminum oxide, has a
better ‘ tooth.’ It can be sand-blasted or roughened in other ways to give a rougher
surface. Gray oxide coatings that are adherent and protective may be applied
chemically (see Edwards, Frary, and Jeffries, II, 471) and electrochemically. L.
McCulloch has published a method for giving aluminum a dead-white finish.
The aluminum is boiled for some time in a mixture of lime and calcium sulphate;
the coating formed is fine-grained, is adherent, and does not separate from the
metal on bending.

Artificial Building Boards. In recent years a number of artificial building boards
have been developed and are coming into extensive use in house construction and
in remodeling. Although some of these may be considered as lumber or plaster
substitutes, others have specific purposes. They may all be divided into three
categories:

1. Fibre building boards

a. Homogeneous type (either porous or compact)

b. Laminated type

2. Mineral building boards

a. Asbestos with cement

h. Paper liner with gypsum core

3. Composite board (wood core with paper liner)

Low-grade vegetable fibres are largely used for the manufacture of the fibre
boards. They may be wood-pulp, bagasse (crushed sugar cane residue), straw,
corn-stalk, or sawmill waste. The porous, homogeneous fibre boards are made
primarily for heat-insulating purposes, but that made from bagasse is also used
for interior wall finishing. The laminated type, made by joining several sheets of
thin cardboard with an adhesive, is used for wall boards almost entirely. Water
resistance is produced in most boards by incorporating rosin size before forming
and, in addition to this internal sizing, many boards have applied to them paints,
gums, oils, or waxes to further increase water resistance. Both starch and water-
glass are used as adhesives in making laminated boards. Frequently, both sides are

Supports

223

finished with a ' liner/ a sheet of paper of better quality than the fibre in the
interior.

One type of fibre building board has aroused a considerable interest among
painters of the present. It is of the compact, homogeneous type and is made by
the so-called ' Masonite process ' from wood chips of the long-leaf yellow pine.
The wood fibres are torn apart by exploding the chips with high-pressure steam.
The natural wood lignins are used to cement the wood fibres together again on
large plattens with the aid of heat and pressure. The finished fibre board, which is
chestnut brown in color, has one smooth side and one rough side; this is caused by
imprints of the wire screen on which the board is formed. The rough side may be
coated with a gesso ground or it may be primed with a white paint. The wire
mesh imprint gives the surface a texture somew^hat similar to that of coarse
canvas. The Masonite product is hard and dense; it does not bend or warp easily.
It is prepared in sheets four feet wide and up to twelve feet in length and from
one eighth to one half inch thick. Three general types of the Masonite product
are available: a thick, porous board for insulation purposes, a semi-hard board,
and a hard or ' tempered ' board.

Artificial building boards have one advantage; they are homogeneous in
physical properties in all directions. They have no grain and, hence, are not
subject to unidirectional shrinking and swelling. In large sheets, unless properly
supported, they are liable to twist and to warp from their own weight.

Besides the artificial fibre boards, there are two types of wall board that are
made almost entirely of inorganic or mineral materials. The first of these is made
chiefly from asbestos and Portland cement in varying proportions. A little sulphite
pulp for binding purposes and some pigment may be added. These asbestos boards
are hard and dense. Weight and brittleness are their chief disadvantages; they are
liable to fracture much like glass. When secured to a wall or properly re-enforced,
however, they offer an excellent surface for the direct application of paint. The
second type is made with a gypsum filler and a paper liner. This is a plaster
substitute; it is fragile, easily broken, and, unless carefully supported and held
at the edges, it can not be carried about and handled. It is of little use as a paint
support unless it has been previously secured to a wall.

A third general class of artificial building board is a composition board made
with a core of thin wood laths (frequently redwood) which are held together edge
to edge with sodium silicate cement and a paper liner on both sides. On the outside
they are similar in appearance to the laminated fibre boards. Boards of this type
have little to recommend them for painting purposes.

Asbestos is a magnesium calcium silicate mineral which occurs in various
combinations as white, grayish masses of long, compact, silky fibres, flax-like and
readily separated. (See also Artificial Building Boards.)

Asb {Fraxinus). The European ash (F. excelsior) is a wood which grows
widely from England to Asia. The American white ash {F, americand) is a little

224

Painting Materials

coarser in texture but is otherwise similar. It is a tough, close-grained hard-wood.
The heart-wood is light brown and the sap-wood is nearly white. The wood is
ring-porous and annual rings are very distinct. In tangential section the rings
produce marked elliptical or parabolic figures. The rays are fine and not conspicu¬
ous. The so-called "mountain ash’ {Pyrus) belongs to an entirely different
family. Ash has not been extensively used as a support for paintings (see also
Wood).

Beech (Fagus), The beech usually found in panel paintings of the West is
Fagus syhatica^ one of the common forest trees of temperate Europe. Fagus
americana is very like it. The wood is not remarkable for either strength or dura¬
bility but has been much used in mill-work and turnery. It is heavy and fairly
straight-grained, and is diffuse-porous though the pores are small. Annual rings
are not distinct. The color, particularly that of the heart-wood, is reddish to
reddish brown. On quarter-sawed surfaces the rays are conspicuous as dark
flakes from one sixteenth to one eighth inch in height. Only a few paintings of the
German school are reported to be on beech.

Birch {Betuld). This genus of tree is widely distributed over the northern
hemisphere. The white birch ( 5 . alba) is the species most commonly used in
Europe. (It is not to be confused with the paper birch [ 5 . Papyraced\ of North
America.) The yellow birch ( 5 . lutea) and the sweet birch ( 5 . lento) are the
species most valued for timber in America. Birch is moderately strong and does
not warp badly. Heart-wood is reddish brown in color. The texture is close and
compact; the pores are diffuse and very small and for this reason the annual
rings are not conspicuous. The rays appear on quarter-sawed surfaces as very
small, Teddish brown flakes. Birch is often stained to imitate mahogany. It has
been little used in panel painting (see also Wood).

Brass, as the term is now used, is an alloy of copper and zinc. The proportion
of the two metals may vary within fairly wide limits, but ordinary brass is about
2 parts copper and i part zinc. In its older use, the term was applied to the alloys
of copper and tin, now known as bronze. The brass spoken of in the Bible was
probably bronze and so, also, was much of the brass of later times until the
distinction between zinc and tin became clearly recognized. Copper-zinc alloys
were known in Roman times. They were manufactured in England in the XVI
century. Although it is probable that brass was used as a support for painting,
along with other metals (see also Metals and, particularly, Copper), no available
evidence for such use appears.

Canvas is, literally, a coarse cloth made from cotton, hemp or flax. This
definition serves well enough to describe the traditional fabric used as a paint
support in Europe^(see also Fabrics), though hemp fibre is rarely found in such
objects. The word ^ canvas ’ has now a number of meanings. It may be used for
artists’ canvas or for a picture painted on canvas.

Cedar {Cedrus), Under the name " cedar ’ is included a number of woods from
different genera, some of which are not conifers, and the name has been applied

Supports

225

rather indiscriminately to woods having a certain fragrant odor characteristic of
the true cedars. The cedar of Lebanon {Cedrus Libani)^ one of the important
woods of liistory^ was used widely in Egypt and the Near East. No true cedar is
indigenous to Europe. The wood is generally reddish brown and light in weight,
but has a coarse and spongy texture; it is easy to work but is liable to shrink and
warp. The annual rings are clearly marked, ilccording to available records, it was
one of the woods used for the support of mummy portraits made in the Fayum
district between the I century B.C. and the VI century A.D. It had occasional
use in the panels of European painting.

Chestnut {Castanea). The sweet chestnut (C. mlgans) is commonly known in
Europe. A similar tree (C. de 7 ttatd)^ before a recent blight, flourished in America.
Chestnut resembles oak and it is mainly the indistinctness of the medullary rays
that differentiates it from that wood. It is a soft, light wood, comparatively free
from warping and shrinking. It is ring-porous and hence the annual rings are
easily distinguishable. Spring-wood vessels (pores) are large and generally arranged
in double or triple row^s; summer-wood vessels are very small and can scarcely be
observed with the naked eye. Because of the large pores, plain-sawed chestnut has
a conspicuous figure. It is subject to attack by worms. Chestnut was much used
for painted panels, particularly in Italy (see also Wood).

Clay is perhaps the oldest of plastering materials. Although one of the poorest,
it has endured surprisingly well as a paint support in certain instances. The
essential components of clay are the hydrous silicates of aluminum which are
widely distributed on the earth's surface. These may be associated with other
substances or impurities to a greater or less extent. Clay absorbs water readily
and becomes plastic, a propertv which has made it pre-eminently useful in the
ceramic arts.

Cloth (see also Fabrics). Although this term might have become general for
the woven stuff that is used, in various kinds, as a support for paint, ‘ canvas ’ or
* fabric ' are more apt to be applied to it. The word " cloth' is frequently used,
however, in this general sense. More specifically it is used to define a kind of weave :
simple cloth, in which warp threads and weft threads pass over and under each
other alternately; and, among others, compound cloth in -which there are multiple
warps or wefts, or both, one warp and one weft being of cloth weave.

Copper. The chemical element, copper, is a yellowish red, soft metal. It is
ductile and malleable and, hence, is easily rolled into thin sheets or plates. It is
prepared by the reduction of sulphide, oxide, and carbonate ores. Most of the cop¬
per that now appears in the market is electrolytically refined; its purity generally
runs better than 99.9 per cent. The electrolytic refining of this metal was first
carried out on an industrial scale in 1869 at Pembrey, Wales.

Freshly-worked copper has a luster and takes a bright polish but it is soon
tarnished when exposed to the air. Under indoor conditions, it is stable for
centuries and the tarnish remains very superficial. Exposed out-of-doors, it

226

Painting Materials

gradupJly acquires a thin, green patina composed of the basic salts of copper.
These are formed as a result of the corrosive action of atmospheric carbon dioxide
and sulphur dioxide. The patina may last for centuries, under these conditions,
even when the metal is in sheet form. When it is buried in the ground, however,
corrosion is fairly rapid, particularly if the metal comes into contact with saline
wmters. Although copper is not acted upon quickly by dilute mineral acids, since
it is belov/ hydrogen in the electromotive series, it is acted upon by acids under
oxidizing conditions. Some organic acids, like acetic acid, corrode it slowly. The
fatty oils and fats also attack copper slowly but the action does not bear any
simple relation to the acid content of the oil. C. W. Volney (Mellor, Comfrehen-
sive Treatise^ III, p. loa) found that several natural fatty oils, including linseed
oil when in contact with copper, dissolved fairly large proportions of the metal.
Individual oils have different effects on the copper surface: some leave it bright;
others tarnish it. The oil itself turns green. It has been observed that spirit-resin
varnishes in contact with copper or brass turn green (' green drip '). Painting on a
copper panel has been observed to have a green-stained, oil-resin film between
the metal and white ground. This indicates a slow chemical reaction between the
copper and the oil or resin while the latter was still plastic or semi-fluid.

Known in prehistoric times, having a continuous history which is much in¬
volved with that of bronze (a copper-tin alloy), copper itself was not much used as
a support for paint until it became cheap and plentiful in sheet form. That was
probably in the XVI century (see also Metals). At about that time, also. Its use
for intaglio printing began to be exploited and it has since been the principal
plate material of etchers and engravers.

By the XVII century mention of the use of copper as a paint support had got
into the literature. The voluminous MSS of De Mayerne (Berger, IV, 416)
include a reference to painting with oil on copper and other metals. The Spaniard,
Palomino, writing in i^ 2 ^\Museo Pictorko, II, 44; see Berger, IV, 83) says that
copper panels are to be grounded with the same oil preparation as that used for
panels of wood. He points out, also, that the smooth surface of the metal will not
give a good bond unless it is first rubbed with garlic.

Cotton is the seed hair of the cotton plant {Gossypiurn); it consists of a single
hair-like cell which, when fully ripe, is flattened and twisted. The length of the
cotton fibre varies from 2 to 5*6 cm. and the diameter from 0.0163 to 0.0215 mm.
The walls are thick and the lumen or central canal is broad, giving the fibre the
appearance of a collapsed, twisted tube (see figure 2, p. 231). Cotton is over 90
per cent cellulose and is one of the most important sources of that material.

Like wool and linen, cotton was being made into clothing in prehistoric times.
It has been for thousands of years the staple fabric of the Orient- The plant grows
generally in tropical and sub-tropical regions, more than half the world’s normal
supply coming from the United States. It is also grown in British India, Egypt,
Russia, and Brazil, Sea Island cottons, grown on islands off the southeastern

Supports

227

coast of the United States, are the best in quality; Egyptian cotton is next.
Cotton fibres, being nearly pure cellulose, are readily affected by acids and by
moderately strong oxidizing agents but alkali hypochlorites, in dilute solutions
and at ordinary temperatures, have little effect on them. In spite of its long history,
cotton has been used comparatively little as a support for paint until recent times
{cf.. Linen). Particularly in the XX century has it been made into a commercially
prepared canvas. As yet it has not acquired the reputation for permanence that
linen has.

Cradle is the term applied to a wood structure fixed to the back of a panel
painting to prevent it from warping with changes of humidity. It consists of
narrow wood strips having slots and being joined with glue to the back of the panel
parallel to its grain. Transverse strips ride freely in the slots and, if conditions are
favorable, keep the panel flat but do not interfere with the normal expansion and
contraction, across the grain of the wood, with changing humidity conditions.
(For illustrations, see Thompson, Tempera Paintingj pp. ii-ic, and De Wild,
p. 90. See also Wood, deterioration and treatment.)

Esparto or Esparto Grass {Stipa tenacissima) is a kind of spear grass used,
particularly in England, for the manufacture of paper (see also Paper). The
plant has long leaves in which the fibres are strong and flexible. To produce a
pulp from the grass, it is boiled in a solution of caustic soda.

Fabrics. As supports for painting, fabrics may be considered according to the
weave and also according to the thread and to the origin of the fibres. Animal
fibres (see Fibrous Substances) may be divided into those which come from animal
hairs (see Wool) and those which come from insects, /.<?., the silk worm (see
also Silk). The animal fibres may be identified by their continuous cylindrical
structure as seen under the microscope, by chemical identification of their protein
content, and by their characteristic odor when burned.

Among the vegetable fibres are three general categories. Vegetable hairs
include cotton—seed hairs from various species of Gossypium —fibres of ‘cotton
trees,’ Bombax cottons, vegetable silks, seed hairs of Asclepiadaceae^ etc. The
second group is made up of bast fibres from the stems of dicotyledonous plants.
Further division of this group would begin, wfith flax and flax-like fibres, hemp,
Gambo hemp, sunn hemp, and yercum fibre. After these come the Bohmeria
fibres, ramie, and China grass. Next come jute and jute-like fibres, then the coarse
bast fibres, and, finally, basts, including linden and other woody materials. The
third group includes fibre bundles of monocotyledoiious plants. First are leaf
fibres, including Manilla, pita, sisal, Mauritius hemp. New Zealand flax, bow¬
string hemp, and esparto. Next are stem fibres such as tiliandsla, fruit fibres such
as coir, and, finally, paper-making fibres—straw, esparto, bamboo, v/ood, paper-
mulberry, etc. Out of the large range of these categories only a few fibres have a
marked place among the paint supports that have been used or are used now.
These are the animal fibre, silk, the bast fibre, flax, and the seed hair, cotton.

228

Painting Materials

The first step in converting fibres into cloth is, of course, the removal of inter¬
cellular matter (see also Linen), leaving relatively pure fibres. These are carded,
combed, drawn out and spun to the correct size. After spinning, single ends are
put together, in such number as will make the size of thread required, and are
twisted together in a direction opposite from that of the spinning. The reversal
of direction keeps the twist from kinking. When wound, the twists are ready for
weaving.

Figure i. Simple weaves of fabric are shown in the drawings taken from figures i, 2,
and 3 in VA Classification of Hand-Loom Fabrics,’ The Pennsylvania Museum Bulletin
(Philadelphia), XX (November 1924), page 27, by Nancy Andrews Reath. At the left
is a simple cloth, each warp thread (vertical) passing under one and over one weft
thread; in the center is a simple cord.in which the warp threads (vertical) are much
smaller than the weft threads; at the right is a simple twill in which a warp thread
passes under one and over three of the weft threads.

Hand-weaving is a simple and familiar process. One set of threads, the warp, is
strung parallel on a frame and another set, the weft, is carried, by a shuttle, in and
out among the warp threads to make the fabric. The different ways in which the
two intersect are differences in weave. According to the descriptions of these
different ways, as given by Nancy Andrews Reath (pp. 8 ff.), those that usually
appear in painting supports can be defined as follows: (i) simple cloth, in which
warp threads and weft threads pass over and under each other alternately
(ribbed cloth or cord, in which one set of threads is heavier than the other, seems
to have had no appreciable use in painting); (2) simple twill, in which regularly
recurrent warp threads pass in echelon over and under the weft threads two or
more at a time, producing diagonal ribs or stepped patterns.

The history of fabrics as painting supports (see also Linen and Cotton) can
be traced back at least as far as the XII dynasty (2000-1788) in Egypt (a small
painted fragment, linen, of this time is in the Royal Ontario Museum of Archae-

Supports

229

ology, Toronto), and the practice must have preceded that by many centuries.
Examples of it are found in Egypt dating from the early years of the Christian
era. A few scrolls from Chinese Turkestan (see Sir Aurel Stein, Serindia [Oxford:
Clarendon Press, 1921]) are from the Six Dynasties (265-581 A.D.). In the Far
East fabric was used with steady continuity by painters and, although important
paintings in Europe do not mark its employment so regularly, there is no reason
to suppose that it was ever abandoned. Pliny (XXXV, 51) remarks that the Em¬
peror Nero ordered a colossal portrait of himself, 120 feet in length, to be painted
on canvas. The icon, the altarpiece, and walls of churches took precedence in
Christian painting, but linen and some silk were probably always used for painted
banners and for other purposes. In the third book of Eraclius, Be Colorihus et
Artibus Romanonrm, chap. XXVI, are directions for preparing linen that is to be
painted on (Merrifield, I, 230). This was written in the XII or XIII century.
Cennino Cennini about 200 years later gives a similar but far more explicit
instruction (Thompson, pp. 103—104).

With the decline of painters’ work on altarpieces and with pictures acquiring
larger scale, during the Renaissance, fabric or canvas was more and more used.
It is today the principal support for paintings executed in the oil medium. The
usual practice is to stretch the fabric on frames to which it is attached by tacks
at the outer edges. Most modern frames, called stretchers, are so mortised at the
corners that they can be slightly extended in outside dimension, and the fabric
tightened, by means of small wedges, called ' keys,’ inserted in the joint. Such a
stretcher is not much seen before the XIX century. Earlier than that, also, it was
the common practice to stretch the fabric before it was sized or had a ground film
applied. Now artists’ canvas is bought in pieces or rolls ready for painting. In
consequence, these latter canvases show a ground film where they run over the
edges of the stretchers. In older paintings the fabric is apt to show no coating at the
edge. Usually linen or cotton has been used alone but there was an old, and very
sound, practice of using it over wooden panels. It was glued directly to the wood
and had a gesso ground laid over it. Probably this originated in Egypt and it was
much followed in Italy during the XIII to XV centuries. Occasionally it has been
combined with paper into a double support, the paper being used actually in
place of a ground film.

Fabrics, conservation and treatment. The deterioration of fabrics is largely
that of the cellulose which, except for silk, chiefly composes them (see Fibrous
Substances). To some extent this is an inevitable consequence of exposure to a
normal atmosphere. It has been found, however, that cellulose itself is extremely
resistant. Probably the principal deterioration of artists’ canvas comes either
from materials that are added in the paint structures or from conditions that could
be avoided in the environment of a picture. Oil in the ground or applied at the
reverse of a fabric is one cause of its weakening. Not only does the oil film em¬
brittle the whole structure; it also tends by its own oxidation to increase that of

230

Painting Materials

the fibre and to change it to oxycellulose. Glues or pastes used in priming encour¬
age the growth of mold and micro-organisms which will attack and destroy the
fabric. They can be kept from growing by conditions in the rooms where paintings
are put. Darkness and dampness (a relative humidity of 65 per cent or more) are
all that such organisms need to start their development in a normal room temper¬
ature. Spores are almost sure to be present.

Treatment of the fabrics used for paint supports may include attempts at
disinfection to remove active micro-organisms. Such treatment is usually made
with thymol vapor or formaldehyde applied either in solution or in closed cham¬
bers. By far the most usual treatment, however, is that known as ^ relining.’
The aim of this is to strengthen the fabric structurally. It is a process of backing
the old canvas with a new one, a film of adhesive material being used to attach the
two. In general, this treatment has been applied for at least 200 years. In its
application usually a light facing is attached to the paint and often this is held on
a stretching frame. The old canvas being removed from its stretcher, it is carefully
cleaned and smoothed on the reverse side. A new canvas is selected, stretched,
and sized or impregnated. The old canvas is sized or impregnated also, enough
of the adhesive material being left to form a bond. The two surfaces are then set
together and the adhesion is established usually with heat and pressure.

Until the late years of the XIX century, the principal adhesive for the rellning
process was glue. It was sometimes used in emulsions with oils or varnishes. Then
wax and a mixture of wax with a small amount of resin came into use and has
sometimes been called the ‘Dutch process ’ of relining. Wax has found much favor.
The general technique is not very different but Impregnation of the old fabric is
more certain and the sealing power against dampness is, of course, far greater than
that of glue. Glue also suffers from its tendency to augment the growth of mold.

Mounting instead of relining has occasionally been done for European paint¬
ings on fabric. In this treatment the principle is much the same but a rigid panel
instead of another fabric is used at the back of the old support. For the most part,
the wax type of adhesive is used, and the development of this and of artificial
boards has greatly encouraged adoption of the method for use with small
paintings. In the East, of course, mounting has been practiced for many centuries
and is used for paintings on silk or paper. It is done largely with paste as an
adhesive.

Fibrous Substances* Fibres are special structural components of animal and
vegetable origin which are utilized in textile, paper, cordage, and brush manu¬
facture. Only one Important fibre, asbestos, is of mineral origin; it is no special
structural component like the animal and vegetable fibres but is an unusual
crystalline form of a mineral substance.

Animal fibres are composed of nitrogenous colloids like the albumins, fibrins,
and gelatins and they are of highly complex constitution. Unlike the vegetable
or cellulose fibres, they are resolved and finally dissolved by alkalis whereas the

Supports

231

former are quite resistant to that agent. Animal fibres are mainly restricted to the
making of textiles. The number used is relatively small and may be divided into
two kinds. One is the epidermal hair of animals of which the wool from sheep is
the most important. Particularly characteristic of aiiimial hairs are the tiny scales
wdiich completely surround the shaft. These are embedded in an epidermal layer
over a cortex or fibre layer. A medulla or pithy center may or may not be present.

Figure 2. Three types of fibres, adapted from photomicrographs at 100 diameters, in
Chemistry of Pulp and Paper Making^ by Edwin Sutermeister, 2d ed. (New York: John
Wiley and Sons, Inc., 1929),^ Plates i, 2, and 6: at a drawing showdng a group of cotton
fibres; at ^ a group of linen fibres; and at r a group of paper mulberry fibres.

Animal hairs are very long as compared with most vegetable fibres. The identi¬
fication of a fibre as a hair is simple because of the scales, but the identification of
hair as to its source is rather difficult. Among the hairs of textile importance there
are slight individual dilFerences -which allow at least a partial identification of the
source; the quantitative measurements of diameter and scale size are of great aid
in verification of qualitative observations as to the shape of scale and kind of
medulla. Silk is another kind of animal fibre. True silk is the continuous filament
that forms the cocoon of the Bombyx moriy the worm that feeds on the leaves of
the mulberry tree. The silk fibre is, in reality, a double fibre bound together with
sericin or silk glue. The two individual fibres are separated by boiling in soap and
water which dissolves the sericin. Tussah or wild silk is the secretion of wild silk
worms; it is difficult to decolorize and, hence, is usually dyed a dark color.

The vegetable fibres are numerous and diversified in characteristics. They are
colloidal bodies made from cellulose and derivatives of cellulose. In all the
higher forms of plant life the living cells consist essentially of protoplasm sur-

^^2 Painting Materials

rounded bv a wall of cellulose. In the woody structure of these higher plants,
where cell growth has ceased, the cells become cemented together with a substance
r.lled Li«. This lig»i°> which is scmcwha. similar to ce Wose and .s generally
■Sen of as one substance, is in all probability made up of several closely related
Sstances. It is the substance that is removed ehem.c.lly .n pmp.r.ng cellulose
for paper. The woody tissue of plants IS made up of cells which exhibit great
Sversity of form, siJ, and markings. The paper maker IS interested in the true
wLd hbres or libriform cells, and in the tracheids. The wood fibres ch*r.cter,ip
tic of deciduous trees or hard-woods are elongated cells with strongly thickened
walls- they are variable in length in different woods, ranging from 0.14 mm. to
To mm. and in all cases they are the longest elements present. The woody tissue
of coniferous trees consists almost entirely of tracheids which are elongated
“apering cells, more or less llgnified and characterized by bordered pits or discoid
markings. These pits are so constant in number and mode of d«trib«tion that they
may bemused as Ltinguishing characteristics for some mods. The "“heids of
Serous woods are considerably longer than the wood fibres of hard-woods,
Subtly reaching a length of 4 or 5 mm. This is one of the reasons that spruce

wood is so valuable for making sulphite pulp. ^ i i r • i

Bast fibres are those fibre bundles found in the inner bark of various plants.
They are held together by incrusting materials and by partial identity of eir
cell Llls. Some kind of chemical or mechanical treatment is necessary to separate
Sem. The commonest method is that of retting. The bast tissues of dicotyledonous
annuals furnish such staple materials as flax, hemp, ramie, and jute. The walls of
hast fibres are generally of considerable thickness and the central canal varies
greatly in different species. The fibres are characterized by irregular thickenings of

the walls at intervals to give joints or nodes. AUhn.mh

In the seed hairs, like cotton, the fibres are individual cells or units. Altho g
they occur as complex aggregates they are not cemented together as are wood
or iLst fibres. This is the reason that cotton is such a valuable source of fibre and

cellubse.^_^^^ and The name of this wood now correctly includes the

spruces (Picea) as well as the true firs (Mies) and is also frequently ^

general term for all of the true conifers (Abietineae). In northern Europe the Nor¬
way spruce (P. excelsd) is the most important timber speaes but in southern
. Europe the silver fir (A. pectinala) is an important rival. This tree appears to be
the true Abies of the Latin writers. It is abundant in most of the mountain ranges

of southern Europe. The wood is inferior to P.eiwr<?/ra but IS soft and easily wor e .

The wood structure of the spruces and firs is similar to the pines except t at t e
resin ducts are smaller, more scattered, and less distinct. Woods from various firs
have no doubt been used rather extensively in the panel paintings of Europe. It
is little mentioned in the records but there perhaps it has been often confused

with pine.

Supports

233

GessOj in its restricted meaning, the white priming or ground made from
burnt gypsum (plaster of Paris) and glue, which was used by the Italian painters
for preparing wooden panels or other supports for painting and gilding. Today
the term is often applied to grounds made from almost any white inert or pigment
like whiting or zinc oxide (see p. 115). Properly prepared gesso is hard and can
be finished to an ivory-like surface with a proper absorbency for paint. It has
been used on carved wooden moldings which were to be gilded and decorated
because it was easier than the wood itself to carve delicately and to prepare for
gilding (see Thompson, The Materials of Medieval Paintings p. 31). Gesso sets
more slowly than plaster of Paris but is harder, more strongly adhesive, and can
be smoothed more satisfactorily. Modern painters and picture framers prefer to
use whiting or gypsum mixed with zinc oxide in place of straight gypsum or
plaster of Paris because this addition gives gesso a greater bulking and hiding
power; fewer coats are required. Such formulas call for about equal measures
(equal parts by volume) of pigment (gypsum, whiting, zinc oxide, or mixture)
and glue solution (i part of glue to 15 parts of water by weight).

Glass is a super-cooled liquid made by the fusion of soda, lime, and silica in
varying proportions. Other metal oxides, such as those of lead, barium, potassium,
and iron, may enter into its composition. Although glass was known in Egypt as
early as 3500 B.C., cast plate glass was not made until the latter part of the XVIII
century. G. W. Morey C A Half Century of Glass , . p. 943) says, ‘ .

during all these centuries only one type of glass was known, soda-lime-silica glass,
modified, of course, by the very considerable amount of impurities that crude
manufacturing methods made unavoidable.^ Hackh’s Chemical Dictionary says
that glass is an amorphous, hard, brittle, often transparent material consisting of
a mixture of the silicates of some of the alkali metals, the alkaline earth metals,
and the heavy metals, and that it is obtained by the solidification of a fused mass
containing:

a. Si02 as quartz, flint pebbles or siliceous sand; this can'be replaced by B 2 O 3 ,

AI2O3 or Mn203.

h. Na20 or K2O as carbonate or sulphate.

r. CaO as limestone or marble; this may be replaced by PbO, MgO, ZnO or
BaO. The constituents and c are finely powdered, well mixed and
heated to 1000-1100° C.; on cooling to 890° C. the pasty glass is worked
by blowing, drawing, casting, or fusing piece to piece and now annealed
by slow cooling or reheating and cooling. The composition of glass varies
between (K, Na)20, (Ca, Pb) 0 , 6 SiOa and 5(K, Na)20, 7(Ca, Pb) 0 ,

36 Si02.

It is well known that glass was used for windows in Roman times. N. H. J.
Westlake (I, 4) cites several examples of window glass from Pompeii and other
Roman sites. Hot glass was probably both cast and blown at that time. Because

234

Painting Materials

of obvious shortcomings, glass has never been an important support for paintings,
although the large and rigid flat surfaces it offers must have suggested its use. It
has, however, been made a support for miniatures and for decorated clock faces.
The smoothness of its surface has presented a certain difiiculty in the application
of paint. Painting on glass or painted glass may be of different kinds: the applica¬
tion of a pigment with a medium, in the ordinary sense, or the kind that is
commonly called ‘ stained glass ’ where vitrifiable pigments are applied and fired
in. J. D. LeCouteur (p. i) contends that the expression ‘ stained glass,’ so often
applied to colored glass whether mediaeval or modern, is misleading and erroneous.
It should be ‘ painted glass.’ He goes on to say, however, that the method, dis¬
covered in the XIV century, whereby glass was colored yellow by painting with a
silver solution and refiring, produces a glass that may properly be called ‘stained.’
Painting and gilding on glass appear to have been frequently done in the Renais¬
sance. In Thompson’s translation of Cennino Cennini, II Libro del? Arte, there is
shown a diptych with panels of gilded and painted glass and Cennino Cennini
describes (pp. iii—113 of this translation) a method for gilding on glass and
scratching in designs with a needle. Leonardo da Vinci made a suggestion (see W.
Ostwald, p. 137) that paintings be made directly on glass and seen through it so
that there would be no diffusion of light from the surface of the paint. The sug¬
gestion may have been acted on; paintings done in this way have survived from
somewhat later times. For the most part, they are of provincial origin, and doubt¬
less large numbers have been lost entirely.

Although glass is generally considered to be a stable material, it is subject to
internal changes and decay which depend upon environment. There is some
tendency to crystallization or ‘ devitrification ’ on the part of the super-cooled
liquid alkali silicates from which it is made, and this causes glass to become more
brittle with age. Ancient glass, which has long been buried in the soil, exhibits a
dull surface with well known iridescence. The cause is said to be the action of
moisture, oxygen, and carbonic acid in the atmosphere. They dissolve the alkali
from the glass and leave silicic acid in the form of minute scales that produce
iridescence by interference. In ancient glasses there was often an excess of alkali
over and above the proportion required to combine with the silica present. The
excess is soluble in water and attracts moisture and carbon dioxide from the atmos¬
phere. There have been seen ancient glazes on pottery where practically all the
alkali has_ been leached out by ground waters to leave only a residue of porous
si ica or silica hydrate. Potash glasses, like Venetian glass, are liable to become
moist ( sweating glass ’) because of the hygroscopic nature of the potassium
carbonate that is formed in the initial stages of decay. The silica does not reach a
powdery condition, for it is dissolved by the moist alkali. It may become opales¬
cent but not iridescent. These are, however, extreme cases of deterioration. A
glass that still retains its paint film has probably changed little in the course of

centuries, except to become more brittle.

Supports

235

Glass Fabric* Molten glass can be blown or drawn into fine fibres which can
be woven into fabrics. Akhoiigh such fabric is used chiefly for electrical insulation^
it is also made into finer stuffs. Its use as a support for paintings has been sug¬
gested and sniali quaiititieSj glass fabric with a prepared ground^ have been
marketed by artists’ supply houses. Because of its inorganic nature and, hence,
its resistance to mildew and other agents of organic decay, it may be good for
special purposes. The fabric has high gloss and sheen, is harsh to the touch, is
not elastic, and is brittle when sized with glue. Glass fibre is marketed under
the trade name, hioerglas,’ by the Oweiis-Corning Fiberglas Corporation.

