AL'UM, n. [L. alumen.]
A triple sulphate of alumina and potassa. This substance is white, transparent and very astringent; but seldom found pure or crystallized. This salt is usually prepared by roasting and lixiviating certain clays containing pyrites, and to the lye adding a certain quantity of potassa; the salt is then obtained by crystallization. Alum is of great use in medicine and the arts. In medicine, it is used as an astringent; internally, in hemoptoe, diarrhea, and dysentery; externally, as a styptic applied to bleeding vessels, and as an escharotic. In the arts, it is used in dyeing to fix colors; in making candles, for hardening the tallow; in tanning, for restoring the cohesion of skins.
