There's plenty of literature about this, E.R. Brumpton and G.I.P. Levenson of Kodak Ltd (Harrow Research) published a paper on it. I don't have my copy of L.A. MAson's Photographic Processing Chemistry handy but there may well be a reference the paper i the book (Levenson was the editor and also contributed, Mason was head of research at Ilford).
I have read the article suggested (The Staining Properties of Ammonium Thiosulphate Fixers, E.R. Brumpton & G.I.P. Levenson, published in The Journal of Photographic Science, 1965) with great interest. The article is not publically available, therefore I will not put in on-line here. However the abstract available to the general public:
http://www.tandfonline.com/doi/abs/10.1080/00223638.1965.11737305
The article deals with the silver stains that are caused by ammonium fixer on cloth
. It was shown there that trouble was encountered when the ammonium fixer contained more than 10 g/1 of silver…and
The severe trouble was caused by contamination of cotton overalls with drops from freshly-fixed films, because the e contained a very high silver concentration, equivalent to 20-30 g/1.
These concentrations are quite irrelevant in practice because the silver concentration in the fixer should be kept below 2 g/l or even 0.5 g/l with papers (with films it can indeed go up to 10 g/l)
Two questions were posed by the authors:
(1) In what way does the behaviour of the ammonium ion differ from that of the alkali metal to induce more rapid decomposition? (2) What is the role of the organic substratum (i.e. cloth, etc.) in the generation of the dark stain ?
A series of qualitative experiments have been conducted to answer these.
Now fast forward to the Conclusion part of the article:
It seems quite clear, however, that the tendency of silverbearing ammonium-thiosulphate fixers to give stains stems from the loss of ammonia which leads to a drop in pH and to the degradation of sulphite and thiosulphate, with the eventual formation of finely divided sulphur: and it is suggested that the increase in the concentration of uncomplexed silver ion, resulting from the fall in the concentration of thiosulphate, allows a reaction between the silver ion and the organic substratum to give metallic silver which reacts with the sulphur to form silver sulphide /…/ and the conclusion from this is to keep the silver/thiosulphate ratio low in the exhausted fixer as was shown earlier.
I seems that following even the most loose criteria of capacity, reaching such a high level of silver concentration is practically impossible, but one never knows – I had been told by Mirko of Fotoimpex that people were complaining about the stains and upon researching the cause of this it has been revealed that they have been using the same fixer over and over again for more than 12 months waiting for the fixer to tell them it’s exhausted! For such practitioners a hypo might be a better option; their paper would end up being under-fixed but with a lesser chance of staining…
Thanks Ian.