Gum Woods red gum, and satin walnut are names given to the Ltquidamhar
styraciflua^ a tree indigenous to North America. Species from other genera, how¬
ever, may be called ' gum wood.’ The w^ood from A. styraciflua is medium hard, is
easily worked, and has no important variations in structure. The heart-wood is
reddish brown and the sap-wood, pinkish white. The pronounced figure in tan¬
gential and radial sections is caused by dark streaks that are distinct from the
annual rings. The pores are small and rings are not easily noticeable in sections,
either cross or longitudinal. Medullary rays are quite dearly marked in strictly
radial sections. The wood is strongly inclined to shrink and warp.

Gypsum Cement. Plaster of Paris is not the only cement or plastering material
made from gypsum. When gypsum is burned at a higher temperature than that
used in making plaster of Paris, it loses all, or nearly all, its water of crystallization
and the product has poor setting qualities. If, however, this product is dipped in
certain salt solutions, such as alum, borax, cream of tartar, etc., and is again
calcined, a very useful cement, slower to set and more easily worked than plaster
of Paris, is obtained. These highly calcined gypsum products are known under
various proprietary and traditional names, such as Keene’s cement, Martin’s
cement, and Parian cement.

Hemp is derived from the bast fibres of the stem of Ccmuabis scitwd. It has
been used for centuries in the making of ropes and sailcloth, and the finer varieties
have been spun into yarns and woven into textiles. It probably originated in
India but will grow almost anywhere, and commercial varieties include Russian,
Italian, Chinese, and Arabian hemp fibres. In a few cases, chiefly during the XIX
and XX centuries, hemp cloth has been used as a support for paintings.

Hydraulic Cement is any cement that hardens by chemical combination with
water, but the term is usually restricted to that kind which is obtained by
burning a mixture of lime and clay and pulverizing the resulting clinkers, and
which, ordinarily, goes by the name, Portland cement (see Portland Cement).
Pozzolanic cements also come within this classification (see Pozzolana).

Iron. Since mediaeval times iron has had first place among the metals. Its
properties and method of preparation are familiar. Although for purposes of
armor-making it was beaten into thin sheets quite early, it probably was not rolled
into sheets to any great extent before the middle of the XVI century. Its tendency

236

Painting Materials

to oxidize or to rust is also well known. Although the art of coating copper vessels
with tin reached a high standard in Roman times (E. S. Hedges, p. 207), the prac¬
tice of coating sheet iron with tin (tin plate) was not general before the XVII
century. The coating of iron with zinc (galvanizing) probably dates from experi¬
ments of Melouin in 1741 (see Hedges, he. cit). Sheet iron and tin plate appear
to have been used only to a limited extent as a support for paintings. It may be
seen more frequently, however, as a support for miniatures, coats of arms, and
signs. Cennino Cennini’s painting on iron with oil (ed. Thompson, p. 57) is prob¬
ably not painting of an iron panel but painted design on an iron object.

Ivory. For commercial purposes the tusks of the walrus, the hippopotamus, and
other animals are considered to be ivory but that, in its strict sense, means the
material of the tusk of an elephant. This is a tooth and the two tusks are the upper
incisors. They grow during the life of the animal, forming themselves out of phos¬
phates and similar substances. Structurally, they lie in layers, the one inside being
the last produced. Ivory is very dense; its pores are close and compact and they
are filled with a gelatinous solution. Arcs of slightly different color show in cross
sections of the tusk. These arcs intersect to make lozenge-shaped spaces in which
lie innumerable minute tubes closely placed and radiating outward in all direc¬
tions. The marking in cross sections is characteristic and is often used to distin¬
guish ivory from materials used for its imitation. Most ivory comes from Africa
but some is Asiatic and a certain amount is still derived from the remains of pre¬
historic animals in Siberia. Unlike bone, ivory can be worked immediately.

This has made it a favorite material for carving from neolithic times. It was
cut and fashioned and often stained long before it came into use as a support for
paint. Lucas (p. 310) reports that in ancient Egypt it was used as a ground for
writing. By the days of Greek ascendancy it was evidently a common practice to
paint on ivory, for Pliny (XXXV, 149) mentions it quite definitely in that con¬
nection. It is probable that the practice continued, though mention of it is rare.
Theophilus (chap. CX) speaks of ornamenting ivory with gold leaf, and Pierre
Lebrun, Recueil des Essaies des Merveilles de la Peinture, 1635, chap. XXXVII
(Merrifield, II, 820—821) says. Ivory must be washed with the liquid which is
found under horse dung, for the colors can not be applied without this secret and
invention.’

Ivory, cut into thin slices, has been much used as a support for miniature
painting. It became popular in England and France for that purpose in the XVIII
century because it was the most suitable material for painting with transparent
colors. Sometimes curved slices of ivory were flattened out by hydraulic pressure,
making unusually large sheets. Some of these, however, have curled and cracked
with age^ (see Encyclopaedia Britannica, 14th ed., on Miniature Painting).

Jute is derived from several species of Corchorus, especially in India and China,
where it has been cultivated for centuries. It was first introduced into Europe in
the latter part of the XVIII century. The commercial fibre is 4 to 8 feet long but

Supports

237

the ultimate fibre is only i to 5 mm. long, and has a large lumen and thin walls.
The tensile strength and wearing qualities are poor. The fibre contains much
lignin and has not good keeping quality. It is made into sackcloth and as such has
found occasional use as a paint support.

Larch {Larix). The common larch {L. europaed)^ a conifer, is a native of
Europe and Asia. The North American representatives, Z. lartcina (tamarack)
and Z. occidentalism are similar. The wood is coarse in texture and tough but,
unless carefully seasoned, it is liable to warp and bend; it is heavy and straight-
grained. The color of the heart-wood is russet to reddish brown. The annual rings
are distinct. In longitudinal section there are no characteristic figure or ray
markings. Although it is a tree well known in Europe, records do not show that
it was extensively used in panel painting except for a small number by the
German schools.

Leather (see also Parchment). This material is made from the skins of various
animals. These are prepared by a process knowm as ' tanning ’ and are so rendered
flexible and imputrescible. (Raw skins, being subject to bacteriological as well as
chemical disintegration, readily putrefy.) The chief ingredient of tanning sub¬
stances is tannin or tannic acid, secreted by various parts of a large number of
plants, and these are usually classified as Pyrogallolsm including chestnut, oak,
sumach, and others, and Catechohm including hemlock, mangrove, larch, birch,
and others. Oak-bark {Ouercus robur) and Vaolnia {Quercus aegilops) contain
both types and, because of the blend, produce excellent leathers. The fibres of the
plants are crushed and are then leached by infusion with water to produce a tan
liquor. (Under modern methods of manufacture this liquor is then tested for its
tannin content and for its acidity.) The skins or hides are cleaned and softened and
have the hair and the scarf skin or epidermis removed- The latter process is
usually one of liming and scraping. Lime, so used, is removed by washing the skins
in weak acid solutions. After that they are hung in successively different strengths
of tan liquor. When tanning and drying are finished, the leather is usually treated
with oils and is pared and smoothed, a process known as ‘ currying,’

Although the tanning of leather is a craft which appears to be very old, the
product has never been in great favor as a support for paint. The reasons for this
are obvious and are both technical and economic. Leather did, however, figure
among the properties of interior decoration as well as for personal adornment and
for numerous accessories of dress and equipment. Paint was used frequently in
the designs laid on such objects. Painted leather harness is often found and a
Roman parade shield covered with leather on which are painted representations
has been discovered at Dura-Europos in Syria. Stamping and gilding leather was
an old craft in the Venetian lagoon and work of this order spread into other re¬
gions of Europe. The Mediterranean basin seems to have remained the principal
source, however, of European leather in the middle ages, much of it coming from
the countries of the Moors and Saracens in Africa, or from Cordova, or Marseilles.

238

Painting Materials

Mrs Merrifield points out (I, cxi) that Eraclius described painting on plain leather
stretched on a panel and that Marco Rizzi is said to have painted on kid skins.
As late as the XVII century there is written evidence for a survival of this
practice. The MSS of De Mayerne (Berger, IV, an) refer to a color for painting
leather and also to a ground for painting on leather (Berger, IV, 277).

Lime Plaster (see also Plaster). Since prehistoric times lime has been one of
the most important plastering materials. Pure lime itself or ‘ neat ’ lime is not
used except for very thin surface washes and then it is usually mixed with salt,
glue, or casein. In thick layers, neat lime shrinks and cracks. To avoid this, it is
mixed with sand, finely crushed stone (frequently marble), or fibrous, organic
matter. Magnesia (MgO) is a frequent impurity in limes, and the so-called
‘ magnesia limes ’ are derived from dolomitic limestones. Cowper (p. 16) says that
Italian limes contain up to 40 per cent magnesia. Magnesia limes slake more
slowly than high calcium limes. They expand less on slaking and shrink less on
drying and they set more slowly. A high magnesia lime is not considered a good
lime.

Lime is calcium oxide (CaO). It is prepared by heating one of the various forms
of calcium carbonate (CaCOs), limestonej marble, or chalk until nearly all of the

carbon dioxide is driven off, CaCOs ^ CaO + CO2 T . When freshly prepared,
the product is called ‘ quicklime ’ or ‘ caustic lime,’ for it readily combines with
water and decomposes organic matter. When quicklime is exposed to the air for
some time, it becomes air-slaked for the above reaction is reversible and the lime
combines with atmospheric carbon dioxide and returns to calcium carbonate.
When water is added to quicklime, the two combine vigorously with the liberation
of much^heat. The product formed, calcium hydrate, is the slaked lime so im¬
portant in the Reparation of mortar, CaO + H^O Ca(OH)2. In fine mortars
and plasters it is essential that lime in this hydrated condition should be thor¬
oughly seasoned, even over a period of months, before it is used. Calcium hydrate
combines with atmospheric carbon dioxide to form calcium carbonate, Ca(OH)2
+ CO2 CaCOa + H2O. This last reaction is essential to the setting of mortar.

There are two stages in the setting of lime plaster. The first is the drying out
of the water and the second is the subsequent hardening by chemical action be¬
tween the calcium hydrate and the carbon dioxide of the air, resulting in the for¬
mation of calcium carbonate and the liberation of a further quantity of water
whmh evaporates. To the formation of a coherent mass of crystals of calcium
cayonate—^aRically limestone—is, then, attributed generally the strength of
old plaster. This process of carbonation is excessively slow and generally in-
complete. Minute traces of quicklime were found in the pure lime plaster on the
walls of the palace at Knossos, in Crete, which had been exposed freely to the
atmosphere for about 3500 years; and free lime is found in the plaster used in the
yramids. In 19 was reported that free lime was present in most of the samples
of mortar taken from St Paul’s Cathedral, London, and as much as ao per cent

Supports

239

was found in one specimen taken from the core of a main pier. Various investi-
gators have observed that little absorption of atmospheric carbon dioxide takes
place until mortar is fairly dry^ but not completely dry. The process of carbonation
is greatly accelerated by subsequent periodic wetting of the mortar. The useful
penetration of atmospheric carbon dioxide is but one twenty-fifth inch in interior
plaster work.

Lime“-Wood {Tilia). The general name^ Tilia europaea^ or European lime or
linden, includes several vrell marked sub-species which are found growing quite
generally over northern Europe. The American bass-wood {Tilia americand) is
similar. The lime sometimes acquires great size and is valuable as a timber tree.
The wood is soft and light and is one of the white-woods ^ of commerce. The an¬
nual rings are only fairly distinct, even though the pores are very small and dif¬
fusely distributed. In longitudinal section neither the figure nor the ray is con¬
spicuous. The color is generally light, often a mellow, creamy brown. Linden has
been reported as the wood in a fairly large number of panel paintings of the
German schools. Pliny (XVI, 30) mentions it and describes the tree as growing
in the mountain valleys of Italy.

Linen is made from fibres (see Fabrics and Fibrous Substances) of the flax
plant {Lhmm usitatisshnum) which yield about 8 per cent of these fibres. They
are ultimately 6 to 60 mm. long and 0.012 to 0.026 mm. wide. The cellulose con¬
tent is between 70 and 80 per cent. The fibres are obtained from the green bark
by rotting or retting, cleaning, and bleaching. They are very tough, can be
bleached wTite, and take, more readily than cotton, a variety of colors by dyeing.
Microscopically, linen fibre may be distinguished from cotton, with which it is
most likely to be confused, by the fact that it shows joint-like cross markings and
does not twist, whereas the cotton fibre shows no cross markings and does have
twists (see figure 2, p. 231).

Specimens of linen have been found in the villages of the Lake Dwellers and in
other prehistoric remains. The wrappings of most of the Egyptian mummies are
of linen, and fine linens have been made from ancient times. Linen has been woven
successfully in x 4 merica only in the coarser forms of crash and toweling. Scotland,
Ireland, and Belgium produce the finest linens of commerce today, and Russia,
the largest amount of flax. So common has this material been as a paint support
that it is almost a synonym for canvas, the usual name for a fabric that is painted.

Mahogany. There are two principal varieties of this wood—West Indies
{Swietenia mahagoju); East coast, tropical America {Swietenia macrophylla )—
and both, among others, are indigenous to tropical parts of America. The former
is harder and darker than the latter, and is a richer red in color. In both varieties
the heart-wood is reddish brown and darkens on exposure to light. The sap-wood
is narrow, and ranges from light brown to white. The growth layers are practically
homogeneous in character, with little variation in pore size, and they are marked
by very thin, light-colored lines. These are the chief marking device, taking the

240

Painting Materials

form of soft, irregular lines in tangential section and straight lines in quarter-
sawed section. The most distinctive feature is the appearance of the various
markings in reflected light; almost all of them appear relatively lighter or darker
than adjacent areas, depending upon the source of the light. As a paint support,
mahogany has been considerably used since the tropical regions of America were
opened to commerce and it is a favorite material for cradles and other accessory
supports. It is said to have been used as a furniture wood in the early part of the
XVIII century. The timbers of several other species of wood are used and known
as mahogany, and those of the West African Khaya senegalensis are known as
African mahogany (see A. Koehler, ‘ The Identification of True Mahogany, Cer¬
tain So-Called Mahoganies, and Some Common Substitutes,' Bulletin No. 10^0,
C 7 . S, Department of Agriculture^ 1922).

Maple {Acer). The common maple of Europe {A. campestri) is not an impor¬
tant timber tree and it appears that the wood is seldom used for painted panels.
In America the sugar or hard maple {A. saccharinum) furnishes timber of eco¬
nomic importance. The sugar maple is hard, does not readily warp or shrink, and
has superior working qualities. The heart-wood of the hard maple is a light
reddish brown; it has no conspicuous figure, and Is diffuse-porous, the pores being
very small. The annual rings are, however, fairly distinct. Rays are conspicuous
as fine dashes in quarter-sawed surfaces and are similar to those in beech.

Metals. Metals have never been used extensively as supports for paint. In
ancient times they were probably too highly valued for other purposes and were
not available in large sheets or panels. Sheets for armor and such uses had to be
beaten out laboriously by hand; methods for rolling metals were not developed
before the middle of the XVI century. Metal-rolling mills driven by water-power
were in use In Europe in the first part of the XVIII century but steam-power for
this purpose, with a subsequent large-scale production, did not come into general
use before the latter part of the same century.

The great difficulty with metal of any kind as a support for paint, at the
present day, is a flexibility greater than that of the paint, ground, and surface
films of the design itself. The value of a support is the strength and firmness it
supplies for those films. To make metal sufficiently rigid is to make it so heavy
that it becomes practically unmanageable. Expansion of metal with heat has been
argued against it, but since the highest coefficient of thermal expansion for any
of the commonly used metals is that of aluminum, given at 24 X 10“®, and that
for oak is 54.4 X io“® with other common woods all above 32 X lo""®, that ob¬
jection does not hold comparatively. The real objection to it lies in its tendency
to be bent, dented, or flexed.

Below are listed some of the physical properties of the metals generally used.
(These are taken from Smithsonian Tables.^ Vol. LXIII, 1914, and from Handbook
of Chemistry and PhysicSy 0 .ot\itA.. [Cleveland: Chemical Rubber Publishing Co.,
^934]- For other properties, see Aluminum, Copper, and Iron.)

Supports

24]

Relative
Hardness
of the
Elements

Coefficient of
Thermal Expansion *
(Approximate)

Specific

Gravity

Copper

3.0

14X10“^

8.9

Brass (66 Cu, 34 Zn)

—

19X10“®

8.4

Iron

4.5

12X10“®

7.8

iliiiminum

2.9

24 X io“®

2.7

* The increase in length per unit length (measured at 0° C.) per degree centigrade.

Mortar (see also Piaster) is a building material prepared by mixing lime^ plaster
of Paris or hydraulic cement with water and with sand and fibrous materials,
giving a mixture which sets by hydration or carbonation. It is used in masonry
and in plaster. The term ' mortar ’ is ordinarily restricted to the mixture of the
components of plaster while they are still in the plastic condition after the addition
of water. The term is also applied to any one of these mixtures when used as a
cementing or bonding material for stone and bricks.

Mulberry or Paper Mulberry. The fibres of the paper mulberry {Broussonetia
papyrijera) are used by the Chinese and Japanese in the preparation'of certain
special papers (see also Paper). They are separated from the inner bark of the
tree by scraping, soaking, and maceration in water, and they may be purified
still further by boiling in weak alkaline solutions. As used in Oriental papers, the
fibres are generally unbroken. They are long and slender and they vary in length
from 6 to 20 mm., with an average width of 0.030 mm. They are nearly trans¬
parent under the microscope and show transverse jointings as well as longitudinal
striae. The central canal generally shows as a well defined line and the ends are
sometimes blunt and rounded, sometimes fringed (see also Fibrous Substances).

Oak (Quercus), The principal European species of oak are Q. pedunculata and
0 . sessiliflora; the two principal species in North America are the white oak
{Q, alba) and red oak {Q, borealis). Oak is a hard wooi, grayish brown in the
heart-wood and white in the sap-wood. The red oak varieties have a reddish tinge
in the heart-wood. The figure is produced by the strongly marked annual rings in
tangential section and consists of straight lines, irregular curves, and parabolas.
In quartered section the figure is produced chiefly by the large medullary rays
which appear as more or less continuous ‘ flakes ’ across the grain of the wood.
The annual rings are very distinct in end surfaces, with marked segregation of
very large pores at the beginning of each yearns growth and very minute pores
in the denser, outer portions. The rays are also strongly marked, being of two types
—large (regular and conspicuous) and very small. The size and distribution of the
large rays make it easy to distinguish oaks from all other woods. Most oak-wood

Painting Materials

contains tannin. In European panel paintings, particularly of the Flemish school,
oak was much used (see also Wood).

Olive-Wood {Olea). This genus includes about ao species widely scattered,
chiefly over the Old World from the basin of the Mediterranean to South Africa.
Among them, the common olive tree ( 0 . europaea) is most important. Although
it grows to no great height, it frequently develops trunks large enough for timber.
The wood, which is hard and-close-grained, is valued by cabinet makers and by
wood-turners; it is yellow or light, greenish brown in hue but is often veined with
a darker tint. It may be polished to give a smooth marble-like surface similar to
that of boxwood. The pores are scarce and obscure. It is known that a few Italian
paintings were made on olive-wood panels.

Panel (see also Wood) is a word generally used to describe all movable or easel
paintings that are not executed on a support of fabric. At times, however, it is
used even more broadly to designate all paintings not done on walls, as, ‘ the
panel painting of the Italian Renaissance.^ Technically, panels are understood
usually to be supports of wood.

Paper. The papers used in the fine arts are commonly made from cotton or
linen but the Far East and the classical world have been used to coarser plant
materials like mulberry and papyrus. When any such fibres are separated by mac¬
erating the plant or other source into a pulp and then are disposed in a thin,
compact web, dried in a sheet, and pressed flat, the product is some class of paper.
The chief material of all paper is cellulose, the pure plant substance which forms the
cell walls, and this, regardless of its source, is made up of carbon, hydrogen, and
oxygen, present in the same proportions (see also Fibrous Substances), Classes of
paper can be distinguished most conveniently for study according to the sources
of the fibre, though in industry it is a frequent practice to classify papers according
to their particular uses. H. N. Lee has made a convenient arrangement of paper-
making fibres: (i) the cotton group, including cotton, linen, ramie, hemp, and
perhaps paper mulberry; (2) the wood-pulp group including the two distinct
classes— {a) ground wood or mechanical wood-pulp, the entire wood substance
reduced to a fibrous form, and {b) chemical wood-pulp in which intercellular
matter has been considerably dissolved away leaving a much purer cellulose; (3)
the grass group, including esparto and bamboos; and (4) the rope group, a kind
used only for rough paper in localities of the Far East.

Although cellulose remains the same, the characteristics of the different
groups of fibres vary to a great extent. Cotton fibre has a natural full length of
20 to 40 mm.; linen runs usually from 25 to 30 mm. and has a less uniform diam¬
eter than cotton. Paper mulberry fibre shows great variation in width and has a
natural full length of about 15 mm. Wood-pulp fibre is much shorter, its natural
full length being about 3 mm. It can be seen that the longer fibre makes a stronger
paper, and it has also been suggested that the fibres of the cotton group contain
a much purer cellulose than those from wood or grass. The latter are more apt to

Supports

243

deteriorate because they are more easily hydrolyzed and oxidized. Certainly
experience with papers has shown that, in general, those made from cotton-group
fibres are far more stable as to color and consistency than those as yet made from
the wood-pulp group.

Besides the fibres themselves or any impurities that go with them, paper may
contain other materials added for the purpose of giving it certain kinds of surface
properties. Loading materials, chiefly China clay, are used mainly in the manu¬
facture of the coated or glossy papers required for some kinds of printing. Sizing
materials are more common and are apt to appear in western products of the
presePxt day except in blotting and filter papers. Glue and starch used for sizing
seem to have only a slightly harmful effect. They are apt to increase a tendency
towards the development of mold and micro-organisms. Rosin, on the other
hand, is definitely a bad addition to paper fibre, for in a fairly short time it be¬
comes dark and brittle.

The most serious and rapid breakdown of paper probably takes place in these
added materials, if the fibre itself is pure. Fibrous cellulose is naturally very
inert and resistant but it has a number of points where it is vulnerable. It de¬
composes at 250° C. and the products are complex and numerous. Evidently it is
able to withstand effects of atmospheric oxygen to a large extent. It is even
resistant to the action of weak oxidizing agents and bleaching solutions. This
resistance has a definite limit, however, and when the strength of the solution
becomes too great, destruction takes place. The products are chiefly oxalic and
carbonic acids and there are left, also, the residues known as oxycelluloses. These
vary according to the oxidizing agent, its strength and temperature, and according
to hydrolytic reactions that occur at the same time. In all cases, they are weak,
friable substances and no means are known for converting them back into cellu¬
lose. Pure cellulose is attacked by dilute acids and produces either dextrins and
dextrose, soluble substances, or insoluble materials called hydrocelluloses. The
effect of micro-organisms on cellulose is well known; bacterial fermentation is the
means by which it is digested. And it has been noticed that many such organisms
are able to develop and to decompose cellulose in a free supply of air. Mold'is an
enemy of paper and is doubtless responsible for much of its discoloration. The
brown spots or ' foxing ’ so familiar in old books, prints, and drawings are ap¬
parently stains caused by the decomposition of mold and micro-organisms. It
seems probable, however, that mold itself is little apt to grow on pure cellulose
and that its first nourishment at least comes from added materials, chiefly animal
glue and casein.

The ancient papyrus was made directly from the paper reed, Cyperus papyrus
of Linnaeus. This was cut into long strips and those from the center of the plant
were found to be the broadest and the most valuable. The strips were laid out,
so that they crossed at right angles, on boards and were soaked in water (tradi¬
tionally that of the Nile). After soaking, the web formed by the crossed strips

244

Painting Materials

was hammered flat and was dried in the sun. The method is peculiar to papyrus.
Paper, as usually defined, is made from the separated fibres reduced to a pulp
and felted into sheets in which the direction and arrangement of the fibres have
no relation to what they were in the plant. In its natural condition plant fibre is
associated with intercellular matters of a glutinous, resinous, or siliceous character
and the removal of these is the first step in producing a workable pulp.

The pulp for Chinese paper came largely from the inner bark of the bamboo
and the paper mulberry tree. Hemp was used, cotton and linen rags were some¬
times added, and occasionally refuse silk may have been put in, though alone it
would not have been suitable. There is a coarse Chinese paper made of straw
but this is used as a wrapping.

Soon after paper-making had come into Europe, it was found that a fine pulp
could be made with relative ease from old rags of cotton and linen, for the cloth
had already gone through a process that had cleared the fibres of most extraneous
materials. Rag papers are still the finest support of the paper kind to be had for
paintings and similar works of art. In modern manufacture, the rags that come to
the mill are dusted, sorted, cut into small pieces, and boiled in an alkaline solution
under low steam pressure during six to twelve hours. They are then washed and
are broken in a machine that contains a roll and a steel plate so arranged that
they tease out the fibres, destroying the textile nature of the material and reducing
it to a pulp. The pulp is then given a chlorine bleach. After that comes beating
which makes it absorbent and felted in its structure. When paper is made by hand,
it is formed from the wet pulp on a wire screen that is the size of the sheet and has
a removable frame, the deckel. The pulp is shaken out on the screen, and when
the sheet is formed, it is laid out on a felt. A number of them are piled and pressed
and then are dried. Such a method is little different from that described by Hans
Sachs, cobbler-poet of Nurnberg, in a German book of trades, 1568 (translation
from Hunter, Paper-Making, etc., p. 22);

Rags are brought into my mill
Where much water turns the wheel.

They are cut and torn and shredded,

To the pulp is water added;

Then the sheets twixt felts must lie
While I wring them in my press.

Lastly, hang them up to dry
Snow-white in glossy loveliness.

When practically dry, the paper is separated from the felts and taken to lofts
where the individual sheets are spread out. Paper at this stage is called ‘ water
leaf or ^ rough and is highly absorbent, but is used directly by some for water
color painting. To overcome the absorbency, the paper is put through a sizing
bath, and is again dried and pressed (rolled) to give the desired finish; cold pressed
(C.P.) is paper of medium absorbency, and hot pressed (H.P.) is denser and has

Supports

245

a smooth^ almost glazed surface* Several materials have been used for screens in
the shallow sieve that collects and felts paper fibres. In a few instances;^ the bottom
has been made of cloth. Fine reeds were generally the material used in the Far
East. In the West paper has been formed usually on wire screens, and papers made
on such a screen usually show, especially when held to the light, a faint pattern
of the screen which is called a ' wire mark." The marks are caused by the tendency
of the pulp to settle more between the wires than over them and so their positions
are faintly marked by an alternation in densities. Special characteristics in the
wire marks sometimes give a clue to the time-and origin of the paper.

\¥ater-marks, probably suggested by the screen marks or wire marks im¬
pressed in paper during hand manufacture, are the initials, names, seals, or sym¬
bols marked in Occidental paper by the same means. They are made by wdres
which are set into the screen on which the sheet is formed. The wet pulp settles
out more thinly along the lines of the design made by these wires and the design
shows as somewhat more translucent than the sheet around it. Oriental papers
have no water-marks, nor have those of Arabian and European manufacture be¬
fore the end of the XIII century. They first appear in the product of either the
mills of Bologna, 1285, or the mills of Fabriano in. 1293 and 1294. Since then they
have been common and the history of them occupies many volumes (see Briquet,
in particular). In the XIV century they are characterized by simple designs and
large wires. For a short time (1307-1320) in Fabriano the maker’s full name was
used, but the mills then went back to simple initials until the XVI century. By
1545 the date of manufacture began to be added to the full name of the maker.
Until about 1790 they were usually called ' paper-marks."

When paper is made By machine, the wet pulp flows out on a moving, endless
mold of fine wire doth. It is beaten, strained, cleaned, and carried to a screen of
brass doth supported by tube rolls. This is forty to fifty feet long. As thin pulp it
goes over suction boxes and through rolls. Then, as paper, it is passed on felts
through other rolls that take out moisture and over heated cylinders for further
drying. The better papers are ‘ tub-sized" with gelatin or animal glue, containing
alum, after they leave the drying cylinders.

The preparation of pulp from materials other than rags is somewhat more
complicated. W^hen raw fibre is taken for paper-making, it is necessary to remove
as much as possible of intercellular material (see also Fibrous Substances). In the
case of esparto the grass is boiled at a high temperature, and for three or four hours,
in a strong solution of caustic soda. A steam pressure of thirty to forty pounds per
square inch is usually maintained. The fibres are drained, washed, and bleached,
intercellular matter being largely digested by the caustic soda. Roots and other
impurities are taken out of the pulp. A method similar to this is used for straw.

Mechanical w^ood-pulp is little more than sawdust matted into sheets. For
chemical pulp three methods of preparation have to be distinguished: the soda
process, the sulphate process, and the sulphite process. The soda process is used

Painting Materials

246

principally for poplar, birch, and similar hard-woods. In its general lines it is not
very different from that used for esparto and straw. (A full description is given by
Sutermeister, pp. 106 ff.) Wood that is to be made into pulp by any of the three
chemical processes is first cut into logs eight to twelve inches in diameter and four
feet long; these are reduced to chips each under an inch in size. The sulphate
process is essentially alkaline, and the chips go into a soda bath. The lost alkali,
however, is made up in this method by sodium sulphate instead of soda ash. The
sulphite process is acid. In this treatment of the chipped wood, intercellular
matter is decomposed by boiling at high temperatures in the presence of sul¬
phurous acid. Magnesia or lime is added and the steam pressure is 90 to 150 pounds
per square inch. Sulphite pulp has strong and hard fibres whereas the soda pulps
are comparatively soft and mellow.

Paper, conservation and treatment. It has been noticed (see also Paper) that
pure cellulose fibre is the most resistant and permanent part of paper. This is
affected only by very high temperatures, by acids, alkalis, strong bleaching
solutions (mainly oxidizing agents) and a few other means of destruction most of
them very rare under normal housing conditions. Added materials in paper are
apt to cause general discoloration, brittleness, and an aggravation of mold or
bacterial growth. To prevent autoxidation of a resin, for instance, is impossible
by any methods known, but many other ill effects on paper can be prevented or
kept safely down by general care in the mounting, exhibition, and storage of such
objects. High humidity is to be avoided and dry, well-aired storage is indispen¬
sable. Similar conditions must be maintained for exhibition. Bad mounting has
been responsible for much loss and many blemishes in paper objects. Drawings,
prints, or water colors, if they have good care, are given mats, usually of the ‘ win¬
dow ’ type, and made of rag pulp. They are attached to the mats only by small
hinges of adhesive at the upper corners, are kept, with slip papers for protection,,
in light, dry boxes, and, when exhibited, are put under glass.

Certain types of treatment for paper have a value that is largely preventive.
They are to stop any tendency toward deterioration that may have been ob¬
served. Fumigation is one of these and is done largely to destroy any mold or
bacterial growth or to kill spores before they develop. Thymol has been much
used for this purpose and so has formaldehyde. Another preventive treatment is
removal of paper from mounting boards that tend to discolor or embrittle it.
Restorative treatment is also frequent. Removal of grease spots or of dirt can
usually be done with organic solvents. Removal of water stains or foxed marks—
discoloration of the fibre itself—^is more difficult and less certain, but there are
now established methods for it. In all methods oxygen is the active agent. It
has been employed as a vapor, the evaporation from a plaster block saturated
with hydrogen peroxide and placed with the paper in a sealed chamber being
enough to effect bleaching. As a rule, however, the paper is immersed in a bath
of bleaching solution followed by a neutralizing bath and careful washing. One

Supports

247

process (Keck, loc. cit,) has for its bleach very dilute sodium hypochlorite and
hydrochloric acid in a practically neutral mixture. Free chlorine is evolved from
this. Hypochlorous acid, formed by the action of chlorine on water, oxidizes the
organic coloring substances that make up the stains. Besides this, of course, there
is a wide range of operations in the mending and repair of paper.

Paper, history in painting. The history of paper as a support for drawings and
paintings is, to a large extent, the history of the development of paper itself. The
oldest papyrus now preserved is Egyptian and has on it the accounts of King
Asa (3580-3536 B.C.). The manufacture of this ancient forerunner of paper was
probably carried on largely In the delta of the Nile, but the use of it became com¬
mon all through the Mediterranean region and reached to imperial Rome. The
plant from which it was made came to be grown rather widely—in Syria, Arabia,
India, Nubia, and as far west as Sicily. By the X century A.D. paper had dis¬
placed it.

