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Thread: A real formula

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    A real formula

    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

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    HI there,

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

    Thanks.

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    When B -> A.

    Sorry.

    PE

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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...
    “Do your work, then step back. The only path to serenity.” - Lao Tzu

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    Quote Originally Posted by Photo Engineer
    Add spectral sensitizing dye and hold at 45 deg for 15 mins.


    PE
    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

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    Quote Originally Posted by bart Nadeau
    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
    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

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    Quote Originally Posted by Photo Engineer
    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.PE
    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.
    Happiness is a load of bulk chemicals, a handful of recipes, a brick of film and a box of paper. - desertrat

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    Quote Originally Posted by desertrat
    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.
    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

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    Quote Originally Posted by Photo Engineer
    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
    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.

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    Quote Originally Posted by Jeanne
    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.
    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

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