noun
: a salt Al 2 (SO 4 ) 3 colorless when pure that crystallizes with 18 molecules of water (as in alunogen) but is commonly desiccated to about 14 H 2 O, that is usually made by treating bauxite with sulfuric acid, and that is used chiefly in papermaking, in water purification, in sewage treatment, in tanning, in dyeing as a mordant, and in flameproofing — called also alum, filter alum, papermakers' alum