PIGMENTS 139
envelope that protects it from moisture and contact with other particles. Some
oxygen-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 minerals of the heavy
metals, but, even so, such brightly colored minerals as cinnabar, orpiment, realgar3