MEDIUMS AND ADHESIVES 61
They found it to consist largely of esters of an acid to which they assigned the
formula, CH3CH2CH2(CHOH)(CH2)7CHOH-COOH, i.e., dihydroxyficocerylic
acid. The melting point is between 77° and 82° C. Gardner and Whitmore have
experimented with shellac in different organic solvents. They conclude that
hydroxyl, carboxyl, and carbonyl groups are probably present.
Shellac is a spirit-varnish resin. When it is commercially prepared, 5 to
7 pounds are dissolved in I gallon of alcohol. Often, oleo-resins are added to
increase the elasticity. Sandarac, mastic, and Manilla copal are sometimes mixed
with it, and dragon's blood or gamboge is occasionally put in for coloring. Shellac
varnish gives a smooth finish and takes a high polish. The film is tough but not
completely water-resistant. It is used as a primer for wood because it prevents
any resin escaping and affecting the paint film and because it is impervious to
the solvents ordinarily used for fresh oil paint. In restoration, it is sometimes
applied as a weak priming over filling gesso in the losses of a paint film, for it
has the property of being wetted with water and so will take an aqueous medium.
Its color and slow solubility keep it from being much employed in connection
with paint.
Shellac Wax (see also Waxes). The wax content of shellac generally varies
between 3^ and 6 per cent. Pure shellac wax is of a light, rich, yellow color and
resembles carnauba in strength and hardness. Its melting point lies between 78°
and 8a° C.; its mean iodine number is 9.2. It is marketed in small quantities,
and is usually derived from bleaching processes of the resin (see Shellac). In
such processes, it has been subjected to saponification by alkalis with detriment
to its color, melting point, and hardness.
Siccative or Drier. Any metallic salts or solutions of them which are added to
drying oils for the purpose of accelerating the rate of drying or oxidation go under
this name. Usually they are derived from lead, cobalt, or manganese (see Oils,
drying process).
Sierra Leone Copal (see also Copal and Resins). This resin is obtained by
tapping the tree, Copaifera guibourtiana> which grows in the British colony in
Africa. The quality is more uniform than that of many copals. Tapping is
permitted every three years, and the resin is collected five or six months after the
incisions are made. It is brittle, hard (next in hardness to Zanzibar copal), has
a light yellow color, and produces a pale, durable, and elastic varnish. Formerly,
a fossil variety was found, but now Sierra Leone resin comes from the living tree,
Silica (see Water-Glass).
Silicon Ester, which is usually tetraethyl silicate, is a clear, fluid, organic
compound of silicon. It has been used slightly as a medium for painting. When
exposed to the air in thin films, this ester hydrolyzes with the formation of
colloidal, hydrated silica that is the film material. It is a modern development and
i$ still in the experimental stage. It is related to water-glass.