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From Anglo-Norman alum, alume et al., Middle French allume, from Latin alÅ«men.

Full definition of alum

(plural alums)

An astringent salt, usually occurring in the form of pale crystals, much used in the dyeing and tanning trade and in certain medicines, and now understood to be a double sulphate of potassium and aluminium (K₂SO₄.Al₂(SO₄)₃.24H₂O). from 14th c.

1991, Felix Gilbert, The Pope, His Banker, and Venice, page 80,Venice also needed alum for trade, since it was the point of departure for overland transportation of alum to southern Germany and its cloth-manufacturing Free Cities.
2011, Thomas Penn, Winter King, Penguin 2012, p. 201:A natural astringent and antiseptic, potassium alum was coveted for its medicinal and cosmetic properties.

(chemistry) Any similar double sulphate in which either or both of the potassium and aluminium is wholly or partly replaced by other univalent or tervalent cations. from 17th c.

1807, William Nicholson (editor), A Journal of Natural Philosophy, Chemistry, and the Arts, Volume XVIII, page 286,With weld and cochineal, which are colouring matters the most sensible to the action of sulphate of iron, the purified alums gave us colours more brilliant, fresh, and in a slight degree lighter; while those with our common alums were all duller, and evidently of a deeper hue.
2000 June, Competition Science Vision, page 486,For similar reasons, aluminium sulphate and alums are used in dyeing cloth....Normally alums are soluble in water and insoluble in alcohols.
2005, Amit Arora, Text Book Of Inorganic Chemistry, page 386,In structure, the alums consist of simple ions, being not complexes, but double salts. Potash alum or potassium alum is the common alum, with the formula KAl(SO₄)₂.12H₂O) which, for convenience, may be written K₂SO₄.Al₂(SO₄)₃.24H₂O

(transitive) To steep in, or otherwise impregnate with, a solution of alum; to treat with alum.

From alumnus and alumna, by removal of the non-native, gender-specific endings.

(plural alums)

1961 Spring, Anchora of Delta Gamma, Volume LXXVII, No. 3, page 59,Evanston-North Shore alums are happy to open their homes to Sigma actives for special social events.
2006, Ted Hart, James M. Greenfield, Pamela M. Gignac, Christopher Carnie, Major Donors: Finding Big Gifts in Your Database and Online, page 47,You'll remember that we're starting with a list of slightly over 7,000 names that are alums (most of them over 50) that we'd like to whittle down to a manageable list of prospects.
2009, Timothy C. Jacobson, Charity & Merit: Trinity School at 300, page 190,All schools that last have alums, and, ancient as it was by American standards, Trinity by mid-century had thousands.