One of the mysteries of the English language finally explained.
1A colourless astringent compound which is a hydrated double sulphate of aluminium and potassium, used in solution in dyeing and tanning.
Chemical formula: AlK(SO₄)₂.12H₂O
- ‘This, washed and then boiled, yielded aluminium sulphate, which, when ammonia was added, became alum.’
- ‘It could also be treated by rubbing salt, brain or potash alum into the surface to produce a very pale leather.’
- ‘I have no idea how it's manufactured, but it's a kind of alum, a double sulfate of aluminum in crystal salt form.’
- ‘Astringent varieties contain alum, which makes your mouth pucker when the fruits are eaten before they're fully ripe.’
- ‘One site said that clarifying agents could control snails, so I sent my son to the chemist for alum.’
- 1.1count 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’
- ‘Aluminium dross tailings were used to produce two types of alums.’
- ‘Several decades later the Englishman Davy attempted to obtain the metal hidden in alums.’
Late Middle English: via Old French from Latin alumen, alumin- related to aluta ‘tawed leather’.
A former pupil or student of a school, college, or university; an alumnus or alumna.‘a fellow Wellesley alum’
Late 19th century: abbreviation of alumnus or alumna.
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