Alum Tawed skins were traditionally used in the 12th century for covers and thongs. They were preferred for bookbinding as the alum treatment rendered them more resistant to atmospheric pollution than tanned skins.Alum tawing is an ancient process of treating prepared animal hides with aluminum salts and other materials. The skins are then left to dry for several weeks to allow the development of stabilization effects. Alum Tawed skins are typically sort and flexible to the touch, and have a high degree of stretch. They are typically white in color, but can also have varying degrees of yellow.