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  1. #21

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    Quote Originally Posted by T-grain View Post
    Ron and Jim, I thought a Thorium salt was needed just to enhance sharpness not to actually hold the dye in the receiving gelatin?
    Jim, when you say using bleach as in the old wash-off relief process-is this an etch-bleach process?
    thanks
    I don't remember what the bleach was, but it was a silver bleach which removed the silver image and hardened the gelatin as a result.

    - Jim

  2. #22

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    Quote Originally Posted by Photo Engineer View Post
    T-Grain;
    Thorium also altered the hue of the cyan dye a tad. Jim probably has more on that.

    PE
    Thorium mordanted paper causes a shift towards green of the Cyan dye, immediately as the matrix contacts the paper. With Aluminum mordant, no shift occurs immediately, but it does hue shift upon drying. The Aluminum mordant (M1) actually seems to have much more capacity to shift the hue in dense areas, whilst the Thorium in the Kodak paper looses its ability to shift hue in denser areas.

    - Jim

  3. #23

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    I wonder if the bleach contained in E6 developing kits would work? Or maybe just plain old ferric chloride?
    Bill

  4. #24
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    I think it might have been the old dichromate bleach.

    PE

  5. #25

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    What is the formula for dichromate bleach?
    Bill

  6. #26
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    AAAAAAAKKKKKK. You had to ask that.

    OTOMH, it would be 50 g/l Potassium Dichromate and 50 - 100 ml / L of 37% Sulfuric Acid (battery acid). Just a guess, but more dilute is ok it just takes more time.

    PE

  7. #27
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    Quote Originally Posted by T-grain View Post
    ...I am afraid these PVA matrices simply won't work. The reason is simple: they don't have the amino groups present in the gelatin. The dyes (in their ionic form) are attracted to the protonated amino groups (gelatin), so that's why they are held in place even when you rinse the matrix in acetic acid solution. When you make the contact with the receiving paper, there is a gradient in pH between the matrix and the receiving gelatin, so the dye molecules are driven there. Simply put, the dye molecules are more attracted to a gelatin environment with pH 6 rather than about pH 4...
    This sounds like a very good assessment of the situation, specifically regarding the amino groups. Not something I know much about, but definitely, the DT mechanism relies on a special relationship between certain types of dyes & gelatin protein.

    And regarding,
    I thought a Thorium salt was needed just to enhance sharpness not to actually hold the dye in the receiving gelatin?"
    That is certainly my understanding as well. I've transferred magenta dye to a fixed-out (non-hardening) FB paper and the density is very good, but diffusion is terrible! The mechanism of imbibition though, relies on this pH gradient and not a mordant.

    Speaking of which... you can buy thorium nitrate on eBay for fairly low prices. Not that I'm even considering such an endeavor..

  8. #28

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    I have personally used uranyl nitrate mordant with and without aluminum, with the combined effect being a bit sharper. Depends on the receiver paper. But I'd have to defer to Jim's experience on that point. Regarding the use of pigment directly in the relief film and opacity, the hypothetical solution is
    much finer pigments which in effect behave as if they were more transparent in relation to saturation. This can be done, but I've already got way to many irons in the fire to do the testing myself per ideal process colors. The carbon printing crowd seems completely unaware of some of the
    significant advances in pigment technology which might be applicable. But nothing at this point has
    been marketed for process work per se, so there are chroma issues (but nowhere near as bad as back which things like alizarin crimson and poison green were used). One more test which will have to
    wait till I retire.

  9. #29
    holmburgers's Avatar
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    You guys are talking about a kind of Duxochrome thing, eh?

    Hexavalent (Ian) has been toying with this idea I think, on the spurring of CMB.

    But if hardening is such an issue with polymers, I just fear that none of these schemes will apply well. It makes little sense to set out learning guitar with one hand tied behind your back, or trying to build a car that doesn't use wheels.

    Of course finding a way to do it with PVA would be a really great contribution, but gelatin & in turn leather have been utlized for centuries because of its unique property to tan. I'm not sure we should expect a polymer to have any of these same characteristics.

  10. #30

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    From the nature of the bleach involved it looks like Duxochrome involved dyes, not pigments. (???)

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