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  1. #11
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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.
    Gadget Gainer

  2. #12
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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

  3. #13
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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.
    Gadget Gainer

  4. #14
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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

  5. #15

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    Quote Originally Posted by Photo Engineer
    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.
    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.

  6. #16
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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

  7. #17

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    Quote Originally Posted by Jordan
    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.
    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.

  8. #18

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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).

  9. #19
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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

  10. #20

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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.

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