Copper Sulfate Reversal Bleach

Discussion in 'B&W: Film, Paper, Chemistry' started by htmlguru4242, Sep 28, 2005.

  1. htmlguru4242

    htmlguru4242 Member

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    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. Jordan

    Jordan Member

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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. Alessandro Serrao

    Alessandro Serrao Member

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    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. htmlguru4242

    htmlguru4242 Member

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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. Donald Qualls

    Donald Qualls Member

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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.
     
  6. gainer

    gainer Subscriber

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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.
     
  7. Donald Qualls

    Donald Qualls Member

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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...
     
  8. Jordan

    Jordan Member

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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. Photo Engineer

    Photo Engineer Subscriber

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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. fhovie

    fhovie Subscriber

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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.
     
  11. gainer

    gainer Subscriber

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    Edwal Single Solution Cleaner contains dichromate, sulfate, sulfuric acid and sulfamic acid. Q&D research shows that diluted 1+15 or more, it will bleach the silver out of fully exposed developed film in less than 2 minutes. The emulsion is not removed, and undeveloped or unexposed halide remains, but I could not redevelop the bleached silver, even in sulfite-free developer, so it would not be suitable for intensification.

    Of course the dichromate is still nasty, but perhaps not as much if you don't have to handle the powder. I have had this solution for quite a while, so don't know if it is still available.

    Ammonium persulfate is not very expensive if you buy it at an electronic supply house like Mousser's. It definitely removes copper, though it takes a while. Heating speeds it up, of course. It occurred to me that one of the copper toners would change the metallic silver image to copper, rendering it more easily removed by persulfate while leaving undeveloped silver halide.
     
  12. Photo Engineer

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    Patrick, a common misconception is that a copper toner makes a copper image. This is not so in the ones I'm familiar with.

    The common copper toner forms copper ferrocyanide, a copper colored salt of copper sulfate and potassium ferrocyanide which is formed in-situ as the silver image is bleached by the ferricyanide. Copper ferrocyanide is insoluable in water and precipitates as a copper (brown) colored image.

    PE
     
  13. gainer

    gainer Subscriber

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    I didn't have a misconception, I had a question. Do you know if whatever is produced by the red copper toner described as "Dassonville T-5 ifor Deep Brown to Red Chalk Tones in the Darkroom Cookbook" is soluble in ammonium persulfate solution? If metallic copper is soluble (not the proper description, I know) in the persulfate, perhaps the ferrocyanide is as well.

    I do know that the tray cleaner bleaches silver and that if the bleaching goes far enough, the silver will not be restored even by a sulfite-free developer, yet the undeveloped silver remains in some form that can be developed, after exposure to intense light, by any developer, preferably an active one such as Dektol.
     
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  15. Photo Engineer

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    Patrick, I don't have that formula so I cannot answer your question.

    I do know that the copper ferrocyanide is soluable in base to a certain degree, and you can clean your trays stained with copper toner, by washing them with alkali.

    I also know that tray cleaner forms silver sulfate, not silver halide and that is why it is not a rehalogenating bleach. Silver sulfate formed in this type of bleach does not react with developers or other reducing agents to form silver metal, and that is why permanganate and dichromate in sulfuric acid can be used in reversal B&W processes as bleach baths. They form a non-reactive silver salt.

    Ferricyanide with bromide forms silver bromide which can be developed again, and that is why it can be used as a rehalogenating bleach and it can also form the basis of a color intensifier for color processes.

    PE
     
  16. Jordan

    Jordan Member

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    Ron, is it that the silver sulfate is non-reactive or that it is water-soluble? I thought that it's all about solubility -- dichromate and permanganate convert metallic Ag to water-soluble AgSO4 which is removed from the emulsion, leaving AgX untouched.

    Would a bleach solution of dichromate bleach containing NaCl re-halogenate? (I'm assuming that KBr in dichromate is unstable with respect to oxidation of Br- by dichromate.)

    One interesting note related to Pat's post is that after bleaching film in the reversal process, the sensitivity of the remaining AgX in the film is somewhat reduced. The use of a clearing bath (sodium sulfite following a dichromate bleach, sodium metabisulfite following a permanganate bleach) restores the light-sensitivity somewhat, in addition to cleaning up the metallic reduction products. So says Grant Haist in "Modern Photographic Processing" in the chapter on reversal processing.
     
