One of the mysteries of the English language finally explained.
Chemistry. Each of two hydrated double sulphates of (ferric) iron and (respectively) potassium and ammonium, belonging to the series of alums and typically pale violet in colour.
Mid 19th century. From iron + alum, perhaps after German Eisenalaun or French alun de fer.
iron alum/ˌʌɪən ˈaləm/
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