Alum tannage is traditionally based on the use of aluminium sulphate.
Alum salts combine with the hide or skin in a similar way to chrome salts. The main difference being that they do not form a complex bond and therefore are not fixed to the collagen.
In simple terms this means that the alum is easy in and easy out and can therefore be washed out readily to give an untanned skin.
The characteristic of an alum tanned hide is that while the aluminium sulphate does not form a complex bond, it does show extremely high astringency and therefore has the ability to tighten the fibre structure making leather. This makes the alum tanned highly desirable for lacing, whips and cricket balls.