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 themselves. 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 lacin, 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 Formulae

For a Colored Varnish.

Gum shellac.....

Alcohol.......qt.j.

Mix and digest over a moderate heat until thoroughly dissolved.

Dental Formulae 1413Dental Formulae 1414

For an Aqueous Varnish. Pulv. shellac . . . partes j Sat. solut. boracis . . . partes ij. Mix by shaking together; it will give a starch gloss.

Dental Formulae 1415

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, add glycerine. Clear shellac varnish may be prepared by first making an alcohol solution of shellac in the usual way, and then adding a little benzole, and the mixture well shaken. In from 24 to 48 hours the fluid will have separated into two distinct layers, an upper alcoholic stratum, perfectly clear, and of a dark-red color, while under it is a turbid mixture containing the impurities. The clear solution is drawn off with a pipe, or may be decanted.