  17. Photo Engineer

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    Jordan, Silver Sulfate is 'slowly soluable in 125 parts of water' in my Merck Index.

    If that meets with your definition of being soluable, then it is ok by me. I call that not very soluable, as there would be enough silver sulfate left behind to develop in the reversal step causing imaging problems if silver sulfate could develop.

    The bisulfite acts as a mild silver halide solvent, cleaning off surface silver sulfate on the silver halide that remains. It acts as a mild grain scrubber. It dissolves the silver sulfate faster than it dissolves any silver halide due to the differences in pKsps.

    Another reaction is that the sulfite scavenges any oxidizer present, such as left over dichromate or the like, thereby preventing stains from oxidation taking place. The use of clearing baths after bleach baths or before developers for scavenging oxidizing agents is quite common.

    The sulfite 'clearing bath' is a multipurpose bath used in both color and B&W processes, and at the present time is the most common use of sulfites. It is found in the blixes and bleaches of color processes, and is present to react with carried over color developer to prevent stains.

    Yes, sensitivity to development is increased in the presence of any 'grain scrubber' and even bleach sensitivity is increased by helping soluabilize silver sulfate that forms around silver metal. It can be used to give a last 'kick' to the bleaching process using the solvent properties of sulfite and carried over bleach to 'zap' the last little traces of silver metal. I've actually watched that happen in the lab under controlled conditions.

    If you go directly from a dichromate bleach into a clearing bath, and there is just a tiny bit of silver left in the highlights of your reversal image, you can see the cloudiness vanish instantly in the clearing bath. I think that is why they named it 'clearing bath' because under the right conditions you can see the highlights clear in the bath. Well, that and the prevention of stains which leave your highlights clear of any discoloration.

    PE
     
  18. Ryuji

    Ryuji Member

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    In presence of Cl- oxidized silver is likely to form AgCl(s) but if far excess NaCl is present, AgCl may redissolve by forming more soluble complex. Same happens with bromide.

    It is not surprising to see sensitivity drop of unexposed, undeveloped, original AgX crystals, after bleach. This is because sensitivity centers (for example, Ag0 specks) are removed and the efficiency of latent image center formation upon light exposure is greatly compromised. Also, sensitizing dye may be desorbed or destroyed by the bleach bath.

    If one is to ensure thorough development in the second developer, the best is to fog grains chemically. Hydrazine derivatives, stannous chloride, dimethylamine borane, etc. can be used. Indeed, any strong reducing agent that can inject electrons to the conduction band of AgX would be good in principle, but these are the fogging agents that are used often. Dimethylamine borane is probably best but it is toxic.
     
  19. Jordan

    Jordan Member

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    Very interesting stuff, Ron. I thought the silver sulfate was much more soluble than that. I need to re-read the chapter in Haist's book, since I don't remember those details being in there (they probably are, though). I occasionally clear my film in a clear container and yes, the clearing action is very dramatic (though I think what I am seeing is mostly the elimination of reduction products, e.g. MnO2 in the case of the permanganate bleach, etc.)

    Ryuji -- E6 processing uses stannous chloride for this step, right? At least that is what I seem to recall. For ordinary B&W reversal I usually "fog" with an alkaline solution of thiourea. The chemistry is not the same but it is still very consistent. Thiourea is toxic, but easier to handle than hydrazine or dimethylamine-borane. The resulting slides have a pleasing tone (with conventional films -- when I tried TMX the results were sickly-yellow, probably due to the different grain structure).
     
  20. Photo Engineer

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    Jordan, Stannous chloride and TBAB (t-butyl amine borane) and the like, work by reducing siver halide to silver metal specks much like latent image formaton and rendering the grain developable. Thiourea and any silver halide solvent works by dissolution of part of a grain (at least to some small extent) along with reduction sensitization. (R-typing in a broad sense).

    Therefore, use of a silver halide solvent (even a weak one) can possibly lead to degradation of the image as some tiny bit of the smallest details can be dissolved.

    It is unfortunately not a good idea to use thiourea, even though it works. It can come back and bite you in loss of fine detail. If it works, that is just great, but I believe that there are situations where it would not do as good a job.

    Stannous chloride in dilute organic acid is quite good. Try the E6 reversal bath.

