ETYM Old Eng. alum, alom, Old Fren. alum, French alun, from Latin alumen alum.
Sulfate of aluminum used in dyeing, as an astringent, etc.
Any double sulfate of a monovalent metal or radical (such as sodium, potassium, or ammonium) and a trivalent metal (such as aluminum or iron). The commonest alum is the double sulfate of potassium and aluminum, K2Al2(SO4)4.24H2O, a white crystalline powder that is readily soluble in water. It is used in curing animal skins. Other alums are used in papermaking and to fix dye in the textile industry.
1. A double sulphate of aluminum and potassium that is used as an astringent (among other things).
2. A white crystalline double sulfate of aluminum: the ammonium double sulfate of aluminum; SYN. ammonia alum, ammonium alum.
3. A white crystalline double sulfate of aluminum: the potassium double sulfate of aluminum; SYN. potassium alum, potash alum.