Gum Bichromate Print

Manufacturing

The photographer chooses his paper based on its texture, he uses a brush to it with a mixture of pigmented gum arabic, potassium bichromate.  After drying, the paper is brought into contact with a negative in a printing frame. The light areas of the negative allow light to pass through, which hardens the bichromate gelatin, the unexposed areas remaining soft. The paper is then washed (or “stripped”) with hot water, which dissolves and removes the gum and pigment in inverse proportion to the amount of light exposure. The photographer can locally accentuate or retain the stripping process, directly influencing the formation of the image with a brush. The sheet is then allowed to dry. Depending on the desired effect, several passes are often necessary. The various steps must therefore be repeated until the result is satisfactory. Once the final image is achieved, the sheet is immersed in a sodium bisulfite bath to remove any remaining bichromate (clarification), then rinsed again with water and dried. It is possible to create two-, three-, or even four-colour images by successively applying multiple pigmented layers.

Identification

The appearance of bichromate gum prints reminds charcoal drawings. The surface of the paper, mostly watercolour paper, tends to be rough. Most often, prints have warm colours ranging from orange to dark brown. Observation under a magnifying glass shows traces of the brushwork process, although these are not always easily visible. 

Deterioration

Since the pigment, component of the image, is stable, the gum bichromate process offers excellent stability, just like other pigment-based photographic techniques.

Glossary