Gum Bichromate

click on images for larger examples

gum bichromate, from a film negative from a black and white film original; one sponge brush-coated layer on graph paper; about a teaspoon of vacuum plated mica for pigment per ounce of sensitizer (fairly thick and crusty); exposed for a few seconds in a blueprint machine (light source for commercial blueprints) (20k jpeg)	gum bichromate, made with film negative; three layers of a mixture of watercolor and vacuum plated mica pigments (about a teaspoon to an ounce of sensitizer per layer); coated on black charcoal drawing paper with a sponge brush; each layer exposed for about 10 seconds in a blueprint machine. (116k jpeg)
gum bichromate made from four paper computer color separations from an original color slide; with four color layers of about 1-1/2 inch of tube watercolor for each ounce of sensitizer per color; coated with a sponge brush on Somerset paper; each layer exposed for about 2 minutes in sunlight.	gum bichromate greenish watercolor pigment on gray paper, with colored pencil accents (15k jpeg)

all images © wendy mukluk

Safety

These chemicals can be harmful or fatal if swallowed.
Avoid getting chemicals on skin or in eyes.
Wash in running water if this does occur, and seek medical help immediately.
Avoid breathing fumes and dust.
If you are sensitive, wear mask and gloves.
Seek medical attention if adverse reactions occur.
Do not eat or drink while working with chemicals.
Work on a clean surface that can be washed or put down newspapers.
Wash hands with soap and water when done.

Simple Step-by-Step Instructions
1	Mix one part gum solution with one part bichromate solution.
2	Coat and dry paper in subdued light.
3	Expose approx. 1 minute in bright sun; approx. 30 units with arc light or platemaker; or leave sitting out 15 minutes or an hour in room lights or outside on a cloudy day. Times are approximate; make test prints first.
4	After exposing, Develop. Place face down in warm water. Carefully turn over and check. Take print out when it looks good to you. Remove or soak more, as long as overnight. Gently splashing or brushing can speed development but emulsion is fragile, so be careful.
5	Dry flat or hang up.
6	Clean up: Rinse out cups, brushes, spoons, trays, etc. Wipe up spills. (bichromate solution is poisonous)

Detailed Instructions

Materials and Equipment needed

Safety Equipment: An apron and rubber gloves are recommended;
mask and goggles for splattering, airbrush
A clean surface to work on that can be washed, or put down old newspapers.
A place for coated prints to dry, in subdued light
Ammonium or potassium bichromate, gum arabic, and pigments
Measuring cups and spoons
A container for mixing the soultion
Plastic spoon for mixing
Brushes for coating sensitizer on paper
Paper
Negatives
A light source and print frame
A timer or clock
Tray, slightly larger than the paper size, and running water
Masking tape
Pencil for registration or notes
A place for prints to dry undisturbed
Sponge, rag, mop, soap for cleanup

Stock solutions

Make two solutions:

gum solution:
1 ounce dry gum arabic to 2 ounces water.
This is an organic substance and will go bad; mix small amounts at a time and refrigerate. 14 degree Baume gum acacia solution used in lithography may be used straight from the bottle instead.

bichromate solution:
1/2 ounce potassium bichromate or ammonium bichromate to 5 ounces water. This should be a saturated solution, but can be diluted somewhat--experiment. Ammonium bichromate is more sensitive to light than potassium bicromate. Bichromate is also sometimes called dichromate.

Label containers clearly.

Sensitizing

Make sensitizer from one part gum solution, one part bichromate solution, and some pigment.

Use any pigments that mix in water, such as watercolors, poster paint, or powdered pigment. Amounts of pigment vary. Some possible starting points are: One part powdered pigment to two parts gum bichromate solution; or a one inch strip of tube watercolor to approximately one ounce of gum. Delicate effects may be obtained with small amounts of transparent colors, and interesting thick crusty images may be made using large amounts of powdered pigment or fingerpaints.

To avoid lumps, mix pigment into a small amount of gum solution first, then add to the rest of the solution.

When mixing exact colors, mix the pigment and gum first, and then add the bichromate after the proper color has been achieved. The bichromate is bright orange, so the mixture has an orange cast. This orange color washes away during development.

In subdued light (darkroom, schoolroom with lights off, shades closed, etc), pour or spoon small amount of sensitizer onto paper, and brush evenly to coat paper. Work quickly; the solution dries quickly, and brush marks will show more after continued brushing. A wide sponge brush works well. A long glass rod can be used instead to spread the sensitizer. Stir solution frequently since pigment will settle.

Dry flat, still in subdued light.

Exposure

Place negative on paper and expose to sunlight, room light, arc light, floodlights. Use small test prints first to determine exposure time. Depending on light conditions, density of negatives, paper, coating techniques, and other factors, exposure can be from a few seconds to several hours. Try 1 - 5 minutes in direct sunlight or 30 - 60 units in violet colored arc light as a starting point. Depending on pigments and papers etc, the image may or may not be visible after exposure.

Processing

Place paper face down in warm water. After several seconds or a minute, turn over carefully to check development. Very careful splashing or brushing on certain areas can bring out details, but the wet image is very fragile and easily destroyed. An overexposed image is quite tough, however.

Development can take only a minute to overnight. With short exposures, the image may be complete as soon as the paper is turned over. Long exposures can produce images which need to soak a long time and possibly be helped along by hosing or splashing to be finished. When the print looks good to you, take it out and dry it flat or hang it up, whichever is more suitable for the image.

