Gum Bichromate

In 1839, Mungo Ponton published a paper on the light sensitive properties of chromates. These are strong alkaline compounds that are available today as potassium or ammonium dichromate (bichromate is the older and more common term). In 1852, Fox Talbot used animal gelatine in combination with dichromate. He observed that the dichromated colloid hardened in proportion to its exposure to sunlight, and applied the principle to an early photogravure process. This work formed the basis for many later photographic and photomechanical processes. In 1894, A. Rouille-Ladevez redefined the process, producing prints for an exhibition at the Photo-Club de Paris. His work inspired photographers whose pictorial work derived from Impressionism in fine art, and great use was made of the gum bichromate process as a method of producing the painterly images so characteristic of turn-of-the-century art photography.

MATERIALS

Gum arabic
Available as prepared liquid gum arabic (avoid acid preservatives) or mix dry gum (acacia) at 1 part to 3 parts water. Dissolve (overnight), strain, and add optional preservative (phenol based such as thymol, never formalin or formaldehyde)

Pigment
Water colour for transparency, or designers gouache or casein for more opacity. Powdered pigment should be ground with a small quantity of liquid gum arabic. Potassium Dichromate (bichromate) Ammonium Dichromate (bichromate) is capable of a more saturated solution, decreasing exposure time, but giving some problems of exposure control.

Paper
A good quality, sized, art paper is recommended.

THE PROCESS

Potassium dichromate: make a 10% saturated solution (10g to 100ml distilled water) The solution will keep in the dark. If crystals form during storage, redissolve by warming the bottle. Mix gum solution and sensitiser in equal parts. Use as soon as possible. Add to prepared pigment.

Coat paper with a soft, broad brush, and allow to dry away from light and heat. The prepared paper will keep for several days in the dark, but works best when used as soon as possible.

Expose the paper to sunlight or a mercury vapour lamp by contact printing under a negative. Exposure time will depend on density of negative and type of light source, but always look for a brown print-out in the shadows and the start of the mid-tones. The colour of the pigment in the coating will also affect exposure time: blue and green requiring less time than red or yellow.

'Develop' the print in a bath of tepid water. Slide the print into the water (face down). Soak (a few minutes to several hours, depending on exposure). Unexposed gum will dissolve, while gum hardened by the action of light and chromate will remain. Gum coating can then be removed locally by the application of a stream of water or gentle brushing. Flatten the wet print against a tilted, waterproof board to dry. The gum print can be overprinted by recoating (once dry), re-exposing and developing. Staining of pigment in the highlights can be minimised by the application of paper size, and the dichromate stain can be removed in a 5% potassium alum bath, or by re-soaking the finished, dried print in warm water.