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  1. #1
    Photo Engineer's Avatar
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    The key to the door of crystal types

    A number of years ago, Herb Wilgus at Kodak came up with this chart. It was used later by Bruce Kahn for his course at RIT. Bruce has given me his permission to use this chart.

    I was one of the last people to 'interview' Herb for the emulsoin modeling and scaling software before he died of cancer in the 80s. He was a great guy and a great researcher.

    Here it is, finally! Thanks to Herb and Bruce.

    This chart taken with my other posts will enable anyone to predict the grain shape of any precipitation of AgBr. Of course there are charts for Chloride and Iodide, as well as mixed crystals. This is a key to the model we used and only works for double run (double jet) emulsions which are made under feedback control.

    For those who ask about the other charts, I ain't tellin'!!!!!

    For emulsions made using a single run of silver nitrate into salt, you get a mix of all possible shapes and these are K grains (klunkers, not Kodak).

    PE
    Attached Thumbnails Attached Thumbnails crystal diagram.jpg  

  2. #2
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    Ron,

    I do appreciate all of your help in this forum. I hope it'll stand for a long time. I fully intend to carry this art on into my generation. It amuses me greatly how you post hints to everything, but not the whole story. Sly person, making us reinvent the wheel!

    ...Can't say it's a bad thing though. At least for me, learning by a bit (or... a lot) of trial and error makes me remember. I hope I can still do all this when I'm 80!
    Vincent Purcell
    Lexington KY Photographer + Media Artist
    http://vincenttpurcell.com

  3. #3
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    Good Grief!! Part 2

    PE,

    When it rains, it pours! Another puzzle piece falls into place.

    I am glad I took notes!!!

    Thank you for sharing this with us...

    (I wonder if the other charts can be derived somehow.....hmmmm)

    Bob M.

  4. #4

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    Quote Originally Posted by Photo Engineer View Post
    A number of years ago, Herb Wilgus at Kodak came up with this chart. It was used later by Bruce Kahn for his course at RIT. Bruce has given me his permission to use this chart.

    I was one of the last people to 'interview' Herb for the emulsoin modeling and scaling software before he died of cancer in the 80s. He was a great guy and a great researcher.

    Here it is, finally! Thanks to Herb and Bruce.

    This chart taken with my other posts will enable anyone to predict the grain shape of any precipitation of AgBr. Of course there are charts for Chloride and Iodide, as well as mixed crystals. This is a key to the model we used and only works for double run (double jet) emulsions which are made under feedback control.

    For those who ask about the other charts, I ain't tellin'!!!!!

    For emulsions made using a single run of silver nitrate into salt, you get a mix of all possible shapes and these are K grains (klunkers, not Kodak).

    PE
    PE,

    I do not have a background in chemistry, but I'm trying to learn.

    Is this chart showing the shape of the crystals when the film is manufactured, or after development? If it is after development, is the scale on the chart the PH level of the developer? Is this chart for bromide emulsions?

    I apolagize for being so clueless about the technical aspects of all this and asking stupid questions.

    Thanks.
    The purpose of art is washing the dust of daily life off our souls. - Pablo Picasso

  5. #5
    Photo Engineer's Avatar
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    This is the pAg of the emulsion when made, and has no relationship to pH or to developed silver. It is for bromide emulsions.

    The only similarity between pAg and pH is this:

    pH = negative log of the hydrogen ion concentration. The lower the number, the higher the amount of hydrogen ion and the more acidic.

    pAg = negative log of the silver ion concentration. The lower the number, the higher the amount of silver ion in solution. This measure is no longer used at Kodak. We use a vAg scale which is the actual measured voltage.

    PE

  6. #6

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    Quote Originally Posted by Photo Engineer View Post
    This is the pAg of the emulsion when made, and has no relationship to pH or to developed silver. It is for bromide emulsions.

    The only similarity between pAg and pH is this:

    pH = negative log of the hydrogen ion concentration. The lower the number, the higher the amount of hydrogen ion and the more acidic.

    pAg = negative log of the silver ion concentration. The lower the number, the higher the amount of silver ion in solution. This measure is no longer used at Kodak. We use a vAg scale which is the actual measured voltage.

    PE
    Thanks for the clarification. So why do the crystal shapes change as the pAg increases? Which of the shapes is ideal?
    The purpose of art is washing the dust of daily life off our souls. - Pablo Picasso

  7. #7
    Photo Engineer's Avatar
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    Quote Originally Posted by PhotoSmith View Post
    Thanks for the clarification. So why do the crystal shapes change as the pAg increases? Which of the shapes is ideal?
    The pAg is the determining factor for what type of crystal forms, or that is to say, the way the silver halide falls out like snowflakes as they condense from the liquid. So, just as you can get sleet, hail and flakes from water in the air depending on the humidity and temperature, you can get cubes, octahedra and t-grains from silver halides.

    The grain shape is used to control speed, sharpness and grain. T-grains are very fast because they are flat and present a huge surface to the light. Cubes can be made very small and are used in papers and in print films for fine grain and sharpness. There is a use for every type of grain, even if it is to be a foundation to grow another type of crystal.

    You could make a cube and then convert it to an octahedral shape by growing something over top of it. This would then become a core shell or converted emulsion.

    PE

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    Very cool. I'm glad someone else has to worry about all of that, so I can use their products to make my images.

    I find it very fascinating. I think the more we know about our materials and chemistry, the more control we have in mastering our art and craft.

    Why are the T-grain films more responsive to variations in processing technique than the more traditional emulsions?
    Last edited by PhotoSmith; 08-28-2007 at 11:21 PM. Click to view previous post history.
    The purpose of art is washing the dust of daily life off our souls. - Pablo Picasso

  9. #9
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    Quote Originally Posted by PhotoSmith View Post
    Why are the T-grain films more responsive to variations in processing technique than the more traditional emulsions?
    I'm not aware that they really are. The film itself might be by design (see below).

    T-grains do have more surface area than cubes or octahedra for equal mass, and that might be part of it.

    OTOH, some conventional grain films are also more sensitive, I have heard. This might be due to the fact that almost all of the films today are made of blends of emulsion sizes and this causes them to be sensitive to some development conditions. Each component develops differently and some effort must be made in film design so that all of them react alike so as to make a smooth curve. Any error in processing can emphasize the differences in the blend.

    PE

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    Thanks for taking the time to explain all this.

    I assume that if we use high accutance developers, the developer doesn't change the shape of the original grain structure very much, but fine grain developers soften the edges of the grain crystals giving the appearance of finer grain. Is this correct?
    The purpose of art is washing the dust of daily life off our souls. - Pablo Picasso

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