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  1. #1
    Photo Engineer's Avatar
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    How to make the best emulsion, or why "Silver Rich" is a myth.

    Ok, here goes a long description of emulsion making.

    Most all emulsions published fall into two classes, SR (Single Run) and SRAD (Single Run Ammonia Digest). These are emulsions in which silver nitrate is run into salt and gelatin either with our without ammonia.

    First, I'll explain some problems inherent in early emulsion making.

    To make a silver halide that is uniform in grain size, and therefore has good contrast and optimum speed, the pAg should be held constant (pAg is the negative log of the silver ion concentration. Elsewhere on APUG, a member has insisted that pX be measured (negative log of the halide concentration), but this is wrong).

    As you add silver nitrate to salt, the pAg changes because the silver and salt concentrations are changing. Therefore, to keep pAg constant, silver and salt should be added to salt and gelatin. This is the only way to control the situation properly. These are called RS emulsions (Run Salt). The only posted example of this is on Jim Browning's web site.

    However, this method is messy and very hard to control because a thing called 'ionic strength' is changing due to the buildup of sodium or potassium nitrate as a by product of the emulsion making. Therefore, the rates must be controlled by a computer and constant measurment.

    Why is this so? Well, a cube is made at one pAg, an octahedra at another and a t-grain at another pAg. If you don't control the pAg, then you get 'K' grains or 'Klunkers' which is a mix of all possible types of grain of all possible sizes.

    So, the 'perfect' emulsion is one type of grain at one size.

    Ammonia digest is an attempt to rectify this in an SR emulsion. Ammonia dissolves the grains and rounds them off and makes them more even in size. However, this creates its own problems such as fog, and spherical shaped grains along with rods and needles, again 'K' grains. It is also hard to control the digestion step. Some modern emulsions use ammonium sulfate with a pH cycle step to carry out a moderate quick ammonia digest. It can be used but will round off any grain. So, a pure cube may end up with clipped corners and an octahedral grain may end up as a sphere. You have to be careful.

    Well, where does silver rich enter into this? Simply that the old SR and SRAD emulsions themselves contain 'dead grains'. These are grains that have no light sensitivity at all. In addition, the old active gelatins did not activate them very well during the finishing step.

    As emulsion scientists learned how to use sulfur or sulfur + gold in a post precipitation step called 'finishing' or 'chemical sensitization', the number of dead grains decreased by a big amount. In addition, making grains all of one size also decreased dead grains. This is because the sulfur or sulfur + gold tended to react more evenly with the more uniform grains.

    Therefore, in todays modern emulsions, you can coat less silver and still get the same density. At one time, as an example, about 50% of the silver might expose and develop, but today nearly 100% will expose and develop. If this is true, then about 1/2 of the silver will give the same image with no sacrifice in grain and will give improved sharpness due to lower turbidity.

    In addition, the curves of the films and papers can be more carefully fine-tuned, and batch to batch variations will be less.

    Modern emulsions are RS emulsions with many controlled jets of salts combining with the silver under computer control. In addition, jets of metal salts and organic chemicals are being added to control raw stock keeping, latent image keeping, and reciprocity. I have seen 5 or more jets cycling on and off during the make of a modern emulsion.

    So, there is a thumbnail sketch of making a better emulsion and then finding you can reduce the level of silver. Silver rich was necessary in the old days simply because of an imperfection in the emulsions due to the methods used in making them. As long as you achieve the same measured dmax with no sacrifice in grain and shaprness, there is really no difference in quality.

    Now, there is a difference, but that is another story. It is due more to modern papers than film or paper curve shape or even to silver level, but silver level plays a certain part in this. You do see that FB paper is better than RC paper in many ways that can only be seen to be appreciated. That is part of the rest of the story which will be told at a later date.

    I'll address this last paragraph after the discussion over this post dies down and it seems that the facts here, if anyone even cares, are digested. There is a difference, but it is not what you think.

    PE

  2. #2

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    Ron, a total novice question: 100% of silver can be used on a too thin of an emulsion. How do you know when you have gone too far in saving silver?
    van Huyck Photo
    "Progress is only a direction, and it's often the wrong direction"

  3. #3
    Photo Engineer's Avatar
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    The Dmax goes down, or contrast goes down, or both.

