Gum Bichromate
An artistic printing process that combines ammonium dichromate (sensitizer), gum arabic (colloid binder), and watercolor pigment. The mixture is coated onto paper, dried, exposed to UV light under a negative, and developed in water. UV light crosslinks the gum arabic, making it insoluble. Unexposed gum washes away, leaving a pigmented image. Multiple coatings with different colors allow full-color prints. The most painterly of all photographic processes, producing results closer to watercolors than photographs.
Mixing Instructions
Prepare three components separately.
Sensitizer: Dissolve 29 g ammonium dichromate in 100 ml distilled water.
Gum solution: Mix gum arabic powder 1:2 with distilled water by weight, let stand overnight, strain through cheesecloth.
Pigment: Use tube watercolor pigments.
To coat and expose:
- Mix 1 part gum solution + 1 part pigment (squeezed from tube) + sensitizer (1-2 ml per 10 ml gum/pigment mix).
- Coat onto sized paper under tungsten light.
- Dry completely.
- Expose under UV light.
- Develop face-down in a tray of water at 20 °C for 5-30 minutes.
Multiple coatings can be registered and exposed successively for color prints.
Ingredients for 1L of Working Solution
| # | Chemical | Role | Qty (1L) | Unit | Note |
|---|---|---|---|---|---|
| 1 | Ammonium Dichromate | Sensitizer | 250.0 | g | (Sensitizer) |
| 2 | Gum Arabic | Sensitizer | 300.0 | g | (Colloid base) |