The Chinese, using plant fibre reduced to pulp, were making paper certainly
in the II century A.D and may have started the practice even before the Christian
era. Introduction of the manufacture to the West is usually dated from a Chinese
attack on Samarkand, 721. Arabs held the place and, according to tradition,
learned the art of paper-making from Chinese prisoners taken at that time.
Karabacek (p. xxi) says that the dissemination of paper through the West began
with the establishment of the second paper factory at Bagdad in 794-795 under
the reign of Harun AI-Rashid. He lists (p. 245) items 917 and 918 in the collection
of the Archduke Rainer, two letters on paper of about 800. A large number of
Arabic MSS on paper dated in the IX century are still preserved. The Arabs
practiced and spread the craft for some centuries. Apparently linen fibre was
principally used in their product, but other materials, raw cotton or rags, were
sometimes added in small amounts. This eastern-made paper of the middle ages
was strong and glossy and it contained no water-marks. Coming through Greece
it got to be known as charta homhyclna^ or charta^ or even papyrus.

By 1100 paper was being manufactured at Morocco. In 1102, King Roger of
Sicily made a deed on paper that is still preserved. The first manufacture in Europe
was by Moorish workmen located largely at Xativa, Valencia, and Toledo.
France had a paper mill at Herault in 1189, Italy had mills at Montefano and at
Fabriano in 1276, Germany, one at Cologne in 1320 and another at Niirnberg in
1390. Italy’s first great center of manufacture was Fabriano and it was here that
the first known water-marks were made in 1293 and 1294. Paper was used in
England as early at least as the first years of the XIV century. A MS. in the
British Museum (Add. 31, 223), a register of the hustings court of Lyme Regis,
is on paper and the entries begin in 1309. This paper is like that made in Spain,
There is no record of English manufacture before the XVI century but probably
there were mills before then.

Until the XVIII century practically all European paper was made from rags.
A publication of a ‘ Society of Gentlemen,’ London, 1716, Essay VI, suggests

248

Painting Materials

using raw hemp for the pulp, and Rene Antoine Ferchault de Reaumur suggested
to the French Royal Academy (November 15, 1719) that wood or plant fibres
would be suitable for the purpose. This suggestion was carried further by Dr
Jacob Christian Schaffer (1718-1790), a citizen of Regensburg, who wrote a six-
volume treatise on the subject. It was well into the XIX century, however, when
wood-pulp was used on any extensive scale for paper-making. The paper machine
was invented in France in 1798 by Louis Robert and was developed there and in
England during the early years of the next century.

Through all of its history paper has been used primarily as a writing material,
but probably parallel to that has been its employment as a support for drawing
and for painting. It stands with silk and linen in the scroll paintings of China and
Japan. An early mention of it In connection with the arts of Europe occurs in the
MS. of the monk, Theophilus (XI-XII century), where there is a reference to
\ , Greek parchment, which is made from linen cloth" (i, XXIV). By the

time of Cennino Cennini, paper was a regular material in the workshop of the
painter. He speaks of its use for drawing (chaps X and XII), about methods for
tinting it (chaps XV, XVI, XVIII, and others), and of how to make tracing papers
(chaps XXIII-XXVI). From this time, c 1400, it had an established place in the
making of European works of art. Those preserved from the Renaissance are largely
prints or drawings but some paintings are found, notably a head attributed to
Memling and now in the Louvre, 2028A. This is a sketch in color on paper that is
tinted red-orange. It is probably from the development of such sketches and of
drawings with added color that paper came to be used so extensively in the XIX
century as a support for the so-called ' water color" painting. Besides its use
independently, paper has now and then been made a part of a more rigid, com¬
posite support. The small cardboard panels .(see also Artificial Building Boards
and Academy Board) are really made of paper-pulp, but aside from these, sheet
paper is frequently combined with wood or with cloth. A number of such construc¬
tions can be found dating from the XVI century in Europe,

^Papyrus (see also Paper). This ancient writing .material was made from the
strips of a reed, Cyperus papyrus. They were crossed on boards and then were
soaked, hammered, and dried.

Parchment (see also Leather). The distinction between parchment and vellum
is a difficult one to draw. According to some authors (for example, Johnston, p.
^73) vellum is a name which belongs to calfskin only, but is generally given to any
moderately good skin that is used for writing or painting. At times Vellum" is made
to designate what is more exactly called ^ uterine vellum —the delicate skins of
new-born or still-born calves, kids, or lambs. Practically, it is impossible to main¬
tain the distinction in any general description of these materials; one term can
conveniently be made to apply to both, for the materials are similar and the
preparation is the same. The skins best for use are those of calves, sheep and lambs,
goats and kids. They are prepared by washing, liming, unhairing, and scraping;

Supports

249

then follows a second washing after which the skins are stretched on frames, are
scraped again, pared, dusted with chalk, and rubbed with pumice. The resulting
skin is very pale in tone, being smooth and nearly white (if a good piece) on the
surface that was the original flesh side, and somewhat rougher and more yellow
on the surface that was the original hair side.

Skins were used as writing materials in ancient times by Near Eastern peoples,
Persians, Phoenicians, Jews, and Ionian Greeks. The word ' parchment' comes
from Pergamum. The story is that the Ptolemies, envious of the growth of the
Pergamene library, refused to export papyrus. Thrown back to the use of skins
for writing purposes, Eumenes II (197-158 B.C.), king of Pergamum, was respon¬
sible for developments in their manufacture. It appears certain that at about this
time preparation was enough improved so that both sides could be used. This
made possible the construction of a codex. Illumination of MSS is a practice that
must be almost as old as writing itself, and in Europe such painting on parchment
forms an important part of the history of the art. Pliny (XXXV, 67 and 68)
speaks of paintings on wood and on parchment, and this may be taken to mean a
kind of painting not connected with MS. illumination. By the II century A.D.
parchment was probably coming into Europe. In a short time its use was well
established and the skins available were thin and firm and had a delicate texture.
It seems quite evident that in the later middle ages and in the Renaissance the
painters who illuminated MSS thought of no other support. Even in the XVII
century there are numerous references to it by De Mayerne (see particularly
Berger, pp. 175, 177, and 216).

Pine {Finns). Although the genus, Finns^ includes many species, the name
‘ pine ’ is often applied to species of other genera, especially to some of the firs.
Modern botanists, however, reserve the name for the spruces and silver firs of the
genera Ficea and Abies. The needles of the true pines grow in clusters or tufts
from a membranous sheath whereas in the firs the needles are placed singly on the
shoots. The red pine (P. sylvestris)^ also known as the Baltic pine or Scotch fir,
is the most important timber conifer of Europe. It is found as far south as Italy
and Spain but prefers more northerly latitudes. The stone pine (P. Fined) is an
important timber tree of Italy; the wood is soft, fine-grained, and easily worked.
In America the white pine (P. strobus) furnishes valuable, white, soft, and even¬
grained lumber. Other valued American pines are the southern yellow pine (P.
mitis) and western yellow pine (P. ponderosd)^ both hard pines. The pine woods
are characterized by a resinous odor and the presence of resin ducts which may
often be seen, even without a lens. The chief structural component of pine wood,
as with other coniferous woods, is the tracheid with its bordered pits. The heart-
wood is usually light orange to reddish brown; the sap-wood is lighter in color.
Pines are divided into two general groups, according to structure: soft and hard.
In the soft pines, like the American white pine (P. strobus) y the wood is relatively
light in color and the annual rings are not particularly distinct, for there is slight

250

Painting Materials

iifFerence in the density between spring- and summer-wood. There is practically
10 figure or stripe in longitudinal section. The hard-wood group, comprising the
■/dlow or pitch pines, is distinguished usually by deeper color, pronounced annual
•ings and by a stronger odor of the resin. Quarter-sawed, hard pine has a striped
ippearance. Pine wood, in general, Is durable, does not warp badly, and is straight-
drained. According to the records, pine was widely used in European panel
)ainting5 but in these records the term ‘ pine ' probably includes woods from many
)ther conifers.

Plaster is the term given to the surface material, of a wall or building, which
las been applied as a plastic mass made by mixing certain dry materials with
vater and letting the mass set by drying, carbonation, or hydration. The term
plaster ’ is broadly applied to its three stages; the dry powder, the plastic mass,
)r the hardened surface. Although plaster may be made from various materials,
nodern usage confines the term to interior surfaces only. Many materials have
)een used in making it; all employ water for reducing the dry components to a
)lastic state. The active or setting components of plaster are commonly clay
vhich sets by drying, lime which sets by carbonation, and gypsum and pozzolana
vhich set by hydration. Inert materials, like sand and finely crushed stone, and
Ibrous binders, like hair, straw, and jute, are important ingredients in plaster,
land and other inert materials are necessary in preparing thick coats of lime
)laster since neat lime plaster shrinks and contracts on drying. This shrinkage is
dmost entirely prevented by the sand grains when these grains are In contact
vith each other. Very fine sand does not allow a proper distribution of the lime,
ince the pores are too small to be filled properly with the particles of lime hydrate.
51 ay plasters usually contain inert materials and fibrous binders to prevent undue
hrinking. Gypsum and pozzolanic plasters are not so dependent upon these fillers
or shrinkage prevention and binding but may have them added for economy.
Plaster, since earliest times, has served as a support for painting and decoration.
The pigment may be incorporated directly in the wet plaster, as in true fresco, or
t may be applied to the dried plaster surface with an organic binding medium,
is in fresco secco.

The wall upon which plaster is laid must be specially prepared for the reception
md retention of the plastic material. A creviced structure through which the
blaster is forced and keyed was, in early times, built up with reeds and saplings
joined to joists with cords; wooden laths, either split or sawn, have been used
For hundreds of years; metal and wire lathing are now common. The durability of
plaster work very often depends upon the sturdiness of the foundation. A brick
and masonry surface is frequently rough and porous enough for the direct ap¬
plication of plaster, provided the joints have been thoroughly raked out. It is
essential that the surface be not dry and porous because then water is sucked
from the plastic mass and poor setting results. The shrinkage and ultimate decay
of wood and other organic materials used as supports are often the beginning of

Supports

251

the deterioration, of plasters. When plaster in put on^ the wall^ whether of stone,
brick, or slate. Is first thoroughly wetted with lime or baryta water. One or more
coats of rough plaster are then laid on. These are allowed to set, and the rough
plaster is wetted with distilled or lime water before the final coat, one eighth to
one quarter inch in thickness, is applied. In the Greek method there were three
steps. The first coats were often composed of slaked lime with rough sand, pounded
brick, or tile, and' sometimes even straw, flax, or cotton. While this rough plaster
was still damp, the second coats of lime and river sand were laid on. The third step
was the application of the final coats. These were three and each w^as allowed to
dry. The plaster w^as composed of lime and fine marble dust. The surface obtained
by this process w^as so smooth and hard that it could be polished.

Paleolithic man undoubtedly knew of burning and slaking lime and, from the
abundant deposits of limestone and gypsum, made plasters that have lasted
through the centuries. In Egypt a day plaster is found, dating from, the pre¬
historic period. Two principal kinds w'^ere employed: the first, very coarse, of Mle
alluvium with generally a small amount of carbonate of lime and occasionally
gypsum. Being unburnt, this had no binding properties. The second, which is
still used as a finishing coat to sun-dried brick, consisted of a natural mixture of
clay and limestone. Preparatory to painting, these rough surfaces were usually
covered with a gypsum plaster, an exception being found at El-x4marna where
paint was applied directly to the first layers. Though there was plenty of lime in
ancient Egypt, gypsum seems to have been used almost exclusively for covering
temple interiors and exteriors preparatory to painting. A theory has been advanced
that gypsum was used because of a scarcity of fuel, for it requires a far lower
temperature for burning than lime. Analyses of plaster from the pyramid of
Cheops show 8a per cent calcium sulphate and about 10 per cent calcium carbon¬
ate. The Egyptians used plaster for coating brick and wood and for covering
mummy cases w 4 ich were subsequently painted. Herodotus speaks of burnt
gypsum and other ancient sources, notably Vitruvius and Pliny, tell of the use of
both lime and gypsum plaster.

There is a difference of opinion as to whether the fresco paintings at Knossos,
Crete (1500 B.C.), are true fresco or secco. It would be the earliest example of
true fresco. The ground plaster for the palace wall paintings has three layers: the
fi,rst is very coarse; the second is a relatively refined plaster; and the final coat is a
fine lime plaster without any intermixture of sand or marble dust. The lime had
probably remained slaked for a long period before being used. As the Egyptians
used fresco secco on their gypsum plaster grounds, it seems probable that the
paintings at Knossos were done In this same method, in spite of having been
executed on a lime plaster. A few centuries later, at Mycenae, a fine, white lime
stucco is found. The Greeks used plaster very liberally to cover both the interiors
and exteriors of temples and public buildings, for flooring pavements, and for
covering statues. The plaster coatings were then painted, though a pavement

Painting Materials

252

where the plaster has been stained with pigment during the slaking process is
found in several temples. The thickness of plaster walls in ancient times was much
greater than now. The Greeks employed a method of beating the stucco with an
implement called a haculi until the texture became so close and fine that it re¬
sembled marble.

Cowper (p. 4) says that there was much plaster work in Rome and that it was
the common external finish for private houses. Examples of the decorative modelled
stucco of Rome and Pompeii are well known. Vitruvius (II and VII) gives ex¬
plicit working instructions for the choice and use of lime^ sand, pozzolana, and
other materials for mortar and plaster, and he gives, also, details for the applica¬
tion of fine six-coat marble dust and lime plaster on reed laths for internal
decorative work. The old Roman wall paintings seem to have been executed on a
very smooth plaster. The pigment has probably been preserved because of the
wax film used for protection. In mediaeval times the art of making fine lime and
hydraulic mortars was lost, although poor lime mortars continued to be used. In
the Renaissance, however, interest in the ancient arts of decorative plaster and
stucco work was revived. Cowper (p. 5) says that Vitruvius was still accepted as
the authority on plaster.

Linen stiffened with plaster was used for decorating purposes in Egypt, and
Cennini, writing in 1437, speaks of fine linen soaked in glue and plaster and laid
on wood as a painting ground. Canvas and plaster were in general use in Great
Britain up to about 1850.

Wall paintings recovered from Central Asia by Stein, and dated from the end
of the III century A.D. to the X century or slightly later, show supporting walls
of a mud plaster containing, in some instances, an admixture of fibre or coarse
straw. The mud surface has been smoothed and a coat of whitewash applied over
it. The thickness of the slabs is from one and a half to two inches. Plaster and
decorated plaster work reached a high development in Asia Minor; Persian,
Moorish, and Saracen work, however, depended mainly on gypsum.

Painted plaster has been found in excavations in both the American continents,
in Asia, and in the Near East. Lime and gypsum plasters and Portland cement
are used today much as they were by the ancients. Modern mural painting, in
either true fresco or tempera, follows the same rules established centuries ago.

Plaster, deterioration and treatment. Clay plasters are perhaps the weakest,
lime and gypsum are stronger and pozzolanic plasters are the strongest. In the
drying-out and shrinking process they are subject to cracking in the order named.
This defect is counteracted by the proper proportion of filler and binder. Plaster
is subject, also, to slight stresses from thermal expansion and contraction. Walls
may suffer disfigurement through large settling cracks which run from corner to
corner. These are caused by movement of the building after the plaster has
set and are not necessarily defects in the plaster itself. Except for the pozzolanic
types, plasters are seriously affected by continued or alternate exposure to water,

Supports

253

with the result that they bulge and crack. If plasters contain soluble salt impuri¬
ties, such as calcium chloride or magnesium sulphate, they may effloresce with
changing humidity conditions. This may happen if sea sand or estuary sand is
used in the preparation. ‘ Pitting ^ or the appearance of small conical holes in the
surface of plaster is often the result of improper slaking in the preparation of the
mortar. i\fter the plaster has set and hardened, unslaked particles of lime slowly
become air-slaked; they increase in volume and drop from the surface, leaving
tiny craters.

In the treatment of deteriorated or broken plaster, one of the first steps is to
provide a proper support if that is lacking. When, plaster surfaces are not too
badly broken and disintegrated they can be repaired and filled up with materials
similar to those from which the plaster was originally made. Lime plaster can be
filled with fresh lime mortar or with plaster of Paris. When plaster is in bad
condition, A. Lucas (p. 185) recommends consolidation with melted paraffin wax.
Artificial resins and lacquers are finding a limited use for this purpose.

Piaster of Paris is the hemihydrated sulphate of calcium, 2CaS04*H20, and
is made by heating finely ground gypsum at a temperature around 145° C, When
mixed into a paste with water, it regains its water lost on heating and sets rapidly
to a moderately hard solid. If gypsum is heated to 200° C., all the crystal water
is expelled and the product will combine with water and set only very slowly.
The theoretical quantity of water necessary to set the piaster is about 18 per
cent of its weight, but, in practice, from 30 to 35 per cent is generally used. To
cut down the setting time glue, vegetable gums, or other colloidal matter are
added as retarders. Plaster of Paris expands slightly on setting and, hence, is
valuable for making casts and reproductions. The surface of set plaster of Paris
is not suitable for the application of paint because of its high absorbency. By
dipping plaster casts in melted wax, paraffin, or stearin, or by the application of
these in solvents, the pores are filled and the surface is made smooth so that dirt
will not adhere and the articles may be washed. Mixed with glue, plaster of Paris
sets to a much harder material which can be sanded and given an ivory-like
surface suitable as a ground for painting (see Gesso and Stucco).

Plywood and Veneer. Within recent years plywood has become an important
material in building construction. That now used is made from ^ rotary ^ veneer
cut in thin sheets from a log of wood. Three and sometimes five or seven sheets
of the ply are glued together with the grain of each extending in a direction at
right angles to that of the adjacent sheet. Plywood can be recognized not only by
the laminations at the edge, but also by the wavy grain of the surface, particularly
in coniferous woods where alternate wide bands of spring- and summer-wood are
exposed by the rotary cutting of the log. Large sheets up to 8 X 15 feet in dimen¬
sion are now available. Frequently, the Tore ’ or middle ply is thicker and of
cheaper wood than the surface plies. For furniture and interior paneling, the
surface ply may be of an expensive hard-wood like walnut or gum-wood, sliced or

254

Painting Materials

sawed. Coniferous woods are used throughout, however, for the cheaper ply¬
woods. Animal glue is perhaps the most extensively used adhesive in making ply¬
wood but fish glues, casein, blood albumin, and, in special cases, sodium silicate
and artificial resin plastics may be used. Obviously, the strength, durability of the
adhesion of the plies, and behavior to moisture and water depend largely on the
character of the adhesive.

Plywood has certain advantages over ordinary lumber. The plywood panel is
stronger, in some respects, than a single board of the same thickness. Large ply¬
wood panels are not likely to check, shrink, and warp so much as solid pieces of
the same size. They do not suffer so much from the grain characteristics of a solid
piece of wood. This is important since tensile strength, compression strength,
binding strength, and stiffness along the grain of the wood are twenty times more
than that across the grain. The main shortcoming of plywood is the liability to
failure of the adhesive between the plies. There are many poorly made plywoods
that come apart, layer by layer, after a short time. When made, however, with
synthetic resins, hot hide glue, blood albumin, or lime-casein adhesives and when
special care is taken in the selection of the woods, they are very durable and
lasting supports.

Poplar {Populus). Wood from trees of this genus are light in color and light
in weight. In Europe that of the white poplar (P. canescens) and the Lombardy
poplar {P.fastigiata) are the principal sources of timber poplar. Poplar wood is
soft, weak, fine-grained and fine-textured. It is diffuse-porous and the pores are
very small. The annual rings are fairly well defined but the rays are very fine and
are not visible without a lens. Poplar was frequently used in the Italian schools
for panel painting. Although poplar is sometimes known in the trade as ' white-
wood ’ this name is also used for the so-called yellow or tulip poplar {Liriodendron
tulipifera). Tulip poplar is frequently used when panel construction is called for
today. It is stronger and tougher than ordinary poplar; it is fine-grained and fine-
textured. The annual rings are indistinct and there is little or no figure in longi¬
tudinal section. The heart-wood is frequently yellowish green but it may be
streaked both white and black. The wood is diffuse-porous and the rays are fine.

Portland Cement is entirely an artificial product but it represents the most
important branch of the modern cement industry. It is a hydraulic cement ob¬
tained by burning a mixture of lime and clay and pulverizing the resulting clinker.
The clinker thus produced is mixed with a per cent gypsum and ground to 200
mesh. The product is a greenish gray powder that consists of basic calcium sili¬
cates, calcium aluminates, and calcium ferrites. When mixed with water it
solidifies into artificial rock. It derives its name from its similarity to Portland
stone.

Pozzolana is a cement derived from volcanic ash. Deposits of it are found in
Italy, near Naples (Pozzuoli), in the islands of the Grecian archipelago, and in
Germany on the Rhine. They are easily decomposable silicates which have re-

Supports

255

suited from the action of volcanic fires and need no further treatment than fine
grinding and mixing with lime. Pozzolanic cements are slow in hardening but have
a considerable ultimate strength. They have been used since the time of the
Romans who w'ere well acquainted with their properties. The term ‘ pozzolanic ’
has become broadened in its use to apply to those materials which may not be
cementitious but which will combine with hydrated lime at ordinary temperatures
in the presence of water to form stable, insoluble compounds of cementitious value.
Artificial pozzolanic materials may be derived from pounded bricks and tiles,
burnt clay, or granulated blast-furnace slag. Pozzolanic cements are very similar
to Portland cement in chemical composition; both set by the formation of calcium
aluminum silicates of complex chemical structure.

Ramie, Rhea, or China Grass are names given to a plant of the nettle family,
Boehmeria nivea, which grows wild within 36 degrees of the equator and has been
cultivated in China, Assam, and Java for centuries. The bast fibre has been suc¬
cessfully cleaned only by hand and can not be spun so easily as w'ool or cotton. In
China it is made into grass cloth by hand, but is not manufactured by machinery
anywhere. It has more than three times the tensile strength of linen or cotton,
has a very pure color, a good luster, fineness, and resistance to climatic conditions.

Relining Canvas is used in the backing of an old painting on fabric. A new
piece of fabric, the relining, is attached to the reverse side of the old by means
of an adhesive (see also Fabrics, conservation and treatment). The purpose is
usually to strengthen the support, but at times this treatment is applied where
the ground film has become loosened from the old fabric.

Silk is the natural product of certain moths. There are four or five main groups
of these, some cultivated and some wild. In external appearance silk is a solid
thread resembling a glass rod, but the fibre is really composed of three layers sur¬
rounding a tube (see also Fibrous Substances). The tube is filled with a fatty
matter which helps to preserve flexibility. No textile fibre has a larger proportion
of waste than silk. The fibre is a double filament and, prior to boiling off or degum-
ming in a hot solution of soap, it is harsh and relatively lacking in luster. In the
degumming about 30 per cent of the weight is lost. The microscope shows the
double filament irregularly coated with masses of sericin, but after boiling off it
appears as a shining, cylindrical, solid rod. Strong acids dissolve silk but weak
acids, such as tartaric and acetic, are absorbed and improve the luster. Dilute
alkalis do not affect it so much as they do wool, but hot concentrated solutions
rapidly dissolve it. Ammonia dissolves the sericin but attacks the fibroin very
slowly. Silk has great absorptive power for dyestuffs. The general method of
weighting silk is by means of solutions of metallic compounds, especially tin.
This results in a great loss in strength, especially upon exposure to sunlight.

The most striking physical property of silk is its luster. It is a highly absorbent
fibre and readily becomes impregnated or wetted with water. It stands a higher
temperature than wool without receiving injury. It can be heated to 110° G.

Painting Materials

56

ithout decomposition but at 170° C. it is rapidly disintegrated. It readily ab-
mbs dilute acids and in so doing increases in luster. Luster is diminished by dilute
Ikalis, however. Ammonia and soaps have no effect beyond dissolving the silk
lue or sericin. Silk absorbs metallic salts, particularly those of tin, and this
ehavior is made use of in the weighting of silk.

It is generally accepted that the silk industry originated in China. J. M.
latthews {Textile Fibers, p. 24a) says that historically it dates back to about
700 B.C. Tradition has it that in early history the cultivation of the silkworms
nd the preparation of the fibres were strictly guarded secrets known only to the
:hinese royal family. Gradually the industry spread throughout China but that
ivilization monopolized it for about 3000 years. In the early period of the Chris-
:an era sericulture was introduced into Japan. The Arabs acquired a knowledge
f the silk industry in the VIII century and it spread under Moorish influence to
pain, Sicily, and the African coast. In the XII century sericulture was practiced
r Italy, and in the XIII century, in France, but it did not become important
here until the reign of Louis XIV.

There is little evidence of silk as a paint support in Europe but it is common
ti the Far East and its history, traced in the paintings found, carries it back to
r’ang times (618-907). Probably before and certainly since then it has been the
hief material on which was executed the series of great scroll paintings made in
;hina and Japan. Painted silk was probably not often pictorial in the practice of
European workshops, though Cennino Cennini speaks of using linen or silk as if
he two were co-ordinate (ed. Thompson, p. 103).

Stone. The three general types of stone, based on method of formation, are;
ledimentary, igneous, and metamorphic. Sedimentary rock varies, in its mechani-
;al structure, according to the degree of consolidation or compactness and
iccording to the materials of which it is composed. One of its most characteristic
eatures is a stratified structure because of its origin in sediments of eroded ma-
;erial that has been deposited under water in horizontal layers. Since it is strati-
ied, sedimentary rock cleaves readily; slates and shales are good examples. The
principal types are: sandstone, shale, limestone, and gypsum. Sandstone is
:omposed of grains of sand, varying from microscopic to large grain, and bound
Dy a cementing medium such as silica, alumina, carbonate of lime, or oxide of
iron. The color, depending on the amount of iron, varies widely—cream, red,
blue, etc. The grains of sand may be composed of mineral quartz, granite, basalt,
limestone, etc. Shale is a soft, brittle rock, actually compact mud, silt, or clay.
Limestone is a dense, compact rock composed mostly of carbonate of lime. It
is porous and is often filled with shells or fossils. Gypsum, called alabaster in
England, is hydrated sulphate of lime, deposited by precipitation from sea water.
As a rock, gypsum is soft, usually fine-grained, and readily cleaved.

Igneous, or primary, rocks are formed by solidification of molten masses from
within the earth. They are coarse- or fine-grained, depending upon the rate of

Supports

257

cooling. They do not contain fossils or stratification but are sometimes made more
or less of mixtures of crystalline materials, fine or coarse. The most important
component minerals are feldspar, quartz, pyroxene, and hornblende. Granite is
most representative of the class. Other igneous rocks are diorite, basalt, etc.

Metamorphic rocks are sedimentary or igneous rocks changed in character by
mechanical movement of the earth's crust, by the chemical action of liquids and
gases, or by heat. They are highly crystalline, with a parallel structure at times
closely resembling stratification. The principal types are gneiss, mica-schist,
quartzite, slate, and marble. Gneisses are rocks containing feldspar, quartz, and
mica, with parallel or laminated structure. Mica-schist is a friable, slaty rock
containing quartz and mica, often dotted with common red garnets. Quartzite is
sandstone so firmly cemented by silica that fractures take place through the
quartz grains instead of around them. Slate rock originates from fine-grained,
sedimentary rocks; it is closely related to shale, and is sometimes called argillite
with reference to its origin from clay. Marble is the name given to the meta¬
morphic condition of sedimentary rocks formed by lime deposits such as lime¬
stone and chalk. Generally, marks of bedding, fossils, etc., are effaced and the
material is converted into crystalline grains of calcite. Dolomite marble is com¬
posed of a double carbonate of lime and magnesia in crystalline and granular
masses. Pure marble is white. The figures and coloring of ornamental varieties
are caused by impurities; the reds and yellows result from oxide of iron and the
neutral tones from organic matter. Marble is hard, compact, and takes a good
polish.

The use of stone as a paint support is largely architectural. That much of
stone, both flat and sculptured, carried paint in classical times is a commonplace
of history and it is to be supposed that much of the technical writing about how
to paint on stone had this end in view. By the XVI century in Europe, however,
it is found to carry occasional independent compositions. Perhaps the painter
most devoted to it was Alessandro Turchi, the Italian (i 582-^ 1648), but northern
painters used it also.

Stretcher. This is a frame, usually rectangular in shape and made of wood,
over which a fabric is pulled and held taut for purposes of painting. Thompson
{Tempera Paintings pp. 15-17) describes the use of stretchers in preparing
canvas. (See also figure 6, p. 286, and p. 312.)

Stucco is a term that has been loosely and broadly applied to a variety of
materials used in the plastic state for the covering of walls, for the preparation
of decorative details on buildings, and for the making of figures and reliefs. The
term is often used synonymously with lime plaster. It has been given to gypsum
and to pozzolanic materials, particularly when these are in figures and reliefs.
It is sometimes given to plaster of Paris that has been hardened by something
like lime water. Modern, technical use generally confines it to a plaster for ex¬
terior walls or other external surfaces of any building or structure. Here it is used

Painting Materials

258

without regard to the composition of the material, but in connection with its
location and purpose. Modern stucco often consists of small stones bound with
Portland cement. Occasionally the term is applied to surfaces prepared from an
inorganic filler and animal glue, in which case it is practically svnonvmous with
the term ‘ gesso ’ when that is used in the broadest sense.

Support, a term used to designate the physical structure which holds or carries
the ground or paint film of a painting. Panels, canvases, walls, and any of the
flat expanses on which paintings can be made, fall under this heading.

Sycamore is the name of a wood which is identified with three species of
tree; (i) Ficus sycomorus, a species of fig tree common in Egypt, Syria, and else¬
where, was widely used for general purposes in ancient Egypt. Lucas (p. 387)
notes that it was used in the construction of coffins, (a) Acer pseudoplatanus has
no connection with the Ficus sycomorus, but is a large species of maple common
in England and on the Continent. (3) Platanus occidentdis is the North American
button-wood or plane tree. The name ‘ sycamore ’ has been applied erroneously
to both of these. In the latter the heart-wood is reddish brown in varying degrees
and is often not clearly defined from the sap-wood, for the pores are diffusely
placed. The wood is heavy, lock-grained, and has a tendency to warp unless
properly seasoned. There is a distinct demarcation between the spring-wood and
the summer-wood although the pores are diffusely placed. The rays are nearly
all broad, dark, and conspicuous and many are more than twice as broad as the
average pore. The sycamore fig was undoubtedly used as a support for painting in
many of the Egyptian mummy portraits from the Fayum district.

Terra-Cotta is a term applied to baked clay products and these may vary
widely in composition. In modern, technical usage ‘ terra-cotta ’ means those clay
products used for structural, decorative work that can not be formed by ma¬
chinery. In sculpture, particularly of the Italian Renaissance, it made a surface
that was usually painted. It seems not to have furnished a support, however, for
independent designs.

Twill, a weave of fabric in which a series of regularly recurrent warp threads
pass in echelon over and under the weft threads, one after another and two or
more at a time, producing diagonal ribs or stepped patterns (figure I, p. 228).

VeHum (see also Parchment), a fine kind of parchment made, according to
some definitions, from calfskin; according to others k is from the skins of new¬
born or still-born calves, kids, or lambs. Generally it is applied to any moderately
good skin that is used for writing or painting.

Walnut is a wood derived principally from two species of the genus.

The European walnut (y. regia) ranges from England to the Far East. That im¬
ported from the Caucasus (Circassian) has always been the finest in quality. The
heart-wood is fawn-colored and numerous dark brown or black streaks are present;
these streaks determine the figure of the wood. It is heavy and straight-grained.
Although the spring-wood pores are large and clearly visible, it is not a distinctly

Supports

259

ring-porous wood. In tangential section the pores appear as well defined grooves
but the ravs are not very distinct. The American or black walnut is darker, more
uniform in color, and not so prominently figured as the European variety; it is
more distinctly ring-porous than European walnut. Most of the pores contain
glistening tyloses. The rays are fine and inconspicuous. The wood is comparatively
free from warping and has good gluing qualides. Walnut was used to a minor
extent by all the important European schools in panel painting.

White-Wood is a name given to woods from various species of the poplar. It
is also frequently applied to tulip poplar and to bass-wood (see also Poplar).

Willow {Salix). The European willow (.S’, alba) furnishes a, soft, light, nne-
grained wood similar in color and texture to poplar. The American black willow
is. nigra) is similar except that the heart-wood is reddish brown. One of the most
outstanding properties of willow-wood is that it does not split easily. The wood
is diffuse-porous but annual rings are distinctly visible. It has no distinct figure
in longitudinal section. Although the records do not show that willow was in any
way extensively used in panel painting in Europe, its valuable properties must

have suggested it for that purpose. ^ ^ ^

Wood. Woods differ in many respects. Some of their characteristics can be
recognized by the unaided eye but others are so fine that they can only be studied
or recognized with the microscope. Dr Eloise Gerry of the Forest Products
Laboratory, Madison, Wisconsin, has discussed the terms used in describing
woods, the distinguishing characteristics which make the differentiation of various
woods and the identification of species possible. The following is an abridgment
of Dr Gerry’s discussion of terms. First are characteristics common to woods of

the north temperate zone: .

