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Thread: Gum Bichromate

  1. #21
    vintagepics's Avatar
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    Quote Originally Posted by BrianShaw View Post
    Sometimes that can be a great personality trait! Good luck with your new venture!

    What are you using for a light source and negative registration method?
    I was going to be using sun for my light. Its worked well with my salt prints and calotypes. As far as a registration system, I have not given that much thought yet. Im open for any suggestons.
    Rick Lanning
    Retired Crime Scene Photog.

  2. #22

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    Quote Originally Posted by artonpaper View Post
    This is true and photoshop does have a CMYK work space. But if one separates the channels in RGB then inverts those to negatives, those negatives can be printed in their complimentary colors, Red to cyan, green to magenta, and blue to yellow. It's also true that without a K or black negative, the shadows may be muddy. A K negative is most easily obtained in CMYK.

    In addition, I think it very good advice to start with monochrome. I don't think gum is for four-year-olds. It is necessary to learn the technique and establish a good exposure range, and pigment to gum mixes.
    If negative cost is a factor (if you are using Pictorico, for instance) you can cheat on the "K" negative by using whatever neg you use for Cyan and substituting black pigment. Use 50-60% of your usual exposure, and the black will only sit in the shadow areas and not affect the "real" cyan in the rest of the print.

  3. #23
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    If I may wax poetically on color theory for a moment...

    Our eye's have rods & cones, and of the cones there are 3 types; sensitive to red, green or blue. These sensitivities cover (or rather constitute) the visual spectrum of light. Nature's infinite & continuous spectrum of light is reduced to 3 discreet & finite channels in our eye. This is color photography's goal as well.

    So how do we transmute a visual image of nature onto a piece of paper?

    First we must replicate the sensitivities of the eye with our recording media, film. Panchromatic sensitivity was the longest held stumbling block to color photography. Color separation filters mimic the sensitivities of our 3 cones, and the 3 layers of a color film strive to do the same, as does a digital sensor.

    Once we have separate records of the red, green & blue light in a scene, we have to assemble these in a way that will reconstitute the color sensation of the original scene. We will always take separations from the red, green and blue light present in the scene, but to make a print on paper (or a slide) we can't use pigments of these colors since the combination of any two primaries would result in black. This sounds contrary, but red absorbs blue & green, and blue absorbs green & red, and so on. If you overlay any 2, the result is complete absorption.

    We have to use the minus colors (the color complements, secondary primaries, primaries in the sense that we all learned in grade school). Cyan is minus-red, magenta is minus-green, and yellow is minus-blue.

    Cyan freely transmits green & blue (in theory at least) and absorbs red as the white viewing light hits the print and is reflected back from the white surface of the paper. In this way only the red light (present in the white viewing light) is modulated by that layer. That's why the red record becomes cyan, the blue yellow and the green magenta.

    CMY allows us to affect the absorption of only 1 of the color sensations at a time. The combinations possible therein can recreate all color sensations that we are capable of experiencing (again, in theory, and in practice it's pretty darn close).

  4. #24

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    Quote Originally Posted by vintagepics View Post
    I was going to be using sun for my light. Its worked well with my salt prints and calotypes. As far as a registration system, I have not given that much thought yet. Im open for any suggestons.
    When using paper as a substrate, the easiest method to register visually through the print over a light table. Even with preshrinking there will be some change in the paper (although at 8x10, not very extreme) and this approach will allow for you to select the place where you want the maximum sharpness.

  5. #25
    vintagepics's Avatar
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    I have read several different methods for sizing the paper, but have any of you ever used albuman for this?
    Rick Lanning
    Retired Crime Scene Photog.

  6. #26

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    Quote Originally Posted by vintagepics View Post
    I have read several different methods for sizing the paper, but have any of you ever used albuman for this?
    I found that albumen wasn't hard enough and softened in the water. The best, easiest and cheapest size is Gamblin PVA. I prefer the matte look of hardened gelatin over the glossier PVA, but I cant deny the convenience factor of a pre-mixed room-temperature size.

  7. #27
    vintagepics's Avatar
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    Quote Originally Posted by GumPhoto View Post
    I found that albumen wasn't hard enough and softened in the water. The best, easiest and cheapest size is Gamblin PVA. I prefer the matte look of hardened gelatin over the glossier PVA, but I cant deny the convenience factor of a pre-mixed room-temperature size.
    Wow! That Gamblin PVA looks great. I wonder how it would do with my calotypes? sure beats breaking all those eggs to make albumen, and its cheap too. Thanks.
    Rick Lanning
    Retired Crime Scene Photog.

  8. #28

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    Another distinct advantage of PVA has is that you can easily apply it in between coat of gum to "fix" things in place. Nothing is more frustrating (well, lets just say it ranks high), than putting on the 6th layer of gum emulsion and watching the previous 5 lift right off the sheet. After the 4th layer, I sometimes use a foam roller to apply 1:1 PVA. Not saying you need 4+ layers, but the option is nice!

  9. #29

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    Gum is easy to do and a lot of fun! I havn't done 3 or 4 color gum, but i have done cmy (and sometimes k) in a few other processes and i almost always prefer printing without the k (black).

    My paper of choice is fabriano artistico (140/300lb hot press). Its non staining, so you don't have to sub it. If you keep your 'development' times short (less than 15 minutes) then you don't even have to preshrink, but i wouldn't risk skipping that.

    Have fun!

    also: i use thumb tacks for registration, one on the top left corner, and one on the bottom right. I realign the print by getting the tacks back into their holes, slap a piece of tape across the top, then remove the pins and put it in my contact print frame (wishful thinking: I dont have a frame i just shove it under some heavy glass in my uv box)

  10. #30

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    Quote Originally Posted by vintagepics View Post
    I was going to be using sun for my light. Its worked well with my salt prints and calotypes. As far as a registration system, I have not given that much thought yet. Im open for any suggestons.
    For registration I have used GumPhoto's method of re-registrating over light table before clamping down the frame. It works OK, as I'll bet the thumbtack method doest too. I always felt better control over any necessary adjustments due to paper growth/shrinkage.

    For exposure I used sun too. I am currently in a hiatus from alt printing (for a variety of reasons) but the primary reason is frustration with sunlight exposure. UV content of sunlight varies by latitude, time of year, and time of day... as you probably are well aware. In Southern California we have the additional complication of heat, sometimes extreme heat. I noticed an affect of heat on gum printing but not on iron-based processes. When I resume I'll build/buy a UV light source for more consistent (and, hopefully, faster) exposures. There are lots of variables in gum printing and that is one that I'd like to get under greater control.

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