Oren Grad said:
Question: Can you cite any empirical evidence of Sistan failing because of humidity cycles? Can you cite specific examples of prints being treated with Sistan, being put through humidity cycles (not being dunked in water, which we know will wash it out), and then silvering out? Or are you simply speculating based on what we know of its mechanism of action? Nothing wrong with the latter - it may be all we have to go on - but if there is evidence I'd like to know about it.
I almost missed this one.
There is no good published test results using Sistan at ANY condition, as far as I know. People at IPI and possibly ENS Louis-Lumiere tested it and there are casual communications reporting their experiences. There is also a report authored by Fuji researcher who investigated thiocyanate (not the product) in this context in 1980s. There aren't much more on this subject.
So I've tested it. Humidity cycle is time consuming and labor intensive without a computer controlled, automated setup, and I'm yet to run an exact test for this.
But as you know thiocyanate is a hygroscopic compound and it is VERY soluble in a small amount of water.
The range of thiocyanate concentration for it to be very effective is rather narrow. If you use too much or too little, you're in trouble. In particular, if the concentration is higher than the optimal value, it's actually worse than no treatment.
Now, if you treat an absorbent paper with a soluble salt and dry. The concentration may be relatively uniform. But as you repeat humidity cycle, the concentration at the center of the sheet will be lower and the concentration near the edges will be higher. This is the problem.
Similar problem occurs in spray treatment of acid paper to make alkaline. Well, water-based approach is only one method and may not be the best option, but if you use soluble carbonate (like sodium carbonate) the same problem as above occurs with humidity cycles. So in reality calcium carbonate, which is only sparingly soluble, is used, to prevent migration during humidity cycles.
So, for final bath treatment like Sistan or Ag Guard, I'd much prefer compounds that are not very water soluble but highly argentophilic, being adsorbed on metallic silver surface with forming a barrier of dense assembly of the molecules, rather than a highly soluble compound floating between gelatin or cellulose molecules. Ag Guard is former, Sistan is latter.