"Alums" are a class of related compounds - usually double sulfates of metals (aluminum, chrome, iron, maybe zirconium?) that form similar crystals. The "common names" are often misleading. Potassium Aluminum Sulphate (avail in the spices section of grocery stores) is the same as what is found in the 'hardener' concentrate of some fixers - it will harden emulsion in an acidic environment.