School reunion attendee, informally (4)

I believe the answer is:

alum

(Other definitions for alum that I've seen before include "White mineral salt used in medicine and in dyeing" , "double sulphate" , "Potassium compound; IoW sandy bay" , "Compound used in dyeing and to stop bleeding" , "Colourless compound used in dyeing and tanning" .)

I've seen this clue in The New York Times.

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