Alum is a double salt comprising sulfates of two metals (one monovalent, one trivalent) combined with 12 molecules of water of crystallization: MIMIII(SO4)2.12H2O. The monovalent metal is commonly potassium, sodium, or ammonium; the trivalent metal may be aluminum, chromium, or ferric iron.
Alums are soluble in water and are usually acid. They are used in astringents (styptic pencils), as a mordant in dyes, and in the manufacture of baking powder, antiperspirants, and fire extinguishers. Potash alum (potassium aluminum sulfate, KAl(SO4)2.12H2O) is used in the sizing of paper, as a mordant in dyeing, and in water purification.