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SHELLAC. 545

SHELI^LAC—SHELLAC.
Source.—Lac is a resinous substance, obtained from several
varieties of trees which grow in the East Indies, and particularly
from the Croton Lacciferum^ and two species of the Ficus. It is
supposed to be an exudation from the bark, owing to the
puncture of an insect belonging to the genus Coccus ; it is also
said to be the exudation from the bodies of the insects them-
selves. Several varieties are known in commerce, the most
common being the stick-lac^ the seed-lac and the shell-lac. Lac,
in its crude state, consists of resin, coloring matter, and a
peculiar principle, insoluble in water, alcohol and ethyl, and
known as /aa«, a little wax, and various saline matters in small
proportion.
Uses.—It is slightly astringent, and has been recommended as
an adhesive substance for dressing ulcers, wounds, etc., being
used by simply spreading it on the bandages after it is dissolved
in alcohol by a gentle heat. Shellac is prepared by melting the
crude lac, straining it, and pouring it upon a flat, smooth surface
to harden. It is in the form of thin fragments of various sizes,
from half a line to a line in thickness, and of a light and also a
dark brown color, shining, hard, brittle and inodorous. It is
insoluble in water, but freely soluble in alcohol, especially with
the aid of heat. It is employed as a varnish. In dental practice
its uses are the same as those of sandarach, but owing to the
dark brown color of the solution, it is not so sightly, and does
not make a transparent varnish.

DENTAL FORMULA.
For a Colored Varnish. For an Aqueous Varnish.
li. Gum shellac ^v R- Pulv. shellac . . • • . partes j
Alcohol qt.j. Sat. solut. boracis . . . partes ij.
Mix and digest over a moderate Mix by shaking together ; it will
heat until thoroughly dissolved. give a starch gloss.
Shellac may be dissolved without the aid of alcohol, by a
saturated solution of borax in water. This, however, does not
give a very strong solution. To prevent cracking when the
shellac is dissolved in alcohol, add a little castor oil ; if in water,
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