I. Heart-wood and Sap-wood. The interior wood of mature trees is usually
darker in color than that lying near the surface. The dark part is heart-wood, the
region where active growth has stopped. In the outer sap-wood, growth is still
active. Trees with characteristically colored heart-wood are black walnut, cherry,
oak, and red cedar. In some trees, however, there is little or no difference in color
between heart-wood and sap-wood.

a. Annual rings. The cross section of a tree trunk shows_ a ring structure.
These rings represent seasonal growth and are called ‘ annual rings.’ Each spring
wood formation begins and it continues until some time in late summer.

3. Spring-wood. The cells formed at the beginning of the growing season have
the largest cavities and the thinnest walls. They make up the spring-wood which
is the softer, lighter-colored part of the annual ring.

4. Summer-wood. The cells formed during the latter part of the growing season
have smaller cavities and thicker walls. The summer-wood is generdly heavier,
darker in color, harder, stronger, and less porous than spring-wood. One layer of
spring-wood and one of summer-wood together constitute a year s growth or

annual ring.

26 o

Painting Materials

5. Rays (medullary rays). These may be observed as radiating lines in the
cross section of a tree. The rays serve the tree for purposes of conduction and
storage. They are particularly conspicuous in oak^ beech, and button-wood. The
ways in which logs are cut up are described with reference to their relation to the
position of the rays and also to the rings. Quarter-sawed, edge-grained, or radially-
cut lumber is sawed parallel with the rays. Plain-sawed, flat-grained, or tangen¬
tially-cut lumber is sawed at right angles to the rays.

12, page 39, of ‘Technology of New York State Timbers/ Technical Publication no. 18^
New York State College of Forestry (Syracuse University, 1926), by C. C. Forsaith.

Apart from these characteristics, all woods may be divided into two main
groups: hard-woods and soft-woods or conifers. The two classes differ in structure
and not in the literal meaning of the terms, ‘ hard ' and ‘ soft.’

I. Hard-woods are structurally more complex than soft-woods. They all have
a certain type of cell, larger than the others, which is called a ‘ pore ’ or ^ vessel’.
The pores are specialized for sap conduction and have open ends. They are fused
together, end to end, and, when not closed by tylosis, they aid in the drying of a
wood or in impregnating it with a preservative. Hard-woods are subdivided into
two groups, based upon the arrangement of the pores. The first is ring-porous
hard-wood, e.g.y oak, ash, elm. These have comparatively large spring-wood
pores which are generally visible to the naked eye. On the end grain of a log the
pores form distinct rings and there is a marked difference between the size of the
spring-wood pores and those of summer-wood. The second group comprises the
diflFuse porous hard-woods, maple, black walnut, yellow poplar. The pores,

Supports

though larger than the other cellsj are not so conspicuous as in the ring-porous
group. There is only a gradual^ not a marked^ decrease in size from the spring-
wood to the summer-w^ood pores. Tylosis is the name given to froth-like growths
which fill the pores of certain hard-v/oods. A. Koehler {Guidebooky etc., p. 5)
says, ' These are formed by ingrowths from neighboring cells and fill the pores

Figure 4. Diagrams adapted from those in Guidebook for the Identification of Woods
Used for Ties and Timbers, by Arthur Koehler (Washington, D. C.; Government Printing
Office, 1917). These are from figures 3 to 6. At a is shown a cross section of ring-porous
wood with a radial arrangement of pores in the summer-wood; at ^ a similar section
with a wavy, tangential arrangement of the summer-wood pores; at r a ring-porous
wood without any definite arrangement of the pores in the summer-wood; and at d a
diffuse-porous wood.

more or less like so many toy balloons crowded into an air shaft/ Parenchyma
cells make up the light-colored tissue that can be seen between the pores of hard¬
woods. The distribution of the parenchyma is useful in identifying woods.

a. Soft-woods or conifers have no pores, specialized cells of the type
distinguishing the hard-woods. Instead, they have fibre-like cells (tracheids)
with closed ends. Not all the soft-woods are actually very soft. These woods come

262

Painting Materials

from needle- or scale-leaved trees, nearly all of which bear cones and have un¬
covered seeds. Annual rings are clearly defined in soft-woods. Scattered among
the tracheids of some soft-woods and absent in others are minute spaces which
contain pitch and these are known as ' resin passages." The presence or absence of
resin passages in soft-woods is not such a distinguishing characteristic as the ring
and diffuse porosity of hard-woods.

A great deal of chemical research has been done on the composition of wood.
The bulk of the wood fibre is cellulose, a compound of carbon, hydrogen, and oxy¬
gen, belonging to that great class of organic materials known as the carbohydrates.
The formula is expressed empirically as (CeHioOs)^ where n is supposed to have a
value, roughly, of the magnitude of 5000. Besides the cellulose, there is a material
called lignin, a cementing substance which holds the cellulosic wood cells together.
With cellulose, it forms ligno-cellulose. There are, also, various minor components:
resins, tannins, essential oils, wood sugars, and waxes.

Wood substance is a typical elastic jelly. It is not soluble in ordinary organic
solvents. It is quite inert to the petroleum hydrocarbons, but it is swollen by
water and the lower alcohols, ketones and esters. It is also swollen by contact
with the vapors of these liquids. Not all the swelling manifests itself in exterior
dimensional changes of the wood; much of the swelling is absorbed in plastic
deformation within the wood structure.

Several properties of a physical nature make wood generally useful for pur¬
poses of art and architecture, and it has certain physical properties which may be
regarded as shortcomings. Compared with many other materials that may be
used for paint supports, wood is a light material. It has limitations as to width of
single boards, but the ease with which it may be worked, joined, and glued allows
construction of fairly large, flat surfaces and panels. Woods vary in texture con¬
siderably. Many of the conifers are fine-textured, because they have no pores,
and there is not much difference in density between the spring-wood and summer-
wood. Woods like oak which have large wood vessels are spoken of as coarse¬
grained or textured woods.

The structure of wood is cellular, not labyrinthine. From one half to three
quarters of dry wood is air space. The structure is nearly like that of the honey¬
comb in which there is no communication between cells. The air enclosed in wood
is mostly enclosed within the tracheids and wood cells and not between the cells.
There is not complete isolation between the tracheids of soft-wood because their
walls are penetrated by small pits (bordered pits) covered by membranes, which
are microscopic openings about 0.00002 mm. in diameter.

Wood readily absorbs or gives up water (moisture) and, consequently, it is
subject to shrinking and swelling. This change in dimension with changing mois¬
ture content may be considered a shortcoming. There are three forms of water in
wood: (i) the water that is in chemical combination with carbon to form the
cellulosic material, (2) the water that is absorbed by the cell walls, and (3) the

Supports

263

water that is free in the cell cavities. Water in chemical combination is a constant
thing and has no effect on the swelling and shrinking of wood. Also, that which is
free in the cell cavities, like sap in growing w^ood, or which is taken up in resoaking,
has no direct effect. It is the water absorbed by the cell walls themselves that has
a pronounced effect upon the thickness of the walls of the wood cells. It is the
water absorbed by the cell walls that is the important factor bearing upon the
shrinking and swelling of wood. When green wood has become thoroughly dry,
it still contains a certain amount of moisture, but the amount varies with that
in the surrounding air. In an atmosphere saturated with water vapor the cell
wails can take up 20 to 30 per cent of moisture (based on the dry weight of the
wood) while the fibre cavities are still empty. This percentage varies with the
species. The condition when the cell walls are saturated with w^ater vapor is
called the fibre saturation point.’ Under normal atmospheric conditions (60—70
per cent relative humidity) air-dried wood contains about 12 per cent by weight of
water. Data on the relationship of average moisture content of wood to relative
humidity of the atmosphere are given in Technical Note No. F-ij, Forest Products
Laboratory (Madison, Wis.), as follows:

Relative Humidity

Average Moisture

of Atmosphere

Content of Wood

(Per Cent)

(Per Cent)

20

5.0

25

5.8

30

6.6

35

7.4

40

8.2

45

9-1

50

10.0

55

10.9

60

11.9

65

13-0

70

14.3

75

15.7

80

17.5

85

19.5

90

22,2

95

25.6

Wood does not shrink equally in all directions. There is practically no longitudinal
shrinkage or expansion. Change in dimension, caused by change in moisture
content, is all transverse and, in general, the tangential decrease is about twice as
great ^as the radial. This explains why logs of wood crack radially on drying,
Forsaith (p. 114) says that, in general, a loss of i per cent by weight in air-dried
timber will cause a radial shrinkage of about 0.2 of i per cent and a tangential
shrinkage of 0.3 of I per cent.

264

Painting Materials

Species of woods differ considerably in the ratio of their radial and tangential
shrinkages on drying. In the case of mahogany, birch, and black walnut, these
differences are relatively slight, and hence these woods are not prone to' crack
extensively on drying. On the other hand, the differences between the radial and
tangential shrinkages ot beech, maple, and oak are large and these woods crack

Figure 5. Relations of quarter-sawed and plain-sawed boards, adapted from figure i,

Identification of Furniture Woods,’ Miscellaneous Circular
no. (Washington, D. C.: Government Printing Office, 1926), by Arthur Koehler. At
the left is a quarter-sawed (radial) board; in the center the log with annual rings showing
at the end grain; and at the right a plain-sawed (tangential) board.

radially on drying. Since wood shrinks more tangentially than radially, quarter-
sawed lumber shrinks less in width and more in thickness than plain-sawed lum¬
ber. When pieces of quarter-sawed and plain-sawed lumber are placed edge to
edge in the same panel, one may become thinner than the other with change in
moisture content. Plain-sawed lumber, also, has a tendency to cup, since the
center side is more nearly radially cut than the outside. A. Koehler (‘ Shrinking
an we ingo Wood, pp. 18—2.0) has published tables giving figures which permit
^ radial and tangential shrinkages of a great many species of

woo . ert an and Haslam have shown that the expansion of summer-wood
IS often greater than the over-all expansion of the wood. Their data indicate that

Supports

265

summer-wood expands so much that the forces are greater than those of the
swelling of spring-wood, with a result that the spring-wood is often compressed.
Paint does not chip and flake from spring- wood so quickly as it does from summer-
wood, which indicates that there is less mechanical movement in the spring-wood.
Its natural expansion is balanced bv the compression effects exerted upon it by
summer-wood.

It is difficult to find published data which give the linear increase in the
swelling of wood with change in the moisture content of wood. S. T. C. Stillwell,
of the British Forest Products Laboratory, supplies the information (private
communication) that a quarter-sawed oak panel i foot in width, increases 0.023
inch for each increase in moisture content of i per cent. A tangentially-sawed
panel of the same width gains 0.038 inch. These movements were measured over a
range corresponding to relative humidity conditions of from 20 per cent to 80
per cent. This laboratory has also made some measurements on the expansion
and shrinkage of old panel paintings with changes in relative humidity. Their
graphs show that certain painted panels may change in width across the grain as
much as 1.5 per cent to 2 per cent in a range of relative humidity varying from
40 to 80 per cent.

Wood, deterioration and treatment. The decay of wood is always caused by an
external, living organism. All such organisms need for their growth food, water,
air, and warmth. In general, they belong to the mass of dependent plants’known
as fungi and, so far as they infest wood, may be considered in three classes:
bacteria—unicellular organisms the action of which is still imperfectly under¬
stood, though it appears that some of them may be destructive to wood tissues;
ascomycetes—chromogenetic fungi known as sap stains and having only a slight
influence on the strength of wood; and basidiomycetes, the highest class of fungi
and the most destructive to wood. These are made up structurally of many
branching, hollow tubes arranged loosely like mold or crowded into cushions.
The tubes grow principally in length. They may be formed into fruit-bodiesj
plate-like, bracket-like, or flat, and assuming a variety of shapes. The character¬
istics of some of the principal basidiomycetes that live on wood can be shown in a
tabular form (this is condensed from the table by Prof. Percy Groom in ‘ Dry
Rot in Wood,’ pp. 6 and 7).

An understanding of the conditions of growth carries with it sufficient indica¬
tion of how to prevent that growth. Many of the fungi that cause dry rot will
attack sound wood. This is particularly true of the Coniophora puteana {cerebelld).
Inoculation occurs from a microscopic spore, and thin films of such materials as
wax would help to prevent this from getting lodged in the surface. None of the
fungi will live without an ample supply of water vapor, or without fresh air.
Extreme and_ prolonged dampness is therefore to be avoided and occasional
furnigation with thymol or some of the stronger gases would tend to kill any
active or dormant growth.

266

Painting Materials

The result of attacks of dry rot may be more or less severe and treatment of
the wood which has decayed will have to be carried out according to the require¬
ments of particular cases. In all cases where wood has been attacked by fungi
some has been used for their nourishment and the structure is weakened. With
this weakening the wood becomes more red in color and more brittle^ loses its
characteristic odor^ gives a dull sound when struck, shrinks, and often warps. It
may or may not have visible fungus on the surface but in advanced cases will
show cross-shakes and cup-shakes—fractures running at right angles to the grain,
with, in the latter, a curling into concave flakes between the fractures.

Name
Merulius
lacrymans
(domes ticus)

Polyporus
vaporarius
(group)
Coniophora
puteana
(cerehelld)
Paxillus pan-
noides {Acher-
untius)
Lenzites sae-
piara and L.
ahietina

Polyporus de¬
structor

Surface Growth

Comments

White to gray strings sometimes as thick
as a lead pencil; brittle when dry; auxil¬
iary growth in dense sheets with large
bracket-like and scalloped fruit-bodies.
White strings and auxiliary growths,
never colored, with fruit-bodies in thin
sheets, all white.

Slender strings becoming red-brown, and
fruit-bodies yellow to olive-brown, sheet¬
like and with minute pimples.

Very slender, yellow to brownish yellow;
fruit-bodies shell or fan-shaped, surface
yellow to brown and with radiating gills.
No strings; occasional sheets or cushions,
tan in color; fruit-bodies from wood cracks
are yellow to umber with radiating gills.

No strings or other growths on surface;
bracket-shaped fruit-bodies, white to gray.

Little moisture needed; rot
runs through and causes
cross-shakes.

Members of this group need
more water than the Meru¬
lius lacrymans.

Needs much moisture; rot
incomplete, often without
cross-shakes.

Takes much moisture; wood
when attacked first takes on
a characteristic yellow color.
Demands much moisture;
fungus remains inside and
resists death from desicca¬
tion; external wood may re¬
main intact except for cross¬
shakes and cup-shakes.
Demands much moisture;
decay internal.

* Since, in the study of conservation of works of art, identification of species is not needed
as a rule, the descriptions of Professor Groom are much abridged here. He points out
that fruit-bodies in particular have various shapes within species.

The so-called ‘ worms ’ which destroy wood are the larvae of beetles of the
order Coleoftera, In this large order only one group needs to be much considered
in relation to objects of art. This is made up of the furniture beetles { AnobUdae)y
insects which inhabit seasoned wood. The larvae of the Anobiid beetles are usually
one tenth to one third inch long, are somewhat cylindrical and elongated in shape,
and are brownish in color. When full-grown, or developed as ‘ worms,^ they are
curved and wrinkled, have three pairs of legs and strong, biting jaws. The entire
life history of them is not known in all its points, but it is certain that, although the

Supports

267

cycie from egg to beetle may be prolonged to two years or more^ it usually lasts
only a year. These larvae infest both coniferous and hard-wood objects and may
invade heart-wood as w^ell as sap-wood.

The common furniture beetle is the Anobium punctatum. It is small (i/io to
1/4 inch) and is reddish brown to dark brown in color. The insects usually emerge
in June or July, leaving their pupal beds or cells in the wood at that time. Fre¬
quently they are found crawling on walls and ceilings. They pair then and the
females lay their eggs in cracks and crevices of wood. The young larvae bore in,
going first at right angles and then along the grain. x 4 s they grow, they work to¬
wards the surface and gnaw out a small cell wTich serves as a pupal bed. From
this the beetle emerges. The largest of the furniture beetles is the Xestobium rufo-
villosum or death watch beetle. It is one fourth to one third inch long and brown
in color. Its pupation occurs in the spring as does that of the Anobium but the
beetles do not emerge until autumn or even as late as the following spring. It is
comparatively rare and its traces distinguish it from the Anobiuin: the exit holes
are larger and the dust is in coarse pellets. Two other kinds of beetles that infest
furniture are so rare as not to have acquired popular names. They are the
Ernobius Mollis and the Ptilinus Pectinicronis,

It appears that the Anobiids prefer old wood, possibly because of chemical
changes that improve it as food, or because of the presence of micro-organisms, or
simply because it offers better places for egg-laying. In panel paintings the tunnels
made by larvae not only bring about a general weakening of the support, but
also establish areas of particular w^'eakness where the paint is apt to collapse and
be entirely lost. The ground and paint film offer no food and it is often found that
the larvae run their tunnels directly beneath them. Pupal beds are made there
and beetles emerge through the paint, leaving behind them tubular openings next
the ground and completely hidden. These empty tunnels afford no resistance to
the general shrinkage of the panel and when that occurs, the paint above them
‘ buckles * or " tents ’ and frequently flakes off entirely. The presence of such
tunnels in a painted panel is indicated by buckled films, by marked or thread-like
cracks following, usually, the grain of the wood, by visible exit holes, or by a
lack of resonance if the tunnels are sufficiently numerous. In many instances
there are but slight indications on the painted surface; probing the panel from the
reverse will give some evidence about, the general state of the wood. Where
ground and paint films are thin and uniform, a radiograph may show the location
of tunneled wood, but as a rule the density contrast of pigments is so great as to
conceal that in the support.

The activities of the termite {reticulitermes flavipes) are similar to those of the
beetle. It appears, however, that prevention or elimination of them is less a
matter for individual objects than for structural changes in buildings. (This is
explained by Morgan Marshall in ' The Termite Menace,’ Technical Studies^ IV
[1936L PP- 729 ff')

268

Painting Materials

Much the larger part of wood treatment as it has to do with the art of painting
is restorative. In the preparation of panels there are only the familiar processes
of joining members or of adding cleats, dowels, or splines to such joins. For pur¬
poses of restoration, or to prevent or arrest the deterioration of wood, fumigation
is often employed. As a rule, the object is placed in an air-tight chamber and there
exposed to the fumes of a nature toxic to larvae and usually to fungi. Such fumh
gants are carbon disulphide, carbon tetrachloride, formaldehyde, ethylene di¬
chloride, and ethylene oxide, among others. Where wooden objects are stored or
exhibited in tight cases repellants like paradichlombenzene are often kept with
them.

The treatment of weakened wood by impregnation is a method as yet little
used for panel paintings. Where it has been used, wax or a combination of waxes
and resins is made the impregnating material. This is forced into the wood by
heat or is carried in by absorption when dissolved in a solvent. In most instances,
such treatment is effective rather in sealing the wood structure and in making
the outside more firm than in actually strengthening it, for the depth of penetra¬
tion is not great and the cavities in wood cells are rarely reached. Thorough
impregnation could only take place with immersion of the panel in the dissolved
material or with keeping it at a temperature above the melting point of the
impregnating material for somewhat prolonged periods. Either of these carries
with it a risk to the paint structure, which has kept it from being generally
adopted.

Various types of accessory supports have been attached to wood panels in an
effort to strengthen them or to keep them in an even plane. The most simple are
cleats or braces either glued to the reverse or mortised in. Occasionally a solid,
checked-out lattice of new wood has been glued on. This has the disadvantage of
putting the panel under great restraint and with the expansion and contraction
that is inevitable from changes of atmospheric humidity, the panel itself is apt
to check or crack. The attachment called a ' cradle ’ is designed to prevent this. It
is composed of longitudinal members set at intervals in the same direction as the
grain and glued in place. On the edge of these members, next the panel, notches of
uniform size are cut so that another member, transverse, can slide in them. These
transverse members are not attached and the cradle as a whole is expected to leave
the panel free to expand and contract without warping. Although effective in
many cases, the cradle must have favorable conditions or it, ,too, becomes fixed
by cramping and endangers the panel as does a lattice.

For any accessory support to be effective, the wood of the panel must be strong
enough to carry it. When the wood is much weakened by worms or by dry rot,
another type of treatment is necessary. Removal of the weakened wood and re¬
placement with a luting material is possible where deterioration is confined to
limited areas. If the panel is largely honeycombed by worm tunnels or desiccated
by fungi, the painting will probably have to be rebacked or transferred. In the

Supports

269

rebacking process the original panel is cut down to a thickness of one eighth inch or
less and this is attached to a new panel, usually three to five ply and with a mid¬
dle core made up of a number of strips. In the process of transfer, the wood is en¬
tirely removed and the ground may or may not be left.

Wood, history in painting. In western civilizations the use of wood as a paint
support can well be supposed to have reached into the earliest practice of the art.
Painted ivood sculptures exist from the time of the Old Kingdom in Egypt, as
early as the I¥ Dynasty (2900-2750 B.C.), and by the Middle Kingdom (2160—
1788 B.C.) sarcophagi with painted representations are known. Duell draws
attention to the practice of easel painting in the VI dynasty, and by the time of
the mummy portraits, executed largely in £he Fayum during the early centuries of
the Christian era, vrood is found as a regular, independent support for pictorial
designs. The kinds employed for this purpose were, according to available data,
sycamore (undoubtedly the sycamore fig [Ficus sycomorus])^ cypress, cedar, and
pine. The cedar, cypress, and pine were all foreign importations but are listed by
Lucas as found In other Egyptian objects made of wood. The sycamore fig was
native. Practically nothing survives in the way of painting on wooden supports
from classical times, though a set of parade shields, pine and covered with rep¬
resentations, have been found by a Yale University expedition at Dura-Europos
in Syria. They are of Roman origin, somewhat before 256 x 4 .D. PHny (XXXV, 77)
says that Pamphilos, IV B.C., taught his pupils to paint on panels of boxwood,
and such a support must have been common for the portable pictures of antiquity.
Except for the fine craft of lacquer painting which was generally exercised on
objects of wood, the Far East was not given to its use as a support for painted
designs. A possible exception is the set of the doors, carrying representations of
religious scenes, of the Tamamushi shrine in the Kondo of the Horyuji monastery
at Nara, Japan, dating from the VI or VII century. It is evident, however, that
by the XVI century, and probably for a long time before that, panel painting was
a common practice in India, and Coomaraswamy has brought together a number
of references to show this.

A summary made from the catalogues of the Munich and Vienna museums,
with a few items from the catalogue of the National Gallery, London, shows the
following distribution of woods in the panels of 1100 European paintings from the
time of the early Renaissance:

Beech—German, 11.

Cedar—Flemish, i; Italian, 4. (Total, 5)

Chestnut—German, i; Italian, 28. (Total, 29)

Fir—‘Flemish, 2; German, 4. (Total, 6)

Larch'—German, 3.

Linden—^^Flemish, 3; German, 85; Italian, 8. (Total, 96)

Mahogany—Flemish, 2.

Oak—English, 2; Flemish, 560; French, 13; German, 80; Italian, 18. (Total, 673)

Olive—Italian, 4.

270

Painting Materials

Pine—Flemish^ 2; German, 131; Italian, 15. (Total, 148)

Poplar—Flemish, i; German, 10; Italian, no. (Total, 121)

Walnut—Flemish, 3; French, 2; German, 4; Italian, 3. (Total, 12)

A further comment on the panels used in European painting is made by Eibner
in his Tafelmalerei (pp. 78 fF.). For the XV and XVI centuries he says that Italian
painters used poplar, walnut, chestnut, cypress, pear, mountain ash, maple, pine,
cedar, and willow; that the French used oak; that the painters of the Netherlands
used oak, mahogany, linden, cedar, maple, pear, and poplar; that the Dutch used
linden, red beech, fir, spruce, alder, and Swiss pine; and that the English used
oak, spruce, fir, and pine.

The joining of pieces to make a wood panel is described by many of the early
treatises on painting; such a description is in the Schedula divers arum artium of
the monk Theophilus (early XII century) and is given by Laurie {Materials^ etc.,
p. 158, from MS. chap. XVII). It calls for making a glue from cheese-casein, with
quicklime to dissolve it. Similar directions are found in the famous Lihro delT Arte^
the Craftsman’s Handbook of Cennino Cennini (early XV century; ed. Thompson,

p. 68).

From very early times other materials, like cloth, leather, and (later) paper,
were combined with wood to make a compound support for paint. Structures of
this kind are found in Egyptian mummy cases where linen with gesso over wood
forms the support of paint and gold. A Roman parade shield of the II century
A.D., found at Dura-Europos, is made of plywood covered with leather on which
the painting is executed. In European panel painting, particularly in Italy, the
practice of applying cloth over the wood was common before 1400, and is speci¬
fically described by Cennino Cennini (ed. Thompson, p. 70). He directs the use of
old thin linen cloth that is cut or torn into strips, soaked in a good size (/.<?., a glue
made from goat or sheep parchment) ,and spread over the panel.

During the Renaissance in Europe cloth supports came to be more common,
particularly for larger compositions, but wood continued in use and is still fre¬
quently employed by painters.

Wood.“Piilp (see also Paper). In the early stages of making paper, cardboard,
and certain building boards, logs are cut, chipped, and reduced to a fibrous mass.
This is usually done either by grinding and soaking, which produces mechanical
pulp, or by chipping and decomposing intercellular materials in the raw wood
through the action of alkaline or acid additions to the water used for cooking.
The latter produces chemical pulp.

Wool (see also Fabrics and Fibrous Substances) was probably the first fibre
spun by man. It is the epidermal hair of lambs and sheep, and consists of a pro¬
tein called keratin. Although the best and greatest quantity of it comes from
sheep, some comes from the angora goat, the cashmere goat, the camel, alpaca,
llama, and other animals. The use of cloths made from this fibre for support of
paint and paintings has been negligible.

Supports

271

BIBLIOGRAPHY

Fred H. Andrews, O.B.E,^ ‘‘Centra! Asian Wall-Paintings/ Indian Art and Letters^ VIII
(1934). PP-

Anonymous, Cleaning and Restoration of Museum Exhibits^ Third Report (London:
H. M. Stationery Office, 1926).

Anonymous, Some Notes on Atmospheric Humidity in Relation to Works of Art (London:

The Courtauid Institute of Art, 1934). Pp. 48.

Victor Bauer-Bolton, ‘Zur Frage der Konservieriing der Tafelmalerei/ Museufnskunde,
V (1933)5 PP- 95—112, and ‘La Conservation des Peiiitures de Chevalet—les Sup¬
ports et les Fondes/ Mouseionj XXIII-XXIY (i933)j PP* 68-91.

Ernst Berger, Beitrdge zur Entwickhmgsgesckichte der Maltecknik^ 4 vols. (Munich: G.
D. W. Callwey, 1901-1912).

R. M. Boehm, ‘The Masonite Process/ Industrial and Engineering Chemistry^ XXII
(1930), pp. 493-497*

Charles Aloise Briquet, Les Filigranes (Paris: A. Picard and Son, 1907).

J.-F. Cellerier, ‘Traitment dXne Statue en Bois contre 1’Action des Parasites/ Mouseion^
XVII-XVIII (1932)5 pp. 128-131.

A. H. Church, The Chemistry of Paints and Pamtifjg (London: Seeley and Co., 1901).
Ananda K. Coomaraswamy, ‘The Technique and Theory of Indian Painting,' Technical
Studies^ III (1934)5 pp. 59-89.

A. D. Covrper, ‘Lime and Lime Mortars,' Special Report No, p, Department of Scientific
and Industrial Research (London: His Majesty's Stationary Office, 1927).

C. F. Cross and E. J. Bevan, A Textbook of Paper-Making (London: E. & F. N. Spon,

1907).

Max Doerner, The Materials of the Artist and Their Use in Painting^ trans. (New York:
Harcourt, Brace and Co., 1934).

Prentice Duell, ‘Evidence for Easel Painting in Ancient Egypt,' Technical Studies^ VIII
(1940), pp. 175-192.

J. D. Edwards, F. C, Frary and Z. Jeffries, The Aluminum Industry, 2 vols. (New York:
McGraw-Hill Book Co., 1930).

A, Eibner, Entwicklung und Werkstoffe der Tafelmalerei (Munich: B. Heller, 1928).

Entwicklufiig und Werkstoffe der Wandmalerei (Munich: B. Heller, 1926).
Encyclopaedia Britannica, nth and 14th editions.

C. C. Forsaith, ‘The Technology of New York State Timbers,' Technical Publication no,
18, New York State College of Forestry (Syracuse University, 1926).

Eloise Gerry, ‘The Structure of Wood,' Lecture Note, U. S, Forest Products Laboratory
(Madison, Wisconsin, June, 1929)*

A. L. Goodale, Chrofjology of Steel (Cleveland: Penton Publishing Co,, 193^)-
Percy Groom and others, ‘Dry Rot in Wood,' Bulletin no, /, Forest Products Research
(London: His Majesty's Stationery Office, 1928).

J. H. Haslam and S. Werthan, ‘Studies in the Painting of Wood, 1 . Influence of Wood
Structure on Paint Behavior,' Industrial and Engineering Chemistry, XXIII (193
pp. 226-233.

E. S. Hedges, Protective Films on Metals (New York; D. Van Nostrand Co., 1933)’

H. O. Hofman, Metallurgy of Copper (New York: McGraw-Hill Book Co., 1914).

Painting Materials

A. L. Howard, Timbers of the World (London: The Macmillan Co., 1920).

Dard Hunter, Paper-Making Through Eighteen Centuries (New York: William Edwin
Rudge, 1930).

F. Hamilton Jackson, Mural Painting (London: Sands and Co., 1904)

K. Jex-Blake and E. Sellers, The Elder Pliny s Chapters on the History oj Hrt (London
Macmillan and Co., Ltd, 1896).

Edward Johnston, Writing and Illuminating and Lettering, 12th ed. (London: Sir Isaac
Pitman and Sons, Ltd, 1922).

Joseph Karabecek, Papyrus Erzherzog Rainer, Fuhrer durch die Ausstellung (Vienna
1894).

Katalog der Alteren Pinakothek zu Miinchen (Munich, 1925).

Katalog der Gemdldegalerie (Vienna: Kunsthistorisches Museum, 1928).

Sheldon Keck, ‘ A Method of Cleaning Prints,’ Technical Studies, V (1936), pp. 117-126
J. A. Knowles, ‘Processes and Methods of Medieval Glass Painting,’ Journal of the
Society of Glass Technology for ig22, pp. 255-274.

Arthur Koehler, Guidebook for the Identification of Woods Used for Ties and Timbers
(Washington, D. C.: Government Printing OfSce, 1917).

‘The Shrinking and Swelling of Wood,’ Bulletin, Forest Products Laboratory
(Madison, Wisconsin, July, 1931).

A. P. Laurie, The Materials of the Painter’s Craft (Philadelphia: J. B. Lippincott Co
1911).

The Painter’s Methods and Materials (Philadelphia: J. B. Lippincott, 1926).

J. D. LeCouteur, English Medieval Painted Glass (London: The Macmillan Co., 1926).
H.N. Lee,‘Established Methods for Examination of Paper,’ Technical Studies,VI Uq-ils
pp. 3-14.

A. Lucas, Ancient Egyptian Materials and Industries, 2d ed., revised (London: Edward
Arnold and Co., 1934).

Harry Miller Lydenberg and John Archer, The Care and Repair of Books (New York;
The R. R. Bowker Co., 1931).

J. Merritt Matthews, The Textile Fibers (New York: John Wiley and Sons, 1924).
eon McCulloch, 'Lime Process for Coating Aluminum,’ Transactions of the Electro¬
chemical Society, LVI (1929), pp. 41-44.

J. W. Melior, Comprehensive Treatise on Inorganic and Theoretical Chemistry (London-
Longmans, Green and Co., 1923).

M. P. Merrifield, Original Treatises on the Art of Painting, 2 vols. (London: John Murray

decorative (London: B. T. Batsford, 1897).

Or. W. Morey, A Half Century of Progress in the Glass Industry,’ Industrial and En¬
gineering Chemistry,WI\\\ (1926), pp. 943—945

T W m' Glass Surfaces,’ ibid., XVII (1925), PP. 389-39^.

j. W Munro, Beetles Injurious to Timber,’ Bulletin no. 9, Forestry Commission (Lon-
don. His Majesty s Stationery Office, reprinted 1930).

Kenneth P. Oakley, ‘Woods Used by the Ancient Egyptians,’ Analyst, LVII (1932), pp.

