The book I am reading (the book of photograhic alternative processes)suggests using Alum as a hardener in gelatin when sizing paper. What is Alum? Is it Aluminum Potassium Sulfate? I hate it when authors are unclear...

Hmmmm, I think it may also be reffering to Chromium Aluminum Sulfate, commonly called Chrome Alum.

I think either version will work, regardless.

If nothing else is specified, "Alum" is supposed to mean Potassium Aluminium Sulfate.

Unfortunately this is not always true in photographic literature.

Read http://en.wikipedia.org/wiki/Alum for more info on the various alums.

BTW, Chromium Alum is not CrAl-sulfate, but KCr-sulfate.