What does KBr do in a bleach formula?

Discussion in 'B&W: Film, Paper, Chemistry' started by PVia, Apr 4, 2008.

  1. PVia

    PVia Member

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    I've seen formulas for ferricyanide bleaching with and without potassium bromide. What exactly does the addition of KBr provide to the bleach solution?

    And what is your favorite formula and method of bleaching?

    Thanks!
     
  2. eumenius

    eumenius Member

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    Bromide is needed for rehalogenating the metallic silver - to silver bromide.
    I use an old ORWO recipe - but virtually any ferricyanide bleach will also do fine :smile:

    Zhenya
     
  3. dancqu

    dancqu Member

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    It is needed to convert silver ferrOcyanide to silver bromide.
    As Mr. Rudman has pointed out a bromide, chloride, or iodide
    can be used with or after the treatment in ferrIcyanide. And
    I'll add; who knows what else? Dan
     
  4. MarkL

    MarkL Member

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    What happens if just a plain pot ferri/water solution is used? The image still bleaches and has to be redeveloped right?
     
  5. tim rudman

    tim rudman Member

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    Hi PVia,
    In a nutshell;
    When you buy your paper the emulsion contains silver combined with a 'halide' - that is a bromide (silver bromide), a chloride or iodide, or commonly a mix of these 'silver halides'. After exposure the developer converts the exposed silver to silver metal (black) and the fix then removes unused silver halides. You now have a black (& grey) image.
    Pot.ferricyanide acts on this metallic silver to convert to silver ferrocyanide - near colourless (straw colour ish). But if a halide (e.g. bromide) is included, it 'rehalogenates' the silver back into a silver halide - i.e. back where you started in the first place, except that it is now only in the image area (the fixer removed it from elsewhere) and it is more stable in room light than the original unexposed sheet was.
    Being a silver halide this can now be redeveloped.
    This might be in a toner (eg sepia) or in another developer of higher or lower contrst, warmer or cooler tone, lith, and so on.
    It can be pretty useful.
    Tim
     
  6. gainer

    gainer Subscriber

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    Except that just redeveloping in a "high contrast" developer cannot change the contrast because the amount of image-forming silver has not increased.
     
  7. tim rudman

    tim rudman Member

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    Well. it depends on the bleach too. using a dichromate bleach followed by a high contrast developer was a favourite process of mine for some time. The result is intensified blacks with higher contrast to the eye and altered toning properties.
    Can be very effective with a suitable paper.
    Tim
     
  8. Photo Engineer

    Photo Engineer Subscriber

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    You have to be careful with a dichromate bleach with sulfuric acid though. It forms silver sulfate which dissolves in water and therefore removes highlight detail or even the whole image. The Ferricyanide bleaches raise contrast by the same general method, removing highlight detail. This is because a ferricyanide - bromide bleach is a mild blix.

    Be careful that there is no ammonium salts in any of these, or you turn them into blixes which of course, destroys the image, at least in part.

    PE
     
  9. CBG

    CBG Member

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    Can you elaborate? I would have got it all wrong, and thought bleaching and redeveloping would have kept the same contrast or lowered it. I'm missing something.

    C
     
  10. tim rudman

    tim rudman Member

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    I agree. I use hydrocloric acid

    Tim
     
  11. tim rudman

    tim rudman Member

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    Not necessarily. Both the redeveloper and the bleach bring properies to the table. Consider for example FSA using different bleaches based on bromide, chloride or iodide. Redevelops to colloidal silver. Not only does the colour change to a wide range of options, but the print density can be increased by up to a couple of stops and contrast can change significantly. Many redeveloping toners shift contrast using other metals, iron blue for example, perhaps the best know contrast increasing, but others have their own effects.
    Tim