Heated water works faster, or you could use a hair dryer set to hot (around 60C - 70C max ) the heat should destroy the gelatine emulsion. I mentioned distilled water because of the effects of water impurities water stains etc.... Formamide also known as methanamide is a gelatin solvent but i don't know what effects it will have on the varnish. This link might be interesting to you: http://albumen.conservation-us.org/l...ozlov1983.html

Good luck