Gum bichromate is one of the most expressive alternative photographic processes. It allows polychromatic prints of unique painterly richness. Here is the complete guide to making your first successful print.
Materials needed
- Gum arabic: in 25-30% water solution
- Potassium or ammonium bichromate: UV sensitizer
- Watercolor pigment: color of your choice (sanguine, black, cyan, magenta, yellow…)
- Watercolor paper 300 gsm minimum, no optical brighteners
- A digital negative printed on transparent film
- A UV source: sun or UV exposure unit (Luminograph)
Step 1: Prepare the emulsion
Mix equal parts of the gum arabic solution (with pigment incorporated) and the bichromate solution. Work under incandescent light or low light — bichromate is sensitive to UV.
Step 2: Coat the paper
Using a flat brush or hake brush, apply a uniform coat on your paper. Allow to dry completely in darkness (30 to 45 minutes at room temperature).
Step 3: Expose to UV
Place your negative in perfect contact with the paper (emulsion side against the paper). Ensure good contact with a glass plate. Expose to UV. With a Luminograph, typically count between 3 and 8 minutes.
Step 4: Develop in water
Immerse the paper in a water bath at room temperature (about 20°C). Unexposed areas will gradually dissolve in the water. Development typically takes 10 to 30 minutes.
Step 5: Drying and additional passes
Once dry, you can coat again with a second color to create polychromatic prints — layering cyan, magenta and yellow to obtain a color print of unmatched richness.
