I took up wetplate collodion a few months ago and have recently run into some contrast problems with it. Part of these problems might be resolved by using potassium cyanide as the fixing agent but I am hesitant to use that chemical because of its toxicity and issues of storage. The historical alternative has been sodium thiosulphate in about a 20% solution.

I'm wondering if ammonium thiosulphate might be substituted for the regular hypo without ill effect. This question has been posed on other (wetplate) forums and the response by practitioners is generally: "Why take a chance? Use the traditional method..." I'm wondering if the rapid fixing agent might produce a brighter, higher contrast image more like the effect of the KCN. I can of course test the chemical fixing potential by running a plate through it, but I'm wondering about issues of longevity, stability, etc.

AFAIK, the wetplate emulsion is a combination of silver iodide and silver bromide salts formed by the reaction of a silver nitrate bath on cadmium, potassium, and ammonium iodides and bromides in the "iodized collodion." Anything here that might be problematic (w.r.t. color, archivalness, tonality, etc.,) if ammonium thiosulphate was substituted as the fixing bath?

Any insight appreciated.