Supports

273

J. R. Partington, Origins and Development of Applied Chemistry (London: Longmans,

Green and Co., 1935).

L. V. Pirsson, Rocks and Rock Minerals (New York: John Wiley and Sons, 1908).

H. J. Plenderleith, 'La Conservation des Estampes, Dessins et Manuscrits/ Mouseion^
XXIX-XXX (1935)^ PP. 8i”I04.

The Preservation of Antiquities (London: The Museums Association, 1934).

F. Rathgen, Die Konservierung von AUertumsfunden (Berlin: Walter de Gruyter, 1926).
Nancy Andrews Reath, 'The Weaves of Hand-Loom Fabrics,* The Pennsylvania Museum
Bulletin (Philadelphia), XX (November 1924).

Helmut Ruhemann, 'La Technique de la Conservation des Tableaux,* Mouseion^ XV
(1931), pp. 14-22.

B. W. Scribner and F. T. Carson, 'Study of Fiber Wall Boards for Developing Specifica¬

tion Standards/ Paper Trade Journal (September 26, 1929), pp. 61-68.

Shorter Oxford Efiglish Dictionary,

Alfred J. Stamm and L. A. Hansen, 'Minimizing Wood Shrinkage and Swelling,*
Industrial and Engineering Chemistry, XXVH (1935), pp. 1480-1484.

William Suhr, 'A Built-Up Panel for Blistered Paintings on Wood,* Technical Studies,

I (1932), pp. 29-34.

Edwin Sutermeister, Chemistiy of Pulp and Paper-Making TNew York: John Wiley and
Sons, 1929).

Theophilus, An Essay upon Various Arts (translated, with notes, bv Robert Hendrie,
London: J. Murray, 1847).

Daniel V. Thompson Jr, 'The De Clarea of the So-Called "Anonymus Bernensis/* *
Technical Studies, I (1932), pp. 8-19 and 69-81.

II Libro del! Arte, The Craftsman's Handbook of Cennino A Andrea Cennini, 2 vols
(New Haven: Yale University Press, 1932, 1933).

Daniel V. Thompson Jr and George Heard Hamilton, De Arte Illuminandi (New
Haven: Yale University Press, 1933).

T. R. Truax, 'The Gluing of Wood,* Department of Agriculture^ Bulletin no, ijoo
(Washington, D. C.: Government Printing Office, 1929).

C. G. Weber, 'Fiber Building Boards/ Industrial and Engineering Chemistry, XXVII

^935)7 PP- 896-898.

C. G. Weber, F. T. Carson and L. W. Snyder, 'Properties of Fiber Building Boards,*
Miscellaneous Publication, Bureau of Standards, no, ij2 (Washington, D. C.:
Government Printing Office, 1931).

N. H. J. Westlake, A History of Design in Painted Glass (London: James Parker and Co.,
1881).

A. Martin De Wild, The Scientific Examination of Pictures, trans. (London: G. Bel!
and Sons, Ltd, 1929).

Agate, a hard, semi-precious stone out of which burnishers for gold leaf are now
commonly made. 1 he word is often used to mean burnisher.

Alembic. Like the oil press and other articles of equipment for the preparation
of mediums, the alembic, a glass vessel with neck bent dowmwards, for distillation,
is occasionally seen in studio interiors of the XV century and shortly after. A
painting of St Luke of the school of Massys, in the National Gallery, London (no.
3902), shows a small alembic on a shelf along with other containers. Leonardo da
Vinci mentions it a few times in his notebooks, largely with respect to the distilla¬
tion of turpentine.

Amassette (see also Slice and figure 24). This name, obviously French in origin
and probably restricted to that locality of Europe, has been used to define a
scrapci made out of horn. This, like the slice, was for piling and collecting paint on
the grinding slab. It is mentioned in the Brussels MS. of 1635 tiv Pierre le Brun
(Merrifield, II, 770 and n.).

Anatomical Figure* Interest in anatomy on the part of painters brought about
extensiye study by means of dissection.^ and later, as a more convenient means of

information for students, various kinds of casts and models were made to show
the positions of muscles and bones. Although occupation with this study began in
the XV century, widespread study of anatomy was a development of the XVI
century. Various drawings and paintings of academies in the late XVI century
particularly show students and teachers at the work of dissection and drawing.
Skeletons and skulls are common models and, more rarely, casts or models of
muscles.

Atomizer. When certain types of drawings require to be fixed in order to pre-

vent their chafing they are sprayed with a resin dissolved in a suitable organic
solvent. The common means for making such a spray is a pair of small metal tubes
placed at right angles to each other so that a stream of air from the larger blows
across an open end of the smaller, the other end of the smaller tube being in the
resin solution. A spray of the diluted resin is thus thrown out. There are many
variations of the construction of this simple tool. Most artists still blow these by
mouth rather than with a pump or bulb. Historically, the atomizer is probably
fairly recent in the arts. As late as the XVII century, large drawings used as
cartoons were dipped into a bath of fixative (Meder, p. 191).

Basin Slant. The porcelain palette with inverse, wedge-shaped depressions for
water color painting is frequently made in the form of a disk which sits like a lid on
a basin for water (figure 26, h). In this lid the depressions are on radial lines and a
hole is put in the center where a brush can be dipped in the water beneath. Some
manufacturers add a separate and removable cup which fits into the central
opening.

Beaker. A glass container for measuring quantities of fluid is now practically
without consideration amongst artists colormen. Beakers are commonly seen,
however, in paintings of artists’ studios from the XV to XVIII centuries.

277

Sr

278

Painting Materials

Blender or Softener, a fairly large, round brush, with the end flat rather than
domed, and made regularly of badger hair. It is used, dry, to grade or soften edges
particularly in oil painting (see Brush and figure 1, h). ’

Figure i. A selection of brush shapes: (a) a pointed sable set in a quill; (i) the same in a
metal fewule; (r) a slightly flat sable with a straight edge; (d) a square end sable flatter
tftan c. Ihese three shapes are common in both sable and bristle brushes and many
ot^ ers are niade as variations on them. At ^ is a very long sable with a flat end, called
a striper ;/, with a somewhat longer hair than is called’ a ‘writer’; both are set in
quills. Heavier brushes are: (g) a mop of camel hair with a wired quill mounting similar
to the brush called a ‘dabber’; (h) a blender or softener made of badger hair, round, and
mounted with wire; (i) a very thin, flat brush, called a ‘fan,’ set in a metal ferrule and
made of sable or bristle.

Breathmg Tube. A small tube made of paper, cane, or occasionally of metal,
a out a an inch in diameter and six or more inches long, has as its principal
purpose the moistening of the bole ground before gold leaf is laid in manuscript
illumination. The breath, blown through it gently, carries enough moisture to

Tools and Equipment

279

make the bole adhesive. A simple instrument of this kind may have had long use
in the past, but it was not much referred to before the time of modern writing
on methods (see Johnston, p. 154).

Bristle, the material, usually hog hair, made into heavier brushes.

Brush. Some kind of fiorous tuft used for holding and spreading paint seems
to be quite as old as the art itself. The brushes chiefly used in the West by the
artists of the present are of two general types: the bristle of hog hair and the sable
or other fine hair. Many different materials are actually used. For auxiliary work
in painting, that is, for blenders, for the so-called "fan,’ for stencilling, or for
varnishing, badger, horsehair, camel hair, and others are added to the two prin¬
cipal types (figure i). The East has known a still greater variety of materials for
brush making (figure a). Ancient Egypt held largely to one tool of this kind—plant
fibres as they occurred in a reed which was macerated at the end to separate them
and produce a suitable tuft. China and Japan have had brushes made regularly of
deer, fox, and numerous hairs of wild animals, and for certain purposes have had
brushes made of feathers or of human hair.

During the past 200 years or more the craft of brush making has been carried
on as a special activity outside of the artist’s studio. Whether the brushes are of
bristle or of a fine hair, the mounting is much the same for those used directly in
the practice of painting. The hairs are cut and sorted, and thorough cleaning with
sterilization is usually part of this preliminary process. The hair itself has to be
kept aligned because it is important in brush making that the tip of the brush be
made up of the natural tips of the hairs that are in it. These are the outer ends,
called the ‘flag’ or ‘split’ ends, whereas the root end of the hair, called the butt, is
usually cut off. Brushes which do not have the flag ends as their tips are brittle
and can not be controlled for fine work. During manufacture the hairs are straight¬
ened, sorted, and cut according to the length and size the brush is to have. They
are then shaken together in small metal cylinders called ‘cannons,’ and shaped by
hand to the form desired. The tufts so made are tied. After this, they are mounted,
usually^in metal ferrules, with a cement such as rubber, pitch, or synthetic
composition. Quills also are used still to a very large extent for the mounting of
smaller brushes such as the sable or the so-called ‘camel hair.’

It seems commonly agreed that the best of the fine, soft brushes are those made
from what is called ‘red’ sable. This is the hair of the kolinsky, an animal which
may be any of several species of Asiatic minks, and the name is from the Russian
ftJt Kolciy a district in northeastern Russia where these animals are found. The pelt
is reddish yellow in color, and the tail supplies the hair used for brush making. The
so-called camel hair, also used for small, soft brushes, is, in the modern production
of these tools, actually the hair from a squirrel. The best of this comes from Siberia
and Russia, and the color ranges from red through gray to black. Fitch brushes
have also been used as a fair substitute for sable. Probably the fine brush described
by Cennino Cennini (C. LXIIII) was rather rare, even in his day. This was made

28 o

Painting Materials

of miniver^ or ermine^ and some of the quality of the brush must have been lost
by the trimming that he suggests^ with scissors (Thompson, pp. 40-41). The
regular brush for modern oil painting, because it is stiff enough to draw out a
fairly heavy and paste-like mixture, is made of hog hair, usually bleached white.

In the Far East, brushes are commonly mounted in bamboo and have some¬
what more specialized selection according to use than is common in the West. The
hairs of deer and wolf will be combined, for example, into a brush for certain kinds

]!>

Figure 2. A few of the Chinese brushes described and illustrated by Sickman: (a) a
small brush of deer hair made for drawing and having two reducing collars, the smaller
one a silver tube; ( 3 ) a plain, pointed brush of pure goat hair with the usual bamboo
handle; (c) a goat hair brush set in an enlarging holder of ivory attached to a handle of
bamboo which carries a loop for hanging—a brush for large ink painting or for inscrip¬
tions; (^) a brush of chicken feathers pressed open; (<?) a stub brush with an enlarging
holder of horn; (f) a wide, flat brush set in a brass frame or ferrule attached to a bamboo
handle—used for removing charcoal or powder.

of writing and painting. A particular brush of deer hair is made for marking such
details as the veins of a leaf. A brush for inscriptions is made of two parts rabbit
and eight parts goat hair. With a few exceptions, these are pointed in shape.

The soft or fine brush used by western painters is ordinarily pointed, except for
a few that have blunt ends and for such special brushes as dabbers, stripers, mops,
fans, and wash brushes. Shapes of the bristle brushes, however, have a large
nuniber and variety. The ferrule in which the hairs are mounted may be round in
section, slightly flattened, or very flat and thin. The flat brush is probably the
inore usual in the modern practice of oil painting. The shapes of the ends and the
differences in the length of the hair make up a various range of brushes that are

Tools and Equipment

281

offered by artists’ colormen. The sizes are indicated by number and commonly
run from o to 12, the last being about an inch across at the end of the ferrule. Fine
sable and camel hair brushes, when mounted in metal ferrules, are numbered in a
similar series. Those mounted in quills have the size indicated by the source of
the quill, as lark, crow, duck, goose, or swan. These, with intermediate entries,
give a series of eleven or twelve sizes.

Although minor changes and greater standardization have come with special¬
ized brush manufacture, brushes themselves as implements for painting are prob¬
ably not far different from those that were made and used by prehistoric man. The
Egyptian brush of reed fibre is an exception to the general rule of East or West,
for animal hair tied to a stick or set in a reed or quill has been the typical primitive
brush. In the British Museum is a short brush made of woolen yarn bound with
fine thread to a small reed handle. It was found at Oxyrh}mchus in Egypt among
objects of the Roman period and, although it is shown with painters’ materials, it
may not have been typical of any brushes used for pictorial work. From the same
site and in the same collection is an unusual brush of the simplest possible design,
being only a very small bundle of coarse, black hair bound through the middle to
leave the ends free. Berger (I and II, p. 172) assumes, largely on the interpretation
of literary evidence, that the Greeks and Romans used bristle for their larger
brushes, and that finer work was done with a soft hair. By the time of the so-called
^Mt Athos’ MS., reflecting methods of the XI and XII centuries in the region of
Greece, it is clear that many kinds of hair were used—pig, bristle, ox, and ass.
Cennino Cennini, representing largely the methods of the XIV century, speaks
of small, fine brushes of miniver, mounted in quills, and large brushes made of the
bristles of a white hog, bound in a bundle around a stick handle. These two
types, doubtless with great variations in the kinds of hair used, seem to have pre¬
vailed throughout Renaissance and early modern painting. Until metal ferrules
came into use, probably in the early part of the XIX century, the fine brush had a
quill mounting and was always round in section. Paintings of artists’ studios
through the XVIII century still show the heavy and the light brushes as round in
section, although a fiat brush could have been made out of bristles and without
a metal ferrule.

Brush Case. As manufactured for the modern painter, this is a long box of
metal with a sliding top, usually of enamelled or plated metal, and with a handle.
Its purpose is to hold brushes for travelling or for out-of-door sketching and to
prevent their being unduly worn or frayed. Some cases have a partition to which
the brushes are fastened by means of an elastic band. Those for oil-color brushes
are about 14 inches long and often more than 2 inches in diameter. The water
color cases are smaller and may be either round or oval in cross section.

Brush Cleaner. Pans or vats of various sizes and shapes to hold oil or turpen¬
tine (see figure 3) have been in use for many years as receptacles for cleaning paint
brushes or to prevent their drying out. Clips are often arranged above a pan so

2.82

Painting Materials

that the brush handle will be caught there and the hairs kept immersed in the
fluid. Such a cleaner was advertised in the catalogue of Winsor and Newton for
1863.

Bumislier* An instrument of hard stone (figure 4) has been used from remote
antiquity for polishing the leaf or foil of gold or other metals. To be burnished^
the leaf has to be laid by what is now called 'water gilding’ over a layer of amor¬
phous earth or bole mixed with size. The usual stone for modern burnishers is
agate, and it is produced by manufacturers in a number of shapes suitable for fine

Figure 3. Brush cleaners or washers as sold for the modern practice of oil painting:
(a) a circular base carrying a beaker of oil or turpentine above which is a metal clip for
holding brush handies; (^) a rectangular container for the turpentine or oil, open at the
topj into which fits a frame (^Oj which is open at both top and bottom and contains
a wire grating. When the grating is submerged, the brush is dragged across it in the fluid.

illuminations and for heavier, broader work. Haematite, a stone recommended
for burnishers by Cennino Cennini (C. CXXXV, Thompson, p. 82), is still some¬
what used in branches of the gilding trade. Various other stones were common in
the Middle Ages and Renaissance, including sapphires, emeralds, and rubies.
Fine burnishers were frequently made from the teeth of animals, and a pointed,
hook-shaped burnisher of agate is still referred to as a dog tooth. A detail of a wall
painting from the Egyptian tomb of Nebamen and Ipuky (Thebes, no. 181) shows
one of the artisans in the scene using a burnisher while another is stamping designs
in the gold. The date of this is ^ 1380 B.C. A MS. of the XIII century A.D.,
De Coloribus et Artihus Romcinorum (Merrifield, I, 220), describes making bur¬
nishers from haematite, a method which includes smoothing on a grindstone, tile,
and whetstone, followed by polishing on a plate of lead, on the hairy side of a
cowskin, and afterwards on poplar wood. The teeth of animals are said to have
been polished in the same way.

Bttmisliiiig Slab, a flat piece of hard material put under parchment when gold
kaf used in illuminations is being burnished. Johnston (p. 153), in his discussion of
modern methods of writing and illuminating, suggests a flat piece of vulcanite,
celluloid, or metal for this purpose.

Tools and Equipment

283

Cabinet, The painter's cabinet as made today is a chest of drawers, ordinarily
low and not more than two feet square, with the top free for palette, oil cups, or
other accessories. This article of furniture is largely for oil painting, tubes of color
being kept in the drawers. Water color cabinets and others made for drafting
have, however, been advertised since early in the XIX century. A small cabinet
for illumination and missal painting is mentioned in the catalogue of Winsor and
Newton for 1863 and another for heraldic blazoning in their catalogue of 1864.
A desk-like object of walnut, very sm.all in size, is seen in the Victoria and Albert

Figure 4. Burnishers. The upper row has a number of shapes for use on gold leaf. These
are adapted from drawings in Thompson, The Practice of Tempera Paintings p. 67.
Usually such burnishers are of agate. One (<?) is represented as made of haematite, and
a is made of flint; / is a copper plate burnisher of steel; g and h are also steel and are used
for the burnishing of copper plates in the engraving process.

Museum, London, where it is described as a miniature painter's cabinet and is
believed to have been used by Richard Crosse, an English painter of 1742 to 1810.
Berger (I and II, p. 186) quotes a remark of Varro which seems to indicate that
painters of classical antiquity kept boxes or large cases in which the various colors
were arranged in compartments (see figure 15, ^ and b). Those representations
which remain, however, of painters' workrooms during the Middle Ages and the
Renaissance have no furniture which resembles the painter's cabinet as that is
known today. By the XVII century a few signs of it begin to appear, as in 'A
Painter's Studio' by Gonzales Coques (Flemish, d. 1684), in Schwerin. Here a
large box, like a small trunk or chest, is shown open In the middle of the floor. The
upper part is divided into compartments. One of these holds brushes, another a
large flask, and another smaller, bottle-necked containers. Below these compart¬
ments, the contents of some of which are not visible, is a set of drawers.

284

Painting Materials

Cabinet Saucer (see Saucer).

Camel Hair, a term inaccurately applied to a fine-haired brush, usually a
small, pointed one but also a large wash brush. This hair is from a squirrel native

to Russia and Siberia (see Bnisli).

Camera Lucida. A prism is used in this instrument to deflect the rays of light
from an object and to throw the image of this object onto a paper where it can be
traced. Meder (p^ai4) mentions it as a modern instrument. It is illustrated in the
catalogue of C. Roberson for about 1848. Perhaps its chief contemporary use is
as^an accessory to microscopes. With it drawings can be made from the minified

Camera Obscura. This differs from the camera lucida in being a more direct
and simple though more awkward apparatus. One type was, in effect, a large box
in which the artist sat and executed drawings (figure 5). In another, the image or
view was conveyed through a small hole in which a double convex lens was placed
so that It threw this image on translucent paper or on ground glass. hL of
course, the linage is seen inverted. According to Meder (p. 550^ the camera
obscura was invented by Erasmus Reinhold of Wittenberg in 1C40 Certain
improvements were made by Porta in 1558, and it was made into a trlnsportable

pSecte?°^ the camera lucida had been

Canvas Pliers. For stretching canvas over the stretching frame, painters fre¬
quently use pliers or pincers made with a wide nose, corrugated in order to hold
well o„ the edp of the fabric (figure 6. i). This toil is Co™ i„ studLs b«
seems to have had only a rather recent use. It is mentioned in catalogues of 1889

Cast. Models in the form of plaster casts have been for many centuries a

wSlZ furnishings of painters and of painting aLdemies At

eta foTmostof'Se^'A^ “ orfragmenTL nt

Clear tor most of the evidence about the use of such material is found in paintings

of tb ^ distinction between the two can not be made. The earliest

“ P"™" ~Il«tion in London. In
this a number of fragmentary figures, probably stone sculptures are seen with
other working materials. By the XVII century casts .re commoSy represented S
studios and freijuently students are shown draing from them. Wo ks by Rtkaert

of the twls fonndt Ih! ■ ““ r ““1“t‘™”>'=»ts. some

lite^trlre of A '’"‘P”'" «f P"”ters and mentioned in the

erature were of a type which might weU have been used in this way. One of

Tools and Equipment

285

these was the cauterium^ a kind of spatula of metal, with a rounded end. It may
have resembled those in figure 23, c and d. Many of the Fayum portraits show in
their conformation the marks of a blade of this sort and have somewhat the look
of modern oil paint that is spread with a palette knife.

Cestnim. Like the cauterium^ this was a metal spatula used for painting in
ancient times. It was, however, different in shape, being pointed and more like
a stylus. Eastlake (I, 154) gives as synonyms for it, viriculum and rhabdion.
Pliny (XXXV, 149) mentions the cestrum and, from the context, it appears that
this must have been an instrument that was usually heated.

FIGURE 5. Two types of camera obscura—drawings adapted from illnstrations in Meder.
At the left (p. 550, fig. 256) is a cross section of an early construction in which the image
falling through the lens is reflected by the mirror {a) onto a drawing glass or tracing
surface {h). At the right is a French camera obscura of the second half of the XVIII
century (p. 551, fig. 257). This is a large box in which the draughtsman sits, closing the
door behind him. The image enters at the side opposite that shown here, through a lens
in the small box at the top. From there it is deflected by the glass {a) onto the drawing
surface {h). At c is the seat for the draughtsman. Heavy metal straps at the side of the
box are to hold poles by which it is carried.

^ Chalk. The regular black chalk used for drawing is a natural deposit, a slaty,
soft, earthy material, very rich in carbon. In Europe it is found mainly in Thurin¬
gia, parts of France, and Andalusia (Meder, p. 109). Other colors, chiefly a range
of earth reds, are common in practice and are available in the stocks of artists'
colormen. The white chalk used for drawing—to be distinguished from the white
chalk that is an inert used for grounds—probably has included a number of dif¬
ferent materials such as sticks of gypsum or steatite (soapstone). As now prepared
for the market, the various colors of drawing chalk are put up in small sticks or as

286

Painting Materials

pencils. Their use goes back to the earliest antiquity of the art, and they were very
common during the Renaissance. Cennino Cennini (C. XXXIIII, Thompson, p
ao) speaks of a certain black stone from Piedmont soft enough to be sharpened
with a penknife, very black, and good for drawing. Meder (p. 122) says that
Leonardo da Vinci was the first to use chalk throughout for a complete and
finished drawing.

Figure 6. Equipment for use with painters’ canvas. At a is shown a corner Section of the
common stretching frame. These pieces, made so that they can fit together to a mortised
and mitred corner, form a complete frame by a combination of four and are sold as
single pieces of various lengths by artists’ colormen. They are usually of pine and are
about 1X3 inches in section. At ^ is a type of pincer or plier used for stretching canvas
over such a frame. The fabric is gripped between the two broad jaws and the spur be¬
neath serves as a fulcrum, acting against the edge of the stretcher frame.

Channel Edge Support. This name is given arbitrarily to a wooden channel
occasionally seen attached to the end grain edges of thin panels in paintings of
the studios particularly of Dutch artists of the XVII century. One is found in a
work attributed to Rembrandt, ‘The Painter in his Studio,’ formerly in a private
collection in England {Burlington Magazine, CCCLXXII [1925], p. 264). This
channel was undoubtedly fitted without nails or screws over the end grain in
order to hold the panel itself flat while the painter was working on it and before
it could be supported in the frame. Painters of this period who chose to work on
wood often had panels that were thin, less than i inch or 6 mm., and these were
in danger of warping or even of splitting while still in the studio.

Charco^. Like chalk, this universal drawing material has probably been used
from primitive times. (See figure 7, B.) The kind usually favored is made from
the willow twig, and the description of its manufacture by the painter himself is
adequately given by the early XV century writer, Cennino Cennini (C. XXXIII,
Thompson, p 19). According to this, a dry willow stick is cut into slips as long
as the palm of the hand and then split or divided like match sticks and these done
up m bundles. The bundles are put into a baking dish and left in a baker’s oven
overnight. The coals should then be quite black and, if they are too much roasted,
will break easily m the hand. Far Eastern painters are reported to have used
charcoal for preliminary outlines, particularly in ink painting on paper (Bowie,

Claude Lorraine Glass. For reflecting a landscape in miniature, a black convex
glass was said to have been developed and used by the painter whose name it

Tools and Equipment

287

carries. Since it was not a silvered mirror, much of the detail of a landscape was
merged together by the relatively weak reflection, and convexity brought a large
scene down to a very small area. Sketches and drawings were done from the
reflections in the glass. It was much used in the XVII and XVIII centuries and
is still occasionally seen in the studios of painters and etchers. Mrs Merrifield
(cxxv) reports that an eminent Italian painter of the XIX century spoke of

Figure 7. Tools for use with charcoal and crayon: (a) a crayon holder of modern
type, so made that the jaws at either end are tightened by a sliding ring and grip the
crayon between them; (^) a similar holder as shown in a drawing by Bouchardon (Meder,
p. 118, %. 45); (r) a lead pencil, apparently a holder for a metallic stylus, described by
Konrad Gesner in 1565 (Meder, p. 140, fig. 50); (d) willow twigs set in a lump of clay
for burning into charcoal by the method described in the Mt Athos MS. (Meder p.
102, fig. 42).

having a black mirror once owmed by Bamboccio in which the subject was re¬
flected exactly like a Flemish landscape. It was said to have gone from Bamboccio
to Caspar Poussin and on to others.

Cloths. The common paint cloth or paint rag of the modern studio has prob¬
ably come in largely with the development of broad-scale oil painting. Frequently
they are called palette cloths, and J. S. Templeton in The Guide to Oil Painting
(London, 1845) speaks of using old linen in order to avoid lint. De Mayerne,
writing in the XVII century (MS., p. 90; Berger, IV, 261), speaks of holding
paint cloths with the little finger of the left hand and of using them to squeeze
out and clean the paint brushes. A cloth for ordinary cleaning purposes must
have been used from very early times. Pliny, however (XXXV, 103), tells how
Protogenes got the effect of foam on a dog’s mouth by throwing his paint-soaked
sponge at the picture.

Compass, the usual drawing instrument for circles (figure 9). This probably
became a painter’s tool during antiquity. It is made of two straight legs, usually
pointed at the tip and hinged at the other end. Meder (p. 187) says that it was

288

Painting Materials

certainly known in Pompeian times. Cennini (C. CXL, Thompson, p. 8 5) speaks of
using compasses for incising the outlines of haloes in the burnished gold of panels.
Modern usage often refers to this instrument in the plural as a pair of compasses.

Copper Point. Though less common than that of silver, the point or stylus of
copper as a drawing instrument to be used on a prepared surface has had occa¬
sional mention in the history of the art. It is said to have a tendency to turn
slightly yellowish as it corrodes on standing. This might depend on the kind of
atmosphere to which it was exposed. A codex in Montpellier (see Meder, p. 74)
mentions the use of a copper point.

Crayon, a small stick for drawing, composed usually of pigment in an oil or
wax. It is smooth and is ordinarily used on paper. Certain crayons of modern
manufacture contain water-soluble dyes and are prepared in an aqueous medium.
Drawings made with these are afterwards washed over with a brush and water to
extend the tones made by the crayon marks. The use of crayons in Europe evi¬
dently began in Italy in the middle of the XVI century (Meder, p. 108). Marks of
such a drawing instrument are found in the work of Tintoretto, the Bassani and
others. They appear in German drawings of the XVI century and are coiUmon in
Europe by the XVII century. One type of crayon was made in the XVII century
by dipping and cooking charcoal in linseed oil.

Crayon Holder. Made as a removable handle for short pieces of charcoal,
chalk, or crayon, this is usually a tube split at both ends and fitted with sliding
rings (figure 7). Into the split part the round chalk or crayon can be placed and
held tightly by the rings slid over the split metal. The common crayon holder is of
brass and is from 4 to 6 inches long, with or without a wooden center. It is ad¬
vertised by most dealers in artists’ materials but probably had a greater use in
times when the pencil as known today had not yet been devised. Paintings of the
XVI century frequently show it in the hands of artists and draughtsmen, and by
the XVIII century it is found with fair regularity in studio interiors. According to
Meder (p. 185), an earlier form of the holder was a split reed, and the metal tube was
introduced only in the XVI century. Catalogues of artists’ materials in the XIX
century list it normally under the name of ‘ portcrayon.’

Cushion. In the process of applying gold leaf, this extremely thin material has
to be laid out where it can be flattened and frequently be cut. Squares of the gold
are usually dropped from the books, in which modern leaf is put up for the market,
onto a gilder s cushion (figure 11, d). The typical cushion is made over a rectangular
panel of wood, five or six inches wide and eight to ten inches long. A thin padding
is placed over this. Stretched across the pad and pulled tightly over the edge of
the wood a piece of soft calfskin is tacked rough side up. Around three sides of this
cushion a shield four or five inches high is fastened, made ordinarily of parchment
or of heavy paper. This is to prevent draughts of air from disturbing the gold leaf.
l he calfskin needs to contain as little oil as possible, and it is frequently powdered
over with haematite or red ochre to prevent the leaf from sticking. Some form of

Tools and Equipment

289

cushion must have been in use since ancient times when the application of gold in
leaf form began. The one described by Cennino Cennini in the early XV century
(C. CXXXIIII3 Thompson^ p. 81) is like that in common use today^ the padding
in his case being made with 'shearings.’

Babbetj a large, round brush, regularly of camel hair (see Brush and figure i).

Desk. This piece of furniture as a particular equipment of the artist is largely
confined to the use of scribes, illuminators, illustrators, miniature painters, and
draughtsmen. The varieties of its forms are innumerable and, according to the
evidence of old paintings and illuminations, follow the changing styles of furniture
in general. Johnston (pp. 49^-51) describes in some amount of detail a simple desk
to be made for illuminating and lettering from a drawing board hinged to the
edge of a table and elevated to the height required by a round tin set under it.
On this board (figure 8) the paper is held by tape or string at the top, and by a
kind of pocket made from heavy paper or vellum fastened across the lower part.
Under this is a light pad of blotting paper. He says that Eastern scribes use a
pad made of fur.

Dipper, a small cup for medium or diluent, made to be clipped onto the edge of
a palette. This is of metal, usually with an opening smaller than the body of the
cup, and frequently is equipped with hinged lids. In Renaissance paintings in
which an artist is represented at work, the container for such liquids is usually
shown as a kind of cup hung over the peg of an easel or in a similar position. The
dipper, or small container, attached to the palette, is probably of fairly recent
origin. A pair of them is seen in a work of the school of the XVIII century in the
Bonnat collection at Bayonne. This is a drawing, said to be a portrait of Boucher.
The artist is shown with a large, oval palette on which is clipped a double oil cup
or dipper of the kind now commonly sold.

Divider, an instrument like a pair of compasses (figure 9) used for comparing
and laying off dimensions (see Compass). The proportional divider has a movable
axis point and is scaled so that the opposite ends can be kept at a certain ratio.

Draughting Instruments. These are used more by architectural and engineer¬
ing designers than by painters, but sets of such instruments are frequently listed
with the supplies of artists’ colormen. They usually include pencil and pen com¬
passes, ruling pens, and dividers.

Easel. A light frame made to hold a painting in a vertical or nearly vertical
position is probably the most universal article of furniture in the workrooms of
painters and certainly is one of the oldest (figure 10). The simplest design is an
arrangement of three legs so that two stand forward in a parallel position and have
pegs or other rests where the painting can be held up. The third leg swings back,
and its position determines the angle at which the painting stands. Although
subject to many variations in details of its construction, such a three-legged easel
with pegs for the picture is a standard type and is somewhat different from an¬
other that is now prevalent. This other has usually been called a 'studio easel/

290

Painting Materials

It is heavier and more complicated (figure ii). The base is broad and rides on
four casters or small wheels, one of which is usually adjustable in height. From
the base rise posts firmly fixed, into which a sliding frame is slotted. This frame
carries the painting in a shelf that has a wooden rail at its front. The top of the
painting is held by another sliding member which comes down over its edge and
is fastened with a set screw. The narrow shelf or rail is usually moved up and down
by means of a worm gear operated by a crank. Another adjustment permits the
painting to be tilted forward, also by means of the crank. Modifications of this
as of the simple three-legged type are innumerable, and some easels have been
made which combine the aspects of the two.

r[.Hsi h.

4

1 1 1 n

0 0

0 0

Figure 8. The desk of a scribe, according to Johnston (p. 50),
which a piece of stout paper or vellum is fastened with thumb
writing pad usually made of blotting paper. The tape or string
board holds the writing paper in place.

