alum in French

An astringent salt, usually occurring in the form of pale crystals, much used in the dyeing and tanning trade and in certain medicines, and now understood to be a double sulphate of potassium and aluminium (K2SO4·Al2(SO4)3·24H2O). [from 14th c.]

(transitive) To steep in, or otherwise impregnate with, a solution of alum; to treat with alum.