MEDIUMS AND ADHESIVES 9
acid has escaped through effervescence. Ammonia casein may be kept in a corked
bottle and diluted in water before it is used. The adhesive power, though not so
great as that of lime-casein, is good. It has the additional advantage that its solvent
is harmless. Commercial caseins are often prepared with potash or soda, and, as
these lyes destroy certain colors, the litmus test (red litmus should not turn blue)
can be applied. It is difficult to keep casein colors in tubes without their harden-
ing and crumbling. Glycerine may be added, but, although this keeps the paint
moist, it destroys the insolubility of casein in water. A great difficulty with this
medium, besides its brittleness in the film, is its tendency to encourage mold
growth.
Castor Oil (see also Non-Drying Oils) is an oil from the seeds of Ricinus
communis which is grown in India and in most hot countries. It is the heaviest
of all the fatty oils, is almost colorless, and is very viscous. Chemically, it is
quite different from the other fatty oils (see Oils and Fats), consisting largely of
the glyceride of ricinoleic acid (CigH^Oa); a small quantity of hydroxystearic
acid and stearic acid also occurs. It is largely used as a plasticizer and in practice
is distinguished from many oils by its ready solubility in alcohol,
Celluloid (see also Cellulose Nitrate) is a pyroxylin plastic that is plasticized
with camphor. In time the camphor disappears and the film becomes brittle.
Celluloid lacquers have been extensively used during the past fifty years in re-
storing and repairing objects of art. Celluloid clippings could easily be dissolved
in acetone or in some solvent mixture.
Cellulose Acetate (see also Cellulose Coatings and Cellulose Nitrate). Cellu-
lose acetate is a white, bulky solid that now finds extensive use as a coating and
lacquer material and as a molding compound. Compared with cellulose nitrate,
it has some advantages and some disadvantages. Although acetylated carbo-
hydrates were known as early as 1865, it was not until 1910-1911 that mention
of products which were like those now called * lacquers' and which contained
cellulose acetate, began to appear in the patent literature. Cellulose acetate is
prepared from some form of cellulose, like cotton linters or paper, with a mixture
of glacial acetic acid, acetic anhydride, and concentrated sulphuric acid. The
product is an ester of cellulose (which may be considered to be a polyhydric
alcohol) and acetic acid. Cellulose acetates of widely different properties may be
made* The low-viscosity acetates are best for lacquers. The chief difficulty in the
way of the commercial development of cellulose acetate has been its limited
solubility. It is dissolved by fewer organic solvents than cellulose nitrate, and
these few are strong—acetone, diacetone alcohol, ethylene dichloride, and the
glycol ether acetates. Even with such solvents, the dilute solutions of cellulose
acetate are viscous. In general, the lacquers have too low solids content for wide
commercial application. Hofmann and Reid have made an exhaustive study of
the solubility of cellulose acetate in single solvents and in solvent mixtures, and,
on the basis of their experimental data, have been able to work out some very
satisfactory lacquer formulas. There has been difficulty from the tendency of cellu-