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  1. #21

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    Quote Originally Posted by Jordan
    it's just like the toning step in an "odourless sepia toner" kit, only on paper, not film.
    Of course, by that I mean "only on film, not paper".

  2. #22
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    Jordan, I got it this time. I understand. Thanks.

    PE

  3. #23
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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.
    Gadget Gainer

  4. #24

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    Quote Originally Posted by Jordan
    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).
    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.

  5. #25
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    Quote Originally Posted by Photo Engineer
    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
    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.
    Gadget Gainer

  6. #26

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    Quote Originally Posted by Ryuji
    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.
    Mmmmmm, this is getting VERY interesting Ryuji, I think I'm going to try it this weekend on a scrap of T-Max 100.
    I'll report asap.

  7. #27

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    Sodium hydrosulfite is very different from thiourea... I think there's a formula for a "fogging agent" based on the hydrosulfite in Grant Haist's book.

    The "sepia toner" method works well for me. I do not see any visible problems with reduced density... in fact, the dark tones are qualitatively "deeper" with thiourea toning than with re-exposure and re-development, probably because the latter does not go to completion. I pretty much use only Pan F and FP4 for reversal, so the disgusting T-grain tone doesn't bother me.

  8. #28

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    Kodak has published two fogging developers for B&W reversal based on sodium hydrosulfite, they are FD-70 and FD-72.

  9. #29

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    I think "hydrosulfite" is a historical misnomer. It's best called dithionite. Dithionite probably fogs grains via reduction. But I don't know how practical this agent is, as it gets oxidized and hydrolyzed rather rapidly.

    If the sepia toner method works without Dmax penalty, I think it would be advantageous in terms of image permanence and probably better granularity.


    Quote Originally Posted by Jordan
    Sodium hydrosulfite is very different from thiourea... I think there's a formula for a "fogging agent" based on the hydrosulfite in Grant Haist's book.

    The "sepia toner" method works well for me. I do not see any visible problems with reduced density... in fact, the dark tones are qualitatively "deeper" with thiourea toning than with re-exposure and re-development, probably because the latter does not go to completion. I pretty much use only Pan F and FP4 for reversal, so the disgusting T-grain tone doesn't bother me.

  10. #30
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    There are two things regarding fogging agents that we are discussing here if I am not mistaken.

    One is that thiourea can fog film and render it developable by a separate reducing agent or developing solution (fog in thiourea and then develop in developer). Another is that thiourea, under the right conditions can act as the entire reducing agent and form metallic silver directly (directly form silver image or silver sulfide image). We must distinguish between these two and the form of silver that can be formed from the two reactions. This is essentially a restatement of what Ryuji said above, but in less technical terms.

    In a similar fashion, dithionite (hydrosulfite) can probably act the same way, but I am not as familiar with it as with the thiourea.

    PE

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