This post is a running record on how I have hardened gelatin for sized art paper. I generally coat my papers with a 10% gelatin solution using an RD-95 coating rod for a .25mm wet hight thickness. To my surprise, I have found coating papers and finding good hardener to be as challenging as making my own carbon tissue. I have currently settled on using formalin as it works wonderfully and is perfectly safe when used outdoors in low concentration.
Know to work well in all ways, but toxic. Formalin hardens perfectly and doesn't leave a cast in the paper or gelatin. For these reasons, I avoided using formalin for a long time, but finally decided to give it a try for my last batch of papers. Following recommendations from others I used a 2% solution of formalin 37. That's a 2% working dilution of a 37% stock solution. I hardened the papers in a tray outdoors and pre-soaked the papers before putting them in the formalin.
The combination of the dilute solution and soaking outdoors made left almost no odor. So, I felt confident that I wasn't putting myself at risk. I soaked each paper for about a minute and put them on a clothesline to dry. The formalin can be reused. I am not sure how to judge when it would be expended. So, there is little environmental impact.
The results were perfect. The gelatin was perfectly hardened and there was no change in the color of the paper. The process itself was also very easy. Combined with soaking and drying outdoors for safety, formalin will be my go to solution for hardening from now on.
It's far less toxic than formalin and hardens well, but does produce some yellow stain in the paper. Much of the stain washes out in development and clearing but not all. My method to date has been to put about four drops in 75ml of gelatin, and this works well. I would like to try brushing it on after coating and drying. This would minimize my exposure to it and keep my gear from getting as gummy while coating.
The key is knowing the difference between potassium alum and chrome alum. Potassium alum is relatively harmless. It's apparently the same stuff they put in the little pencils used to stop bleeding from shaving knicks.
However, it does not harden well and is difficult to wash out of paper. So, it can be good for hardening paper that will be used as a final support in a double transfer carbon print. However, when I tried to use it for a single transfer, it hardened the gelatin in the tissue during the mating to the support and ruined my prints.
Chrome alum apparently hardens well but is far more toxic. I have been told it can also leave a nasty blue cast to the paper.
Never used it, but given it's toxicity, I would probably just use formalin.