I am doing B&W reversal processing and just tried using thiourea to develop sepia-toned slides. SInce I use a permanganate bleach, I'd like to harden the emulsion after the thiourea redevelopment and just before washing. A test clip dipped in Kodak Rapid Hardening Fixer darkened the film and led me to believe that I shouldn't drop my whole roll into this. Is Fixer a no-no for silver sulfide? If I wanted to mix a hardening solution without fixer in it, how would I do this? Potassium Alum? Chrome Alum? Can I use diluted acetic acid to acidify the solution? Any other ingredients? I'd like to avoid formalin unless that is really the best option. I'm trying to keep my darkroom limited to acutely toxic chemicals only ;-) Thanks!