Alum

Alum, in its general sense a double salt produced by the combination of the sulphate of an alkali metal with the sulphate of a triatomic metal of the aluminium group. As a class the alums are marked by identity of crystalline form, ready solubility in water, astringent taste, and acid reaction. They also contain the same quantity of water of crystallisation. The term alum, in its special sense, invariably denotes ordinary potash alum.