Alum

Alum ( /ˈæləm/) is both a specific chemical compound and a class of chemical compounds. The specific compound is the hydrated potassium aluminium sulfate (potassium alum) with the formula KAl(SO₄)₂·12H₂O. More widely, alums are double sulphate salts, with the formula AM(SO₄)₂·12H₂O, where A is a monovalent cation such as potassium or ammonium and M is a trivalent metal ion such as aluminium or chromium(III).