Alum in the form of potassium aluminium sulphate or ammonium aluminium sulfate in a concentrated bath of hot water is regularly used by jewelers and machinists to dissolve hardened steel drill bits that have broken off in items made of aluminum, copper, brass, gold (any karat) and silver (both sterling and fine). This is because alum does not react chemically to any significant degree with any of these metals, but will corrode steel. When heat is applied to an alum mixture holding a piece of work that has a drill bit stuck in it, if the lost bit is small enough, it can sometimes be dissolved / removed within hours.