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  1. #11

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    Careful with saturated solutions if your darkroom temp fluctuates a lot, the saturation point can change a bit and effect the sensitivity.

  2. #12
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    Quote Originally Posted by Colin Graham View Post
    Careful with saturated solutions if your darkroom temp fluctuates a lot, the saturation point can change a bit and effect the sensitivity.
    The saturation point does change but it has been my experience over many years that for a saturated solution of AD or PD there seems to be little effect from temperature changes as long as a large enough volume of dichromate lays on the bottom of the container.
    Don Bryant

  3. #13

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    My ambient temp varies on average 15C from morning to afternoon, which can represent a 30% change in saturation for ammonium dichromate according to one of the sources I could find. Maybe real world results aren't as dramatic as this table suggest, but I figure gum is capricious enough already without throwing in that variable on top. I do know I had difficulty getting consistent exposures when I did use saturated solutions.

    Anyway, to find the best concentration for my darkroom I just got some water as cold as my space gets, and kept track of how much would go into solution at that temp, then rounded down to the next whole number for good measure. Since I established this concentration my exposures have been constant.

  4. #14
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    Quote Originally Posted by Colin Graham View Post
    My ambient temp varies on average 15C from morning to afternoon, which can represent a 30% change in saturation for ammonium dichromate according to one of the sources I could find. Maybe real world results aren't as dramatic as this table suggest, but I figure gum is capricious enough already without throwing in that variable on top. I do know I had difficulty getting consistent exposures when I did use saturated solutions.

    Anyway, to find the best concentration for my darkroom I just got some water as cold as my space gets, and kept track of how much would go into solution at that temp, then rounded down to the next whole number for good measure. Since I established this concentration my exposures have been constant.
    Which dichromate do you prefer? AD or PD?
    Don Bryant

  5. #15

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    I've had much better luck with AD, for both gum and carbon transfers. I know the opposite is true for many though!

  6. #16

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    FWIW, I use ammonium dichromate. Can't compare the two, since I've never used potassium dichromate.
    Good luck,
    Steve
    www.scdowellphoto.com

  7. #17

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    I'm with Colin, I prefer to use *known/stable* solution strengths. I use ammonium dichromate at concentrations: 2.5%, 5%, 10% and 15% which are all stable (w/o crystallization) in reasonable room temperatures. (That is >= 18C/64F...) Not that it will definitely give you much more consistent results (since there are so many different critical variables in gum printing), but mind is easier that way...

  8. #18

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    I suppose if you work in a garage or a basement that is not climate controlled, then you should be more diligent about your concentrations. But my darkroom is in my house where the temp is pretty consistent throughout the year. I live in Houston, so humidity is a different story entirely. I'm not sure how, or if humidity affects your concentrations. Maybe someone else can interject.
    Steve
    www.scdowellphoto.com

  9. #19

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    Even if there are substantial temperature fluctuations, I don't think it will affect the dichromate solutions strength too much, especially for a large volume like Don said. The effect would be negligible indeed. OTOH, why not nailing down a variable where you can? Gum is already pretty tough to control anyway...

    I also don't understand the practice of using saturated AD for making gum prints; even 15% is extreme in my experience. My usual dilution is 5% and it works like a charm with inkjet transparencies. I use 10% for single layer bleach developed prints where exposure times should be around 2-3x stronger than normal and 15% for non-oiled paper negatives . Strong dichromate concentrations tend to kill shadows too. (I use negatives with a relatively high density range BTW; negatives calibrated for trad. cyanotype.)

  10. #20

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    I standardized on 20% AD, but count drops like any other process. I also started pre-making dispersions of watercolor pigment and gum in known and repeatable concentrations. While that's hard to measure by dropper because of the viscosity, it does measure accurately in one of those graduated cough syrup spoons and gives much more consistent results than weighing- unless you have a very sensitive scale.

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