alum

Meanings

1. 1
   noun

   any of a class of crystalline double sulfates, especially potassium or ammonium aluminum sulfate, used in dyeing, tanning, water treatment, and as an astringent

   The dyer added alum as a mordant to help the color bind to the cloth.

From Middle English alum or alun, through Anglo-French from Latin alumen, 'alum.' The substance was historically important as a mordant in textile dyeing and as an astringent. The unrelated informal alum meaning 'former student' is a nineteenth-century shortening of alumnus or alumna.

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