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  1. #1
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    Details of emulsion types and how to get there

    In my continuing effort to pass on knowledge to you all for the future, here are some additional pieces to the jigsaw puzzle of emulsion making.

    Emulsions may be broadly classified into 5 types. They are:

    1. Cubes
    2. Rounded cubes
    3. Octahedra
    4. T-Grains
    5. K grains (klunkers or amorphous grains) this is ancient technology.

    Considering only bromide emulsions here are details.

    Cubes form at a pAg of about 6 - 6.5 or about 0.00014 m/l of NaBr.

    Rounded cubes form between a pAg of 7.5 and 8 or about 0.0008 m/l of NaBr.

    Octahedra form at a pAg of about 8.5 or about 0.0039 m/l of NaBr.

    T-Grains form at a pAg of about 9.5 or about 0.1845 m/l NaBr.

    K grains form at pAg falues above 9.5. Or, they form if you run just silver into salt and let the pAg vary during the precipitation.

    Now, this means that you must control the pAg to those values when making the emulsion, so this basically means that we cannot make anything but K-grains with a formula that adds silver into salt + gelatin. The forms listed above (other than K-grains) require the addition of silver nitrate + salt to salt + gelain with sophisticated control apparatus for keeping the pAg at the proper value.

    pAg is a way of measuring salt content. It is the negative log of the silver halide concentration. You must not use pX or the negative log of the halide ion because there may be more than one salt and each one has a different pX but give the same pAg. Kodak does not use any of these. They use a far more sophisticated system with more precision.

    Further, emulsions may be divided into the following types (not an all inclusive list, just a mere glimpse of the types)

    1. SR = single run of silver into salt
    2. SRAD = single run of silver into salt with ammonia for digestion
    3. SRC = single run converted
    4. RSTK = run silver and salt with the digestion aid "T"
    5 RSEK = run silver and salt with the digestion aid "E"
    6. RSTP = run silver and salt with the digestion agent "T" using method "P"
    7. RSAKAS = run silver and salt, digestion agent "A" and methods "K" and "S"

    So, without resorting to running two solutions, it is very difficult to make a modern emulsion. It is not impossible, just difficult, and the results are probably going to be more variable if you do use double runs without precise machine control.

    Among other things, the pAg changes as a function of dilution and as a function of the buildup of the nitrate ion, and so if you don't have control equipment or measuring equipment, you pAg will drift, sometimes very badly, with unexpected results. There are several ways to solve this problem, but they are mostly beyond our means.

    Hope this helps you out there intetersted in this sort of things.

    PE

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    Dear PE,
    I alwais read you posts with a uge interest, as this part of photography is wastly unknown to all of us photographers.
    Have you ever thought collecting them together into a pamphlet ok booklet (in some sort of electronic way) in order to colect all the information you spread here into one piece, growing and adapting at your pace ?
    It could even turn into a "emulsion making 101" book.

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    Georges;

    Yes, I have considered it, but there is not much interest in publishing a book on analog photographic chemistry like this anymore. In addition, I have no access to any Kodak facilities and could not do the book the justice it deserves such as photo micrographs, electron micrographs, approved formulas for publication, and detailed calculations. Therefore, the book would be rather superficial compared to what it could be.

    Last but not least, I feel that there are better people out there to do this sort of job. Unfortunately, they are totally disinterested in this type of endeavor.

    PE

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    Perhaps you could get a grant to pay for your time, so effort would not be dependent on book sales for compensation.

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    Grants are not easy to come by for this type of topic. Even the big organizations like George Eastman House are having problems.

    PE

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    Quote Originally Posted by Photo Engineer View Post
    Among other things, the pAg changes as a function of dilution and as a function of the buildup of the nitrate ion, and so if you don't have control equipment or measuring equipment, you pAg will drift, sometimes very badly, with unexpected results. There are several ways to solve this problem, but they are mostly beyond our means.

    PE
    Although pAg control may be very difficult and require feedback loops and computer control in a production run of an emulsion in industrial quantities, does the "pipette and syringe" level of small scale production in an amateurs darkroom make alternative strategies viable?

    I mean, if you are making 100ml of emulsion you can literally add solutions drop by drop...

    Steve

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    Steve;

    This works just fine, but you cannot be assured of getting a repeatable material or an emulsion as good as the manufacturers do. I was pointing out the double jet method as what allows precise control, not as something mandated by the Deity.

    So, as you see on many postings the use of a burette or pipette or dropper will work, but with limitation and with only moderate success.

    A prime example would be making a cubic emulsion which might turn out very nice but polydisperse giving you a lower contrast than you wanted, or a t-grain which would be really a k-grain due to imprecise control.

    Also, ramping up the flow rate to grow the emulsion would be a problem with using a burette, pipette or dropper. For controlled growth, you would have to figure out how to follow the equation noted in anohter posting.

    PE

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    Well, let me say first of all that I know nothing about this subject. I am very interested in making my own emulsions, but besides collecting a few old formulas I haven't started actually making my first emulsion yet so this is just arm-chair musing on my part.

    And it is all way over my head!

    But, I was never one to let total ignorance stifle my enthusiasm

    I've read up on how to make an 1880s style rustic emulsion, but I was thinking about how an amateur could control crystal growth without the feedback loops and computer controlled jets.

    I was thinking that if you knew the theoretical flow rates you required, if you scaled the whole thing right down could this become a 'drop rate'? Say, if you had a graph of a theoretical flow rate calculated by the formula could this be resolved to so many drops per second, timed with a second hand or a metronome or something? Assuming each drop is the same volume and that you had a good tap on your burette in theory you could get quite precice control of flow rates by counting drops per unit time. (as opposed to just opening the tap and letting it trickle in at some unknown rate). Ramping up the flow rate is just simply (!!!!!) a matter of tweaking the tap open a bit wider whilst furiously counting drops against your timing device. Or opening your second burette. Or... something?

    I was just wondering if it might be possible to work out a low-tech 'folky' way of attempting a control crytal growth, was all.

    Or is this just a daft idea?

    Either way, I will shut up now, at least until I've made my first Victorian style Klunker emulsion!

    Steve

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    Steve, your idea is quite reasonable.

    The salt flow rate should be identical to silver (if they are equal in concentration) plus about 2% and that difference should increase with time due to the increase in 'buffer capacity' of the emulsion kettle due to excess ions such as nitrate building up.

    Drops however, can introduce an unwanted pulsation in the make that might ruin it. It should be a pair of slow steady streams.

    Also, beware that 1880s style emulsions assume that you are using active gelatins which are not sold anymore. These had sulfur impurities in them.

    You would have to use some sort of method to sensitize the emulsion using sodium thiosulfate which I have described elsewhere.

    Also, the absolute rate of both flows has to ramp up for proper growth.

    PE



 

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