Selenium forms silver selenosulfide (boiling selenium in sodium sulfite
creates a selenium sulfo/complex; sodium sulfide will dissolve selenium to
form a similar material). It's a true toning process, not a
displacement/plating mechanism. Silver selenosulfide is very stable, and is
not reactive with atmospheric gases, which are the main antagonists in
causing silver image deterioration.
Just a guess: the thiosulfate/thiocyanate would be added to prevent gelatin
stain; for alternate processes without the gelatin base, such use may be
redundant. In a private message to Dick Sullivan, I suggested that perhaps
tetrasodium EDTA would be a more modern ingredient for stain prevention.
This might be a great area for experimentation, and I would like to do it,
but unfortunately I'm neither set up to do so, nor do I have time at the
moment. Tempting, though!
Sil Horwitz, FPSA
Technical Editor, PSA Journal
silh@iag.net