In some cases, some images might bleed or slide off the paper if hung up to dry, while other times, water may pool up and distort images on prints left to dry flat. This can be caused by many things. Sometimes a short exposure time can result in a fragile image, or the type of pigment and paper, humidity of the air, and other things can make wet images more unstable than usual. Putting down a thinner coat of sensitizer in the beginning, or using longer exposure times might help. Also try hanging up the print for a few minutes to drain most of the water, and then check it. If the image is starting to bleed or slide, take it down when it is damp but not dripping and let it finish drying flat. Check it a few more times while it is drying, and if the image is pooling up or oozing away, use a fan or a blow dryer to speed up the drying process. A fan or blow dryer is especially helpful on damp days or if you are having a lot of problems with sliding, distorted images.

A Small Amount

20 ml of sensitizer will make approximately five to ten 8"x10" prints.
Use 0.8 gr (about 1/8 tsp) bichromate to 10 ml water,
and mix with 10 ml gum solution and pigment as desired.
Coat, expose and process as above.

Paper

Use any paper that can stand soaking, such as many watercolor, etching, and charcoal papers, commercial offset cover stock, or heavier sketchbook pages.
If unsure, test a small paper sample by soaking it in water at least an hour, or overnight, then pick it up and turn it over several times. If it tears easily or falls apart, it is unsuitable.
Colored papers may bleed or fade.

If the paper is very absorbent or unsized, size the paper first:
Mix 1 teaspoon of cornstarch or arrowroot with a small amount of cold water to evenly wet it, then add 1 cup boiling water. Brush evenly on paper. Let dry.
Spray starch may also be used to size paper.
Many commercial papers do not need sizing.
If unsure, make a small test print. If sensitizer immediately soaks into the paper or if the image is very faint or looks like it sank under the surface of the paper, sizing is needed.
Put sensitizer on same side of paper as sizing.

Negatives

Continuous tone or line negatives and positives may be made in a copy camera or by projecting small negatives or slides onto sheet film in an easel under an enlarger. Any graphic arts film, line film or copy film may be used, such as Rapid Access, Kodalith, LPD4, QPD4, Kodak EL. See the high contrast film page for more details. These may be obtained from a lithographic supply company.
Process film according to manufacturer's instructions, OR For continuous tones on high contrast film, use a diluted paper developer such as Dektol (mixed 1:1 with water, or even more diluted, such as 1:4 or 1:10), or use halftone screens when making the copy negatives. (see the halftones page for more information.)
When making large negatives in an enlarger, it may be necessary to project onto a wall or floor, and to make an easel. Use a big piece of cardboard with the film size drawn on it, and some masking tape to hold film if necessary.
Ortho films require a red safelight. Red cellophane may be used over a yellow darkroom safelight.

Other methods for making negatives

Photocopy images or make computer printouts onto clear plastic (transparencies, overheads, mylar).
Piece together several 8"x10" sheets for larger images. Overlapping edges will show up as less exposed areas on print. If this is undesirable, cut pieces to fit exactly. Secure with tiny pieces of clear tape in unobtrusive places. Or, use overlaps and pieces of tape as part of the design.
Thin paper printouts may also be used, but will need long exposures, approximately twice as long as needed for clear film. The paper grain might show up in the image.

Draw or paint on clear plastic or thin paper. Try marking pen, India ink, film opaque. Shadings in pencil may or may not not come out well.

Use paper or rubylith and cellophane cutouts or make photograms. Opaque materials will block the light and appear as blank spots; transparent and translucent materials will make areas of tones.

Special effects

Make layers of multiple coatings. Make four color separations and print in yellow, magenta, cyan and black, or use any colors and as many layers as you want. After exposing, processing and drying one layer, coat and process additional times. Use the same negative with different colors, or try different exposure times for different layers, different thicknesses or transparancies of solutions for different layers, or use different negatives. For best results with registration, preshrink paper by wetting it in warm water for several minutes and letting it dry before making the first coating. Make registration marks with pencil or tape before coating next color. (my page about registration for offset lithography may be illuminating, or confusing, and the halftones page has a short section on color separations.)

Coat sensitizer over wax resists or crayon drawings.

Try coating small areas, spattering, stenciling, or painting designs with sensitizer. Spattering may be done with an old toothbrush. Use aprons or goggles or other protection if necessary to keep spatters off skin and out of eyes; wipe up spatters in area when done.

Draw or paint on finished print, or print over an existing image. Use transparent pigments and coat sensitizer over a waterproof drawing, painting, or other image with a surface which will accept the sensitizer. Make small test prints first on a test drawing or painting or print similar in materials and execution to your good image, in case the image fades or smears, or colors change, or other odd reactions with the sensitizer occur.

Cloth, wood, birch bark, leather, some kinds of plastic, bisque ware ceramics, and other porous, water-immersible materials may also be used. Exposure and processing times may vary; make small test images first. Dip in sensitizer or brush on sensitizer as on paper. Tape negatives to three dimensional objects that won't fit under glass for exposure.

For more information:

The first two are technical books, available at some libraries. The other books are for a more general audience.

John Carroll, E.J. Wall, Franklin Jordan, Photographic Facts and Formulas, Prentice Hall, New Jersey, 1975

L. P. Clerc, Photography Theory and Practice, Pitman Publishing Co., NY, 1954

William Crawford, Keepers of Light, Morgan & Morgan, Dobbs Ferry, NY, 1979

Robert Hirsch, Exploring Color Photography, Wm C Brown Publishers, Dubuque Iowa, 1989

Kodak Creative Darkroom Techniques, Eastman Kodak Co. Rochester NY, 1973

Links to alternative photo sites and sources for chemicals and film on the Oatmeal Box Camera main page.

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ekelana at gmail dot com

8-4-03