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Thread: A real formula

  1. #11
    htmlguru4242's Avatar
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    Can't the additives be removed from the Knox gelatine?

  2. #12
    David A. Goldfarb's Avatar
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    Even if they could, you can get photo grade 250 Bloom gelatin from Artcraft for $19.90/lb.
    flickr--http://www.flickr.com/photos/davidagoldfarb/
    Photography (not as up to date as the flickr site)--http://www.davidagoldfarb.com/photo
    Academic (Slavic and Comparative Literature)--http://www.davidagoldfarb.com

  3. #13
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    Quote Originally Posted by David A. Goldfarb View Post
    Even if they could, you can get photo grade 250 Bloom gelatin from Artcraft for $19.90/lb.

    The additives cannot be removed without specialized equipment.

    Inactive (oxidized) photo grade gelatins are available in 3 grades. They are 75 bloom, 175 bloom and 250 bloom.

    PE

  4. #14

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    PE, here's a copy of a gelatin formula I found an the web as a lab experiment a couple days ago. I'm curious how you would rate it, especially for sensitivity, quality, and longevity.

    With the lights on, dissolve 2.4 g of potassium chloride and 1.2 g of potassium bromide in 100.0 ml of distilled water. Place this solution on a stirrer hot plate and bring the temperature to 35ºC. Add 20.0 g gelatin. Set this aside to thicken. In another beaker dissolve 6.0 g of silver nitrate in 60.0 ml of distilled water. Assemble the constant temperature apparatus shown below. Run the hot water for a while to ensure temperature is about 50ºC. Heat the potassium chloride-gelatin mixture in the beaker to about 50ºC.
    The remainder of this experiment must be done with a safety light as the photographic emulsion is extremely light sensitive once it is prepared. Add the silver nitrate solution slowly, with continued stirring, to the gelatin-halide mixture. The rate of addition should be no greater than 1 mL per 3 seconds. Ripen at 50C for 2 hours while stirring. At the end of this digestion period, add another 10.0 g of gelatin. Once this has dissolved, you can chill the emulsion with an ice bath. If necessary, the experiment can be stopped at this point. Wrap the emulsion beaker with foil, label and store in the refrigerator.
    If the emulsion is shredded and washed, then cold tones will be observed. If it is left unwashed, warm tones will be obtained. The emulsion should now be melted and with continuous stirring, held at approximately 60ºC for 1½ hours. At the end of this second digestion period. 0.125 g of chrome alum (dissolved in a minimum of H2O) can then be added as a hardener, but the alum will make the gelatin very thick.

  5. #15
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    I see a few things to comment on.

    The gelatin level is rather high compared to what I make. The silver level is a bit low compared to what I make.

    The emulsion will be a normal Cl/Br paper with a rather slow speed, say about 2 - 6 stops slower than current enlarging papers. I can't really tell without testing it, but that would be my guess due to the lack of sulfur sensitization. If the formula assumes active gelatin, then I would not care to predict the contrast, but it could be quite low.

    Most published formulas omit telling you that they use active gelatin or that they use a sulfur sensitization step.

    I would use 10% glyoxal instead of chrome alum.

    PE

  6. #16
    OldBikerPete's Avatar
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    G'day PE.
    Your formula mentions a spectral sensitising dye - can you be more specific?

  7. #17
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    These dyes are hard to get and run about $100 - $200 / gram (US). To save money and effort, I use erythrosine, which is a food dye and is about $20 / gram and can be bought from most chemical supply houses.

    It is very very low in toxicity.

    It will give you an ortho sensitive emulsion when coated on film.

    If you want to know about modern dyes used by Kodak and other companies, there is a good description in Mees, or Mees and James. They include structures and spectra. I have posted some of the spectra here and mentioned erythrosine several times. If you search for it, you will find the wedge spectrograms.

    PE

  8. #18

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    Gee, I see Pfaltz & Bauer has xenocyanine listed at $180/10mg

    How many 120 rolls will 10 mg get us?

  9. #19
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    You use a dye at about 20 - 100 mg / mole of silver.

    Go figure.

    PE

  10. #20

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    PE

    Are there any major differences between the formulas for gelatin film emusions and gelatin paper emulsions? I'm guessing, if anything the gelatin percentage and silver content in film emulsions would be higher than in paper.

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