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Writer / Sam Wang
Photography / Sam Wang

Tri-colour gum printing

Always be careful when handling chemicals. Read the health and safety instructions.

Gum printing, or gum-bichromate printing, is a historic photographic printmaking method that is enjoying a revival.

"It is not hard to do but quite difficult to master".

The vast number of variables that affect its outcome means one needs to expend a great deal of time and energy to adapt it to his or her own working environment and to his/her own aesthetic taste. This will be an initial guide, so please be aware that "your mileage" will probably vary. A beginner to the process would benefit from reading the basic gum recipe.

Image left: Unititled 1 , 1980, one of Sam's earliest casein prints, color separated in-camera from a Polaroid.

Basic concepts

Gum Arabic is normally soluble in water. It can also hold pigment - tubes of watercolor are usually just that, pigments in gum, with a little something else thrown in. When ammonium or potassium dichromate is added, gum begins to harden. This hardening is increased also in proportional to exposure to light, especially UV light. So, when a coating of pigmented and dichromate-sensitized gum is dried and exposed under a negative, after "development" (soaking in water), what's left will be a reverse image of the negative. Since a negative of a negative equals positive, you know the rest.

Essentially, the gum's insolubility is proportional to the amount of exposure received under the negative. This of course presumes that the gum was good, that it did not receive a dose of overly powerful preservative, that the exposure was correct for the amount of dichromate sensitizer, that the pigment used did not stain terribly, that the paper had the appropriate quality, plus a zillion other little factors, including what Mark Nelson often referred to, "whether you wore your lucky shorts that day".

Though possible, a single layer of gum rarely gives enough tonal richness to be very satisfying. With multiple printing, a variety of colors can be used. Some of the best examples from the turn of the last century received numerous printings from single negatives, often with a little hand "retouching" for added color variety. Many of these have a Rembrandt oil painting appearance due to the layers of gum on top of one another (the models for such Pictorialist work were often classical oil paintings, but that's another story). Considering that sensitometry was not very developed or understood, that films were mostly orthochromatic, and temperature and humidity were generally not under rigid control, it's nothing short of amazing that good prints could and were made way back then.

Image right: Untitled 2, ca. 1982, color-separated in the darkroom from a color slide.

To increase the color range and perhaps even to mimic the colors of the natural world, it's necessary to separate colors of light into different colors of pigment. My early color gum and casein prints were made from "in-camera" color separation negatives on panchromatic film: with stationary subjects, I would make 3 monochrome negatives, each with a different color filter in front of the lens, first a #25 red, then a #58 green, and last with a #47 blue filter. Filter factor had to be calculated, as well as the film development necessary to get exactly the required negative density range for gum, plus some method to identify which negative was shot under what filter - plenty of chances for something to go wrong. If everything did go OK though, then these negatives were used to print gum with the primary colors of yellow, magenta, and cyan, one on top of the other, in perfect registration. The difficulty and the amount of work involved may explain why there had been very few tri-color gum prints made, recently as well as "way back in the old days."

Today, with digital tools, the color separation as well as the negative making parts have all become very reachable. The aesthetic challenges, however, remain as elusive and hard to master as ever.

Basic digital color separation

Regardless of the origin of your image, digital camera capture or a scanned color file, it is already in RGB colors. The R channel resembles a monochrome image shot through a deep red filter, the G a green filter, and B a blue filter. In Photoshop, just click on one of these channels to see it. Invert the picture and separate the channels (Split Channels in Photoshop) and you have 3 black and white negative images ready to be printed on your inkjet printer.

One might ask why not to use CYMK conversion. My understanding is that it could work, but CYMK is primarily tailored for the printing industry so it carries with it dot gain info and other factors affecting press runs, so RGB is simply quick and easy for personal gum printing use. Also, in printing industry, it is customary to print yellow first, followed by magenta, cyan, and then a black. In my gum printing, I do not feel it's necessary to get the highest fidelity of tones and usually bypass the black.

I have additionally been printing in a reverse order of cyan first, with cyanotype, followed by yellow, then magenta or red. So, all I usually would use are 3 negatives.

Making the negatives

Almost any recent model inkjet printer would do fine for making negatives. For the best results, one really should study Mark Nelson's PDN method. However, I've successfully used many cheap inkjet printers. Teresa VanHatten Granath, when she was studying with me, used a 300 dpi laser printer to make negatives that produce beautiful gum prints.

