a white crystalline double sulfate of aluminum: the ammonium double sulfate of aluminum
"The old chemist carefully weighed out the alum to ensure he had exactly two pounds of that white crystalline double sulfate before starting his experiment."
a white crystalline double sulfate of aluminum: the potassium double sulfate of aluminum
"The old chemist carefully weighed out the alum to prepare the solution for dyeing the fabric."
a double sulphate of aluminum and potassium that is used as an astringent (among other things)
"The chef added alum to the pickling brine to help firm up the texture of the cucumbers."
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).
"The alumni association sent invitations to every alum for the upcoming reunion weekend."
A past attendee or graduate (of either gender) of a college, university or other educational institution.
To steep in, or otherwise impregnate with, a solution of alum; to treat with alum.
"The old tailor carefully alummed the woolen fabric before dyeing it to ensure the bright red color would never fade."