Formula for Hardener?

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!

DO NOT DO IT! Hardening the film in mid process is not part of the system. How did thiourea become the second developer?

Alessandro, thank you.

By what means does the thiourea harden the emulsion?

I did notice it was much more resilient than using a standard light exposure + redevelopment.

Also, do you have any experience in altering the tone of the sulfide by changing the alkalinity of the solution (i.e. varying the amount of sodium carbonate)?

Thanks!

Your thiourea toning positives process sounds interesting. As you work through the various parts, would you be amenable to posting some sort of summary of the steps - formulas, times, temperatures etc?

Thanks!

C