Gum Bichromate

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Shopping For Pigments

Posted by on 12 Nov 2008 | Tagged as: Gum Bichromate

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:

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Maimeri powdered pigments

    These were purchased at the local art shop over ten years ago. I only tried them with a few gum prints at that time, but they have kept well in tightly closed glass jars. I planned to make 4-color gum prints, and chose the pigments as close to CMYK process colors as possible. The pigments are:

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)

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Lefranc & Bourgeois Linel watercolours

    High quality French watercolors. Hard to find at art shops – I had friends buy them whenever they visited France. I’ve got a few 5 ml tubes left, but I can’t find them any more on Lefranc & Bourgeois web site 🙁 …
    I have used these Linel pigments for tricolor gum:

Hortensia Blue
Ruby Red
Helios Yellow

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Maimeri Venezia watercolours

    There are 12 watercolor tubes in a plastic box. We didn’t like these at all, so we quit using them for the time being. Let’s see what we can do with them later…

Hard Gum

Posted by on 05 Nov 2008 | Tagged as: Gum Bichromate

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.

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Here are two of my first 4-color gum prints, made in 1996 or 1997. Different brands of pigment (which I can’t remember any more) were used in each print, hence the differences in color balance. I was using sort of hybrid negatives – CMYK channels of the original image file, split to grayscale images, were printed on paper with Canon Bubble Jet technique of that time. The prints were then photographed with a process camera on continuous tone negative material – analog negatives from digital files. The neg size was reduced to 50% to diminish inkjet graininess.

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.

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Paper: Bergger COT-320, Pigment: Maimeri Ivory Black, Light: Osram Ultra-Vitalux (from 1 m.), Exposure: 6 min., Development: 50 min.

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.

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Paper: Guarro, Pigment: Maimeri Venezia: Raw Umber, Light: Osram Ultra-Vitalux (from 1 m.), Exposure: 20 min., Development: 40 min.

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.

kumi-050908-2.jpg
Paper: Guarro, Pigment: Maimeri Venezia: Raw Umber (concentrated), Light: Osram Ultra-Vitalux (from 1 m.), Exposure: 12 min., Development: 30 min.

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…

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