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  1. #1

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    Gum Bichromate Reality Check

    Hi,

    I have been working with gum for several months and have gotten to the point that I can make a decent monochrome/duotone print but am not yet at the Tricolor level.

    What I am interested is if I am more or less right in how I percieve the relative importance of the printing variables. Since gum is a tricky process to learn, having a decent vision of what is going on can help to make it more predictable.

    I am using potassium dichromate and lana aquarelle cold press paper. I use compact fluorescent UV lamps to expose. I make digital negatives and apply Christina Z. Anderson's simple gum curve found in alternativephotography.com. I don't see the need to consider more sophisticated curves until I gain more skill with the process.

    The most important variable appears to be time as exposure time determines the ability of the gum to stick to the paper and the extent to which the highlights are colored.

    The second most important seems to be humidity. Here in North San Diego County, CA wild shifts in humidity can occur rapidly. I have to watch the National Weather Service and my hygrometer pretty carefully since humidity seems to dramatically affect printing times, and low humidity can make coating an awful experience. Below 40% I humidify my paper in a water based humidification chamber before coating and before exposure and I run a humidifier. This seems to bring a modicum of predictibilty to the exposure process (low humidity=long exposure-high humidity=short exposure.

    Pigment and pigment concentrations seem to be the third most important factor and since each pigment varies in how it bleeds out, tends to affect exposure and God(dess) knows what else, I ran some simple exposure time test strips and use them as a rough guide as I slowly get to know each pigment.

    Temperature of wash water and the ambient temperature seem to be the least important variables. There are variations with temperature and as summer approaches I expect to notice changes to the whole process. However if I were to compare temperature and humidity I would guess that humidity has a strong linear effect on the process while temperature has a weak linear effect.

    Anyhow those are the large factors I try to conceptualize and my way of thinking about them. I would be interested in any comments or suggestions for a better approach

    Thanks!

  2. #2

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    Oops, I managed to forget development as a variable :-) It occupies its own special place in the list of variables as it can correct and compensate for errors in the others so I would probably list it as the most important of all. So much for missing the obvious.

  3. #3

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    Double Oops, I also managed to leave out the issue of pigment concentration/exposure as high concentration, short exposure prints toward shadows while low concentration, long exposure prints toward highlights. I'm not sure this even fits into a hierarchy since it is a way of working with time and with pigment to affect the layers that make up a print. Perhaps the complexity of this and the difficulty of reducing the process to simple concepts points to one of the great beauties of the process, that is, how one must work intuitively yet methodically to handle it at all. Well, each successful print is a "small miracle" at least to me at this stage in my relationship to Gum.

  4. #4

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    Quote Originally Posted by Jon Harwood View Post

    The most important variable appears to be time as exposure time determines the ability of the gum to stick to the paper and the extent to which the highlights are colored.
    i was wondering about that lately. some time ago i forgot a print under the uv-unit and accidentally gave it a 20minute exposure, instead of my normal 6.
    i was really surprised, because i didn't notice any differences in the image, nor in the developing times (maybe it went up from my normal half/three quarters of an hour up to maybe an hour or a little longer for the overexposed one).
    i was really surprised that the print wasn't ruined, but perfectly useable.

  5. #5
    donbga's Avatar
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    If you will post this question on HybridPhoto.com I can discuss this topic with you since it involves digital negatives.
    Last edited by donbga; 01-31-2009 at 03:03 PM. Click to view previous post history. Reason: Typo
    Don Bryant

  6. #6

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    Yes, I get phenomena like that which is why I am trying to figure out the relative importance of the variables. When something unexpected occurs I try to figure out which variable was changed and what impact it had.

    I would have been quite surprised at a 3x increase in exposure yielding a usable print with a relatively small change in development.

  7. #7
    donbga's Avatar
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    Quote Originally Posted by Jon Harwood View Post

    I would have been quite surprised at a 3x increase in exposure yielding a usable print with a relatively small change in development.
    Me too, and I expect any experienced gum printer would be also, unless ones standard development time is extremely long.
    Don Bryant

  8. #8

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    Quote Originally Posted by Jon Harwood View Post
    Double Oops, I also managed to leave out the issue of pigment concentration/exposure as high concentration, short exposure prints toward shadows while low concentration, long exposure prints toward highlights. I'm not sure this even fits into a hierarchy since it is a way of working with time and with pigment to affect the layers that make up a print. Perhaps the complexity of this and the difficulty of reducing the process to simple concepts points to one of the great beauties of the process, that is, how one must work intuitively yet methodically to handle it at all. Well, each successful print is a "small miracle" at least to me at this stage in my relationship to Gum.
    Jon, I don't really have any comments, except to say that I enjoyed reading about what you're doing and that your approach is the one most likely to achieve lasting results, in my experience. And I really liked the way you put this above, very cogent and insightful. The only small thing I'd suggest is that I wouldn't recommend using someone else's curve, but that's a small suggestion; everything else looks good to me on first pass. It would be great if we could see some of your prints as you progress.
    Katharine

  9. #9

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    Thanks Katharine. I am coming to the realization that I will be needing to develop my own curves and that will be the next frontier I embark on with gum.

    As I start getting some quality prints I will post them. At present I only have one show grade image.



 

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