One mentioned in "Class Notes," informally (4)

I believe the answer is:

alum

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

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

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