MEDIUMS AND ADHESIVES 59
He describes amber and copal varnishes and the use of turpentine as a thinner
(pp. 221, 240, and 259). In giving the proportions of oil to resin, he comments
on the varying effect which different proportions had on the hardness and rate of
drying of the varnish (pp. 232 and 237). In the last century there has been much
investigation, notably by Tschirch and his co-workers, into the nature of specific
resins.
Rosin (see also Colophony and Balsam) is the residue left after spirits of
turpentine has been distilled off from the balsam collected from pine trees. Rosin
is a resin. The similarity of the two words has caused much confusion, and it is
preferable to call rosin by its other name, colophony.
Rubber, chlorinated. Chlorinated rubber is an odorless, non-inflammable,
yellow-to-white solid; it is soluble in the coal-tar (toluene) and chlorinated ali-
phatic hydrocarbons (ethylene dichloride), and in these solvents it may be applied
as a coating. Ellis (Chemistry of Synthetic Resins, p. 116) says that the film is
hard, tough, glossy, and translucent with a light yellow color, and that it exhibits
a high resistance to the action of acids, alkalis, and oxidizing agents. Where
decorative or protective coatings are to be exposed to a very unfavorable environ-
ment, such as smoke, salt spray, and fumes, or extreme weathering, the use of
chlorinated rubber as a medium or surface film might be considered.
Safflower Oil occurs in the seeds of the Carthamus tinctorius to the extent of
30 to 32 per cent. It is a relatively slow-drying oil with an iodine number of 130
to 147. Polymerized safflower oil (see Polymerized Oils) ia prepared by heating
the oil to about 250° C, a treatment which turns it into a jelly-like mass. In
India this has been prepared by natives for many centuries and is known as
* Roghan ' or € Afridi wax.' (See also Oils and Fats.)
Sandarac (see also Resins) is produced by a coniferous plant, Callitris quadri-
vahtS) which grows in Africa on the Mediterranean coast and also in Australia.
It now comes chiefly from Algiers, but in ancient times it was apparently ex-
ported in considerable quantities from Benghazi, then known as Berenice. (See
Varnish.) The resin first called * sandarac * was probably juniper resin, having a
dull reddish color and yielding a dark brown varnish. The term was given, also,
to a variety of red pigments. The present sandarac resin occurs in somewhat
elongated, pale yellow lumps, being dusty on the surface and crumbling into
reddish powder when chewed. It is soft and brittle and similar to mastic. Its
melting point is 135° to 145° C. and its specific gravity, 1.078 to 1.088. It is soluble
in alcohol and in ether, but is only partially soluble in chloroform, oil of turpen-
tine, and petroleum ether. Like the hard resins, it is readily soluble in hot tur-
pentine, if it is first fused; if it is gradually heated with turpentine, it will soften
into a stringy mass without dissolving. With alcohol or turpentine, sandarac
gives a white, hard, spirit varnish, the resulting film being harder than a mastic
film. With age, however, it becomes darker and redder. Sandarac varnish is
used particularly for coating metals, as it gives a lustre when applied thinly. Its