[alt-photo] How does alum hardening work?

[Mike and Jan Andrews]

I have been trying to find out how alum or chrome alum harden gelatine, but with no success!
 
My guess is that alums form colloidal hydroxides and that these hydroxides have the opposite charge to gelatine. So they coagulate. This explanation ties in with the fact that alum hardening only works within certain pH ranges.
 
I also guess that the same thing occurs when ferric chloride is added to gum arabic and hardens it. In this case the colour changes from yellow to brown, which I guess is ferric hydroxide. And ferric hydroxide is known to exist in a colloidal state.
 
It would be helpful if someone could either confirm my guesses or shoot them down in a constructive manner! 
 
Michael Andrews