DERIVATION.--From alum slate, shale, schist, a native mixture of
aluminum silicate and iron sulphide. This is roasted and exposed to the
air, when the sulphur is oxidized into sulphuric acid and combined in
part with aluminum and iron to form sulphates. The mass is lixiviated
with water, and aluminum and iron sulphates together with sulphuric acid
are recovered in solution. The solution is concentrated and to it is
added potassium chloride. The double sulphate of potassium and aluminum
(alum) is formed, which crystallizes out on cooling, while potassium
sulphate and ferric chloride remain as by-products. Alum is purified by
recrystallization.
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 nine
parts of water, insoluble in alcohol.
DOSE.--Horses and cattle, 2 to 4 dr.; sheep and pigs, 20 gr. to 1 dr.;
emetic for dogs, ¹⁄₂ to 1 dr.