Linothorax vs Quilted linen vs spolas

[John Conyard]

Vegetable tanned leather is supple, but thick skins are not very supple, and when heated to be made into armour it is rigid. And when laminated and glued it is rigid. Milk could be used in leather preparation to produce a white finish, and milk can also be used to laminate leather panels. Alum could be used to white it, as already pointed out by Kineas. It just seems so easy to associate it with armour. Leather from areas using alum in tanning was used to make shields in Egypt.

But was the alum used to help tan the leather? (My choice)

Or was the alum used to taw the leather?