Gum Bicromate from Fred Endsley

GUM BICHROMATE PRINTS

Historically, the gumprint is an offshoot of the old style of carbon printing. Gumprinting has survived among both technicians and artists because of the variety of possibilities in the technique and quality of the obtainable image ranging from very fine color separation to a controlled Sumi or loose painterly application. The comparative inexpensiveness of the materials, the ease in applying the colors of your choice, both generally and locally; and the ability to work into the surface of the image with other media have maintained the gum prints popularity, although it is a far more difficult process than Blue or Van Dyke printing.

Materials: Any paper which will stand repeated soakings in water and has a slight tooth; coarser papers result in more break-up of the image and also tend to trap unexposed emulsion (a good grade watercolor or rag fiber etching paper - e.g., Reeves, D'Arches, Strathmore - works well).

• Negatives: As this is generally a contact print process, enlarged ortho film or thin paper negatives are required; negatives should be slightly higher contrast with good blacks and clear whites, although care should be taken to include as wide a range of middle tones as possible because the gum process can reproduce a fine grey scale from a properly made negative.

• Brushes: The type of brush you use will determine the kind of marks you make; traditionally it should, above all, be very soft. A 2-inch camel hair sizing brush works fine. (A $75, 5-inch Grumbacher badger hair blending brush is nice, but this seems a little extravagant). A smaller brush is necessary for mixing and more detailed application, and the emulsion can also be applied by sponge, rag, fingers, etc.

• Pigment: Tube water colors seem to be the easiest method of measuring and mixing color; I recommend getting an inexpensive brand as you will use them quickly (A fifteen tube set of Guitar watercolors cost 2 or 3 dollars; better brands, Windsor Newton, and Permanent Pigment work better but cost much more). Todd Walker grinds and mixes his own dry pigments, and his measurements for those appear below; if you're energetic enough to try this, you will also need a mortar and pestle.

• Palette Knife: notched at fractional-inch intervals is the easiest method of accurately measuring out tube colors.

• Graduate: should be small and divided into single "cc" or "ml" measurements .

• Sizes: (Optional - see procedure below.) Liquitex gesso, spray starch, or more traditionally Knox gelatin.

• Size Hardener: (Optional - see procedure below.)Potassium Alum as in clearing bath below (avoid using formaldehyde hardeners).

• Sensitizer: One of the Dichromate salts:Ammonium Dichromate is relatively light sensitive and most widely used (Potassium Dichromate is about half as sensitive; some people use Sodium Dichromate as it doesn't stain as much).

• Hardening and Clearing Agent: (Optional) - Potassium Alum

Procedure:

1. (Optional) Pre-shrink the paper by soaking it for 15 minutes in hot water, and allow it to dry completely. This will prevent your paper from shrinking between coatings and throwing off the registration.

2. (Optional) Size the paper. Use gesso, spray starch, or best:mix three packets of Knox gelatin (21 grams) per one quart of warm water; allow the gelatin to swell. Pour the warm gelatin into a tray and immerse the paper until it is fully soaked. Lift the paper gently out of the solution and pull it over a glass rod, broom stick, or the side of a tray; repeat this with the other side of the paper as well. This procedure removes any air bubbles and lumps which may be present. Allow to fully dry. Sizing is a good idea as it preserves the paper throughout repeated soakings, and provides for smoother coating and easier washing. Sizing can be done twice for optimum results.

3. (Optional) Harden the dry, sized paper by soaking it for 2 minutes in a hardening/clearing solution of 25 grams Potassium Alum to 1000 cc water, rinse in cool running water for 1-2 minutes, and allow to dry.

NOTE: The above 3 steps will have to be done well in advance, and require a lot of time and energy, but are well worth it if you are after a classical, well-registered gumprint. However, if you are willing to sacrifice a little "quality", acrylic spray workable fixative or spray starch are decent sizes if not sprayed on too heavily, and they dry in minutes.

4. Coating. This should occur under safe light conditions. (Buglight at 5 feet or more will do well.)

• Pre-mix and set aside ammonium dichromate sensitizing solution:
  • 25 (250) grams dichromate
  • 100 (1000) cc water

• Mix:
  • 10 cc of gum arabic solution.
  • 1/10 gram of tube pigment (or 1 to 2 inches of tube-squeezed pigment.) This varies according to color and quality of pigment -experience is the best teacher here.