There seems to be no historical limit to the simpler easel. It is manufactured
and sold now and probably has been the property of the painter since separate
and portable pictures were first made. It is found in studios as these are shown in
the works of the Renaissance and until quite recent times. Often, in the easels
represented, a rest board is laid across the pegs, and these pegs vary in shape and
in ornamentation. The swinging leg is hinged in different ways and the top is
finished in a variety of shapes. Far more complicated means for holding pictures
are, however, occasionally illustrated during the Renaissance and later times.
One striking example is also the earliest instance known of the representation of
an easel. This is in a relief on the wall of the mastaba of Mereruka (see Prentice
Duell). It is an upright pair of posts which Duel! considers to have been fixed in
a base to permit the entire frame to be moved. The painting was supported on
notched members which swung forward at right angles to the post or swung back
out of the way if other such members at a different height were needed. Pliny
(XXXV, 81) says that when Apelles went to call on Protogenes, he found a soli¬
tary old woman keeping watch over a large panel that was placed on an easel.

The studio easel, as described above and as shown by artists^ colormen of the
present, was devised and largely developed during the XIX century. The earliest,
said to have been of French manufacture, had worm gears made of wood.

is a drawing board to
tacks. Under this is a
around the top of the

Tools and Equipment

291

Eraser. Probably the common means of removing marks of charcoal, chalk, or
graphite from paper or parchment has been, until a century ago, with crumbs or
pellets of soft bread. Feathers as brushes are mentioned in connection with
charcoal; soft leather like chamois skin has been used extensively in the past; and
Cennino Cennini (C. XII, Thompson, p. 8) speaks of making erasures in silver-
point drawing, also with bread-crumbs. In the MSS of Jehan le Begue (Merrifield,
I, 63) an alum paste is suggested to be used for erasures in drawing. Abrasives
such as pumice and cuttlefish bones, as well as metal scrapers, served this same
purpose. Rubber seems to have become an eraser for drawings in the latter part
of the XVIII century (Meder, pp. 190—191). It was then very expensive and evi¬
dently so hard that it was apt to scratch the paper. A manual on miniature paint¬
ing,^ written by Constant-Viguier and published in 1839, warns against this.
Indian rubber in different shapes and sizes is listed in the catalogue of C. Roberson
and Company in about 1840. It was then sold by the pound. Methods of plas¬
ticizing crude rubber and of introducing various grades of abrasives have given
to erasers now available on the market a very wide range of cleansing properties
from soft, pliable materials to others which act almost like sand-paper.

Fan Brush, a soft brush of fan shape, very flat, and set in a metal ferrule. It is
made in either sable or bristle and is used for special finishing, as of foliage or hair,
in oil painting (see Brush and figure i, f).

Finder, a device usually made by the painter himself for locating the area of
his composition in a natural landscape. It is merely an aperture of the size and
shape required, cut out of a thin cardboard or similar material. Occasionally cross
lines of thread or wire run through the center, (See also Sight Measure.)

Folio (see Portfolio).

Gallipot, a small cylindrical vessel made of porcelain and used for holding
diluent or medium. The name has now gone out of use in catalogues of artists'
colormen, but appears in those of Winsor and Newton for 1868 and 1870.

Gilder^s Knife, a steel-bladed knife, fairly heavy, only a little flexible, and
about eight inches long (figure 12, 3 ). This is used for cutting gold leaf into pieces
smaller than those in which it is regularly sold. Cutting is done on the cushion
after the leaf has been thoroughly flattened.

Glass Frame or Tracing Apparatus. One method of drawing much in use during
the Renaissance was in effect a tracing of the person or object represented, on a
frame which was placed between the draughtsman and his subject. The frame
contained a sheet of glass and evidently some kind of soft crayon was used on it.
Diirer illustrated this method with woodcuts and in one of these a fixed position
for the draughtsman's eye is shown (figure 27). Squaring the area of the glass
with lines is indicated by him in other representations. Meder (p. 467) says that
Holbein, Clouet, and Ottavio Leoni used an apparatus of this sort. It appears
that paper could be placed over the tracing which was on the glass, and the
drawing could be worked up from that.

2g2

Painting Materials

Gold Point (see also Silver Point). To what extent a wire or point of gold has
been used in the practice of drawing is a matter of some doubt. Enough has been
said about it, howeverj to indicate that it may have had at least occasional em¬
ployment along with the very common silver point as an instrument to be used
over a slightly abrasive ground. Meder (p. 8i) mentions it, and a modern writer
on drawing (Harold Speed, Tke Practice and Science of Drawing [^Philadelphia:
J. B. Lippincott Co., 1925], p. 275) says that gold point gives a warmer line
than silver, but can be used in much the same way on paper that has been treated
with Chinese white (zinc oxide).

Figure 9. Compasses and dividers: {a) a type of compass with a lead stylus as shown
by DGrer (from Meder, p. 77, fig. 29); {b) two sketches of proportional dividers as shown
in the MSS of Leonardo da Vinci (from Meder, p. 188, fig. 68); {c) a proportional
divider, closed, of modern manufacture. This opens to an x-shape on^ the screw near
the center and can be set for the proportional relations desired.

Graphite Pencil. This, usually called a ‘lead’ pencil, is the common implement
for casual writing and for a great deal of drawing in modern times. It is made
almost entirely of pure graphite held together by firing at high temperatures, with
some addition of clay. Such pencils are now commonly sold as wooden sticks with
the so-called ‘lead’ in the center, or the leads are put in a permanent holder where
they can be moved in position as they wear down.

Lead itself was certainly used in the same way as other metals (see Silver
Point in particular), and the confusion of names makes it difficult to know at
what time graphite came in to take its place. R. Borghini, writing in 1586 {II
Riposo in cut della Pittura e della Scultura si Favella^ Florence, p. 139), describes
the use of a piomUno which may be graphite instead of lead. Meder (pp. 140-141)
quotes Johann Mathesius (1564) with regard to a new writing instrument of

Tools and Equipment

293

mineral origin and for use on paper. This, as judged by the context, might be
graphite. It was not until the XVIII century, however, that anything like the
graphite pencil as that is now known had come into regular use as a drawing
instrument.

Figure 10. Painters^ easels as used in the past: («) an Egyptian ease! represented in a
tomb relief of the Old Kingdom (from a reconstruction by Prentice Duel! [VIII, 181,
fig. 4]); W an easel in a Roman tomb relief (from Berger, I and II, 175, fig. 33); (c) an
easel in a Pompeian wall painting of a pygmy's studio (from Berger, I and II, 174, fig.
31); (^f) an easel of the XVII century, shown in a painting by Rembrandt, ‘The Painter's
Studio,' in the Art Gallery, Glasgow; (e) an easel in a French miniature painting of the
XV century (from Berger, III, 231, fig. 16).

Grinding Slab, a flat piece, usually of glass or stone, on which color is ground
from a coarse to a finely divided state, frequently with the medium that is to bind
it as paint (figure 13). Materials for this purpose available on the market today
are usually of glass and are small in size. They are used little, except by tempera
painters or by illuminators who are working with colors not previously prepared.
Many catalogues of artists^ materials do not even list these small slabs amongst
their supplies. Machine grinding and preparation of artists' paint, a development
which has taken place largely since 1825, have slowly removed from the studio
the grinding equipment which was invariable in its furnishing before that*

294

Painting Materials

Probably many different hard stones were adopted by painters for this pur-
pose^ but porphyry seems to have been a favorite since the Middle Ages. Thomp¬
son {The Practice of Tempera Paintings pp. 88-89) porphyry or granite

are still the best materials for slabs and mullers but that glass is a simple substi¬
tute. This or marble has to be resurfaced when it gets smooth and that can be
done with moderately fine emery powder, wetted, and ground between muller
and slab. Sir Aurel Stein {Serindia^ I, 772) reports the discovery at Tun Huang in
Chinese Turkestan of a wooden block which he thought had been used as a grind¬
ing slab. It was D-shaped in section, and one end and the adjacent side still had
wrappings of linen. On it was thick, black paint. A second similar block was found
in the same site. No mention has appeared in the technical literature of the color¬
grinding methods used by ancient Egyptians, but it can be assumed that the hard
mineral pigments, particularly the well known Egyptian blue, must have been
ground on a slab or in a mortar. One indication of such a utensil is seen in the
British Museum. It is a piece of dark stone about seven inches across, rather flat on
the bottom, rounded at the edges, circular in plan, and less than half as thick as its
diameter. The top has a shallow, saucer-shaped depression. This seems to be
Egyptian and is something between a mortar and a grinding slab. Little is known
about other ancient practices of preparing paints, but an anecdote by Pliny
(XXXV, 85) suggests that in the studio of Apelles there were boys who ground
the colors. By the early Middle Ages in Europe, the fairly standard equipment of
the painter’s work-room appears to have been a heavy slab mounted on a heavy
block. The MS. of Theophilus speaks of it, as does that of Eradius (see Laurie,
The Painteds Methods and Materials, pp. 25 and 27). Cennino Cennini mentions
porphyry (C. XLII, Thompson, p. 25) and so does the author of the much later
Brussels MS. in 1635 (Merrifield, II, 77^)* By the XVII century there are many
representations of the grinding tables or blocks, usually with circular or polyg¬
onal, heavy slabs.

The hand-grinding of colors undoubtedly continued long after fairly large-
scale commercial manufacture was developed, and some amount of it is still done
by artists colormen or by painters themselves. In the early XIX century, a form
of hand-operated color mill was developed, and shortly after that a larger mill
with ^stones propelled by hand, horse, or steam power. The common modern
mill is operated by a motor and is of rotating cylinders. Water colors, as first
manufactured for sale, were usually in small, hard cakes which, like the Far
Eastern Ink stick, had to be ground with water on a small slab in order to bring
the color out into the fluid. Such grinding material and equipment were sold
commonly after the middle of the century and, although little mentioned or
advertised, can still be bought.

Hog Hair, the name formerly applied to the heavier artists’ brush, now usually
referred to as bristle (see Brash).

Tools and Equipment

295

Holder (see Crayon Holder).

Inkpot. Primarily the property of scribes, the inkpot, ink-horn, or inkstand,
simply a vessel for holding fluid ink, was doubtless also a regular part of the
furnishings of the painter’s studio, even in those times when writing was com-

Figure II. Modern easels: (s) the studio type with screw adjustment of height and of
tilt; (i) a folding easel for sketching; (c) a tilt board or table easel.

paratively rare. As illustrated in Renaissance painting, the inkpot is apt to be
either a kind of well like the modern ink-well or a simple, cylindrical vial with a
flange at the top. The latter is perhaps slightly more common. It frequently ap-
psars on the desk of St Luke, set into a board at the edge (flgure xq, ^), the flange
or lip of the vial catching on the wood, and the main part going through a hole.
As a rule, a number of vials, probably for different colored inks, are seen together.
Classical scribes, it appears, were in the habit of using inkstands or inkpots, and
many of these in bronze and terra-cotta and in various shapes are still preserved.
At the Niya site in Chinese Turkestan were found oval, trough-shaped pieces of
horn which Stein thought to be probably inkstands {Serindia, I, 225 and 256).

Kolinsky, the name usually given to the hair from which the fine sable brushes
are made. Furriers apply the name to the red sable or tartar sable or any of several
Asiatic minks. The tail is used for brush making and that of tht Putorius sihiricus
is said to be the most favored for this purpose.

296

Painting Materials

Lay FigtirCj a mechanical figure of human shape, usually of natural size and
covered with a knitted fabric. The armature is jointed, even to the fingers, and
can be put into postures like those taken in life. Such a complex mechanism has
been used for the arrangement of draperies for study purposes and has had a wide
utility in professional portrait painting because it permitted the artist to work on
costume without a sitting. The lay figure probably superseded models and
manikins, and it is doubtful if its use was at all general before the XVIII century.
It is listed in the catalogues of artists’ colormen in the early XIX century and is
still occasionally seen.

c

Figure la. Parts of a gilder’s equipment (after Thompson, The Practice of Tempera
Painting^ pp. 56 and 57): {a) the cushion of leather with a shield of parchment or paper
and straps underneath for holding; {b) a knife for cutting the leaf on the cushion; (r)
tips of fine hair in different lengths used for picking up the leaf. (These are out of scale,
the size of the cushion being represented as too small for the others.)

Lead Point. Lead is among the various soft metals which, drawn over an
abrasive ground, produce a distinguishable and clean mark (figure 7, c). Its place
in the recorded history of drawing is vague because of the natural confusion of
this metal with silver and its later confusion with the graphite pencil which,
almost from the beginning, was called a Tead’ pencil.

Mahistick or Maulstick, a light stick or rod of wood, with a soft leather-
covered ball at one end; it is to rest and support the painting hand. The ball rests
against a part of the easel, or at times against a part of the painting. The opposite
end is held by the hand which holds the palette, and the working hand is rested
on the stick itself. The sticks made commercially are usually of hard wood or
bamboo, jointed with brass ferrules to give them a length of four feet. Early
catalogues of artists’ supplies speak of this as a rest stick, and it still carries that
name. It was probably not much used before the time of oil paintings on a large
scale and does not appear to be mentioned or referred to in the Mediaeval and
Renaissance treatises on painting. A few instances of it appear in representations
of artists made during the XVI century and It has a regular place in the hands of
the painters as they are shown in works of the XVII century and later.

Tools and Equipment

297

Manikinj a jointed figure of human shape but with only a general resemblance
to the human form, used by painters as models, particularly for arrangement of
costume and drapery. The manikin, as manufactured and sold today, is different
from the lay figure in having a much more mechanical appearance, being of wood
and openly jointed. There is no distinction of sex and no description of muscles
or of any superficial refinements of form. The modern commercially made manikin
for painters is of wood and varies in size from a height of about a foot to more
than five feet.

Although collections of ancient art show many jointed figures, particularly
those of small scale, there is no definite indication that these were ever used as
manikins. Meder (pp. 551 ff.) speaks of the use of manikins and refers to a wooden
female figure, carefully carved and so jointed that it was obviously used as a
manikin, made in the XVI century and now at Innsbruck. Another is shown in
a drawing by Adrian van Ostade. Michael Sweerts, active in the second quarter
of the XVII century, made many paintings of studio interiors. One in the Cook
collection (Richmond, England) shows a student making a copy. Around him is an
assembly of paraphernalia, including what seems to be a life-size manikin.

Metal Point. The stylus of metal (figure 25) was a common tool for drawing
and for writing In ancient times, and the discovery that a metal point drawn over
an abrasive surface left a fine line, must have been made centuries before any
historical evidence appears for its general use. Drawing with a point of silver or of
lead, copper, or gold depends upon a surface which has been constructed in such
a way that it will abrade away and hold a small deposit of such a metal when that
is moved over it. A great variety of means has been found for producing surfaces
of this kind on wooden panels to be used for practice by apprentices, or on parch¬
ment or paper for the work of their masters. Chalk and calcined bone bound with
a weak size and coated over such a surface were among the many in common use.
Some of the metals, particularly silver and copper, leave a mark that grows darker
with time as the abraded grains of the metal corrode. It is probable that the over¬
whelming reference to silver point in descriptive literature gives it slightly too
great a prominence. Frequently there is no distinction among metals and drawing
with any one of them Is referred to as silver point.

Miniverj a hair formerly used in brush making. The word has been connected
with the ermine, but apparently referred also to other plain white furs used in
trimmings of ceremonial costumes. Cennino Cennini (C. LXIIII, Thompson, p.
20) says, "In our profession we have to use two kinds of brushes: minever brushes,
and hogVbristle brushes.’

Mirror. Commonly employed by etchers in order to have a reflected image of
the model or landscape and so to avoid transposition in the final print from the
plate, the mirror is also used extensively by painters. It has the value of giving a
kind of distance and isolation to the work when a painting in progress is studied

298

Painting Materials

in this mirror image. Probably such reflecting surfaces have been useful to painters
since they were first invented. Leonardo da Vinci {Treatise on Paintings Rigaud
translation^ C, 350^ p. 150) says that a mirror is useful in reflecting the object that
is being painted in a position for comparison with the painting itself. Both then
have a flat appearance^ "an even superficies/

Figure 13. Grinding implements: {a) mortar and pestles of alabaster from a late classical
find (Berger, I and II, 215, fig. 47); {b) a table with slab and probably a jug of oil, in a
French miniature of the XV century (from Berger, III, 231, fig. 16); {c) stone slab and
muller set on a wooden block, as shown in a painting by G. Dow, ‘An Old Painter’
(private collection, London).

Model. Objects used by draughtsmen and painters to serve as models from
which to work are, of course, innumerable, and any object In a room or studio
could become such a model. Certain kinds of objects, however, have been taken
up so regularly and so exclusively for this purpose that it is possible to consider
them as furniture of the painter. (Among these are inanimate figures; see Cast,
Anatomical Figure, Lay Figure, and Manikin.) Renaissance literature on painting
indicates that before that time hills, mountains, grottoes, and similar features in
the landscape were drawn from rocks which could be studied at close range or
set on a worktable. It appears, also, that foliage may have been described by
repeating small areas drawn from branches brought indoors. From the Renais¬
sance on, direct representation of small objects has continued, and painters^
studios are represented as containing a variety of fabrics, costumes, armor, and
similar things clearly to be used as models. A more affected type of small model,
made and sold as such, seems to be a device of the XIX century. By 1870 the
catalogue of Winsor and Newton advertises J. D. Harding^s "Drawing Models/
These were architectural blocks made up in cubic shapes, for, as the catalogue puts
it, the cube was the unit and basis of all solid and rectangular figures, including

Tools and Equipment

299

architectural construction. Somewhat earlier than that, the catalogue of Rowney
and Coni,pany had listed and itemized what were known as Rustic Models. Among
these were a Dutch windmill, gates, pumps, stiles, railings, wells, cottages, water¬
wheels, boat-houses, and castles, ‘carefully studied from Nature, for the use of
sketching classes.^

Mop, a large, round brush, usually of camel hair (see figure i, g).

Mortar, a grinding vessel of cup shape, often made of hard stone (see Grinding
Slab and figure 13, ^).

MuEer, the moving part of the usual device for grinding color, of which the
grinding slab itself is the fixed and lower part (figure 13). Generally the muller
is a stone rounded at the top and at the bottom, completely flattened so that it
has full contact with the grinding slab. The top is curved to fit the hand. Probably,
in most cases the material of the muller was the same as that of the stone because
any difference in hardness w^ould have resulted in undue wear on the softer; but
the Brussels MS. of 1635 (Merrifield, II, 770) mentions a muller of flint or whet-
stone to be used on a porphyry slab. As hand-grinding of colors has gone out of
practice, the large slab and heavy stone muller are now very rarely seen. Occa¬
sional grinding of small quantities of pigment is usually done on a ground glass
slab with a glass muller.

Needle, the name usually given to the fine-pointed steel instrument used in
etching and in dry-point on copper plates.

Oil Cup (see Dipper).

Painting Knife (see Palette Knife).

Palette. This word, perhaps as much used as the name of any utensil con¬
nected with the art of painting, has a figurative as well as literal meaning. By the
former, it denotes arrangements of color, mixtures and assortments of pigment, or
the scheme of tone relations in a given work or in the work of a painter or school.
Literally, in its present use, it has to do with the surface on which a painter lays
out and mixes his colors before applying them to the painting itself. As a rule, the
palette is an object that can be carried in the hand, but many painters prefer
to use the top of a painting cabinet or table which can be brought into a position
conveniently near to the easel. Such a mixing surface is said to have been preferred
by Whistler and by other painters of recent times. It is seen also in an ancient
studio, that of a pygmy in a Pompeian wall painting (Berger, I and II, 174, fig.
31). For the most part, however, the palette is a thin slab of material made in
such a way that it can be held securely in one hand and provide a fair amount of
area in which plastic paint can be placed in lumps or mixed together with medium
or diluent.

The standard material for palettes during many centuries has been hardwood,
cherry and walnut having been particular favorites. Other materials, however, are
not uncommon—'porcelain or enamelled ware, often with cups or slants in them
for water color glass, and aluminum for decorating or for wax painting. At

300

Painting Materials

presentj three shapes of palette are usual (figure 17): the oval, the oblong or
rectangular, and the studio or arm palette. The two former are from nine to
sixteen inches long and from six to twelve inches wide. The oval palette has a
thumb hole set not far from the center, and the edge is cut out in an irregular
curve so that the fingers can grasp the edge of the palette. The oblong palette
has three right angles, and the thumb hole is set fairly near the fourth corner which
is cut out in a curve suitably shaped for a grip. The studio or arm palette is larger
than the other two types, and varies from about twenty to nearly thirty inches in
length and from about fourteen to about eighteen in width. The thumb hole is

Figure 14. Containers for ink: (a) ink or color pots set in holes of a board at the edge of
a scribe's desk, as shown in a painting of the German school, 'St Luke/ Castle Rohoncz,
Hungary; (^) an inkwell and stopper in a 'Portrait of a Young Painter,' by PL Burgkmair,
late XV century, from a private collection in Berlin; (c) a pen case and ink bottle to be
carried on a scribe's belt (from Meder, p. 62, fig. 26).

set well back from the edge, and that edge which is held towards the painter is
cut out to fit around the elbow. This palette in particular is of a varying thickness
from one half to three quarters inch at the thumb hole side to about an eighth inch
at the opposite. This provides greater strength where the strain is greater and
gives a certain weight to the end which is shorter, allowing a balance to the whole.
Although these are the standard shapes commercially produced, there are doubt¬
less variations according to the taste and invention of painters for whom the
standard product is not quite suitable.

In the past such variations of shape are too numerous for a specific record.
They are indicated by the paintings of studio interiors and by a few actual palettes
that have survived. The time at which a slab of wood for disposing and mixing
colors may first have been used is impossible of statement. There has probably
been some confusion between slabs used for mixing and those used for the grinding
of colors. It has been suggested, for instance, that the Egyptians had a palette
as early as predynastic times, but it appears that this was a piece of slate on which
face paint was ground. Berger (I and II, 27) saw a wooden palette in an Egyptian
painting which represents a painter at work. He does not, however, consider this

Tools and Equipment

301

to have been at all usual, for other representations seem to indicate that Egyptian
painting was done with fluid paint carried in small pots. The same author draws
attention to a reference in the Mt Athos MS., a painter’s handbook supposed to
reflect practices of the XI century and later, in which is mentioned a palette
with a hole for the thumb of the left hand. By the XV century (figure 16), paint¬
ings of St Luke had begun to show him holding what is evidently a wooden palette.
The shapes of the palettes in these paintings are widely varied, usually oblong
however, and with thumb holes differently placed. The corners are apt to be cut
in complex curves. In an engraving from a self-portrait by Jacopo Bassano, there

Figure 15, Paint boxes: (^) a box, evidently containing paints in small jars, shown in a
representation of a woman painter in a Pompeian wall painting (from Berger, I and II,
175, fig. 32); (i?) a bronze box with a sliding cover and hinged lattice lids under that,
containing irregular lumps of pigment and found in the tomb of a late classical Gallo-
Roman painter at St Medard-des-Pres (from Berger, I and II, 214, fig. 46); (r) a modern
sketch box for oil painting with a palette, metal hooks in the top for holding canvas or
academy board, and compartments below for brushes, tubes, and bottles.

is shown a paddle-shaped palette having three straight sides and a fourth extended
to form a handle. This shape seems to have continued, though it is rarely seen, for
It appears again in a palette formerly used by Sir Joshua Reynolds and now shown
as exhibit no. 332 in the Royal Academy, London. During the XVI century oval
palettes seem to have come more into use and are common in the XVII and after.
Perhaps a transitional shape is that seen (figure 17, in the drawing by Peter
Breughel the Elder, 'A Painter in his Studio,’ Bayonne, wEere an oval with one
straight side is seen. A similar palette, oval but cut off straight at the thumb hole,
is held by Gerard Dow in his self-portrait at the Cheltenham Municipal Art
Gallery.

Palette Cup (see Dipper).

Palette Knife. A spatula, usually smaller and slightly more flexible than the
kind used for domestic and laboratory purposes, serves to mix the oil paint in
modern practice (figure 23). When the paint is worked to the consistency required,
it can then be piled on the palette. As prepared for the artists’ trade, palette knives

302

Painting Materials

of steel are presented in a variety of shapes. The straight blade or simple spatula
usually has a slightly wider shoulder than handle and tapers to a rounded tip.
The length varies from about 2| to 6 inches. A trowel shape with offset blade is
frequently longer. Broader blades with a longer shank having slightly tapered
edges or diamond or triangular shapes are sometimes called ‘painting knives/

Figure i6. Some shapes of painters* palettes in the past: (a) a long-handled palette in a
French miniature of the XV century (from Berger, III, 231, fig. 16); (^) a small palette
shown in a * St Luke* attributed to Wolgemut (late XV century) in the Germanisches
Museum^ Nuremberg; (r) a palette from a ‘St Luke* attributed to Heinrich Diinwegge {c
1500-1523) and in the Landes-^Museum^ Munster; {d) a paddle-shaped palette, shown in
an engraving from a self-portrait by Jacopo Bassano; (<?) a small palette seen in a‘St
Luke* attributed to Colin de Cotter (late XV century) and in Vieure near Moulins; (/)
a XVII century palette seen in a studio representation (Vyvyan Sale, Christie, 1935)
David Ryckaert (Flemish, XVII century); (g) a palette of Sir Joshua Reynolds, now
exhibited in the Royal Academy, London (no. 332); (A) the palette in a work attributed
to Quentin Matsys (^r 1500) and called ‘Quentin Matsys Painting the Portrait of His
Mother,* in a private collection in England; (i) one of the three palettes in the painting
by D. Ryckaert, referred to above.

In the earlier centuries of oil painting the knife used by the painter for scraping
the grinding slab or for transferring paint to the palette seems to have been of a
different kind. Probably a slice was the most common tool for this purpose
(figure 24). Where any kind of knife in connection with the grinding and prepara¬
tion of paint is seen in the studio paintings of the XV to the XVII century, it has
a heavy blade and the appearance of an ordinary stiff knife of moderate size.

Tools and Equipment

303

By the XVIII century^ however, this had somewhat changed. An engraving from
a selAportrait by Hogarth shows a palette knife of the spatula type. In ‘A
Painter's Studio' attributed to Francois Boucher, in a private collection in Paris,
a broad spatula kind of knife is seen beside the grinding slab. A dagger-shaped
tool, much like the so-called 'painting knife/ can be noticed in the thumb hole of

Figure 17. Modern shapes of the painter's palette: (a) a rectangular palette with a block
for grip near the thumb hole, used by John Constable and now exhibited at the Royal
Academy, London (no. 330); (if) this shape is seen as early as the XVI century in the
work of Pieter Breughel the Elder (particularly in a drawing, ‘'A Painter in his Studio,
Bayonne) and later in the work of G. Dow, * Portrait of the Painter (Cheltenham
Municipal Art Gallery). Contrary to the custom in those seen in figure 16, where the
thumb hole is either in a kind of handle near the middle or is in the upper part, in these
more recent types of palette, the thumb hole is placed nearer the center of one edge or
slightly below the center, leaving the upper edge of the palette for distribution and mix¬
ing of paint. This is a slight change, and the palette shapes throughout the Middle Ages
and Renaissance seem to have been very numerous. At c is a standard modern palette,
generally rectangular in outline; at d is the oval palette; and at e the so-called ‘arm'
palette.

the palette lying on the floor in Turner's 'Watteau in his Studio' in the Tate
Gallery. When artists' colormen began to list palette knives, there were two or
three materials used for them. C. Roberson and Company mention steel, ivory,
and iron in a catalogue of about 1840, and the same materials are mentioned in

304

Painting Materials

the catalogue of Winsor and Newton for 1870. Ivory palette knives or spatulas
are still available but are now little used.

Pantograph. Operating on a kind of steelyard relation of scale, this is an instru¬
ment by means of which copies in larger or smaller size can be made by a process of
tracing over the work to be copied (figure 18). The usual construction is of four
strips of wood set as overlapping sides of a square, the position of which, from the
worker, is with one corner towards the bottom of the page. A downward projection
of the upper left strip is fixed in place, the lower corner of the square is moved
over the work to be copied, and a downward projection of the upper right strip
marks the enlarged copy. The scale of the enlargement can be changed by adjust¬
ing the relations of the overlapping strips. The pantograph does not appear among
the catalogues of Winsor and Newton, artists’ colormen, until the year 1892, and
has not been found in other catalogues before that. Its age, however, is much
greater than this. Meder (pp. 189-190) says that it was described by Scheiner in
1635.

Pen. In present-day usage, this word has been attached almost entirely to a
metal instrument for writing or drawing that has been developed largely since the
middle of the XVIII century. Recent usage, however, has not yet established itself
in the history of art, particularly of writing and illuminating, where the word refers
principally to the pen made of a quill, though it may also have to do with a reed or
with a metal slip. Very possibly, the reed has the greatest antiquity, the hollow
joints of bamboo or tubular stock of coarse marsh grass {calamus) having been
used by Greek and Near Eastern writers from Roman times. And the reed or
cane pen is probably the most easily made (figure 19). According to the descrip¬
tion given by Johnston (pp. 51-54), it should be cut from a stalk about 8 inches
long, one end of which is tapered obliquely and shaved back to the hard, outer
shell The nib or writing edge is then cut across squarely and is slitted longitudi¬
nally for about I inch. To retain ink in the pen a thin strip of metal the width of
the nib and about 2 inches long is bent into the reed in an S-shape and sprung up
under the nib. Resides its use in the classical world, the reed pen undoubtedly
had a common employment, particularly for large writing, throughout the Middle
Ages and the Renaissance. It was known in Egypt. In a bronze pen case from
Oxyrhynchus, thought to be of Roman times and now in the British Museum, are
two reed pens cut in the way just described. In the Far East a number of examples
from the III to the IX century was found at Niya, at Lou-Lan, at Miran in the
Tarim Basin, and at Tun Huang. The pens found at Miran were accompanied by
written records and were of wood and of reed; those from Tun Huang were
roughly cut from the twigs of tanierisk. Although they are little used today, reeds
ready cut are available from a few artists’ colormen. They are of bamboo or
what is called‘Indian reed.’

The common pen, however, from classical times until very recently has been
the quill. Although one of the earliest specific allusions to the quill is said to have

Tools and Eq.uipment

':i 30s

been made by St Isidor of Seville in the VII century^ it was probably for
many centuries before that. In the West the quills of the goose and the
seem to have been most taken for the manufacture of pens, and Johnston (pp.
54-60) recommends a wing feather quill from a turkey as strong and suitable for
ordinary writing, x^ccording to his directions, the barbs are first stripped from the

Figure 18. A pantograph, an instrument for making enlargements or reductions in scale
of drawings: (a) that described by Christ. Schemer in 1635 (from Meder, p, 189, fig, 69);
^ and c are modern pantographs produced commercially; ^ is a French one of the XVIII
century (from Meder, p. 189, fig. 70).

shaft and then this shaft is cut and pared at the larger end which is to be used for
the nib. The extreme tip is cut off on a slab and a slightly oblique, chisel-shaped
nib is left. As with the reed, modern scribes have found that a thin strip of spring
metal looped into the shaft and under the nib will hold a fair quantity of ink.
Meder (p. 34) considers that the pen has been the drawing instrument of greatest
use in Europe since the time of Early Christian art, and the history of its use as
summarized by him includes much of the drawing of the Renaissance and most
of the great names through the XVIII century.

Painting Materials

306

Pens of metalj although they dominate the writing means of modern timeSj are
by no means modern in origin. Among writing instruments exhibited in the
British Museum is a bronze pen found in the Tiber at Romoj in its shape much
the same as the steel pen of today. Another found at Pompeii is in the Naples
Museum. There seems to have been little use of metal for writing purposes during

Figure 19, Diagrams showing the cutting of pens from quill and reed (largely adapted
from Benson and Carey, pp. 42-48). In the upper row is shown the general sequence of
cutting a quill, including the position of the metal spring for holding ink; in the middle
row, the cutting of a reed, including the angles given to the nib; and at the bottom, a
series of steel pens used for lettering.

the Middle Ages or the Renaissance or in any general sense until the XIX century.
Samuel Harrison in Birmingham^ England, made a steel pen in 1780 but such
devices were not on the market until about 1803 when it is known that they were
sold in London. These pens were in a tubular shape, so formed that the edges
made the slit, and the nib was cut and tapered as was that of the quill. Machine-
made pens did not appear until about 1822,, and James Parry is said to have been
the first manufacturer of the steel slip pen. When pen making was established as
an industry, the material used was rolled sheets of cast steel made from Swedish
charcoal iron. Suitable strips were cut from the sheets and these were annealed,
pickled, and rolled to a thickness of about i/i60 inch. From the ribbons of thin
steel the pen blanks were punched, annealed again, and were tempered, scoured.

Tools and Equipment

307

and polished. Stainless steel is now frequently'used for non-corroding qualities. A
few other materials have appeared in the history of pen making—tortoise shell
and glass—but they have never had great use as drawing instruments.

Pen Case. This minor piece of equipment seems to have had no currency during
the last century or morcj and w^as probably limited, for the most part, to the time
when the scribe was a special and somewhat Itinerant craftsman. The case"proper
w^as a long, usually tubular, container made frequently of leather or of horn and
attached with thongs to the scribe’s belt where it hung along with the ink-horn
(figure 14, r). During the later Renaissance, painters evidently took up this kind
of case for carrying brushes, and artists’ colormen still shoW'^ boxes of a similar size,
ordinarily of metal, in which brushes can be carried along with other sketching
materials.

