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Thread: Fog control

  1. #1

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    Fog control

    I have been away from here for a while but now I am back. Oh shure, you only come around when you want something!

    Here is what I want. How much and what percentage solution of Benzotriazole do I use and at what stage of making the emulsion?

    Is it based on the silver content?

    The furmula I am using is from the Kodak AJ-12 instructions.

    I have gotten back to making the gelatin tintypes but this time I am going to use a chemical answer to knock out the fog problem, hopfuly.

    Thank you.

  2. #2
    Ian Grant's Avatar
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    The formula in AJ-12 shouldn't need any Benzotriazole, it is normally quite fog free, in visible terms. OK it was 1976 when I last made up taht exact formula but it works well and is fine for enlarging although slow for negative use. Tweaking the digestion and ripening times will greatly increase the speed and contrast, but that means experimenting & testing.

    Ian

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    You probably don't want to use the benzotriazole in the emulsion - I think tetraazaindene (TAZ) or phenyl-5-mercaptotetrazole (PMT) are more appropriate. PE will most certainly follow with suggestions on how much to use, and it will be based on the amount of silver in the emulsion - on the order of milligrams per mole of silver.

    I've bought TAZ and PMT from the Photographers Formulary, already prepared into solution. You need to ask them about it, as I don't think they have it listed online anywhere. It was pretty inexpensive.

    If you have benzotriazole, have you tried adding it to your developer to reduce fogging?
    Kirk

    For up from the ashes, up from the ashes, grow the roses of success!

  4. #4

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    Or "TAI"?
    Bill

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    Bill - I think I've seen it both ways, but I bet your more right.
    Kirk

    For up from the ashes, up from the ashes, grow the roses of success!

  6. #6
    dwross's Avatar
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    Potassium bromide (KBr) added as a 'final' before coating, is the most common recommendation to reduce fog. A few mls of 10% solution is usually enough. Best, though, is to try to stop fog from coming in at all. Fog usually happens as a result of reaching for increased speed. High heat and prolonged ripening can up the chances, particularly in recipes with ammonia. Over-washing the noodles, or washing with distilled water can be a cause. Too much sulfur in the sensitizer, usually as plain hypo, is a common cause of fog. Unless you are using inert gelatin, you can't be sure you're not compounding the total amount of sulfur in your recipe.

    Pierre Glafkides in 'Photographic Chemistry, Vol One', 1958, mentions benzotriazole has an addition to emulsions, usually as a final, but sometimes as a replacement for a small amount of the bromide before digestion (p.380). The amount of antifoggants can range anywhere from 20mg to 300mg per liter of emulsion, determined by trial and observation.

    Happy Cooking!
    d

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    Thanks Denice

    This emulsion has always had a slight fog problem from the start. Everything has been tried to solve it including lower temp, longer or shorter washing, etc. I did get a little better result by adding a very small amount of rapid fixer to the developer, but I ordered ammonium thiosulfate to add to it, the fixer contains acetic acid and that shuld not be used with cyanates. I am still here.

    Again this is for tintypes, they are more suseptible to showing fog than negatives, just like wetplate tins.

  8. #8
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    During making, use of TAI (sold by the Formulary) is useful to reduce fog. See the formula by Jim Browning here or on his web site for an example of how to use it. It is measured in milligrams / mole of silver. If your fog forms during or after making in spite of TAI or without TAI, use of PMT will work, but in very very low levels. This is usually at the level of 0.1 to 1.0 mg/mole of silver. Very low. BTAZ usually does not work well added to emulsions.

    If you have fog with AJ-12, you have probably over washed it and there is not enough residual halide left to buffer the vAg. Use the above solutions I propose. Remember though that you will lose speed. Every milligram of PMT might cost you 1/2 stop or 1 stop in speed so be careful.

    PE



 

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