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  1. #1
    htmlguru4242's Avatar
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    Copper Sulfate Reversal Bleach

    I'm working on my Autochrome project, but I realize that I have not yet figured out the reversal processing part of the development procedure. I hav done it a few times, but I'm not entireley familiar with production of the chemicals, which is what I want to do here (I'm not going to spend a lot on the commercial reversal chemicals).

    Does anyone have a working formula for a copper sulfate reversal bleach (copper sulfate is the easiest to obtain out of the reversal bleach chemicals)?

  2. #2

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    There was a thread here about a month ago on the topic of CuSO4 bleaches. A poster found a recipe for a reversal bleach based on CuSO4 on the Web and found that it completely failed. The only CuSO4 bleach recipe I have found came from "Modern Photographic Processing" by Haist where a solution of acidic CuSO4 and hydrogen peroxide selectively strips Ag-rich emulsion from lith film.

    I don't know what your project entails, but potassium permanganate and potassium dichromate are much more reliable as bleaches for B&W, with permanganate being the less toxic but dichromate working more quickly and with far less emulsion damage.

  3. #3

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    Quote Originally Posted by htmlguru4242
    I'm working on my Autochrome project, but I realize that I have not yet figured out the reversal processing part of the development procedure. I hav done it a few times, but I'm not entireley familiar with production of the chemicals, which is what I want to do here (I'm not going to spend a lot on the commercial reversal chemicals).

    Does anyone have a working formula for a copper sulfate reversal bleach (copper sulfate is the easiest to obtain out of the reversal bleach chemicals)?
    Jordan, that's me (the poster) and...
    ...yes I found the Copper sulfate bleach completely a failure on my part.
    I would recommend the permanganate based bleach or, if you dare, even the dichromate based bleach (more toxic - take care of yourself).
    Which one of the two you decide, based on quantities, costs, toxicity and prompt availability.

    I can say that dichromate is the most reliable in terms of negative integrity, second comes the permanganate but I think, if all solutions are kept at THE EXACT TEMPERATURE, avoiding even the slightest drift, and if the films is GENTLY agitated in the permanganate (even better would be a dip-and-dunk style of agitation), even the latter would do.

    The drawback with the permanganate is that it softens the gelatine more than the dichromate, which tends to tan it.
    Maybe a hardener pre-bath can help.

  4. #4
    htmlguru4242's Avatar
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    Dichromate (from B&H or Photo. Formulary) honestly doesn't seem to be that expensive, and I'm aware of the safetly precautions that must be followed. However, I am still unfamiliar with the quantities. Would I be correct in assuming that its a weak (maybe 1 - 5 grams per liter) aqueous solution of dichromate with a slight amount of soemthing to make the bleach acidic? Does anyone have specific proportions?

  5. #5
    Donald Qualls's Avatar
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    I've been suggesting copper sulfate bleach, because copper sulfate is cheaper and easier to obtain than either potassium permanganate or potassium dichromate, and less toxic than either; I saw the reference to it in a page I found on B&W reversal processing. Unfortunately, I'm not finding it again now; Google is giving too many irrelevant hits and not showing what I want to see.

    The copper sulfate-peroxide mixture is not a proper bleach; it's a bleach-etch, suitable mainly for mordancage type processes; it will destroy or greatly soften the gelatin wherever there is developed silver present. That is not the intent for a reversal bleach, and is primarily an effect of the peroxide. Copper sulfate in a sulfuric acid or sodium bisulfate solution is a true bleach, which I believe originally came from holography. Worth noting that holographers do NOT want their emulsion eaten away; rather, it needs to shrink or expand in proportion to density in order to induce phase change in the viewing light.

    I might note that while searching for the original page where I saw this formula, I found references from Kodak about a ferricyanide/persulfate reversal bleach, but I don't know if this is usable for B&W or if it rehalogenates like C-41 and E-6 bleaches. I don't see how a plain ferricyanide bleach could rehalogenate in the absence of halogen ions in solution, yet Farmer's Reducer seems to do just that; perhaps the persulfate would convert the silver to soluble sulfate instead of an insoluble halide.

    I did find a formula for a rehalogenating copper bleach, pretty much like permanganate bleach with KBr added (and leaving out the KBr should give a non-halogenating bleach), including the sulfuric acid. If you have a permanganate formula, use the same amount of sulfuric acid, and subsitute the same number of moles of sulfate in the form of copper sulfate, as there were of pemanganate in the form of potassium permanganate.
    Photography has always fascinated me -- as a child, simply for the magic of capturing an image onto glossy paper with a little box, but as an adult because of the unique juxtaposition of science and art -- the physics of optics, the mechanics of the camera, the chemistry of film and developer, alongside the art in seeing, composing, exposing, processing and printing.

