A skin prepared in an aqueous solution of a double salt of aluminium and potassium sulphates. The process, which is of great antiquity, produces a white skin which is dried (crusted) and then staked, or worked over a blunt metal knife, to produce a soft, supple skin, qualities which could be enhanced by the addition of flour and egg-yolk to the tawing solution. If wetted again, as many blind-tooled German bindings of the late fifteenth to the mid-eighteenth century were before tooling, the skin becomes once more hard and horny, as though it had reverted to the raw, untanned state. Alum-tawed skin is generally more durable and resistant to deterioration than tanned skins.