Definition of alum in English:



  • 1[mass noun] A colourless astringent compound which is a hydrated double sulphate of aluminium and potassium, used in solution in dyeing and tanning.

    • ‘It could also be treated by rubbing salt, brain or potash alum into the surface to produce a very pale leather.’
    • ‘Astringent varieties contain alum, which makes your mouth pucker when the fruits are eaten before they're fully ripe.’
    • ‘I have no idea how it's manufactured, but it's a kind of alum, a double sulfate of aluminum in crystal salt form.’
    • ‘One site said that clarifying agents could control snails, so I sent my son to the chemist for alum.’
    • ‘This, washed and then boiled, yielded aluminium sulphate, which, when ammonia was added, became alum.’
    1. 1.1[count noun] Any of a number of analogous crystalline double sulphates of a monovalent metal (or group) and a trivalent metal:
      ‘artificially grown crystals of one of the alums’
      • ‘Several decades later the Englishman Davy attempted to obtain the metal hidden in alums.’
      • ‘Aluminium dross tailings were used to produce two types of alums.’


Late Middle English: via Old French from Latin alumen, alumin- related to aluta tawed leather.