Inorganic compound, any of a class of hydrated double aluminum sulfate, water of hydration (an essential part of the crystal makeup), and the sulfate of another element.
The most important alums are those of potassium sulfate (potassium alum, or potash alum, K 2 SO 4 ·Al 2 (SO 4 ) 3 ·24H 2 O), ammonium sulfate, and sodium sulfate. Alums occur naturally in various minerals and can be prepared and purified by crystallization from their solutions. Most are white crystals with an astringent, acid taste. They are used as paper-sizing agents, flocculating agents in water treatment, mordants in dyeing, and in pickles, baking powder, fire extinguishers, and medicines.