How does Sistan work?

Discussion in 'B&W: Film, Paper, Chemistry' started by AgX, Oct 3, 2007.

  1. AgX

    AgX Member

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    To my understanding the main ingredient of Sistan® is Potassiumthiocyanate.
    From this I conclude that it works as a scavenger for Ag+ ions:


    Ag (oxidized within time) > Ag+ > AgSCN [low solubility] + K+ (Sistan-action)


    Is this true?
    And… is this the whole story?
     
  2. Photo Engineer

    Photo Engineer Subscriber

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    No, and no. There are a few articles on it, but it apparently seems to work the same way that leaving a very tiny amount of hypo behind works. This is covered by the Ctein article I have referred to before.

    Both hypo and thiocyanate decompose to sulfur and form sulver sulfides.

    PE
     
  3. Michel Hardy-Vallée

    Michel Hardy-Vallée Membership Council Council

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    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?
     
  4. Photo Engineer

    Photo Engineer Subscriber

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    Well, it is known that thiocyanate forms silver sulfide under the correct condition. This was once used to sulfur sensitize emulsions. I have not seen any good analytical information on this though.

    Formation of silver sulfide is like sulfur toning in that sense then if this is what is going on. Every time silver and thiocyanate interact, silver sulfide forms, gradually forming a black speck of silver sulfide and protecting the rest of the image.

    PE
     
  5. Rich Ullsmith

    Rich Ullsmith Subscriber

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    So, if I wanted to stabilize a print, or sulfur tone it in this manner, what would be a reasonable concentration of K thiocyanate? 5g/l? I can't imagine that a print processed with fresh fix, particularly an alkaline fixer, along with a hypoclear bath and adequate wash . . .how much soluble silver can remain to stabilize?
     
  6. Photo Engineer

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    Rich;

    You are talking about 2 things, stabilization and toning. These are different.

    For sulfur toning follow the instructions on sulfur toning and for use of Sistan follow those instructions. The concentration and formulation of the sulfur compounds are very different, and the results are quite different.

    After fixation, no soluable silver should remain. The formation of oxidized silver compounds is one of the reactions leading to image degradation.

    PE
     
  7. AgX

    AgX Member

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    This all is very interesting. The theory PE refers to would explain why the action of Sistan is often referred to as an `colourless toning´ with those tiny amounts of silversulfide in mind.
    And mhv’s post reflects what I thought of. (Perhaps I should have put it clear that I thought the Ag+ to came from a newly formed salt.)

    Perhaps both reactions can take place…
     
  8. gainer

    gainer Subscriber

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    IIRC, the main ingredient used in so-called stabilization processing was a thiocyanate. I used to use a lot of it when I was in a hurry. I had a stabilization processor with which I was able to do a lot of prints quickly. In most cases, I did this to allow batch fixing and washing of the whole bunch. Once in a while, I let one sit without fixing and washing. After a while, the appearancs was somewhat like sulfide toning, but not good.

    That was, I have heard, a lot different from proper fixing and washing followed by thiocyanate treatment for protection. I don't do it any more anyway. Nowadays, I no longer photograph guest artists at Symphony dress rehearsals and come back the next day to play the concert and present the finished pictures, usually working a full day at Langley Research Center in the meantime.
     
  9. Rich Ullsmith

    Rich Ullsmith Subscriber

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    Do you by chance recall a ballpark time and dilution for stabilizing with K thiocyanate?
     
  10. dancqu

    dancqu Member

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    The IPI should chime in now and then. This Sistan discussion
    reminds me of one of their findings. Sodium sulfide at a 1:9,999
    dilution was found to give microfilm complete protection without
    'toning'. In effect Sistan may be no more than a very expensive
    drop of sodium sulfide. Dan
     
  11. Photo Engineer

    Photo Engineer Subscriber

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    Stabilization of the image to destruction involves one method or mechanism, whereas stabilziation of the residual silver halide in a piece of paper or film from printout with light is another matter.

    Which type of stabilization must be defined first.

    Stabilization of the image to deterioration requires a low concentration. This is what Sistan uses.

    Stabilization to printout requires a very high concentration. These processes are no longer in wide use and the print is normally not washed afterwards so both silver metal and silver halide remain behind along with thiocyanate.

    The former uses grams or milligrams per liter, the latter uses hundreds of grams / liter.

    PE