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.
Origin
Mid 19th century. From iron + alum, perhaps after German Eisenalaun or French alun de fer.
Definition of iron alum in US English:
iron alum
nounˌʌɪən ˈaləm
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.
Origin
Mid 19th century. From iron + alum, perhaps after German Eisenalaun or French alun de fer.