MEDIUMS AND ADHESIVES 27
used in quantity, Doerner (p. 303) says that glue-color painting is very useful
for decorative work. Glue-color is easily soluble in water, but spraying it with a 4
per cent formalin solution will render it less soluble (see Formogelatin), Wax
soap is often applied for the same purpose. For use in painting, glue can be
emulsified alone or it can be used as an addition to egg and gum tempera.
Among the stone carvings of the ancient city of Thebes, at the time of
Thothmes III, the Pharaoh of Exodus, on a stone at least 3,300 years old, there
is a representation of the process of gluing a thin piece to a yellow plank of syca-
more. A glue pot and brush are shown, together with a chunk of glue that has a
characteristic concave fracture. Such a piece of glue, which had originally been
rectangular but had dried, was found in the tomb of Queen Hatshepsut (XVIII
dynasty). Glue is mentioned in the Bible in Ecclesiastes, XXII, 7: 'He that
teacheth a fool is like one that glueth a potsherd together.' Pliny refers to it with
gums, milk, eggs, and wax, as a vehicle for the paints of the ancient Egyptians.
During the Middle Ages glue was used extensively, and old MSS give directions
for its preparation. In the MS. of Jehan le Begue (Merrifield, I, 148) is given a
detailed recipe for the preparation of glue from the skin of an ox or cow. It is
described as a mordant for powdered tin. In the Bolognese MS. (Merrifield, II,
466) it is mentioned in connection with gilding. Cennino Cennini (see Thompson,
The Craftsman's Handbook, p. 67) gives recipes for its preparation and speaks
of a glue which is made of the clippings of the muzzles of goats, feet sinews, and
many clippings of skins.
The earliest practical manufacture of glue that can be directly traced from the
present day was in Holland at the time of William III. It was evidently made there
in 1690, and shortly after was introduced into England and established as one of
the permanent industries by about 1700. In France, the industry started in the
vicinity of Lyons, and for many years these factories were the most important
of their kind in Europe.
Glycerides are esters of glycerol. Oils and fats are glycerine esters of the
fatty acids, *'.<?., they are glycerides (see Oils and Fats).
Glycerine, or Glycerol, is the trihydroxy alcohol (CsH5(OH)3) which is com*
bined with various fatty acids to form oils and fats, i molecule of glycerine
being combined with 3 molecules of fatty acid to form i molecule of an oil or fat
(see Oils and Fats). Free glycerine, or glycerol, is a syrupy, hygroscopic liquid
used as a plasticizer for aqueous mediums (see also Gum Arabic).
Gouache (see also Gums) is actually a water color or a gum tempera and the
word is used more to describe the opacity obtained with such paints than to
define any particular difference of original material. Like water color, it is ordi-
narily applied on a paper support, but less thinly than water color and with
mixed tints of white and color rather than with transparencies of color.
Gum Arabic (see also Gums and Tempera) is produced by several species of
Acacia growing in Africa, India, and Australia. The gum from Acacia arabica