Alum, or styptic, is the common name for the double sulfate salt of potassium-aluminum [KAl(SO4)2]. The form in which it is best known is its hydrated salt [KAl(SO4)2·12H2O]. It is a salt extracted from bauxite.

It appears in the form of large, transparent crystals, with a sweet taste that becomes astringent after a while. It dissolves slightly in cold water and much in hot water. It also dissolves in glycerin, while it is insoluble in alcohol.

When heated, it melts at 91°C and if cooled, it turns into a mass that resembles glass. If we continue to heat it above 91°C, it becomes anhydrous, expands, and turns into a light, spongy white mass, the "burnt alum."

USES:

• as a natural deodorant
• in water purification
• as an astringent medicine
• as a hemostatic
• as a disinfectant
• for gargling
• in dyeing as a mordant
• in tanning
• in papermaking
• in glue preparation
• in photography
• as a waterproofing agent