PIGMENTS
early as an article of commerce, and Church says (p. 153) that it was used by
the early Flemish oil painters. It has been principally a water color or a color for
spirit varnishes and gold lacquer. Mixed with Prussian blue or indigo, it makes a
rich green that was formerly a water color paint (see Hooker's Green). It is
drawn from trees by means of artificial incisions from which it runs as a yellowish
brown, milky juice that hardens in the air (Beam, p. 172). It is marketed in the
form of yellow cakes and lumps which are rather brittle and are often covered
with yellow dust. Gamboge, when powdered and ground in oil, has a rich golden
hue and in this medium it is fairly permanent. As a water color, it is less per-
manent and fades rapidly in sunlight; but in manuscript painting where it is well
protected it has, in some cases, lasted for centuries.
Gamboge burns with an odor of burning resin. It first turns deep orange or
red in dilute caustic soda and then dissolves; since it is a resin, it is partially
soluble in alcohol and in some other organic solvents. Microscopically, it is not
particularly characteristic; the particles are deep yellow or orange and are fairly
transparent by transmitted light.
Gesso (see also Gypsum), in its broadest meaning, is any aqueous, white
priming or ground material that is used to prepare wooden panels or other sup-
ports for painting or gilding. The word is Italian for gypsum. The white ground
for an Italian panel painting was usually a mixture of glue and burned gypsum
(plaster of Paris). Among the Italian painters, two distinct kinds of gesso were
recognized. As described by Cennino Cennini (see Thompson, The Craftsman's
Handbook^ p. 70), gesso grosso is the coarser and thicker undercoating for a panel;
it was made directly from sifted plaster of Paris and applied with size. Gesso
sottile, however, was the fine, crystalline gypsum made by soaking plaster of
Paris for some weeks in excess water so that it did not set. The residue, after the
water was poured off, was made up into small loaves, was dried, and was kept
for future use. Mixed with glue size, it served for the final coatings over the
foundation of gesso grosso (Thompson, op. cit., pp. 71-72). Today the word,
gesso, has taken on even wider meaning and may include grounds made from
chalk (whiting), zinc oxide, or any other inert white. Among modern sculptors,
the word is used for plaster of Paris alone, without glue binder (see Thompson,
of. cif., p. xvi).
Gold Leaf and Gold Powder have been valued in painting for making back-
grounds and details. Because gold is the most malleable of the metals and may
be beaten into leaves of extreme thinness, it can be used quite economically even
to cover large surfaces. Gold beating is a special craft; the foil is placed between
sheets of parchment called 'gold-beater's skin/ and these are stacked, one upon
the other, and beaten until the metal seems almost to lose its third dimension.
In mediaeval times, gold coins were the direct source of most of the leaf, and its
thinness was reckoned in terms of the number of leaves that could be made from
a ducat (Thompson, The Materials of Medieval Painting, pp. 190-229). The color