Gum Bichromate Printing — Painterly Handmade Photographic Prints

Gum bichromate printing is one of the most expressive and painterly photographic processes ever invented. Practised since the 1890s, the gum print uses watercolour pigment suspended in gum arabic, sensitised with a dichromate salt, exposed under UV light through a negative, and developed in water. The result is a handmade print with the texture and colour palette of a watercolour painting — but born from a photograph. Every gum bichromate print is unique: variations in coating, exposure, development, and paper texture ensure that no two impressions are identical. This guide covers the history, materials, step-by-step process, multi-layer printing, troubleshooting, and creative possibilities of gum bichromate photography.

A Brief History

Mungo Ponton discovered the light-sensitivity of dichromated colloids in 1839. Alphonse Poitevin patented the gum bichromate process in 1855. But it was the Pictorialist photographers of the 1890s–1910s — Robert Demachy, Constant Puyo, Edward Steichen — who elevated gum printing into a fine art medium. The Pictorialists loved gum because it allowed the photographer to intervene in the print: brushing on the emulsion, manipulating development with brushes and warm water, even combining multiple coatings with different pigments to build colour and tone. This hand-involvement was a deliberate rejection of the mechanical precision of silver gelatin printing, asserting photography as an art form on equal footing with painting and drawing.

Why Gum Bichromate?

Gum bichromate offers creative control that no digital process can match. You choose the pigment colour — any watercolour pigment works — so your palette is unlimited. You can print in a single colour for a monochrome effect, or layer multiple exposures in different colours to build a full-colour image. The paper texture shows through the image, integrating the photograph with the substrate in a way that inkjet or silver printing cannot achieve. Gum prints are extraordinarily permanent: the pigments are the same lightfast artist pigments used in museum watercolour paintings, and gum arabic is an archival binder. A well-made gum print will survive for centuries.

Essential Materials

You need gum arabic solution (14° Baumé or prepared from gum arabic powder), watercolour pigments (tube pigments are easiest to measure), ammonium dichromate or potassium dichromate as the sensitiser, heavy watercolour paper (Fabriano Artistico 300gsm or Arches Aquarelle are excellent), a UV light source (the sun, a UV exposure unit, or a bank of UV-A fluorescent tubes), large-format digital negatives printed on inkjet transparency film, and basic darkroom chemistry for sizing the paper (gelatin hardened with formaldehyde or chrome alum). A coating brush (Hake brush), mixing containers, measuring implements, and a contact printing frame complete the kit.

Step-by-Step Process

Step 1 — Size the Paper

Watercolour paper must be sized before coating to control pigment stain and improve image clarity. The most common sizing method is to soak the paper in a gelatin solution (15–20g gelatin per litre of warm water) with a hardener (chrome alum or glyoxal). After sizing, the paper is hung to dry and then shrunk by soaking in hot water. This pre-shrinking is critical — unshrunk paper will expand and contract during the multiple wet stages of gum printing, causing layers to misregister in multi-coat prints.

Step 2 — Prepare the Emulsion

Mix equal parts gum arabic solution and saturated ammonium dichromate solution. Add watercolour pigment — the amount determines the saturation and density of the print. Begin with a ratio of roughly 1:1:1 (gum:dichromate:pigment) by volume, then adjust based on test prints. The emulsion must be mixed fresh for each coating session. Work under subdued tungsten or LED lighting — the sensitised emulsion is sensitive to UV and blue light.

Step 3 — Coat the Paper

Using a wide Hake brush, apply the emulsion to the sized paper in smooth, even strokes. Brush in one direction, then perpendicular, then diagonal — working the emulsion into the paper surface. The coat should be thin and even — thick coats produce muddy prints with poor highlights. Let the coated paper dry in the dark (a darkened room with a fan works well). The dried emulsion has a slight orange-yellow tint from the dichromate.

Step 4 — Expose

Place the digital negative emulsion-side down onto the coated paper in a contact printing frame. Expose to UV light. Exposure times vary enormously depending on the UV source, the dichromate concentration, the pigment load, and the desired contrast — from 2 minutes under a powerful UV unit to 20 minutes in direct sunlight. UV light hardens the gum/dichromate/pigment mixture in proportion to the density of the negative. Where the negative is clear (corresponding to shadow areas of the image), the emulsion hardens fully and resists water. Where the negative is dense (highlights), the emulsion remains soft and washes away.

Step 5 — Develop

Development is the magic of gum printing. Float the exposed print face-down in a tray of clean water at room temperature. The unhardened emulsion slowly dissolves, releasing pigment into the water and revealing the image. Development can take 15 minutes to several hours. You can accelerate development by gently spraying water over the surface, or by using warmer water. You can manipulate the image during development — using a soft brush to selectively remove pigment, or masking areas to retain more tone. This hands-on development is what gives gum prints their characteristic painterly quality.

Multi-Layer Printing

The true power of gum bichromate lies in multiple coatings. After a single-layer print is fully developed and dried, you can recoat with a different pigment colour, re-expose through the same negative (carefully registered), and develop again. Each layer adds depth, complexity, and colour. A full-colour gum print typically uses three or four layers — cyan, magenta, yellow, and sometimes black — each exposed and developed individually. Registration requires precise alignment of the negative to the print between layers. This is achieved using registration pins, punched holes in the negative and paper, or a registration jig. Some printers deliberately allow slight misregistration for a looser, more expressive aesthetic.

Creative Possibilities

Gum bichromate is endlessly versatile. Print with unconventional pigments — metallic powders, pearlescent pigments, even coffee or tea. Print on unusual substrates — fabric, wood, handmade paper. Combine gum with other alternative processes: a cyanotype base layer with gum colour layers on top, or a platinum/palladium print combined with gum for selective colour toning. Use brushwork during coating and development to create textures and gestural marks that merge photography with painting. The only limit is your imagination and your willingness to experiment.

Troubleshooting Common Issues

Pigment staining is the most common problem — caused by insufficient sizing, over-exposure, or using staining pigments (phthalo blue and some alizarin crimson pigments stain heavily). Solution: improve sizing, reduce exposure, or switch to non-staining pigments. Flaking or lifting of the image indicates insufficient adhesion — typically from coating over a previously developed layer that was not fully cleared of residual dichromate (clear the print in a dilute sodium bisulphite bath between layers). Uneven coating produces blotchy prints — practice your brushwork and experiment with different emulsion viscosities and brush types.

Digital Negatives for Gum

Modern gum printers use digital negatives printed on inkjet transparency film. The negative must be calibrated to the specific gum process — different pigments, paper textures, and dichromate concentrations require different negative curves. Use Dan Burkholder's or Mark Nelson's calibration methods to create custom curves in Photoshop. Print the negative at the highest quality setting on a good inkjet printer (Epson P900 or similar) using Pictorico OHP Premium transparency film. The negative should be the same size as the final print — gum printing is a contact process.