HereĀ“s an evolution of a multilayered gum print. The final print actually became an exercise of a single negative technique, where the final tone range is achieved by printing several layers on top of each other, using the same or different pigments each time.
Once again I’m using my long-term “standard test image” of the old tree. There’s been a Photoshop correction curve applied to the positive grayscale image. This time the curve isn’t based on any particular measurings, but is more or less guesswork – its meaning is to flatten contrast, because gum emulsion cannot produce very long density range in a single layer.
The paper was Guarro Casas 250g, sized with gum arabic and potassium dichromate 1+1 exposed for 3 minutes. The light source used in exposing was Osram Ultra-Vitalux, from one meter distance. Pigments were powder pigments by Maimeri. The sensitizer was 13% potassium dichromate solution.
The proportions of pigment/gum/dichromate and the time for the first exposure were mainly guesswork. All that came out were the darkest shadow tones. The goal was to get a full scale image (more or less), but the pigment concentration and the exposure time probably were not suitable for this. On the other hand, gum printing is such a “progressive” process that often the print is not ruined after making an unsuccessful layer, but corrections can be made to the next layers.
Not much better – deeper shadow tones, but middle tones and highlights lacking.
By using substantially less pigment and exposing much longer the tone range gradually extended.
Warming the general tone with a red layer (mix of carmine & yellow).
A little more depth with another layer of black.
Finally the shadow tones were deepened and cooled down slightly with prussian blue and a shortish exposure. As the print was considered finished, the dichromate stain was cleared in a bath of 1% potassium disulfite.
Normally you wouldn’t need a stack of six gum/pigment layers to make a print like this. I bet a more experienced gum printer would do the same with three layers. Anyway, the fun lies in interactivity – after each layer you can see what you got, and plan the following layers accordingly.
Paper meant to carry a gum bichromate image will need shrinking and sizing – at least if more than one print layers are planned. To accurately register the layers the paper should keep its dimensions in each consecutive printing. This can be achieved by shrinking the paper before printing – we do this by soaking the paper in hot water for an hour or so.
The more printed layers (and washes in between), the greater the probability to weaken the paper’s inherent sizing. Finally pigments will adhere to paper fibers and stain them permanently. So most papers need some sort of extra sizing when printing multiple gum.
Perhaps the majority of gum printers use gelatin for sizing, but there are other possibilities too. We thought why not use the chemicals that we have on hand anyway: We coated paper with a mixture of gum arabic and potassium dichromate (with no pigment) in 1:1 proportion and exposed 3 minutes with an Osram Ultra-Vitalux lamp, from a distance of one meter. This sensitized gum arabic hardens with exposure, and after washing the paper in water for half an hour, all soluble gum/dichromate should be washed away. The sized paper will then stand several layers of printing without pigment stain.
After the exposure the dichromate tends to stain paper slightly – if so, water can be added to the size. With proper gum/dichromate/water mix, suitable exposure and development, the paper tone should keep unchanged.
We tested this “exposed sizing” with our three UV light sources: Eiri 2000W UV exposure unit, array of Sylvania 20W BL-350-UV tubes, and Osram Ultra-Vitalux 300W lamp.
When printing gum bichromate we need high quality pigments to be mixed with gum arabic. Normally folks doing gum are using artists’ watercolors or powdered pigments for this. When shopping for pigments at our local art store (in a small town) the selection seems somewhat limited – here’s what we have found so far:
Prussian Blue (PB27)
Alizarin Carmine (PR83) I understand this pigment has poor lightfastness, and it doesn’t show any more in Maimeri catalogs.
Cadmium Yellow Medium (PY37)
Ivory Black (PBk9)
Lefranc & Bourgeois Linel watercolours
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Gum printing (Gum Bichromate, Gum Dichromate) is a photographic printing process where pigment is mixed with liquid gum arabic, and dichromate (ammonium, potassium or sodium dichromate) is added as a sensitizer. This solution is the gum sensitizer to be spread on a sheet of paper. Exposure to UV light through a negative makes the sensitized gum arabic harden. The soft, unhardened gum (and the pigment it contains) is washed away in water, and what remais is an image formed by pigment and gum arabic. Here is a description of the process.
Since gum printing is practiced with so many variations today, we will concentrate here on describing our methods, difficulties in printing and different solutions to overcome problems.
I guess the images below will throw some light on the difficulties often experienced by a novice gum printer. I tried some gum printing in late 1980s (more or less monochromatic work), and later on some 4-color prints from separation negatives. Since then many things have changed – digital negatives have revolutionized the practice of gum printing, as they have done with so many other alt-processes too.
Well, I feel like a beginner again when starting to re-learn gum printing. Our intention is to prepare a study module on gum printing for art students, and I need to revive my printing skills. In the beginning there are some important decisions to make, such as choosing the paper stock and pigments to use. In my case the choices made didn’t quite match up, and I need to recheck them.
My first paper was Bergger COT-320, a fine paper for many processes like cyanotype, vandyke, platinum… but obviously not so great for gum printing. I first tried it unsized, and achieved a terrible, blotchy image. Probably the paper should be sized with gelatin or something… The pigment was Maimeri Ivory Black powder.
So I thought I’d try another paper; took a sheet of good old Guarro, which has worked well in any process we have tried so far. I sized the paper lightly with a mixture of gum arabic, potassium dichromate, and water – and hardened it with an exposure with UV light. I also changed the pigment to Maimeri Venezia series (yes I know the rule: change only one variable at a time, but…) because I wanted to try the only tube watercolors available at the local art supply shop. There are 12 tubes in a plastic box, 15 ml each. I chose Raw Umber as my first pigment.
What I expected to see when squeezing the tube was deep brown umber, but this hue seemed somehow watered or thinned down, it was too light and powerless, in my opinion. Earlier, years ago, I had used Linel pigments by Lefranc & Bourgeois (recommended by Stephen Livick), which, as I recall, were much stronger straight from the tube. Anyway, I weighed 2 grams the Maimeri pigment, and added 10 ml gum arabic (VARN 14 Baume). This mix was an estimate of what I was used to with Linel pigments. I then sensitized this solution by adding 13% potassium dichromate in 1:1 proportion, and coated a sheet of paper with it. My negative was quite low in contrast, printed on transparency with an Epson 1800.
As expected, the result was quite weak. I suspect these pigments cannot produce any deeper tones (as a single layer), but nevertheless I tried to increase the sensitizer’s pigment concentration. On the next try I used the pigment with no added gum, adding just the dichromate.
Worse still! The concentrated pigment partly stained the paper base, partly flaked off during water development. Looks like the first thing to do is to find another brand of pigments.
To be continued soon…