ng 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, ytyos, 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 2oo-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 o° 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 Painting, 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 50 and loG are used for the preparation of greenish yellow lakes good in