Pestle, an elongated piece of hard material, usually stone, for grinding pig¬
ments or other materials in a mortar.

Poona Brush. This name indicated a shape and utility of brush rather than
any particular hair. It was ordinarily a rather small implement of bristle, some¬
times of badger, and was blunt-ended and round in cross section. It appears in
catalogues of artists’ colormen chiefly during the middle of the XIX century.
As shown there, it was mounted in a quill and bound around with thread. Prob¬
ably the stencilling brush has largely taken its place.

Portcrayon or Porte-Crayon (see Crayon Holder).

Portfolio, a protecting and carrying cover for drawings and similar works on
paper. It is usually made with two sheets of board, covered and hinged at one of
the long sides. The other three sides are provided with tapes for tying the port¬
folio shut, and flaps are sometimes attached for protecting the edges of the
contents.

Pounce Bag, any loosely meshed cloth through which a colored powder could
sift, has been used over pricked drawings as a means of transferring these to
another surface. This has been done for duplication of drawings or for the transfer
of the design to a ground suitable for painting. The antiquity of the method is
indefinite. Stein (Serindia^ I, 484) found what he considered to be a pounce bag at
Miran Fort, occupied by Tibetans at the end of the VIII and during all of the IX
centuries A.D. This was a felt pad stitched around the edge and filled with
powdered charcoal. He thought it might have been used for transferring designs
to fabrics before painting or embroidering.

Pouncing Apparatus (see Tracing Apparatus).

Press, Oil. Before the time when oils were regularly prepared and sold by
apothecaries, the oil press and similar heavy equipment for preparing and cooking
this medium were the usual properties of the artist’s studio. Little is left to show
what the oil press may have been. Laurie {The Painter*s Methods and Materials,
p. 25) translates a comment from the MS. of Theophilus, c 1200, to the elFect

3 o8

Painting Materials

linseedj which has been dried over the fire and ground to a powder, is heated with
water, wrapped in linen, and put into such a press as was used for extracting the
oil of olives, of walnuts, or of the poppy.

Proportional Divider (see Compass, Divider, and figure 9, c).

Ptuich or Stamp, During the centuries when painters were occupied with
making altarpieces that had backgrounds of gold leaf, there was a varying but
general use of small instruments for stamping figures in the haloes and other details
that were described in the gold. Few references to the nature of these instruments
occur in contemporary literature, Thompson {The Practice of Te7npera Paintings
p. 69) suggests that a variety of punches could be made with brass rods as the
shafts, the ornaments to be filed or incised at one end.

QuiE, the tube or barrel of a feather; this has had much use in painters' tools
(see Brash, Pen, and figures i and 19).

-——t p

Figure 20. The ruling pen: {d) as seen from the side, showing the general shape of the
blades; {b') a compass with a ruling pen, showing the slot between the two blades of the
latter.

Rack. Small strips of ivory or wood, cut with short, semi-circular grooves in
the upper edge, have been used as a place to lay brushes during painting, par¬
ticularly of water color and miniatures. The purpose is to hold the fine tip of the
brush away from the work-table and to keep it from rolling. These no longer ap¬
pear in catalogues but were frequent in the middle of the XIX century and prob¬
ably had been used for an indefinite time before then.

Reducing Glass, a double concave lens which diminishes the appearance of
size in objects viewed through it; often used by painters as an artificial means of
getting a distant point of observation of their work.

Rest Stick (see Mahlstick).

Ruling Pen.-Developed evidently during modern times as an instrument of
precise draughting, this differs from the typical pen made from reed, quill, or
steel. Instead of being a section of tubular shape, tapered to a point or nib, it has
two parallel sides and the drawing edge is between the ends of these (figure 20).
The width of the line described is regulated by their distance apart, and this is
adjusted by a small set screw. A variety of sizes is manufactured. Most ruling
pens are straight, but a curved pen for particular purposes is available, and ruling
pen attachments for compasses are now the usual means of drawing circles with
ink.

Tools and Equipment

309

Sable^ the hair of one of a number of Siberian minks, used in the making of fine
brushes. Sables are ordinarily prepared as round brushes, fairly long, and with a
tapered point (see also Brush).

Saucer. In modern use the saucer is a porcelain disk with a rounded depression
in the top for holding thin paint, usually washes in water color painting and
architectural rendering. In size, saucers range from one inch to nearly four inches
In diameter. They are made with covers and, in the type ordinarily used by
architects, are constructed so that one saucer forms the cover for another in a
tier of five or six.

Scraper. Some kind of blade has perhaps always been used for smoothing
paint or grounds, and it is impossible to indicate the varieties of such a took
Eastlake (I, 375) says that a wooden or horn scraper called stecca was used during
the Renaissance to spread the first coat of gesso on a wooden panel In the modern
application of gesso, Thompson {The Practice of Tempera Paintings p. 30) de¬
scribes scrapers for dry gesso to be made from steel strips or from the blades of
carpenters’ planes. For scraping paint, smoothing, or removing it in the process of
painting, curved knives with a point have been made and listed by dealers in
artists’ supplies. Also for this purpose, Solomon (p. 74) recommends a ‘wire plush
mat’ which can be bought in lengths at tool shops.

Screen Frame. As a means of laying off the area of a subject in squares cor¬
responding to those marked on the paper where the draughtsman was to work, a
screen containing a series of wires so set as to form the squares has occasionally
been used. It is illustrated in woodcuts by Diirer and is to be compared to the
glass frame or tracing machine as one of the devices for getting sight proportions
common in the practice of the XVI century and later (see also Glass Frame and
Tracing Apparatus), At times a combination of the glass and screen frames seems
to have been made.

Shade and Shelter, When landscape painting reached a point at which much
of the finished work was done out-of-doors, weather conditions began to be met
by the construction of various shades or shelters for the use of the painter. One of
these, common today, is the so-called ‘sketching umbrella.’ This is a folding
device. The canopy cloth is on a collapsible frame of small steel ribs; the stalk is
in two or three parts, with removable joints. Such umbrellas have a diameter of
about 3 1 feet. The stalk is fitted with a steel spike at the end so that it can be
set firmly in the ground. Probably the use of a shade or screen of some kind has
been common in studios for a long time. A rather strange Oriental umbrella is
seen attached to easels in a number of paintings by Gerard Dow, particularly a
portrait of the artist, signed, and bearing the date of 1652, in the Czernin collec¬
tion, Vienna. During the XIX century, and perhaps earlier, small tents were used
by painters and some of them were regularly sold by artists’ colormen. They appear
in catalogues between 1870 and 1890.

310

Painting Materials

SheU From earliest times shells of clams and mussels have evidently been
favorite containers for paint mixtures. Prentice Duell says that shells were the
forerunners of the palettes of the Egyptian painters. They are frequently men¬
tioned in the technical literature on painting for the Middle Ages and the Renais¬
sance and are seen as appurtenances in the painting rooms that are represented in
pictures as late as the XVII century. Small mussel shells are used still for the
so-called ‘shell gold,’ powder gold in an aqueous medium, as it is prepared in

^^^Sight Measure. Developed from the simple finder or window, which is an
opening cut into a piece of cardboard, the sight measure is made to establish
certain relations and measurements in the rectangles formed by such an opening
(figure ai). The purpose of the finder is to select out of the field of vision those
isolated areas which are to be the subject for representation. The sight measure
has the same purpose. It is a thin metal window, usually painted dead black, and
with a sliding adjustment fixed automatically at lo units of width and running to
about 30 in length. As the slide is moved away from the end, a window is opened,
the measure of which, with respect to the width of the slide, is indicated by a scale
marked at the edge of the opening. The sight measure has probably not been
much used as a standard piece of equipment, but appeared in the catalogues of
artists’ colormen towards the end of the XIX century.

saver Point (see also Metal Point). This has undoubtedly been the most com¬
mon of the metal points used for drawing. In all of these the principle is the same.
The support, usually parchment or paper, is coated with a thin ground of material
that has an abrasive character: calcined bone, chalk, fine pumice, or, more recently,
zinc white. Usually these grounds are held by an aqueous medium like glue size
and are very lean in their structure. When the silver point is drawn over a ground
like this, fine particles of the metal are taken off and left in the line that has been
described. It is a pale gray mark which in time corrodes slightly to a darker value
and a warmer tone. Probably the use of such a point developed from the stylus
which was a common instrument for writing or scratching on tablets of various
materials in ancient times. Certainly the silver point and various other points
were well established in use in the Middle Ages and, although the graphite pencil
has largely displaced them in recent centuries, the silver point particularly is still
favored by a few draughtsmen. In The Practice of Tempera Painting (pp. 44—45))
Thompson suggests silver point for modern drawing on papers tinted with zinc
white and other colors, and he speaks of an easily made point of silver wire set into
a propelling pencil holder.

SketcMng Easel (see also Easel). Designed primarily as a light, folding, and
easily portable unit for holding a canvas or panel in out-of-door painting (figure
II , b)^ this type of easel has a number of different styles. All those advertised by
artists’ colormen, however, are made with three legs, adjustable in height and
hinged from a small block where they come together at the top. Some have cross

Tools and Equipment

311

pieces which can be fastened in place; many have tilting devices to allow the
canvas to be tipped forward at the top; others are fitted with sliding trays that
can be adjusted at different heights. The wood of such easels has to be light but
well seasoned and carefully selected; it is usually a light hardwood. When closed^
most easels are about 30 to 36 inches long and open to a height of 5 or 6 feet.

In a sense, this is the same as the traditional three-legged easel used during the
Middle Ages and the Renaissance. In a drawing by Jan Asselijn (1610-1652),

‘ Dutch Painters in Rome,’ a regular easel with pegs is seen in use by a landscape
painter. On it is a large canvas. Another of the figures is sitting on the ground,
evidently drawing on a board. When out-of-door sketching became a widespread
practice for amateurs, the number and variety of sketching easels increased. One
model even supplied a combination in a single mechanism of sketching seat and
easel together, and this in 1849 was described as k . . the most convenient and
pleasant apparatus ever introduced for the Lady Sketclier.’ Such combinations
were still shown in 1889, but the models now offered are less complicated in design.

Sketching Frame, a light, wooden, rectangular frame or panel made for holding
water color paper. It is provided with strips that go along the edges and are held
in place with metal clips. Usually the paper is wetted, stretched over the frame,
and held in place by the strips and side clips. These frames are rectangular, as
produced for the market, and vary in size from 6 to 20 inches.

Sketching Stool, a light, portable stool with collapsible frame of wood or metal,
used for out-of-door sketching.

Sketching Tent (see Shade and Shelter).

Sketching Umbrella (see also Shade and Shelter), an umbrella with a diameter
of about 3I feet, made with a staff er stalk in two or three parts that can be fitted
together to a length of 6 or 7 feet. The upper end of the staff has a set screw so
that the canopy can be adjusted at different angles. The lower end tapers to a
metal spike that can be set into the ground.

Slab (see Grinding Slab), a flat stone or glass used for the grinding of pigments,
the word is applied, also, to a porcelain tile with small wells used in tempera
painting, illumination, and similar fine work. These slabs are different from tiles
and slants only in minor details (see also Tile).

Slant (see also The), a porcelain container, usually for a number of colors,
made with depressions in the upper side (figure 26). The typical slant has three or
six inversely wedge-shaped divisions and is rectangular in its general outline. The
depressions are rectangular in plan and taper down from the level of the surface
to a depth of a quarter inch or more. There are other shapes which are circular,
the deeper part of the depression being towards the center.

Slice, a tool with a blade of metal or wood, used principally as a scraper (figure
24). Although the word has been applied to various kinds of spatulas, it is usually
intended to indicate one with a scraping edge at right angles to the handle like
that of the putty knife. Thompson {The Practice of Tempera Paintings p. 37)

312

Painting Materials

speaks of a wooden slice or slip of hardwood with tapered, straight edge for
spreading a heavy gesso on panels. He translates as ‘slice/ a steccha di legnio
mentioned by Cennini {The Craftsman s Handbook^ p. 21). This was used to
scrape up color from the grinding slab.

Softener (see Blender).

SpatnlEj a blade, usually of flexible metal, used in painting largely to mix,
spread, and transfer a somewhat thick paste of oil color (see also Palette Knife
and figure 23). The use of the spatula probably has a very long history in this art
but tools of such a general kind are little referred to in the literature.

r

i:. . .

A

s fo /S -?£> SS So

1 1 n 1 ( M 1 ! I.. I ! 1 1 1. L , , 1 .l1...

/e

IS

Figure 21. A sight measure with an adjustable slide to be used as a finder, principally
in landscape painting. The aperture is set to the proportions of the support to be used
for a particular work. This was shown in the catalogue of Winsor and Newton for 1889
and for a few years following.

Sponge (see also Cloths). This, as a cleaning and wiping material, has probably
been used in painting from ancient times. It is referred to as part of the painter’s
equipment by Pliny the Elder, and Cennino Cennini (Thompson, p. 99) speaks of
a little piece of soft sponge for spreading varnish. At present it is used largely
with water color for thinning washes on paper. Brushes, so-called, made of small
bits of sponge and mounted in handles, are supplied by some artists’ colormen.
They are for picking out highlights.

Stamp (see Punch).

Stencil, a cut-out pattern, usually from a thin metal or cardboard sheet, and
so made that paint can be put into the areas left open when the stencil itself is
held tightly against the surface on which the paint pattern is to be applied. This
Is used largely for repeating abstract elements in interior decoration. Books of
stencils are regularly sold by artists’ colormen. A form of stencil permitting more
flexible treatment is made with a silk screen on which a ground of gesso-like com¬
position takes the place of the paper or metal of the ordinary stencil. In this type
the paint in an oil medium is pressed through the fine silk gauze which covers the
more open part.

Stencillmg Brash, a short, stiff brush, usually round in section and flat across
the end. It is made of bristle and is intended for use in pressing the paint into the
cut-out part of a stencil.

S^etcher. Though properly part of the support of a painting rather than a
tool in its manufacture, this device at times has been also a piece of equipment

Tools and Eq.uipment

313

and not a permanent part of the finished picture. In earlier days of painting on
canvas, provisional stretchers were used to which the linen was laced while it
was in the studio. Such arrangements are evident in many paintings, particularly
of the Dutch school during the XVII century. One of these in the Hermitage
collection, 'The Painter in his Studio,' by Peter Codde (Amsterdam, c i6oo~
1678)3 shows the canvas attached by a heavy cord with pins or pegs at the edges
of the stretcher pieces. A space of about 6 inches was left between the linen and
the wooden stretching frame. In the XVIII century evidently the practice of
attaching the canvas to the stretcher by tacks was usual before the painting began
rather than after, and such practice has continued to the present. In the XVIII
century, also, or perhaps in the early XIX, stretchers with keys, small wedges
which could be tapped up at the corners to extend the outside dimensions and
make the canvas more taut, were introduced. Later in the XIX century and
prevalent still came the stretcher with mitred corners and with double keys at
the corners (figure 6, a). These w^ere made commercially with a rather complex
mortised fitting so that a piece of any length could be used universally with any
other pieces, the mitred ends being all the same. In the restoration of paintings
for the process of relining, a heavy provisional stretcher is common, being much
larger than the outer dimensions of the painting and allowing for manipulation
of the edges during this process. After the painting is relined, these edges are cut
back to a suitable space for stretching.

Striper, a flat brush of moderate size and wdth a very long, fine hair of sable
or of squirrel. This is used largely in industrial rather than in pictorial painting
(see Brtish and figure !,<?).

Studio. The wmrk-room of a painter, which now commonly goes by the name,
studio, has had a varied history and has undergone changes of character with
the different centuries during which it can be seen in painted representations.
At present it is apt to be a rather large room with high ceilings and high sidelight,
preferably on the north, and including some amount of ceiling light. Sloping,
large windows are frequently used for this purpose. Generally it is a place of simple
furnishing, rather bare, and aimed almost entirely at utility. This rather special¬
ized type of work-room was developed in the latter part of the XIX century.
J. S. Templeton, in The Guide to Oil Pahttingy 3d ed. (London, 1845), speaks of
the painting room L . . which is sometimes affectedly styled a Studio, or Atelier,'
as a room of ordinary and moderate dimensions, and the fashion of that time,
seen in paintings of the period, indicates that it was a rather sumptuous apart¬
ment. Turner's painting, 'Watteau in his Studio,' in the Tate Gallery, represents
a rich, salon-like interior, somewhat confused in its appointments, with a number
of pictures and a clutter of draperies and small objects. This type of room is
commonly seen in the studio interiors of the XVIII century. During the XVII,
particularly in the north of Europe, two types of studio are found. One is a normal
interior of a house, a room like a drawing room; the other is plainly a rough work-

314

Painting Materials

shop type of interior, frequently barn-like, often In disrepair, and sometimes
shown as having only an earthen floor. In the XV and XVI centuries, a normal
kind of room interior seems to have been the working place of the painter.

h

^3cz:

C

d

Figure 22. Stumps or soft instruments to be used with chalks or powders: (a) an
elaborate one, shown in the catalogue of Winsor and Newton for 1870, with a curved
handle and a whalebone foot that carried a leather cover; (^) a French stump of chamois,
of about 1740 (from Meder, p. 185, fig. 65); (c) a modern leather stump; (d) a small
instrument of the same type as the stump but made of paper only and called a ‘ tortillon.’

Stump and TortiUon. This is a piece of leather, felt, or paper, tightly rolled
into the shape of a small stick and tapered at one or both ends (figure 22).
It is used for softening edges and smoothing tones in drawings that are made with
chalk, pencil, charcoal, crayon, or any material that can be moved about by
light abrasion after it is applied. The tortillon is made only of paper, according
to the usage of the word by artists^ colormen, and is relatively small, 3 to 5 inches
long. The paper may be gray or white. The stump may also be of paper, usually
gray, and somewhat softer than the tortillon. It is also made and sold in yellow
leather and, occasionally, has been made of cork. The stump is usually pointed
at both ends and comes in a variety of sizes which run according to number, the
largest being somewhat less than an inch in diameter. Felt stumps are still softer
and have about the same sizes as the leather ones. The simple cylindrical stick
shape with the conical-pointed ends is the only one offered by modern manu¬
facturers. In the catalogue of Winsor and Newton for 1870, there was shown,
however, a more elaborate shape, a kind of fine trowel-like instrument with a
handle of whalebone and an elastic foot. This was covered with leather and
allowed for a fine line to be drawn with its edge. Cork stumps were also listed at

Tools and Equipment

this time by a number of manufacturers. The leather ones were either white or
yellow and paper was in use for this purpose at least as early as 1846. Meder
(p. 185) suggests that the stump was a normal development of rolling pieces of

Figure 23. Spatulas and palette knives: {a) a group of palette knives typical of those
m use at present; {b) painting knives, also modern; (r) a bronze spatula in the Naples
Museum (from Berger, I and 11, 264, fig. 54); id) a bronze spatula of Pompeian origin
also in the Naples Museum (from Berger, I and II, 266, fig. 56).

leather in order to get a fine point on them, and says that soft paper stumps
were in use as early as the XVIII century. In the Victoria and Albert Museum
London, is the work-box of a pastel painter, Mrs Elizabeth Cay (lyyc^iggi).’
She had been a pupil of the Scottish miniature painter, Archibald Skirving (1749-
1819). The box contains small bottles of relatively pale pigment in powder form
apparently tints from mixture with white, and with them a set of leather stumps’
in various sizes. The indication is that the pastel in the case of this painter was
taken as a powder rather than as a stick and was applied entirely by means of
stumps.

Stylus or Style, a metal point, usually rather blunt and not a cutting point,
used for various accessory work in connection with painting; for making lightly
ruled lines, for locating points, and for incising on gold ground. This is an ancient
instrument and one of general utility outside of the arts. It was a frequent means
for common writing in classical times when it was employed on tablets of black¬
ened wax or on pieces of broken pottery. In European painting there are many
and various uses to which it has been put; it is mentioned in connection with
incised ornament on gold, with the marking of outlines in wet plaster during fresco
painting, and with light ruling on parchment or paper. The distinction between
the entirely mechanical stylus and the metal point of silver or copper used for
drawing is sometimes hard to make. (See also Metal Point and Silver Point.)

3i6

Painting Materials

Table Easel (see also Easel)^ a small rack or stand usually of hardwood and of
very light and compact construction, made and sold under this name, particu-.
larly during the middle of the XIX century. The catalogue of Winsor and Newton
for 1849 shows such an easel in three sizes. At about this same time. Reeves and
Son had a small folding easel which could go into a pocket. It was of walnut
about 12x16 inches opened, and with two or three angles of adjustment. A
somewhat heavier easel of this type is still available (figure ii, c).

Figure 24. Scrapers for paint: {a) a slice or stucco spatula of the type used for grounds,
this one made of bronze and in the Naples Museum (from Berger, I and II, 264, fig. 54);
{b) a wooden slice (from an illustration in Thompson, The Practice of Tempera Painting,
P- 37); {c) a knife used for laying grounds (from the MS. of De Mayerne, Berger, IV*
102, fig. 2).

^ Tablet The drawing tablet of modern use is a sheaf of papers, always fastened
with an adhesive at one or more edges, and backed by stiff paper board. It is
made in a great variety of sizes and of many different papers. Its name doubtless
comes from the actual tablet of wood commonly used for drawing during the
Middle Ages and until the XV or XVI century in Europe. These, according to
descriptions of Renaissance writers, were used largely by apprentices, though
Meder (p. 165) says that before this they had been used by both masters and
pupils. They were usually coated with a white ground like gesso. Drawings were
made on them with metal points or with charcoal as studies and for practice.
With slight abrasion, the drawing could be effaced.

^ Tile (see also Slant). This name has been given to a variety of porcelain con¬
tainers used largely in water color painting, wash drawing, miniature, and tempera
painting (figure 26). The tile is usually a block of porcelain containing in its upper
face a number of depressions, wedge-shaped or round, the former generally carry¬
ing the name, slant. Some tiles have both, that is, rounded depressions and slanting
or^ wedge-shaped ones. Small tiles are about 2|x 4 inches and larger ones, con-
taming 2o and more wells, are about 4x8 inches. In the early part of the XIX
century, the tiles were called saucers and, in some cases, particularly in the XVIII

Tools and Equipment 31-7

century equipment of the water color painter, they were accompanied actually
y very small saucers or independent dishes of porcelain. Containers of this kind
that IS, divided dishes, are well known in the Far East and, as a kind of painting
slab, were familiar articles of equipment in the ancient world. The British Museum
has a number of such divided dishes that are Egyptian in origin, one of alabaster
with a pedestal, and another with divisions on radii and inner circles in its gen¬
era y circular shape, is of terra-cotta. A pan, circular in outline and rather flat,
with a Partitira across the center, is occasionally seen in representations of
studios m the XVII and XVIII centuries.

Figure 25. Three shapes of stylus or metal point (from Meder, p. 77, fig. 20) ; (a) the
Sliver point of Hans Kranach; (&) the silver point of Hans Baldung; (r) the silver point

Of Jan Mabuse. These are instruments shown in paintings by those masters.

_ Tip, a gilder’s instrument used for lifting the leaf from the cushion and laying
It upon the bole (figure 12, c). It is made of camel hair, usually set between paper
cards, and varying in length. The thickness of the brush is hardly more than that
of two or three hairs. The tip is placed over the leaf that is to be taken up, having
first been made very slightly adherent by being rubbed on something that is a
htde oily. Most gilders rub it across their heads. The technical literature of the
iddle Ages and the Renaissance makes no reference to such an instrument as the
glider s tipj but does speak of other means for lifting and handling gold leaf. The
necessity for a tip came in with the much thinner gold that is used now.

_ Tool. This word is here applied miscellaneously to any implements used by the
painter. During the XIX century and somewhat earlier, it was given to the brush
alone. Brushes are frequently listed in old catalogues as painting tools.

Tortillon (see Stump and figure 22, d).

Tracing Apparatus. It appears that this was first described in any adequate
detail by Leonardo da Vinci, and the descriptions are found in his Notebooks
(II, 253 “ 2 . 54 ; MS., 2038, Bib. Nat., 24r). The machine was later described and
illustrated by Diirer (see Meder, p. 466, and figs 204 and 206). The principle of
such a device lies in tracing the visual image as this is intercepted on a pane of
glass (figure 27). It shows on such a glass only when it is described by some
instrument like a wax crayon, piece of soap, or a similar soft and fatty marking
material. When highly developed, this tracing machine was so made that squared

Painting Materials

318

cross lines of wire were placed behind the glass so that the outlines traced there
could be transferred to paper which had a similar linear screen.

The word;, 'tracing machine/ or 'tracing apparatus/ has also been applied
to a glass plate so arranged that a mirror beneath it throws the light through the
glass. Over the glass is placed a drawing and over that the thin or tracing paper
on which such drawing is to be followed. Transmission of the light through the
paper clarifies the details and makes the tracing operation a relatively easy one.

Figure 26. Tiles and slants: (a) an ancient container of crystal, found in a late classical
tomb at St Medard-des-Pres and containing gold powder mixed with a gum-like sub¬
stance (from Berger, I and II, 214, fig. 46); (^) a circular slant, with cup and basin; (r)
a divided slant tile; (d) a tile or slab, with cover; (<?) a slant and well tile.

Tube, Both oil paint and water color are now largely put on the market in
collapsible metal tubes with screw caps (figure 28). These are made of tin^ as
other metals are apt to have a slightly deteriorating effect on pigments or even on
mediums. A variety of sizes among these tubes is now available. Certain pigments,
usually white, are put up in pound tubes and many in half pound. The smallest
tube in ordinary use is 2 inches long by about | inch in diameter. Three- and 4-
inch tubes of the same diameter are common. There is a double 4-inch tube about
I inches in diameter, and a studio tube which is somewhat larger. The smallest
size is used mainly for water colors. The development of collapsible tubes as con¬
tainers for color occurred largely during the middle years of the XIX century.
Somewhat earlier than this there had been rigid metal tubes with pistons into
which oil colors were put and which could be refilled at the colorman's. Small
bladders were a common container for commercially sold oil paints in the early
part of that century, also, and these bladders of paint are listed along with col¬
lapsible metal tubes in the catalogue of C. Roberson and Company for about
1840. Tubed water colors were slow to displace those put up in cakes or in pans,
but had appeared in the trade by 1850,

Figure 27. Tracing apparatus or glass frame: (a) that designed hj
Fried. Christ. Muller in 1776 (from Meder, p. 547, fig. 253a); (^) that
shown in a sketch by Diirer of 1514 and said to be an exact representa¬
tion according to the description by Leonardo (from Meder, p. 546,
fig. 253); (r) an illustration of the use of the apparatus sketched in h
(from a woodcut by Diirer).

320

Painting Materials

WasliBmsli. This, also called a 'sky brush,' is usually made of can-^l hair *
is a fairly large tool set in a moderately flat metal ferrule and shapes . che

end. It is used for applying broad areas or washes of water color (see Brush),

Figure 28. Containers for oil paint: (a) the skin or bladder in which the mixed and
ground paint was kept, with a tack-like piece of bone used to puncture the skin; (i?)
the firm metal tube with a piston filled and refilled with oil paint; (c) the collapsible
metal tube in common use today.

Whiskj a mediaeval tool of the illuminator used for beating up egg white as a
medium. It appears to have been a wooden stick with a flat, thin, circular loop
at one side, this being also of wood. It is illustrated and described by Thompson in
'The De Clarea of the So-Called "Anonymus Bernensis"’ {Technical Studies^ I
[193^1 PP- 13 and 17).

BIBLIOGRAPHY

John Howard Benson and Arthur Graham Carey, The Elements of Lettering (Newport,
Rhode Island: John Stevens, 1940).

Encyclopaedia Britafinica^ nth edition.

Ernst Berger, Beitrage zur Entwicklungsgeschichte der Maltechnik^ 4 vols (Munich,
' 1901-1912).

Henry P. Bowie, On the Laws of Japanese Painting (San Francisco: Paul Elder and Co,,
1911).

Cennino Cennini, Thompson edition.

Prentice Duell, ‘Evidence for Easel Painting in x 4 ncient Egypt/ Technical Studies^ VIII

(1940), pp. 175-192.

Charles L. Eastlake, Materials for a History of Oil Painting^ 2 vols (London, 1847-1869),
K. Jex-Blake and E. Sellers, The Elder Pliny's Chapters on the History of Art (London;
MacMillan and Co., Ltd, 1896).

Edward Johnston, Writing and Illuminating and Lettering^ 2d ed. revised (London: J.
Hogg, 1908).

A. P. Laurie, The Materials of the Painter s Craft (Philadelphia: J. B. Lippincott Co.,
1911).

The Painteds Methods and Materials (Philadelphia: J. B. Lippincott Co., 1926).
Leonardo da Vinci, MacCurdy and Rigaud translations.

Edward MacCurdy, The Notebooks of Leonardo da Finely 2 vols (New^ York: Reyna! and
Hitchcock, 1938).

Joseph Meder, Die Handzeichnung: ihre Technik undE^itwicklung (Vienna: Anton Schroll
and Co., 1923).

M. P. Merrifield, Original Treatises on the Art of Painting^ 2 vols (London: John Murray,
1849).

Reeves and Son, Ltd, miscellaneous catalogues.

John Francis Rigaud, Leonardo da P'hiciy A Treatise on Painting (London: George Bell
and Sons, 1906).

C. Roberson and Company, Ltd, miscellaneous catalogues.

George Rowney and Company, Ltd, miscellaneous catalogues.

Laurence Sickman, ‘Some Chinese Brushes,’ Technical StudieSy VIII (1939), pp. 61-71.
Solomon J. Solomon, The Practice of Oil Painting (Philadelphia: J. B. Lippincott Co.,
1910).

Sir Mark Aurel Stein, Innermost Asia^ 3 vols (Oxford: Clarendon Press, 1928).

Serindia, ^ vols (Oxford: Clarendon Press, 1921).

Daniel V. Thompson Jr, IlLihro del!Arte^ The Craftsman's Handbook of Cennino d'Andrea
Cenniniy 2 vols (New Haven: Yale University Press, 1932-1933).

The Practice of Tempera Painting (Ntw Haven: Yale University Press, 1936).
Winsor and Newton, Ltd, miscellaneous catalogues.

The definitions of the few terms which follow here are added to the general data
on painting mateiials in order to clarify somewhat fu?nher technical words used in
the foregoing text. Most of those not defined here will he readily located in a dictionary.
The defifiitions given may not completely explain the meanings of all the terms^ hut
it is hoped that they may be of some kelp. They are taken^ with a little modificatiofty
f7 0771 staftdard reference works in the various branches of science.

A, a symbo! for Angstrom unit which is a
measure of length, usually of the wave¬
length of light. I A = io“" millimeters or
io“^® meter or o-. 000,000,0001 meter;
10 A = I millimicron (nip,).

A.S.T.M., x4merican Society for Testing
Materials.

Acictiiar, needle-like or slender in shape.

Acid, a chemical compound which contains
hydrogen as a positive radical and in solu¬
tion gives hydrogen ions it neu¬

tralizes bases, yielding a salt and water;

HCi (hydrochloric acid), H2SO4 (sul¬
phuric acid), and CH3COOH (acetic acid).

Acid Number or Value, a measure for free
fatty acid in animal and vegetable fats;
expressed as the number of milligrams of
potassium hydroxide required to neutralize
the free fatty acids in one gram of the fat.

Acid Salt, a compound derived from an acid
and a base in which only part of the hydro¬
gen of the acid is replaced by a basic
radical; an acid carbonate, NaHCOa
(sodium bicarbonate), or an acid phosphate,
NaH2P04 (sodium dihydrogen phosphate).

Aliphatic, belonging to or derived from fat;
belonging to that group of organic com¬
pounds having an open- or straight-chain
structure as opposed to a ring or cyclic
structure (see Aromatic). The aliphatic
compounds include not only the fatty acids
and derivatives of the paraffin hydrocarbons
but also unsaturated compounds of the
ethylene and acetylene series; e.g., butane,
CH3CH2CH2CH3; ethylene, HaC = CH2;
and acetylene, HC ^ CH.

Alkali, essentially the hydroxides of the
metals, lithium, sodium, potassium, ru¬
bidium, and caesium, but also the car¬
bonates of these metals and of ammonia;

NaOH (sodium hydroxide) and K2CO3
(potassium carbonate).

Aitim, ordinarily the double sulphate of alumi¬
num and potassium, K2SO4* A!2(S04)3*
24H2O; also the generic name for other
double sulphates of monovalent and triva-
lent metals which are isomorphous in
crystalline form and contain 24 molecules

of -water of crystallization, like ammonium
alum and chrome alum.