With whatever printer, it is crucial to develop a curve for your own working environment. The primitive trial and error way to arrive at a curve:

1. Print a gradient on transparency material (I use black ink only on inexpensive overhead transparency film specifically made for inkjet printers)
2. Use this to make a gum print
3. Compare the print to the gradient on the computer and create a curve to compensate for any difference
4. Use this curve to print with gum and then go back to tweak the curve
5. Repeat till the gum resembles the screen in tonal distribution

Be aware that a change of any variable (paper, water, sizing, etc.) will throw all your results off. So, do everything the same way!

In printing the actual color separation negatives, it is helpful to include color identifications of C, or M, or Y. Otherwise it can be hard to tell one from another.

The gum emulsion

These 3 primary colors are used to achieve a complete range of colors:

1. a light yellow
2. a magenta or red
3. a cyan or blue (I use cyanotype)

Most good grade watercolor pigments or powder pigment would work. I dissolve my own gum but any "Gum 14" or liquid gum from photographic supply houses should work, unless, as noted above, it contains too much preservative. So, always test a new batch of gum without adding dichromate to see if it dissolves well. Paper choice is very important also. Most papers designated for watercolor would work well. Be aware that one batch of paper may vary a great deal from another. Again, test to make sure before investing too much time working with it.

Image above: Dead Bird #2, © sam wang 2002, gum print using cyanotype for the blue layer.

For best fidelity, yellow should be printed first. In that case, it is very important to make registration marks on the back of the paper to line up with marks on the negatives or negative masking sheets. If blue or cyan is printed first, then no registration device is necessary since negatives can be placed in register visually. I usually print with cyanotype first, then yellow and magenta. After that I often would go back and print another layer of one or more of these colors to bring them into balance and/or add richness.

Papers shrink a great deal especially during first "development." To avoid severe out of registration problems, preshrink them by soaking in hot (say, 125 degrees F) water for an hour. Let dry thoroughly before use. Sizing with gelatin is also very helpful. However, it is not absolutely necessary if you use a good quality watercolor paper. I usually do not add sizing. The white areas on my prints may not be as clean as if they were additionally sized, but not to the degree to render them unacceptable to me. So, look up and use the readily available sizing and other gum information you can find elsewhere if you wish to size your papers.

The ammonium or potassium dichromate sensitizer is added after the gum/pigment ratio is determined. I use ammonium dichromate in powder form while many others use a liquid concentrate. In either case, keep track of the dilution rate. Make sure that no more than equal amount of water is added to the Gum 14 stock solution. (In my case, I add 1 part Gum Arabic powder/granule by weight to 2 parts water by volume to make my stock gum solution, and a small amount of 100% Thymol to preserve it).

Image above: Them Apples Combo is a combination of 3 gum images of strange apples - gum/cyanotype, 2007.

Coating

Coating can be done in a somewhat dim room light. Coat rapidly and evenly with a soft, springy Hake type brush. For larger surfaces, a house paint roller may be used. Lay the print to dry flat for a few minutes, after which it can be hung on a line to dry in the dark.

Exposure

Expose with UV lights immediately after the emulsion is dry for best results. If absolutely necessary, dried paper can be put in plastic bags and stored in the refrigerator for up to one week before exposure, though quality may suffer. Measure and record exposure by using Stouffer or the Mark Nelson Step Tablet, timer, or a light integrator.

Development

Float-develop the print faced down in room temperature water. Transferring it to fresh water after a few minutes will accelerate clearing. Proper developing time may vary from 10 minutes to hours. Gently rinse the print with cool water and hang to dry.

Abbreviated work-flow

• Prepare image in Photoshop
• Invert and size the image
• Split channels and identify each by adding a letter or two to the edges
• Apply curve
• Print negatives with the appropriate inkjet Overhead Transparencies
• Preshrink paper
• Add pigment to gum to arrive at right proportion
• Add sensitizer
• Coat paper and let dry
• Expose under the appropriate negative with UV light source
• Soak/develop
• When dry, coat with the next color and so forth
• After all 3 colors have been printed, determine in good light whether further printing is necessary to balance or enhance some of the colors
• Last, throw out the failures and write down what to do next time for better results

Hopefully a few prints will be good enough to keep. Allow for surprises. It's not a predictable process and don't expect to make print to compete with inkjet printed images or C-prints. Don't lower your standards, but some of the technically bad prints may be beautiful in their own ways. Try not to overlook them.