MEDIUMS AND ADHESIVES 81
possible, also, that oils and resins were added to the wax. One such combination,
called Zopissa, a mixture of wax and balsam, was familiar in ancient industry.
Laurie (Greek and Roman Methods of Painting Cambridge [1910], p. 65") argues,
however, that fatty acids detected by analysis in some ancient wax could have
been caused by oxidation, and considers the medium of encaustic painting to be
wax alone. This is probably the general opinion. The melting point has sometimes
been found to be high in specimens from Fayum portraits and there remains a
possibility that some other medium was added.
Although wax serves very little now as a painting medium, it finds many
other applications in the arts. Because of its easy solubility in weak solvents and
because of its protective strength and its permanence, its use as a surface coating
for pictures has increased in recent years and it is much used in restoration
(see Waxes).
Wood Oil (see Tung Oil).
Wool Wax (see also Waxes) is the natural grease from the fleece of sheep.
It is a pale yellow, translucent substance with a distinctive odor and an unctuous
consistency. Purified, it forms, together with about 25 per cent of water, the
lanolin of commerce. Its chemical composition is not fully known. It consists of
a mixture of neutral esters and free alcohols, among which occur cholesterol and
isocholesteroi (C27HUOH). Although insoluble in water, it emulsifies it (see
Emulsions), and it can easily be made to take up 80 per cent of its weight of
water. It is used extensively in the treatment of leather, as a rust preventative,
and for medicinal purposes.
Zanzibar Copal (see also Copal and Resins) occurs either as a resin from a
living tree, Trachylobium verrucosum Oliv. (of the Papilionaceae family), as a semi-
fossil in the ground beneath these trees, or as a true fossil deposited by a tree no
longer standing. The fossil resin has a brown crust which, when scraped away,
exposes a transparent, yellow mass on which are small, round elevations called
* goose skin/ The exudation from the living tree is not so hard as the fossil resin,
but has a smooth, glossy surface. Very little of this resin is now actually collected
on the island of Zanzibar; it is sent directly from the mainland. It is sometimes
also called ' anime.' Zanzibar copal is the hardest of the copal resins, has a very
high melting point (240° to 360° C), and makes a rather dark, oil varnish, used
chiefly for industrial purposes.
Zapon, a lacquer or varnish containing highly viscous nitrocellulose (see G.
Zeidler and F. Wilborn, ' Application of Zapon Lacquers to Metallic Surfaces/
Paint and Famish Production Manager^ XIX [December 1939], pp. 358-363,373).
It is sometimes mentioned in works of Continental origin that deal with restora-
tion of museum materials.