Amorphous, a term applied to materials
which apparently do not have crystalline
form; unorganized; without definite form;
structureless.

Amphoteric, said of a substance that has both
acid and basic properties; aluminum
hydroxide, Al(OH) 3 .

Anhydrous, said of a compound which is with¬
out or has lost water, in particular, its water
of crystallization.

Anisotropic, having different physical proper¬
ties in different directions, like crystals; as
regards the transmission of light, a sub¬
stance -wffiich is doubly refracting (see
Isotropic).

Aqueous, containing water; watery; Aqueous
Solution, one in which water is the solvent.

Aromatic, belonging to or derived from that
series of organic compounds, many of whi ch¬
are odorous, having a closed-chain-or ring
structure of the benzene type (see Benzene)
as opposed to the aliphatic (see Aliphatic)
and alicyciic or saturated ring compounds;
e.g.,

OH

C

\

HC CH

I II

HC CH

V

H

phenol

O

H 11 OH

c c c

/• \ / \ / \

HC C C C—OH

1 II 11 1

HC C C C—H

\ / \ / \ /'
c c c

H 11 H

Q .

alizarin

325

Painting Materials

326

Atom, the smallest subdivision of a chemical

element which remains unchanged in chem¬
ical reactions; composed of electrons, pro¬
tons, and other structural units.

Atomic Weightj the number which indicates
the relative weight of an element as com¬
pared to hydrogen (1.008) or oxygen
(16.000).

Autoxidation, a complex type of slow oxida¬
tion at ordinary temperatures by oxygen of
the atmosphere.

Base, (i) a compound which yields hydroxyl
(OH~) ions in aqueous solution; a com¬
pound which reacts with an acid to form a
salt and water; an alkali; e.g., NaOH
(sodium hydroxide) and Ca(OH)2 (calcium
hydroxide); (2) an inert material upon
which a dye is struck in the manufacture
of a lake pigment.

Basic Salt, a compound derived from an acid
and a base in which not all of the hydroxyl
radical of the base has been replaced by an
acid radical; e.g., white lead, sPbCOs*
Pb(OH)2.

Binary Compound, a compound that consists
of only two elements; e.g.y KCl (potassium
chloride) and Fe203 (ferric oxide).
Birefracting, doubly refracting; a character¬
istic of transparent anisotropic crystals.
Birefringence, the strength of the double re¬
fraction of a material. Birefringent, doubly
refracting.

Blancb, (i) to make white; (2) a pale, misty
cast that comes over a film of varnish or
paint; it can be used, in distinction to blush
and bloom, as a name for the change which
comes in old films when a solvent has been
put on them and has evaporated, leaving a
milky look, usually irregular in distribution.
Bleeding, the property of a color in a dried
paint film to diffuse and spread away from
the layer in which it is applied to other
paint or varnish layers.

Bloom, the white or cloudy appearance of the
surface of aged varnish films, caused by
the presence of minute cracks or pores
which diffuse light. Such a film has a faint
bluish cast; it is not milky white like a
blanched film.

Blush, the white appearance or turbidity that
sometimes develops in lacquer or varnish
films almost directly after they are applied.
The phenomenon usually occurs only in
humid weather. It is caused by rapid
evaporation of solvents, which produces
cooling at the surface and which, in turn,
causes condensation of moisture within the

film. The effect on light is like that in an
emulsion. Blush is distinct from blanch and
bloom, which affect films previously dried.
Boiling point (b.p.), the temperature at which
a liquid boils; the temperature at which the
vapor pressure of the liquid reaches the
pressure of the surrounding atmosphere.
(The b.p. of water at normal atmospheric
pressure is 212° F., 100° C.)

Calcareous, containing calcium; said of com¬
pounds such as calcium carbonate or lime¬
stone.

Carbohydrate, a type of organic compound
containing carbon, hydrogen, and oxy¬
gen, the latter two in the proportion of
water; e.g., sucrose, Ci2H220n, or cellulose
(C6HioOs)n.

Catalyst, a substance which increases the rate
of a chemical reaction but is itself un¬
changed at the end of the reaction. Some
catalysts are merely contact agents acting
mechanically, but others appear actually to
take a direct part in the chemical reactions
they accelerate, forming unstable inter¬
mediate compounds from which the catalyst
is regenerated as they are decomposed.
Caustic Soda, sodium hydroxide (NaOH),
usually an impure form or technical grade.
Cellulose, a carbohydrate, (CsHioOc)^, which
is the main structural ingredient of plant
and woody tissue; hence, also, the principal
constituent of wood, fabric from plant
fibres, and paper. It is almost pure in ab¬
sorbent cotton and filter paper. Cellulosic
plant cells in woody tissue are cemented
together with another plant material called
' lignin' and, hence, wood is sometimes
called * ligno-cellulose.’

Chemical Compound, a substance in which
the molecules consist of unlike atoms and
in which constituents can not be separated
by physical or mechanical means. In a
chemical compound the constituent ele¬
ments are combined in definite proportions
by weight. In a physical mixture, on the
other hand, there is no fixed relationship
among the components.

Chemical Element, a substance which can not
be decomposed by ordinary types of chem¬
ical change or made by chemical union of
other substances; matter which consists of
atoms of one type. There are at least 92
possible elements, from hydrogen, the
lightest, to uranium, the heaviest, but of
these only 90 are known. About a third of
all the elements are comprised in the

Glossary

327

common classes of paint materials. These

are listed with their symbols.

Aluminum

A 1

Antimony

Sb

Arsenic

As

Barium

Ba

Cadmium

Cd

Carbon

C

Chromium

Cr

Cobalt

Co

Copper

Cu

Gold

Au

Hydrogen

H

Iron

Fe

Lead

Pb

Manganese

Mn

Mercury

Hg

Molybdenum

Mo

Nitrogen

N

Oxygen

0

Phosphorus

P

Potassium

K

Selenium

Se

Silicon

Si

Sodium

Na

Strontium

Sr

Sulphur

S

Tin

Sn

Titanium

Ti

Zinc

Zn

Clieniically Pure (c.p.), of the highest grade
but not necessarily 100 per cent pure; some¬
times applied to commercial pigments to
describe a grade free from extender or added
inert pigment; e.g., c.p. cadmium sulphide.

Chromophore, a structural arrangement of
atoms which endows dyes and other related
organic compounds with color; the

quinoid structure in dyes.

Colloid, a state of subdivision of matter which
consists either of single, large molecules
(proteins, organic polymers, etc.) or of ag¬
gregates of smaller molecules (colloidal
gold, sulphur, etc.). There are eight recog¬
nized classes of colloids: solid sols (solid in
solid) such as alloys; suspensions (solid in
liquid) such as paint; smokes (solid in gas)
such as carbon smoke and zinc oxide smoke;
gels (liquid in solid) such as glue gel and
fruit jelly; emulsions (liquid in liquid) such
as miIk;/o^5 (liquid in gas) such as clouds
or visible steam; solid foams (gas in solid)
such as sponge'rubber or pumice;/oamj
(gas in liquid) such as soap lather. The
colloidal particles are called the disperse

phase and the surrounding medium, the
continuous or external phase.

Compound (see Chemical Compound).

Conchoidal Fracture, the type of fracture in
which the broken edge of a material has
shell-shaped depressions and elevations.
This fracture is often shown in amorphous
or vitreous substances like glass, cold
asphalt, and resins.

Condensation, in organic chemistry, a process
in which molecules of the same or different
substances combine to form molecules of
greater molecular weight, accompanied
usually by the loss of water; e,g.^ phenol-
formaldehyde condensation to form bake-
lite; the transformation from gaseous to
liquid state, as steam to water.

Consistency, the viscosity or fluidity of a
liquid or paste; the resistance of a product
to deformation or flow.

Co-precipitate, an intimate combination of
substances which are precipitated simul¬
taneously in one chemical reaction; e.g.^
ZnS (zinc sulphide) + BaS04 (barium sul¬
phate) to form iithopone.

Coupling Agent, a solvent which will cause
two immiscible liquids to mix; e.g,^ the
addition of butyl alcohol to effect the mix¬
ing of petroleum spirit and ethyl alcohol.

Covering Power, the extent to which a certain
volume of paint will spread on a surface in
normal thickness, usually expressed in
square feet per gallon.

CrjTptocrystalline, very finely crystalline,
microcrystaliine.

Crystal, a form of matter in which atoms or
molecules regularly aggregate by forces ^of
molecular affinity; it is a solid with a definite
internal structure and an external ^form
enclosed by a number of symmetrically
arranged plane faces. The granular texture
of solids is caused by the irregular massing
of crystal aggregates. Crystals are usually
classified in six systems: isometric ox cubicy
in which the crystal axes are of equal length
and at right angles to each other (sodium
chloride, NaCl); tetragonaly in which the
three axes are at right angles to each other
but one is of unequal length (nickel sul¬
phate, NiS04-6H20); rhombic or ortho-
rhombiCy in which there are three unequal
axes all at right angles to each other (barium
sulphate, BaS04); monoclinicy in which the
three axes are of unequal length, two at
right angles to each other and a third at
right angles to one but inclined with refer¬
ence to the others (gypsum, CaS04-2H20);

Painting Materials

328

triclinic, in wliich all axes are of unequal
length and all inclined toward each other
(copper sulphatCj CuS04*5H20); hexagonal^
in which three equal axes in the same pkne
intersect at angles of 60° and a fourth is at
right angles to ail of these (quartz, Si02).
Deliquescencej the property of certain sub¬
stances to become moist or liquid when ex¬
posed to the air; as sodium hydroxide and
calcium chloride. It is a property of solid
substances whose hydrates have lower
aqueous vapor tension than the air.

Density, the weight of matter per unit volume
of a substance; it is conveniently expressed
as the number of grams per cubic centimeter
(see Specific Gravity).

Dicotyledon, having two cotyledons; a plant
whose seed develops two leaves on germinat¬
ing, as the bean seed.

Diffraction, the bending of light as it strikes
the edge of an opaque body.

Double Bond, in chemistry, a method of
representing a certain type of unsaturation
in organic compounds where two single
valence bonds connect two atoms, as
in H2C = CH2; same as ethylenic linkage.
The position of the double bond in the
carbon-to-carbon chain is a point of activity
in the molecule (see Unsaturated). ^
Efflorescence, the property of certain sub¬
stances, particularly crystalline salt hy¬
drates, to lose water when exposed to the
air and to crumble to a powder {e.g.^ wash¬
ing soda, Na2C03-ioH20); a property of
solid substances in which hydrates have
greater aqueous vapor tension than the air.
Element (see Chemical Element).

Empirical Formula, in chemistry, a formula
which shows the number and variety of
atoms in a molecule but does not indicate
the way in which the atoms are linked to¬
gether; e.g.^ the empirical formula of verdi¬
gris [Cu(C2H302)2* 2.Cu(OH) 2] is CU3C4H10O8
(see Structural Formula).

Emulsifying Agent, a material which reduces
interfacial tension between immiscible
liquids to aid in the formation of an emul¬
sion, as soap.

Enzyme, a catalytic substance produced by
living cells which has a specific action in
causing the decomposition or synthesis of
compounds into new ones; e,g.^ the yeast
enzyme, zymase, which splits sugar into
alcohol and carbon dioxide.

Essential Oil, one of a group of volatile oils of
characteristic odors, distinguished from the
fatty oils by their volatility, non-greasiness,

and non-sap-onifying properties. They fall
into two main classes: (i) those existing in
plants as such, the odoriferous constituents
of leaves, flowers, or woods like oil of
turpentine, peppermint, and lemon; and (2)
those developed from plant constituents by
enzymic action or heat, like oil of bitter
almonds and creosote oil, respectively.
Essential oils may include as constituents
hydrocarbons like pinene, alcohols like
menthol, phenols like thymol, aldehydes
like geranial, ketones like camphor, acids
like hydrocyanic acid, and esters like
methyl benzoate.

Ester, an organic compound formed from an
alcohol and an organic acid by elimination
of water; hence, sometimes the name
* ethereal salt e.g,^ ethyl acetate (CH3-
COOC2H5), formed by the action of acetic
acid upon ethyl alcohol.

Esterfication, the formation of an ester from
an alcohol and an organic acid, with the
aid of dehydrating or catalytic agents.

Ethylenic Linkage, in organic compounds, an
unsaturated linkage or double bond, as
contained in the substance, ethylene,
H2C = CH2 (see Double Bond).

Fat, a solid or liquid oil consisting of the
glyceryl esters of the higher fatty acids, as
tristearin (stearin) which is contained in
beef fat, and tripalmitin, contained in
vegetable fats and oils such as palm oil.

Fibril, a small fibre or filament.

Film, a thin, usually continuous layer or skin
of any substance. It may be more or less
homogeneous in structure like a varnish
film or heterogeneous like a paint film.

Flash Point, the lowest temperature, under
specified conditions, at which the vapors of
a liquid can be momentarily ignited; a con¬
stant for many organic liquids.

Fluorescence, a kind of luminescence which
is the property of certain substances of
absorbing light of one wave-length and
emitting or radiating it as another wave¬
length, usually greater. It is thought to be
caused by the return of electrons displaced
by the exciting radiation to a more stable
position. Fluorescence radiation is the im¬
mediate result of, and takes place only dur¬
ing, the absorption of radiation from some
other source- Ultra-violet Hgbt and x-rays
are the most commonly used exciting radia¬
tions for producing fluorescence.

Fraction, in distillation, those liquids which
distill at a certain temperature or temppa-
ture range. Fractionate, to separate into

Glossary

329

parts, as liquids of a certain boiling point or
boiling range, hj distillation.

Fungicide, an agent which destroys fungi; as
mercuric chloride or Bordeaux mixture.

Fungus, a cryptogamous plant, destitute of
chlorophyll, deriving nourishment wholly or
chiefly from organic compounds; included
are bacteria, molds, rusts, smuts, and
mushrooms.

Gel, a jelly; same as jell; the colloidal suspen¬
sion of a liquid in a solid; the solid phase of
a colloidal system as opposed to the sol, or
liquid, phase; a colloidal system in which
matter is dispersed in a solid dispersion
medium and in which the particles of the
disperse phase are at a standstill in contra¬
distinction to those in a sol (see Sol) where
they are in motion.

Glyceride, an ester derived from the tribasic
alcohol, glycerol. The animal and vegetable
fats and oils are mainly triglycerides of the
higher fatty acids, as linolein.

Grain is the longitudinal arrangement of
fibres or particles in wood, stone, leather or
a fabricated substance; it may be the plane
of cleavage along which a material can be
split into two or more parts. In substances
that have grain, physical properties such
as moisture absorption, heat conduction,
and thermal expansion are likely to be
different with the grain than they are across
it. Tensile strength, flexure breaking
strength, and compressibility may also be
affected by the direction of the grain. Ma¬
terials with grain may be said to be hetero¬
geneous in physical properties.

Ground, the film or stratum in a painting
which lies between the support and the
paint or design proper. The materials in
the ground, paint, and surface layers have
many points of identity, but can be con¬
sidered separately because they represent
different structural units in a picture. All
contain, except in true fresco and in some
gypsum grounds, a film material, or ad¬
hesive, or binding medium. This occurs
alone in a surface film and often in a prim¬
ing and acts as a vehicle or carrier of inert
pigment particles in the paint film and
usually in the ground. The ground is a
mechanical preparation on which the design
of the picture is executed. The surface film
is likewise mechanical and often is not
applied by the artist himself. Differences in
the function and in the treatment of these
three parts of a painting make it expedient

Hiding Power, the capacity of a paint to ob¬
scure or hide the surface on which it is
applied; the degree of opacity in a paint or
pigment.

Homologous Series, a series of compounds,
the members of which differ in composition
by a constant amount and in which physical
constants change uniformly; e.g.^ the
paraffin series of hydrocarbons, the mem¬
bers of which differ regularly by the addi¬
tion of (—CH2) but have each the general
formula, CnH2n+2.

Hydrocarbon, any compound consisting of
hydrogen and carbon, as benzene, CgHs.

Hydrolysis, (i) a decomposition reaction in¬
volving water; (2) the partial reversal of a
neutralization or condensation reaction.
Hydrolyze, to cause hydrolysis.

Hydrophobic, said of a substance which does
not absorb or is not wetted by water.

Hydrophyiiic, said of a substance wffiich ab¬
sorbs or is wetted by water.

Hydrous, containing water, as opposed to
anhydrous. Hydrous Salt, a salt contain¬
ing water of crystallization, as gypsum,
CaS04-2H20.

Hygroscopic, said of a substance which ab¬
sorbs moisture from the atmosphere, as
calcium chloride or sodium hydroxide.

inclusion, a foreign body, gaseous, liquid, or
solid, usually of minute size, enclosed in a
mineral or other body.

Infra-Red, the invisible part of the, spectrum
between radio waves and the red portion of
the visible spectrum, consisting chiefly of
thermal rays; the wave-length region be¬
tween 0.03 centimeter and 8000 A units.

Inorganic, not organic; pertaining to chem¬
icals or substances which do not contain the
element, carbon; designating or composed
of matter other than animal or vegetable.
Inorganic acid, same as mineral acid; a
compound of hydrogen, with a non-metal
other than carbon, or an acid radical con¬
taining no carbon; HCl (hydrochloric
acid), HNO3 (nitric acid), and H2SO4
(sulphuric acid).

Interface, the boundary between two phases;
e.g.j the boundary between the dispersed
phase and the continuous phase in an
emulsion of immiscible liquids.

Interfaciai Tension, the forces that orient
molecules and keep them together at the
boundary of a phase; the surface tension of
a liquid/iiquid boundary surface.

Iodine Number or Value, the amount of
iodine absorbed by a fat or oil under sped-

330

Painting Materials

fied conditions; the amount of iodine in
grams as iodine monochloride (Hiibl solu¬
tion) or iodine trichloride (Wijs solution)
absorbed by lOO grams of the oil or fat;
the measure of unsaturation of an oil or fat.

Ion, an electrically charged atom, radical, or
molecule; a positively charged ion is a
cation; e.g,^ Na"^; a negatively charged ion
is an anion; e.g., Ci~.

Ionization, electrolytic dissociation; the break¬
ing up of molecules into two or more op¬
positely charged ions; a phenomenon that
happens when polar compounds like inor¬
ganic acids, bases, and salts are dissolved
in water, as NaCl-^ Na"*" + Cl'~.

Isomer, any compound having the same num¬
ber and kinds of atoms as another but differ¬
ing in molecular arrangement as well as in
chemical and physical properties; having
the same empirical but different structural
formula; e.g.y butane, CH3CH2CH2CH3,
and isomeric butane, CH3CH(CH3)CH3.

Isotropic, having the same physical properties
in every direction (c/., Anisotropic).

Laminated, made up in thin plates, scales,
layers, or plies, as mica or plywood.

Levigate, to reduce a substance to a powder
by grinding in water, followed by fractional
sedimentation in order to separate the
coarser from the finer particles.

Lignin, the cellulose-related material which
lines the vegetable cells of wood; it acts as
a cement for the cells and imparts rigidity
to woody substance. Ligno-Cellulose, any
of several closely related substances con¬
stituting the essential part of woody tissue.

Lixiviate, to extract and separate a soluble
substance from its mixture with insoluble
matter; to leach.

Lumen, the bore or inside passage of a small
tube, vessel, or duct.

Lye, a solution of sodium or potassium hydrox¬
ide or the alkaline solution (potassium
carbonate) obtained in leaching wood ashes.

Melting Point (m.p.), the temperature at
which a crystalline solid changes to a liquid.

Micelle, a unit structure built up from com¬
plex molecules in colloids. It may have
crystalline properties and is capable of
increase or diminution in size without
change in chemical nature. The cellulose
micelle, for example, is a bundle of parallel
chains composed of polymeric molecules
derived from the simple unit, CeHioOg.

Micron, or mu, a unit of length equal to one
millionth part of a meter or one thousandth
part of a millimeter.

Micro-organism, any minute animal or plant
visible only through a microscope, as a
bacterium.

Mildew, any whitish or spotted discoloration
caused by parasitic fungi like mold on cloth,
leather, or other vegetable matter (see
Mold).

Mineral, any inorganic or fossilized organic
substance found in nature. Minerals differ
from rocks in that they have definite chem¬
ical composition and usually definite crystal
structure. Their composition can generally
be expressed by a chemical formula.

Mineral Acid, inorganic acid, as hydrochloric,
nitric, or sulphuric acids.

Miscibility, the property of certain liquids to
mix with each other in all proportions, as
alcohol and water.

Mold (mould), a variety of fungus growth,
usually filamentous, which grows on damp
vegetable material (see Fungus).

Molecular Weight, the relative mass of a
molecule referred to the mass of the hydro¬
gen atom. It is calculated by adding the
atomic weights of all the elements and their
multiples indicated in the chemical formula;
e.g.^ molecular weight of water, H2O, is
18.016, which is the sum of the atomic
weights of two atoms of hydrogen (at. wt,
1.008) plus that of one atom of oxygen
(at. wt, 16).

Molecule, the chemical combination of two
or more like or unlike atoms. It is the small¬
est quantity of matter that can exist in the
free state and retain all the properties of
the original substance.

Monomer, the simple unpolymerized form of
a compound, having low molecular weight
as distinguished from the dimer or polymer
(see Polymer).

Mutual Solvent, that which acts as a common
solvent or coupling agent; that which brings
about the miscibility of substances not
otherwise miscible with each other; e.g., the
action of acetone in producing a mixture of
certain oils with water (see Coupling Agent).

Keutralization, in chemistry, the process of
combining an acid with a base (hydroxide
or alkali) in proportions to form a salt and
water, leaving no excess of free acid (hy¬
drogen ions, H*^) or base (hydroxyl ions,
OH""); the opposite of hydrolysis. The end
point of neutralization is shown by indi¬
cators like litmus or phenolphthalein or by
measurement of electrical conductivity.

Normal Salt, a salt in which all the hydrogen
atoms of the acid have been replaced by a

Glossary

331

metalj or ail the hydroxyl radicals of a base
have been replaced by an acid radical; e.g,y
sodium carbonate, Na2C033 normal salt,
and sodium bicarbonate, NaHCOs, acid
salt.

Occlusioii, the adhesion of a finely dispersed
substance to larger solid particles or their
retention inside a solid mass.

Organic Acid, a compound containing one or
more carboxyl radicals, —• COOH. A mono¬
basic acid contains a single carboxyl group;
e.g.^ acetic acid CH3COOH; a dibasic acid
contains two carboxyl groups; e.g.^ oxalic
acid, (COOH)2, etc.

Oxidation, the act of combining with oxygen
or any electronegative element or radical,
as the addition of oxygen, chlorine, or
sulphur; increasing the positive valence of
an atom or ion by loss of electrons; e.g.j
!?£++ _ j electron—>• Fe'^'^'^.

Oxycellulose, oxidized cellulose, a degradation
product of cellulose formed by natural
oxidation or by bleaching processes.

Paint Film, a thin, continuous layer of medium
and pigment combined.

Particle Size, the average diameter of par¬
ticles, as those of pigments or of colloids,
usually expressed in microns (ju).

Paste, in general, a glutinous or other tena¬
cious substance used as an adhesive; in paint
technology, a thick, putty-like mixture of
medium (usually oil) and pigment.

pH, a measure of acidity, neutrality, or
alkalinity in aqueous solutions; the symbol
for the logarithm of the reciprocal of the hy¬
drogen ion concentration; pH = log i/Ch+.
Solutions with pH 1-6 are acid, pH 7 are
perfectly neutral, and pH 8-14 are alkaline.
pH is measured electrometrically or colori-
metrically with the use of indicators.

Phase, any homogeneous substance, either
solid, liquid, or gaseous, that exists as a
distinct and mechanically separate portion
in a heterogeneous system, as in an emul¬
sion; any homogeneous parts of a system
that are separated from one another by
definite physical boundaries.

Pleochroism, a change in color exhibited by
certain optically biaxial colored crystals
when rotated in polarized transmitted
light. If only two extremes of hue are ob¬
served, the substance is said to be dichroic;
if three, trichroic.

Polarized Light is that in which the light
waves vibrate unilaterally, parallel to each
other in the same plane, elipse or circle,
whereas in non-polarized light the waves

vibrate in a number of planes. Light may be
plane polarized by reflection or refraction
at non-metallic surfaces or by transmission
through crystals showing double refraction.

Polyhydric Alcohol, an alcohol that contains
more than one hydroxyl group, as glycerol,
(CH20H)2CH0H.

Poljnner, in organic chemistry, a compound
formed by the combination of two or more
molecules of the same substance to produce
a new compound with the same empirical
formula but with higher molecular weight.
The polymerization process is usually ac¬
companied by a change in state (as liquid
to solid) and a transfer of energy. A poly¬
merized substance is often named with the
prefix, poly-; e.g.y polyvinyl acetate.

Precipitate, the deposit of an insoluble sub¬
stance in a solution after the addition of a
chemical or precipitating agent or on
evaporation, cooling, or electrolysis. Pre¬
cipitation takes place in a solution when
conditions are such that the solution con¬
tains more of the component than is re¬
quired for saturation and there is an excess
of the component to be thrown out of the
solution.

Priming, in painting construction, a thin,
continuous layer between the ground and
the paint film, sometimes confused "with
‘ ground.’ A priming layer may consist of
pigment in medium but is usually medium
alone.

Protein, one of a group of nitrogenous organic
compounds of high molecular weight that
occurs in vegetable and animal matter.
Examples of protein-containing substances
are animal glue and egg albumen.

Pyrogenetic, produced of or by heat; made by
a furnace process.

Radiography, photography with x-rays.

Rectij&cation, the redistillation of a liquid for
the purpose of purification.

Reduction, the act of depriving of oxygen or
any electronegative element or radical; in¬
creasing the negative valence of an atom or
ion by addition of electrons; e.g.^ -f i

electron—> Fe"^"^; opposite of oxidation but
both reactions take place concurrently.

Refraction, the bending or deflection of light
rays when passing from one transparent
medium to another of different density.

Refractive Index, the ratio of the velocity of
light in a certain medium compared with
its velocity in air under the same conditions;
it is expressed as the ratio of the sine of the
incident angle of light to the sine of the

332

Painting Materials

angle of refraction; t/ = sm//sinr. The
refractive index (w) of water is 1.33; gyp¬
sum, 1.52; zinc oxide, 2.00.

Roentgen RaySj same as x-rays (see X-Rays);
a form of radiant energy discovered by
Wilhelm Konrad Rontgen in 1895,

Salt, one of the group of substances that re¬
sults from the reaction between acids and
bases; the product, in addition to water,
formed by the neutralization of an acid by
a base; e.g., sodium chloride (common salt)
formed by the action of hydrochloric acid
on sodium hydroxide.

Saponification, the conversion of an ester into
an alcohol and an acid by hydrolysis or into
an alcohol and an acid salt by means of an
alkali. It is the process by which soap is
made by action of alkali on vegetable or
animal fats and oils.

Saponification Ntmiber or Value, the quantity
of potassium hydroxide (in milligrams) re¬
quired to saponify one gram of fat or oil;
the measure of the amount of true fat or
fatty acid in a substance.

Saturation, the complete satisfaction of the
valence bonds in a molecule; also the com¬
plete or maximum absorption of a substance
by a solvent.

Single Bond, a single linkage or valency be¬
tween atoms.

Slake, (i) to slack or loosen; (2) the addition
of water to quicklime to form calcium
hydroxide.

Sol, a colloidal solution or the liquid phase of
a colloidal solution; a colloidal system in
which matter is dispersed in a liquid dis¬
persion medium and in which the dispersed
particles show independent movement, as
Brownian movement (see Gel).

Solution, the combination of a solid, liquid, or
gaseous substance (called the ‘ solute ') and
a liquid (called the ‘ solvent') to form a
homogeneous mixture from which the dis¬
solved substance can be recovered un¬
changed by evaporation and crystallization
or by other physical processes.

Specific Gravity, the ratio of the density of a
substance to the density of some other
substance chosen as standard. In the case of
solids and liquids, the standard is usually
water and, if metric units are selected,
specific gravity is equal to the density
(grams per cubic centimeter); e.g., s.g. of
water is i.oo; gypsum, 1.36; vermilion, 8.09.

Spectrum, a variously colored band of light
showing in succession the rainbow colors or

isolated lines or bands of color when light
is refracted by a prism or diffracted by a
grating.

Spectrogram, a photographic plate, film, or
print on which a spectrum is recorded
together with a comparison spectrum.

Stria, a minute groove or channel or a narrow
line or band, as of color; one of a series of
parallel lines or grooves.

Structural Formula, a representation on a
plane surface of the atomic arrangement of
a molecule, as that of benzene:

H

H—C

C—H

H-C C-H

\

I

H

Sublimate, a solid substance which, on heat¬
ing, passes into the vapor state and, on
cooling, returns to the solid state without
passing through the liquid state in either
direction; e.g., mercuric chloride, iodine,
and red mercuric sulphide.

Supernatant Liquid, the liquid standing above
a sediment or precipitate.

Surface Film, the thin, transparent film,
usually of varnish, spread as a protective
coating over the surface of a painting.

Surface Tension, the contractile surface force
of a liquid by which it tends to assume a
spherical form and to present the least
possible surface; its value is measured in
dynes on an instrument called a tensiometer.

Tack or Tackiness, stickiness, as the condition
of a surface of partially dried varnish.

Technically Pure (t.p.), pure enough for tech¬
nical or industrial uses but not pure enough
for analytical or pharmaceutical purposes.

Temper, to mix in proper proportions; to
compound or blend; to soften or mollify; to
combine with a liquid medium; to make a
paint or to make a material brushable; to
harden, as of metals, by heating and rapid
cooling.

Thermoplastic, capable of being softened and
made to flow by heat and pressure; a term
commonly applied to artificial resins and
plastics which are resoftened by heating.

Thermosetting, a term applied to artificial
resins and plastics which are molded and

Glossary

set by heat and pressure but which do not
^return to the plastic state on reheating.

Tincturej a dilute extract of a drug or chem-
icalj usually a plant principle in alcohol.

Tinting Strengtli, the ability of a coloring
material like dye or pigment to impart
color; same as * tinctorial power.’

Toothj the roughened or absorbent quality of
a surface which favors the application and
adhesion of paint coatings.

Top Tone (or Mass Toiie)^ the full strength
and color of a pigment or paint when
viewed by reflected light (see also Under¬
tone),

Tufa, a sedimentary rock composed of silica
or calcium carbonate deposited from waters
of lakes, rivers, and springs.

Ultra-Violet (or Ultra-Violet Rays), that por¬
tion of the invisible spectrum that lies
beyond the violet or on the shorter wave¬
length side of the visible spectrum; that
portion of the light spectrum between
4000 A units and 120 A units.

Undertone, the color of a pigment or paint
when viewed by transmitted light or when
spread thinly over, or mixed with, much
white.

Unsaturated, (i) designating an organic com¬
pound having double or triple bonds or
linkages between carbon atoms; ethyl¬
ene, H2C = CH2, or acetylene, HC s CH;
(2) said of a solution which is capable of dis¬
solving more solute.

U.S.P., letters commonly affixed to the name
of a material indicating that it conforms in
grade to the specifications of the United
States Pharmacopoeia and that it is ap¬
proved for use in medicinal preparations;

333

it does not necessarily mean, however, that
the material is chemically pure.

Valence, a value or number which expresses
the capacity of an atom to combine with
other atoms in definite proportions; It is
measured with the combining capacity of
hydrogen taken as a unit; chlorine in
HCi (hydrochloric acid) is monovalent;
oxygen Jn H2O (water) is divalent; and
carbon in CH4 (methane) is tetravalent.

Vapor Pressure, the pressure at which a liquid
and its vapor are in equilibrium at a definite
temperature. If the v.p. reaches the pre¬
vailing atmospheric pressure (i atmos¬
phere), the liquid boils. V.p. is usually ex¬
pressed in millimeters of mercury; e.g,^ v.p.
at 20° C. for water is 17.5 mm.; for alcohol,
44 mm,; and for acetone, 178 mm.

Viscosity, the internal friction of a fluid which
influences its rate of flow or causes it to
exhibit slight resistance to change of form;
the state of being glutinous or sticky or
resistant to flow.

Volatile, designating a substance that evapo¬
rates rapidly.

Water of Crystallization (Water of Hydration),
water that is combined with certain crystal¬
line salts in definite proportions by weight
and which may be completely removed by
heating; e.g,^ CaS04*2H20 (calcium sul¬
phate dihydrate) and CuSO^ *51120 (copper
sulphate pentahydrate),

X-Rays, a group of invisible light rays of
extremely short wave-length, ranging from
0.06 to 20 Angstrom units, produced in an
exhausted tube (called an * x-ray tube ’) by
fast-moving cathode rays impinging upon
a metal surface.