    PE

  4. #4
    Michel Hardy-Vallée's Avatar
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    So in essence "silver rich" really means "silver halide rich" not "metallic silver rich," right?

    If a modern and a less modern emulsions are matched for dmax and contrast, they will end up with the same amount of metallic silver in the final image, after fixing, for similar exposure (I'm just speaking very broadly here!).

    Therefore, with respect to film (looking forward to part 2 about paper), the difference in taste between modern and older films is not caused by a bigger amount of metallic silver, but rather by the qualitative aspects of the emulsion (spectral sensitivity, imperfections, "gestalt" effect, etc).

    Guess you'll have to educate a few resellers here....
    Using film since before it was hip.


    "One of the most singular characters of the hyposulphites, is the property their solutions possess of dissolving muriate of silver and retaining it in considerable quantity in permanent solution" — Sir John Frederick William Herschel, "On the Hyposulphurous Acid and its Compounds." The Edinburgh Philosophical Journal, Vol. 1 (8 Jan. 1819): 8-29. p. 11

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  5. #5
    Photo Engineer's Avatar
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    Michael;

    Gestalt would be the right word.

    But, silver rich does indeed only end up referring to the amount of silver in the final image. That is all we see. We cannot see the silver from the silver halide that is fixed out.

    Now, I must add that the older emulsions were coated thicker, as noted above and this affects sharpness, so todays films are sharper for a number of reasons, including thickness and amount of turbidity.

    Maybe this sharpness is part of the gestalt, in that older films were subtly less sharp.

    PE

  6. #6
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    I am thankful you brought one of my pet peeves up in more detail, PE.

    Could the Bruce Kahn emulsion recipe be termed an SR emulsion? Or is the BK recipe even further down the rung in terms of crudeness?
    “Do your work, then step back. The only path to serenity.” - Lao Tzu

  7. #7
    Photo Engineer's Avatar
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    It is indeed an inefficient SR emulsion, and if you compare it with the one recently posted here from the coffee site, it is close to that. It is a slow contact grade chloride emulsion.

    PE

  8. #8
    Michel Hardy-Vallée's Avatar
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    Quote Originally Posted by Photo Engineer View Post
    But, silver rich does indeed only end up referring to the amount of silver in the final image. That is all we see. We cannot see the silver from the silver halide that is fixed out.
    Ok, but in reference to your statement "Therefore, in todays modern emulsions, you can coat less silver and still get the same density. At one time, as an example, about 50% of the silver might expose and develop, but today nearly 100% will expose and develop," I thought you meant that in old emulsions, only 50% of the sliver halides reduced to metallic silver, hence the existence of "dead grains," grains of silver halide not being reduced to metallic silver.

    Thus what I understood is that for a given density in a piece of processed negative, both modern and old emulsions would have the same amount of g/cm2 of metallic silver, only that to achieve this amount, an older emulsion would need extra halide given its low yield.

    (You knew it wouldn't be easy to explain to us hare-brains! )
    Using film since before it was hip.


    "One of the most singular characters of the hyposulphites, is the property their solutions possess of dissolving muriate of silver and retaining it in considerable quantity in permanent solution" — Sir John Frederick William Herschel, "On the Hyposulphurous Acid and its Compounds." The Edinburgh Philosophical Journal, Vol. 1 (8 Jan. 1819): 8-29. p. 11

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  9. #9
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    Michael, you are correct.

    You can coat 50% of the silver, if the emulsion is twice as efficient and get the same density. The metallic silver yield is low in the old emulsions.

    I should add that I have taken out of this differences caused by developer and differences in the morphology of the developed silver which both enter into the arguments I presented above in the OP. The argument assumes identical developer and identical silver metal morphology.

    PE

  10. #10
    Michel Hardy-Vallée's Avatar
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    Thanks Ron, I was unsure for a moment of what I understood.
    Using film since before it was hip.


    "One of the most singular characters of the hyposulphites, is the property their solutions possess of dissolving muriate of silver and retaining it in considerable quantity in permanent solution" — Sir John Frederick William Herschel, "On the Hyposulphurous Acid and its Compounds." The Edinburgh Philosophical Journal, Vol. 1 (8 Jan. 1819): 8-29. p. 11

    My APUG Portfolio

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