Alum

An alum (ˈæləm) is a type of chemical compound, usually a hydrated double sulfate salt of aluminium with the general formula XAl(SO4)2·12 H2O, where X is a monovalent cation such as potassium or ammonium. By itself, "alum" often refers to potassium alum, with the formula KAl(SO4)2·12 H2O. Other alums are named after the monovalent ion, such as sodium alum and ammonium alum. The name "alum" is also used, more generally, for salts with the same formula and structure, except that aluminium is replaced by another trivalent metal ion like chromium^III, and/or sulfur is replaced by another chalcogen like selenium. The most common of these analogs is chrome alum KCr(SO4)2·12 H2O. In most industries, the name "alum" (or "papermaker's alum") is used to refer to aluminium sulfate, Al2 (SO4)3·n H2O, which is used for most industrial flocculation (the variable n is an integer whose size depends on the amount of water absorbed into the alum). In medicine, "alum" may also refer to aluminium hydroxide gel used as a vaccine adjuvant. The western desert of Egypt was a major source of alum substitutes in antiquity. These evaporites were mainly FeAl2(SO4)4·22 H2O, MgAl2(SO4)4·22 H2O, NaAl(SO4)2·6 H2O, MgSO4·7H2O and Al2(SO4)3·17 H2O. The Ancient Greek Herodotus mentions Egyptian alum as a valuable commodity in The Histories. The production of potassium alum from alunite is archaeologically attested on the island Lesbos. The site was abandoned in the 7th century CE, but dates back at least to the 2nd century CE. Native alumen from the island of Melos appears to have been a mixture mainly of alunogen (Al2(SO4)3·17 H2O) with potassium alum and other minor sulfates. A detailed description of a substance called alumen occurs in the Roman Pliny the Elder's Natural History. By comparing Pliny's description with the account of stypteria (στυπτηρία) given by Dioscorides, it is obvious the two are identical. Pliny informs us that a form of alumen was found naturally in the earth, and calls it salsugoterrae.