• Inspect your color under white light as this cannot be done accurately once the sensitizer has been added. Gum/pigment solutions can be stored indefinitely until the sensitizer is added.

• Mix well the 10 cc gum and pigment solution with 10 cc of the Dichromate sensitizing solution.

• Apply evenly and thinly onto the desired area of the paper, and allow to dry completely in the dark (should dry to a matte surface within 20 minutes - best results come from exposing promptly as soon as the emulsion is dry).

  More than one color can be applied in one coating, however, too thick a coating through overlapping colors will tend to be underexposed and will flake off in the washing. Also some of the darker pigments tend to be coarser, so less is needed in the coating mixture. Wash out your brushes well after using, and remember that the Dichromate salts are poisonous - DO NOT GUMPRINT UPON YOUR BODY!

5. Exposure: A variety of light sources are adequate including sunlight, photoflood, ultraviolet (unshielded), fluorescent, Nuarc, etc. UV at 5 inches should take around 5 to 7 minutes; open shade works very well for about 10 to 15 minutes; direct sun works less well in 2 to 5 minutes. Make sure your contact printing apparatus is tight otherwise the light will seep around the negative.

6. Development: This part is somewhat touchy; the main problem is getting the highlights to clear. Ideally, with perfect coating and exposure, the print should develop itself in gently running water with no agitation in 10 to 20 minutes.

My own method is to float the print face down for a few minutes in room temperature water, allowing most of the unexposed emulsion to dissolve; then warm up the water 10 or 15 degrees and wash the print gently (not directly) with a hose (this should soften up the gum and allow the highlights to wash clear); if there are still some sticky highlights, gentle brushing with a soft brush sometimes help, although care should be taken not to brush off the delicate gum image. Finally return the water to a cooler temperature, float the print face down for 15 to 30 minutes, then allow it to dry.

Once the paper is dry, repeat the process, if necessary, to build up the image. Think of the process as a multiple printing process - rather than trying for a final image in one run, try to build up the densities and image layers gradually.

7. Clearing: (OPTIONAL) When the print is completed, and dried, soak it for 2 to 15 minutes in the hardener/clearing solution of 25 grams Potassium Alum to 1000 cc water, wash in running water for 30 minutes, and allow to dry.

Todd Walker's Dry Pigment Measurements (Weight of dry pigment per 50 cc of gum arabic)

Can be mixed and stored for 2 to 4 weeks without ammonium dichromate.

Ivory Black 5.0 grams
Van Dyke Brown 4.0 grams
Burnt Umber 4.0 grams
Pthalocyanine Blue .6 grams
Pthalocyanine Green 1.0 grams
Alizarin Crimson 1.2 grams
Cadmium Red 3.0 grams
Hansa Yellow .5 grams
Chocolate Brown Oxide 4.0 grams
Thioindigo Violet .6 grams
Ferrite Lemon 4.0 grams
Titanium Dioxide (white) 5.0 grams

These are some good and bad water color pigments for gumprinting in terms of permanent vs. fugitive - somewhat in order of preference:

GOOD:

Cadmiums - Red, Orange, Yellow
Cobalts - Yellow, Blue, Violet
Chinese White - Zinc White
Cerulean Blue
Yellow Ochre
Burnt Umber
Alizarins - Red (Crimson, Lake, Scarlet)
Ivory Black
Lampblack
Manganese (Blue, Violet)
Naples Yellow
Permanent Carmine
Pthalocyanines - Blue, Green
Raw Sienna
Burnt Sienna
Red Oxide
Vermillion - Heavy, sometimes turns black
Viridian
Strontium Yellow

BAD: Carmine - fugitive
Chromes - anhydrous
Ultramarine - changes
Prussian Blue - erratic
Hooker's Green - course, fugitive
Magenta - fugitive
Moss Green - chromete
Olive Green - course, chromete
Rose Madder - thin
Emerald Green - poison
Van Dyke Brown - course, fugitive

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