Chrome alum is the preferred hardener for gelatin on glass regardless of preparation according to two experts, one of whom coated glass plates commercially.
Usually, you use either chrome alum or glyoxal at 10% in water, but remember that they both decompose over time. Also remember that glyoxal is nominally 40% in water so a 10% solution of that is actually 4%. I use the same liquid measure of either one, but they are both sensitive to other ingredients and I have had melts harden up instantly with both based on the pH and other ingredients present. And, due to the mix of ingredients and the expense, I was unable to identify the precise nature of the problem.
Maybe someday I'll revisit the issue, but for now, I know that Chrome Alum or Glyoxal at equal liquid measure at nominal 10% dilution works. There is more precise information on this in your class notes.