A real formula

Discussion in 'Silver Gelatin Based Emulsion Making & Coating' started by Photo Engineer, Sep 3, 2006.

  1. Photo Engineer

    Photo Engineer Subscriber

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    Here is a real emulsion formula. At Kodak, we called this an SRAD emulsion (Single Run Ammonia Digest). This is adapted from one by Baker in his 1940s textbook on emulsion making. Comments will follow.


    A solution
    Potassium Bromide 132 grams
    Potassium Iodide 4.5 grams
    Gelatin 30 grams
    Water 1 liter

    B solution
    Silver nitrate 130 grams
    Water 500 ml

    Heat A and B to 45 deg C

    Add 28% ammonium hydroxide to B with stirring until a clear solution results.

    Add B -> A over 10 minutes

    Hold for 30 minutes at 45 deg C.

    Let stand for 2 hours or until at room temperature.

    Shred into noodles and wash. (make sure all salts and ammonia are removed)

    Remelt and adjust gelatin percent to the desired level (5 - 10%)

    Add spectral sensitizing dye and hold at 45 deg for 15 mins.

    Coat with a hardener and surfactant.

    This can achieve up to ISO 40 speed.

    Now, here come the caveats. This formula assumed, as they did at the time, that you were using standard (ACTIVE) photo gelatins, and you will be lucky if you get ISO 3 - 6 with it using modern oxidized photo grade gelatins. You cannot get active gelatins that are any good today, for the most part.

    The only way to get speed is by chemical sensitization, or finishing. This involves the addition of any one of a variety of ingredients. The original was allyl thiourea, another was thiourea, and then finally they added sodium thiocyanate. Modern emulsions use either sodium thiosulfate or sodium thiosulfate plus a gold salt. It is done after the wash step, as excess halide represses this sensitization. This finishing step varies for every emulsion and sometimes for every batch of every emulsion.

    The problem is that the quantity, time and temperature must be determined by trial and error as it is based on the surface area of the emulsion. This is a very complex procedure. This type of emulsion varies from batch to batch quite a bit in speed, contrast and fog.

    BTW, this emulsion is polydisperse and the iodide that would otherwise be in the core and not very useful is churned by the action of the ammonia to be more uniformly distributed and therefore increases speed. This gives a rather high speed negative film with a long latitude and an upwardly bowed mid scale. (sound familiar?) Yes, this curve is very similar to some very revered products of the 40s, 50s and 60s.

    Enjoy and have fun.

    PE
     
  2. phfitz

    phfitz Member

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    HI there,

    At what point does this need to be done in total darkness?

    Thanks.
     
  3. Photo Engineer

    Photo Engineer Subscriber

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    When B -> A.

    Sorry.

    PE
     
  4. Jerevan

    Jerevan Subscriber

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    Film speed of ISO 3-6, that's about what I need for using my Rodenstock Aplanat with the lenscap... *grins*

    BTW, I prefer having all the ingredients of the formula at the top, so I can see what goes in at a glance. Nevertheless, I have nowadays the habit of reading a formula, at least three times, just to be sure...
     
  5. bart Nadeau

    bart Nadeau Member

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    So is this where an emulsion starts as blue sensitive and is then made to be orthochromatic or pancromatic?
    Sorry if this is an ignorant question but it must happen somewhere in the process.
    Also, another question I've wondered about is making silver nitrate. If you disolved as much silver as could be in nitric acid and then air dryed it down to silver nitrate crystals would this give you good enough silver nitrate for an emulsion such as this.

    bart
     
  6. Photo Engineer

    Photo Engineer Subscriber

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    Bart;

    Yes, the addition of sensitizing dye can make an emulsion sensitive to any particular portion of the spectrum you wish including infra-red. A raw emulsion is mainly UV and Blue sensitive.

    Most silver has impurities in it that will make it unusable for photographic emulsions. It would depend on the silver and on the nitric acid. Reaction of silver metal with nitric acid, IIRC, requires red fuming nitric acid as silver is a noble metal. RFNA, as it is called, is rather hard to get and gives off very toxic fumes.

    PE
     
  7. desertrat

    desertrat Subscriber

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    Can an active gelatin be made from the gelatins we can get today? I'm guessing it would be very difficult, but I'm curious how it would be done.
     
  8. Photo Engineer

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    No, it cannot be done.

    The active part has been removed (oxidized generally) from the emulsion. It is also a variable quantity from batch to batch of raw gelatin. We used to used what were termed Hard, Medium and Soft grades of active gelatin to achieve the various speeds and contrast grades. That is now achieved in a precise manner by addition of an exact quantity of an active sulfur compound such as was removed from the gelatin.

    The original component was allyl thiourea.

