Alum, << AL uhm, >> is the name of a particular group of double salts. Double salts consist of two simple salts that form crystals together in fixed proportions. Common alum is a double salt of hydrated (water-containing) potassium sulfate and aluminum sulfate. This compound is also called potash alum or potassium alum. Ammonium alum, sodium alum, and potassium chrome alum are some of the other alums. Most are manufactured from bauxite (aluminum oxide ore).
Alums are used to make glue, dyes, baking powder, flame retardants, and leather tanning agents. Industry also uses alums to purify water, to harden plaster of Paris, and to preserve fruits and vegetables. Potassium alum helps stop bleeding.
