Use chrome alum in emulsions for glass coatings. For film coatings use an aldehyde hardener or use chrome alum but with a longer drying time before use. Chrome alum is slower to harden. The old writers note that emulsions can be stored in the cold with chrome alum for quite some time.
Use of Chrome Alum or Glyoxal are best done at the same quantity per unit of gelatin and so I use 10% Chrome alum or I dilute commercial 40% Glyoxal to 4% (I call it 10% Glyoxal) and use either at the rate of 5 ml / 120 grams of 10% gelatin. The Glyoxal will harden in 4 - 8 hours, but the Chrome Alum requires about 24 - 36 hours.