A technical dictionary of printmaking, André Béguin.

The Basics
(copied from the web site (mentioned at the end of this article) of Mr. Hamish Steward)
The gum print works on the principle that an organic colloid (in this case gum Arabic) when combined with ammonium 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 coated with new gum and exposed again. Anything from four 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 effect the number of coats possible.

A Brief History of Gum Bichromate Printing
No one person can be credited with having discovered the gum process.
Rather it was the continual investigation of several pioneers who evolved the process into its current form. Today gum printing continues to evolve as each practitioner brings their own interests and sensibilities to the process.
Mongo Ponton is credited with discovering the light sensitivity of dichromate in 1838, and a significant photographic pioneer, Fox Talbot noted that soluble organic colloids, when combined with dichromate became insoluble. One of the continuing problems for the early photographers was the lack of permanence of early silver processes, providing a strong incentive for the exploration of alternative methods to produce photographic prints.
While Mongo Ponton and Talbot are credited with the understanding the chemical reaction that makes gum prints possible, it was a Frenchman, Alphonse Louis Poitevin, who in 1855 added pigment to the gum arabic/dichromate mixture. In the 1890s Poitevin's process was revived by the Pictorialists who were attracted by the ease with which the wet emulsion could be manipulated. In 1898, Van Hubl introduced the practise of re-sensitising the image and then reprinting it in registration under the same negative. This adaptation to the process was probably one of the most significant, for it allowed the gum printer to build up tone and texture through repeated printings. Single printings in gum tend to be rather flat with poor detail and tonal separation. But the multiple gum print allows the print-maker to build up considerable tonal range and detail. Perhaps the most noted and technically proficient of the Gum printers was Robert Demachy.
While photographers continued to work with the process up till the 1920s, by the early years of the century the process had been discredited by those who sought to establish the "straight" silver print as the photographic standard. From the 1970s onward gum printing along with other early and non-silver photographic processes have been revived by photographers seeking to expand available print-making options.
Web site http://perso.wanadoo.fr/hamish/gumphoto.html
related subjects:graining, phototype, photoglyph.