  6. #6
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    I haven't tried any of this. I have made etched printed circuit boards using ammonium persulfate to etch away the copper where there is no resist. Perfulfate is also used in a reducer for silver images. I don't remember if that reducer also uses ferricyanide. The bleach of chromium intensifier converts silver into a compound that can be dissolved in a sulfite solution. That is the reason for the admonition to use a low sulfite developer when intensifying. I'd better quit. I don't know what I'm talking about.
    Gadget Gainer

  7. #7
    Donald Qualls's Avatar
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    Am I correct in thinking I can make a persulfate solution by combining a sulfate with hydrogen peroxide, similar to the perborate post-sensitizer sometimes used for extreme film pushing? If so, it might be pretty simple to make a ferricyanide/persulfate bleach (though obviously the solution shouldn't be significantly acidic). Ammmonium persulfate is expensive, but sodium sulfate is cheap, as is potassium ferricyanide, and 3% hydrogen peroxide is about $2 a quart at the drug store...
    Photography has always fascinated me -- as a child, simply for the magic of capturing an image onto glossy paper with a little box, but as an adult because of the unique juxtaposition of science and art -- the physics of optics, the mechanics of the camera, the chemistry of film and developer, alongside the art in seeing, composing, exposing, processing and printing.

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    Donald, you're right that the copper-peroxide bleach system is really a "bleach-etch". I have seen the page on reversal processing quoting a CuSO4 bleach, probably the same one you're referring to... IIRC it was a page describing reversal processing of HIE.

    Alessandro (yes, I knew it was you) tried the bleach from that page and it failed. I agree with Donald that a bleach based on CuSO4 would be cheaper and less toxic than one based on Cr or Mn. But I think (though this may be a discouraging thought contrary to the spirit of APUG) that if it did work well and reliably, it would be used and cited all over the place, and it is not.

    There was a patent filed by Agfa (I think someone here on APUG pointed me to it) describing work to make a reversal bleach almost catalytic in Cr or Mn by using persulfate as an added oxidant. They tried a whole bunch of conditions, many of which involved the use of a permanganate bleach with persulfate and a catalytic amount of AgNO3. I had no idea what for at the time, but I just checked my "Mellor's Inorganic Chemistry" and found that this reaction does have a name, and that in the presence of AgNO3, persulfate oxidizes MnO2 (the Mn product of permanganate bleaching) back to permanganate. This is a cool way of making the Mn the "catalytic" oxidant and persulfate the "stoichiometric" oxidant. Neat stuff.

    Persulfate on its own won't oxidize silver, AFAIK. Mellor and the Merck Index both say that persulfate salts are generated by electrolysis of cold, mildly acidic solutions of the corresponding sulfate salts (do not try this at home). Under these conditions, adventitious peroxide is probably generated electrochemically. There is probably a surface reaction going here.

    I was very hesitant to use dichromate for years. It is certainly much more toxic than permanganate, but because it works so quickly and cleanly, I almost find it safer to use than permanganate -- for small-tank reversal processing I don't even really have to invert the tank at all, unlike for permanganate (therefore less spillage), and the acidic solution of dichromate is stable, unlike acidic solutions of permanganate (therefore less handling of the solution each time). Dichromate bleaches also partially harden the emulsion, as Alessandro says, and I saw very clear improvements in overall film quality on going from permanganate to dichromate bleach.

    My formula for dichromate bleach is 65 g/L sodium bisulfate (available as "pH Minus" swimming pool additive -- purists may scoff but it works for me) and 10 g/L potassium dichromate. Handle both with great care.

    I have some reversal chemistry notes at http://www.photosensitive.ca/BWslide-formula.shtml.

  9. #9
    Photo Engineer's Avatar
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    I posted the Dufay reversal bleach in the other thread on Autochromes.

    Work with copper bleaches for a number of purposes failed to perform properly in any of the work we did at EK. The only satisfactory bleaches were Dichromate, Permanganate, Ferricyanide, Ferric EDTA (and analog - homologues) and similar type chemistry. This was either rehal or non rehal types.

    Copper and even other ferric salts failed miserably and organic bleaching agents were not much better.

    Peroxide and persulfate are known and Kodak has published a formula for a rehal reversal bleach using them IIRC.

    PE

  10. #10
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    Bleaching with Copper Sulphate is part of the Mordancage process - the mix would have to be adjusted so as not to etch. I have used it to make Mardancage but not for simple bleaching.
    My photos are always without all that distracting color ...

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