MEDIUMS AND ADHESIVES 13
shortcoming of cellulose nitrate, the inadequacy of plasticizing materials now
available for it, has already been touched upon. Loss of plasticizers, however,
may not be secondary to the effects of light and heat. Camphor and such plasti-
cizers escape eventually because of their inherent vapor pressures. The incorpora-
tion of natural and synthetic resins tends to lessen some of these shortcomings.
The high flammability of cellulose nitrate compositions is well known. There is
much greater danger attendant upon application of the lacquer than there is
from any possibility that the dried film will ignite.
Cellulose nitrate, particularly in the form of a celluloid lacquer, has played
some part in the restoration of museum objects in the last quarter century,
principally as an adhesive and as an impregnating agent (see Lucas, Antiques, etc.,
index). In the Third Report of the British Museum on the Cleaning and Restoration
of Museum Exhibits (p. 20) is a record of the employment of a celluloid varnish
for coating baked clay tablets prior to washing them in distilled water. Plender-
leith (p. 15) says that a celluloid lacquer is useful for coating the powdery surface
of decayed wood; advantage is taken of the great contraction of the celluloid to
re-enforce the surface,
Cement Frequently adhesives, and film materials generally, are referred to
by this name if they are used for the purpose of joining objects or parts of objects.
For such a purpose, a number of types of film material may be used (see Glue,
Resins, and Synthetic Resins).
Ceresin (see also Waxes and Ozokerite) is obtained from Galician * earth
wax/ ozokerite. It is harder than paraffin, is dazzling white in appearance,
inodorous, and transparent at the edges. It consists of a mixture of hydrocarbons
and differs from paraffin wax in being plastic and non-crystalline in character
(Fryer and Weston, p. 208). The melting point varies between 65° and 80° C.
It is not attacked by acids, either cold or hot, or by alkalis, which do not saponify
a trace of it. It is entirely volatilized at a high temperature without alteration.
It is employed as a substitute for beeswax which it resembles in plasticity. It is
often adulterated with paraffin wax, many so-called * ceresins * being, in fact,
entirely paraffin.
Cherry Gum (see also Gums) is from the cherry, mahaleb-cherry, apricot, and
plum trees. It swells in water, and about 10 per cent is enough to form a thick
substance. The solution is pressed through a cloth. It may be emulsified with
fatty oils and balsams. It gives great transparency to color, but is inclined to
chip easily if used alone or if applied in thin glazes. When added to an egg or
casein emulsion, it is said (Doerner, pp. 223-224) to give a brilliant, enamel-like
effect. It is mentioned as a painting medium in some treatises, particularly of
northern origin, and probably had occasional use as late as the XIX century.
Chinese Insect Wax (see also Waxes) is the deposit of an insect. Coccus
ceriferus, which is a parasite on certain Asiatic trees. The wax is obtained by
placing the larvae of the insect on certain selected trees UD which it creens, and