A chemical compound, typically potassium aluminum sulfate, used in medicine, dyeing, and water purification. Also informal term for alumnus/alumna (graduate of a school).
From Latin 'alumen' (bitter salt), from Proto-Indo-European root meaning 'bitter'. The chemical was known to ancient civilizations for its astringent properties and use in leather tanning and dyeing.
Ancient Romans used alum to set dyes in fabrics and preserve leather, making it one of history's most valuable trade goods - entire economies rose around alum mines, and the Pope once monopolized its trade to fund Renaissance art projects.
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