Alum

Alum is the hydrated potassium aluminium sulfate (potassium alum) with the formula KAl(SO4)2·12H2O. Alums are useful for a range of industrial processes. They are soluble in water; have a sweetish taste; react acid to litmus; and crystallize in regular octahedra.

They are astringent and acidic. Alum in block form (usually potassium alum) is used as a blood coagulant. Styptic pencils containing aluminium sulfate or potassium aluminium sulfate are used as astringents to prevent bleeding from small shaving cuts.

Alum may be used in depilatory waxes used for the removal of body hair, or applied to freshly waxed skin as a soothing agent.