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'MedievalPainting, p. 106) ho wits 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.
VertEmeraude (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's Handbook^ p. 22).
Viridian (pert emeraude, Guignet's green, transparent oxide of chromium) is the
transparent, bright green, hydrous oxide of chromium and is usually given the
formula, Cr2Or2H2O. 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 Beam, p. 101). 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 (c Chromoxydhydrat-
griin,' Farbtn-Zeitung,'XLI [1936], 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