> Alum cannot be used in place of gelatin size. It can be used to harden (or
> tan) gelatin and then can be washed out but it cannot replace gelatin since
> it is not a colloid and it is for this purpose that we need gelatin in the
> first place.
Maybe I understand the way it works incorrectly. Somehow I thought that
since the gelatine was hardened, it trapped the alum in it so the alum is
no longer water soluble? How can the alum be washed out?
Thanks.