Definition of iron alum in US English:

iron alum

noun

  • 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.

Pronunciation

iron alum

/ˌʌɪən ˈaləm/