In my copy of Post Exposure, p. 160, Ctein argues that Sistan works as such:

"If soluble silver salts form in the emulsion, the potassium thiocyanate immediately reacts with them to form silver thiocyanate, which precipitates back onto the silver particle being attacked. This compound is insoluble, immobile, transparent, nearly insensitive to light, and resist oxidation."

So Sistan is reacting with silver salts in the emulsion, but what does it mean to say that they form?

Is it the case that after fixing, some of the solid silver in a print may be converted to a silver salt because of aerial pollution, or because of the effusions from the RC base, as he shows in the same chapter?