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.
Chromium Oxide Green, opaque. This is the most stable of the green pig-
ments; it is the anhydrous oxide of chromium (C^Os), 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 unaffected 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 Viridian)
because it is dull. Vauquelin, the discoverer of chromium (1797), 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 Viridian).
Chrysocolla was a classical name to indicate various compounds that were
useful in the hard soldering of gold (Greek: xpwbs = 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-^H20), 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 Spurrell as a pigment on certain Twelfth Dynasty
tombs at El-Bersha (see Lucas, p. 288), and at Kahun (see Spurrellj p. 227).
Cinnabar (see Vermilion).
Cinnabar Green (see Chrome Green).
Clay is any plastic, variously colored earth consisting, essentially, of hydrous
aluminum silicate, HaA^SigOe, formed by the decomposition of feldspar or other
aluminum silicates (see also Kaolin a*nd 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.