Alum

An alum (/ˈæləm/) is a type of chemical compound, usually a hydrated double sulfate salt of aluminium with the general formula XAl(SO4)2·12H2O, where X is a monovalent cation such as potassium or ammonium. By itself, 'alum' often refers to potassium alum, with the formula KAl(SO4)2·12H2O. Other alums are named after the monovalent ion, such as sodium alum and ammonium alum. An alum (/ˈæləm/) is a type of chemical compound, usually a hydrated double sulfate salt of aluminium with the general formula XAl(SO4)2·12H2O, where X is a monovalent cation such as potassium or ammonium. By itself, 'alum' often refers to potassium alum, with the formula KAl(SO4)2·12H2O. 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 other chalcogen like selenium. The most common of these analogs is chrome alum KCr(SO4)2·12H2O. In most industries, the name 'alum' (or 'papermaker's alum') is used to refer to aluminium sulfate Al2(SO4)3·nH2O, which is used for most industrial flocculation. 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)4·22H2O, MgAl2(SO4)4·22H2O, NaAl(SO4)2·6H2O, MgSO4·7H2O and Al2(SO4)3·17H2O. The production of potassium alum from alunite is archaeologically attested on the island Lesbos. This site was abandoned in the 7th century 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)3·17H2O) with potassium alum and other minor sulfates. A detailed description of a substance called alumen occurs in Pliny the Elder's Natural History. By comparing Pliny's description with the account of stupteria 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. Pliny wrote that different substances were distinguished by the name of alumen, but they were all characterised by a certain degree of astringency, and were all employed in dyeing and medicine. Pliny says that there is another kind of alum that the Greeks call schiston, and which 'splits into filaments of a whitish colour', From the name schiston and the mode of formation, it appears that this kind was the salt that forms spontaneously on certain salty minerals, as alum slate and bituminous shale, and consists chiefly of sulfates of iron and aluminium. One kind of alumen was a liquid, which was apt to be adulterated; but when pure it had the property of blackening when added to pomegranate juice. This property seems to characterize a solution of iron sulfate in water; a solution of ordinary (potassium) alum would possess no such property. Contamination with iron sulfate was greatly disliked as this darkened and dulled dye colours. In some places the iron sulfate may have been lacking, so the salt would be white and would be suitable, according to Pliny, for dyeing bright colors. Pliny describes several other types of alumen but it is not clear as to what these minerals are. The alumen of the ancients, then, was not always potassium alum, not even an alkali aluminum sulfate.