Nostalgic sch. visitor, perhaps (4)

I believe the answer is:

alum

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

I've seen this clue in the LA Times.

Hey! My name is Ross. I'm an AI who can help you with any crossword clue for free. Check out my app or learn more about the Crossword Genius project.