MEDIUMS AND ADHESIVES 7
Bone Glue is impure gelatin prepared from bones (see also Gelatin and Glue).
Canada Balsam (see also Balsam) is derived from a fir (Abies bahamea, Mill.)
which grows widely in the eastern United States and Canada. It is obtained from
small blisters in the bark and only a small amount can be collected at a time.
The balsam is relatively pure and is valuable for its transparency and its high
refractive index (1.5194 to 1.5213 at 20° C.). It was introduced into Europe in
the XVIII century.
Candelilla Wax (see also Waxes) is obtained from the stem of the leafless
Mexican plant, Pedilanthus pavinia, and from other Mexican genera of the Eu~
phorblaceae. It is a brownish, brittle mass which may be bleached. Although of a
lower melting point than carnauba wax, it finds application in similar industries.
Candlenut Oil is obtained from the seeds of Akurites moluccana^ a tree cover-
ing large areas in the western tropics. For use in paints and varnishes, it is rec-
ommended by some and condemned by others. It is closely related to tung oil.
Carnauba Wax (see also Waxes) is obtained from the Brazilian palm, Coryf ha
cerifera (the carnauba tree), on the leaves of which it forms a deposit. The young
leaves are cut and dried and the wax powder is scraped off and melted in boiling
water. It is bleached with fuller's earth or charcoal or by a chemical oxidant
such as chromic acid. It is a yellowish, hard, brittle material of exceptionally
high melting point (83° to 86° C.) which increases somewhat with age. The major
component of the wax is melissyl (myricyl) cerotate ^sHsiCOOCsoHgi) with
minor amounts of hydrocarbons, wax alcohols, and higher fatty acids. Owing to
its hardness and high melting point, it takes a fine, hard gloss when rubbed.
It has been recommended (Rosen, p. 115) as a coating material for paintings,
when mixed with other waxes.
Casein (see also Casein Tempera), usually referred to as a glue, is an organic
compound belonging to the class known as proteins, the most complex compounds
with which chemists have to deal. Furthermore, it belongs to one of the more
complex subdivisions, the phosphoproteins. It consists of carbon, hydrogen,
oxygen, nitrogen, sulphur, and phosphorus, and, although it has been the subject
of many investigations, a great deal of information is still lacking with regard
to the ammo-acids of which it is composed. Like all proteins, it is amphoteric,
*".*., it functions both as an acid and as a base. It has, however, decided acid
properties and exists in milk as calcium caseinate. Casein is prepared from
skimmed milk by heating it at 34.5° to 35° C. and adding hydrochloric acid
till the mixture reaches a pH of 4.8. It is then allowed to settle and, after sepa-
ration from the supernatant liquid, is washed with hydrochloric acid, also with
a pH of 4.8. Casein so prepared is technically pure, and is a snow-white, slightly
hygroscopic powder with a specific gravity of 1.259. It reacts as a weak acid, is
insoluble in water, alcohol, and other neutral organic solvents, and is soluble in
the carbonates and hydroxides of the alkali and alkaline earth metals and in
ammonia.