Alternative B&W Bleaching Formulas/Methods

Discussion in 'B&W: Film, Paper, Chemistry' started by Athiril, May 6, 2013.

  1. Athiril

    Athiril Subscriber

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    Been working on this for a little while.

    Dichromate bleach is excellent and reliable. Of course people have concerns with it.

    Permanganate bleach is unreliable and damages film, but is much safer for you to handle. And as it reduces, MnO2 precipitates onto the surface of the film, and also inside of the gelatin. Emulsion can peel off as well.


    Anyway, one example, which worked well for me, ended up with a dMax of 2.12 of unexposed area (old base fogged film, so it's pretty good), was potassium permanganate (enough to get that 'opaque' look in solution) in 250g/L of citric acid.

    Silver Citrate isn't soluble in water, but it is in a good concentration of citric acid. Citric acid also keeps MnO2 from building up on or in the film. And it takes the edge of the permanganate bleach, so that it can be run at elevated temperatures and long times without damage.

    It is however one shot, as the citric acid slowly reduces the permanganate and eventually clears it completely.

    The time it took me to bleach a film with this was approx 16 minutes or so. I added some more potassium permanganate solution to it near the end (just a little) to finish it off.




    The other thing I have done is copper sulphate with sodium chloride. Followed by a 300g/L Ammonium Chloride bath to dissolve the silver chloride selectively. Bleaching was done quickly, but the clearing time in the ammonium chloride I used was 30 minutes, but it becomes hard to tell when to finish. Think I also had the absolute thinnest neg left over stacked on top of the positive, basically unnoticeable.

    But some of the silver bromide was also dissolved. dMax on the same film and developer process with this ended up being just 1.52 down from 2.12, but otherwise did work.
     
  2. Gerald C Koch

    Gerald C Koch Member

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    Manganese dioxide should not form if the bleach is used properly. It is important that the bleach is acidic and then the permanganate will be reduced all the way to Mn+3.

    The only bleach that I have had trouble with is the copper one which caused severe softening and frilling of the emulsion
     
  3. Athiril

    Athiril Subscriber

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    This happened in another test I was doing, it was certainly acidic, but it wasn't sulphuric acid, it was acetic acid.

    Haven't had that, have only ever had trouble with permanganate + bisulphate and sulphuric acid bleaches. I haven't tried the CuSO4 + H2SO4, as that seems unlikely to work.
     
  4. Gerald C Koch

    Gerald C Koch Member

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    When bleaching is stopped at the manganese dioxide point half the bleaching capacity of permanganate is lost.
     
  5. Athiril

    Athiril Subscriber

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    The citric acid on it's own will reduce permanganate over time, it's not fast like sodium sulphite for example, but over the course of an hour or so.
     
  6. Gerald C Koch

    Gerald C Koch Member

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    You really must use sulfuric acid in the permanganate bleach. Acetic acid is not strong enough.
     
  7. Alessandro Serrao

    Alessandro Serrao Member

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    Never had a problem whatsoever when I began using permanganic acid at 18°C
     
  8. Rudeofus

    Rudeofus Subscriber

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    How about Sulfamic Acid? It is so much easier to handle than Sulfuric Acid ...
     
  9. stefan4u

    stefan4u Member

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    Dear Athiril

    Well, at least the stability of the permanganate bleach can be improved easily.
    Add about 20g Calgon per liter and the result should be much better.

    Look for this agfa link for further explanation. www.google.com/patents/EP1006408A1?cl=en


    The final formulation will look this way. The text is quite promising, but I must admit that I haven’t used it till now…

    Water 700 ml
    M 19 (Natriumkaliumhexametaphosphat) 20 g
    Potassium permanganate 2 g
    Sulfuric acid (20 wt -%) 27 ml

    Fill with water to 1000 ml (pH = 1.4)


    In this movie forum is roughly the same formulation, just slightly different sulfuric acid concentration.
    http://movies.groups.yahoo.com/group/movieprocessing/message/891

    Regards Stefan
     
  10. Hexavalent

    Hexavalent Subscriber

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    Sulfamic acid will merrily dissolve silver; it might just do a little more bleaching than desired.
     
  11. Gerald C Koch

    Gerald C Koch Member

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    Since sulfuric acid is used all over the world as battery electrolyte it is probably the easiest of all the strong acids to obtain. So why worry about substitutes.
     
  12. Alessandro Serrao

    Alessandro Serrao Member

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    A substitute for sulfuric acid is pH- used for pools. It's sodium bisulfate. Perfect! Use 55g/l.
    Don't use a chelating agent as water softner in the bleach, it would chelate the permanganate rendering it useless...
     
  13. Rudeofus

    Rudeofus Subscriber

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    AFAIK Sulfamic Acid is used in Ilfochrome dye bleaches to get the low pH needed, so it must be compatible with Silver somehow...

    I am not worried about availability of Sulfuric Acid, but it's a dangerous liquid in the hand of imbeciles like me. Sulfamic Acid is cheap, easy to obtain and so much simpler to measure and use.

    I like the idea ...

    Look at the patent Stefan linked to. Most chelating agents will get oxidized by Permanganate, but evidently Polyphosphates don't, so they can be used to great advantage.