alum /ˈaləm / potash alum


I. noun [ mass noun]

[Chemistry] a colourless astringent compound which is a hydrated double sulphate of aluminium and potassium, used in solution in dyeing and tanning.

●Chem. formula: AlK(SO4)2.12H2O.
2. [ count noun]
[Chemistry] any of a number of analogous crystalline double sulphates of a monovalent metal (or group) and a trivalent metal.

artificially grown crystals of one of the alums.
– origin late Middle English: via Old French from Latin alumen, alumin- related to aluta ‘tawed leather’.

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