Gum bichromate is a deceptively simple process which involves just a single light sensitive chemical, is developed in water, and can be printed on a variety of papers and other surfaces.
The gum print works on the principle that an organic colloid (in this case gum Arabic) when combined with a dichromate becomes light sensitive. Exposure of this dichromated colloid to light causes the organic colloid (gum arabic) to harden in proportion to the light striking it. Adding watercolour pigment to the gum provides the colour. Development is achieved by floating the exposed print in water for 30 minutes or more. The unhardened gum is washed away, leaving the hardened, exposed gum to form the image on the paper.
The gum print, unlike most traditional photographic prints, can be physically manipulated to a considerable extent. During development, when the print is wet and fragile, details can be rubbed out with the use of a brush or water jet. Images can be re-sensitised and re-exposed several times, either to deepen tone or to achieve definite colour shifts. Tone, texture and colour can all be altered and manipulated.
Most photographic processes are essentially the product of chemical interaction, but gum printing is a balance between chemical and physical interaction.
Outline of the procedure of gum printing
Creating a gum (or gum bichromate) print involves applying an emulsion of watercolour and gum arabic, combined with an ammonium or potassium dichromate sensitiser onto sized paper. After drying, the emulsion is exposed by contact with a UV light source. Available sources include; sunlamps, UV BL fluorescent tubes, Mercury Vapour lamps, or sunlight. Development of the image is achieved by floating the print on water. The water penetrates the gum and permits the un-hardened gum to dissolve. Development takes around 30 minutes. After one layer has dried the paper may be re-coated and exposed again. Anything from three to sixteen printings is possible, depending on the extent of staining of the paper by pigment. Paper choice, intensity of pigment and other factors all affect the number of coats possible.