    PE
     
  9. Jeanne

    Jeanne Subscriber

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    This -- as well as other postings -- has got me wondering if there is a source for soft grade gelatin here in the US. Seems like most suppliers (B&S, PF, Artcraft) carry 250 bloom only.
     
  10. Photo Engineer

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    Jeanne;

    All of their gelatin comes from either one of two suppliers in the US AFAIK.

    The only active (non oxidized) gelatin is Knox unflavored food gelatin, but it has so many additives that it is about 30% gelatin and 70% additive. It is virtually unusable IMHO for emulsion work, as it varies from batch to batch.

    PE
     
  11. htmlguru4242

    htmlguru4242 Member

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    Can't the additives be removed from the Knox gelatine?
     
  12. David A. Goldfarb

    David A. Goldfarb Moderator Staff Member

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    Even if they could, you can get photo grade 250 Bloom gelatin from Artcraft for $19.90/lb.
     
  13. Photo Engineer

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    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
     
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  15. robopro

    robopro Member

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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.
     
  16. Photo Engineer

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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
     
  17. OldBikerPete

    OldBikerPete Subscriber

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    G'day PE.
    Your formula mentions a spectral sensitising dye - can you be more specific?
     
  18. Photo Engineer

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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
     
  19. 25asa

    25asa Member

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

    How many 120 rolls will 10 mg get us?
     
  20. Photo Engineer

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

    Go figure.

    PE
     
  21. robopro

    robopro Member

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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.
     
  22. Photo Engineer

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    Robo;

    Yes, film emulsions are almost always bromoiodide emulsions from about 0.3% - 10% iodide and very large in grain, or from 0.2 - 5 microns in size.

    Paper emulsions are usually bromide, chlorobromide and chloride (Azo). Grain size varies from about 0.1 micron to about 0.3 microns but there are exceptions, even with film emulsions.

    Paper silver varies from about 500 mg/ meter square to about 5 g/meter square, and film varies from about 1 gram / metre square to about 20 or 30 grams per meter square (X-Ray is the highest).

    I hope I got that right. At EK we used a mixed english metric system and I was converting from mg/ft square to mg/meter square and rounding.

    PE
     
  23. robopro

    robopro Member

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    OK, give me a day or two to digest that. In the meantime, how do you feel about multiple coatings on glass plates? I guess my question is, using a 'primitive' process with gelatin and hand coating on glass plates, would more than one coat really be of any noticable benefit? Right now I'm using albumen with 4 coats. I've never tried less or more -- just doing it because that's what the formula my friend gave me said to do, and it makes sense because you drain the excess from a different corner of the glass with each coat, so common sense tells you 4 coats would give you a more even total coating. If I switched to gelatin, would I see any real advantage to multiple coats?
    I have to ask these stupid questions because I'm not a chemist and I need things explained to me in 'easy' terms. If you say mix 6 grams per 100 ml and add 4 ml of 1% solution, I understand that. When you start saying add 26% of 2% strength per 1.5 moles -- factoring in surface area and total volume...uhm, OK, what was that again?
    I asked a question once about building a 'simple' lens for my camera and people went off on discussions about refraction coefficients and diopters. What the heck is a diopter? (Do you believe I can actually develop my own prints? I just follow the formulas and procedures that come with the materials -- I really have no idea how they work...) :smile:
     
  24. Photo Engineer

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    A simple answer on glass plates then. With silver halide in gelatin, you only need one coat. Pour the emulsion onto the center of a warm plate and rock the plate until it is evenly coated. Let the excess drain evenly from each corner by tipping all for corners down in turn and then let dry. That should do it.

    Do not coat unwashed emulsions on glass plates. They will not adhere. Use an aldehyde hardener for best hardening and adhesion. If you have problems, a gelatin under coat will help.

    Make sure plates are dry and free from oil before coating.

    A bit of Everclear (ethyl alcohol) and spreading agent will help the coating.

    PE
     
  25. robopro

    robopro Member

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    I think I'm about ready to give this a try. I have some experience coating glass with albumen -- which is probably thinner than gelatin. I think the rest I'm just gonna have to learn by doing. Time to start rounding up suppliers!
    It'll be interesting to see how handmade gelatin negs compare to albumen.

    This whole adventure began because I had this crazy idea about sensitizing albumen to infrared, but the speed is just too slow with ULF pinhole. I put the IR idea on the back burner for now, but I would like to be able to do 11X14 pinhole portraits, and looks like gelatin is the only way I'm going to get the necessary speed.
    I'll keep the forum uprised of my progress, if anyone's interested...
     
  26. dwross

    dwross Subscriber

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    robopro:
    re: if anyone's interested...

    Very much so. Thank you for sharing your explorations.