Origin. - Prepared by a complicated process from a mixture of aluminum silicate and iron sulphide by roasting, lixiviating with water, concentrating the solution, and, while hot, mixing with potassium chloride. Upon cooling the alum separates as a crystalline powder, which is purified by one or two recrystallizations. It should contain not less than 99.5 per cent. of pure aluminum and potassium sulphate Al.K(SO4)2 + 12 H10.

Description and Properties. - Large, colorless, octahedral crystals, sometimes modified by cubes, or crystalline fragments, without odor, but having a sweetish and strongly astringent taste. On exposure to the air the crystals are liable to absorb ammonia and acquire a whitish coating. Soluble in 9 parts of water and 0.3 part of boiling water; also freely soluble in warm glycerin. Insoluble in alcohol.

Dose. - 5-40 grains (0.3-2.60 Gm.); as an emetic, 1-2 drams (4.0-8.0 Gm.) [7 1/2 grains (0.5 Gm.), U. S. P.].

Official Preparation

Alumen Exsiccatum - Aluminis Exsiccati - Dried Alum (Burnt Alum). - Origin. - Alum heated until it is deprived of its water of crystallization.

Description and Properties. - A white, granular powder, without odor, possessing a sweetish, astringent taste and attracting moisture from the air. Very slowly but completely soluble in 20 parts of water, and quickly soluble in 0.7 part of boiling water.

Dose. - 1-5 grains (0.06-0.3 Gm.).