Chrome alum can be used to harden both raw emulsions before coating and as a hardener during processing. This solution is quite toxic. Handle with care.
For coating, I make a 10% solution in water and add about 5 ml / 200 ml of 10% gelatin. The hardness is about the same over that range, but the length of time needed to achieve good hardness varies from 24 hours to about 96 hours.