    PE
     
  21. Jordan

    Jordan Member

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    Ron, I'm not using the thiourea for reduction sensitization. I'm using it in alkaline solution to directly convert the remaining AgX to Ag2S. AFAIK this is extremely rapid and the resulting Ag2S positive image is very stable. It is the final step in my reversal process -- it's just like the toning step in an "odourless sepia toner" kit, only on paper, not film.
     
  22. Jordan

    Jordan Member

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    Of course, by that I mean "only on film, not paper".
     
  23. Photo Engineer

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    Jordan, I got it this time. I understand. Thanks.

    PE
     
  24. gainer

    gainer Subscriber

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    There is a household product that does this fogging. It's trade name is "Iron Out" IIRC. It may in fact be mostly thiourea. I think another name is sidium hyposulfite or hydrosulfite. I'll have to look on the bottle. It definitely blackens film, exposed or not.

    Perversion of this kind is fun.
     
  25. Ryuji

    Ryuji Member

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    Alkaline thiourea solution to convert AgX to Ag2S is also a usable technique. One problem is that, because this conversion (toning) process does not involve regular chemical development process, the image forming silver will take the form of sphare-like or tablet-like shapes instead of fine filaments forming steel wool grains. One potential problem here is that, when the grain morphology is sphare-like, the covering power of the grain will be much lower. That is, you may get a lot lower density with the same amount of silver. Also, when the tabular grain has very high aspect ratio, you'll get yellow image. This is probably what you get with TMX. According to a tabular grain expert in the industry (he's now in academia), in tabular grain emulsions, image hue is mostly determined by the grain thickness, not the diameter. Tabular grain emulsions tend to preserve grain shape when developed chemically, and this presents a serious problem when they use tabular grain technology to make document films, because users prefer black images. (Ironically, one of the first t-grain emulsion product was Kodak's microfilm, but at that time, grain wasn't as thin as today's emulsions.) So I can understand that TMX forming yellow image. For document films as well as slow medical films, there are numerous patents that deal with the issue of ensuring good blue-black image with very thin tabular grain emulsions. However, the mechanism of how those work is not understood. (We have ideas of what kind of compounds work, though.)

    Another approach is to use a weak solution of alkaline thiourea or any other suitable labile sulfur compound to make Ag2S specks on grains (do a partial conversion) thereby fogging them, and then transfer those fogged films to the second developer bath. By doing this, you are making only small specks of Ag2S on grains, instead of converting the whole grain, but this is sufficient to lower the energy level for the conduction band of the AgX crystal in the neighborhood of the Ag2S speck, where developer can inject electrons, thereby rendering the grain developable in the second developer. This is conceptually similar to purposely overdoing the sulfur sensitization in emulsion making. Similarly, you could do the same with a noble metal toning bath. Au0 specks, for example, does the same to the energy level of the conduction band.

    This is getting more academic, but there are dyes that fog emulsions as well. (a lot more expensive approach than other options here.)

    In commercial reversal processing, using reducing agents that are a lot stronger than hydroquinone or ascorbate is easier because they can be mixed in a single bath to minimize the number of processing steps. But in a hobbist darkroom, stannous chloride, hydrazine derivatives and DMB are probably more pain in the azz because they are really easily oxidized by air. So thiourea or any suitable labile middle chalcogen compounds could be used in a separate bath prior to second developer. There are a large class of organic selenium compounds useful for this purpose as well (though, again, very toxic).

    I've also experimented by mixing a small amount of thiourea in developer. It surely fogs the emulsion and develop them, but I had a bit harder time in obtaining uniform results. I didn't pursue that direction to figure out why.
     
  26. gainer

    gainer Subscriber

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    I wasn't trying to trick you up. I'm sure you would have known that toner if I had presented the ingredients. It is one of several that have copper sulfate, potassium chloride, potassium ferricyanide and potassium citrate and should as you say produce a copper compound. I wasn't surprised that the sensitivity was much reduced. That used to happen with Anscochrome, which is the first one I developed. I remember exposing the film to a very hot photoflood for several minutes before the second developer. If your tank did not have translucent reels, you had to remove the film.

    It slipped my mind that chromium intensifier is acidified with hydrocloric acid, whereas the tray cleaner uses sulfuric. Chromium intensifier would not be such a good bleach for reversal, although with a strong enough sulfite bath and enough time, It might work.

    I have used the ferricyanide-bromide or chloride bleach to rehalogenate for redevelopment by pyro. It was one of the things I described in my article "More Pyrotechnics" in PT magazine.