Alum in a gelatin mix

Hello all! I was looking to add a hardener to gelatin/cyanotype mix for glass. Ive read on a few forum posts about alum.. but there were a few questions i felt were unanswered.

When does the alum get mixed in? When you initially make the gelatin solution?
Do you add it in at a later date when you are ready to pour the gelatin onto a plate? Etc?

Thanks for your help!

When I was coating paper with gelatin for carbon transfer printing, I always added alum just before pouring. From one litre of gelatin solution, I'd pour out 100ml, and to this I'd add the required amount of alum. Mix well.

I am a rank novice here as well, but in my recent attempts to make dry plates, I have been adding a 5% chrome alum solution immediately before coating, at the rate of 1 drop per 5ml of emulsion.