PIGMENTS 93
Aluminum Stearate, A1(C18H3502)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,
CaSO4, which is often associated in nature with calcium sulphate dihydrate or
gypsum (see Gypsum). 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 (0 = 1-575) than 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, Sb2C>3, 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