Een encyclopedie over grafische technieken, André Béguin.

De Basis
(vertaald van de web site (genoemd aan het einde van dit artikel) van de heer Hamish Steward)
Het basisprincipe: een organische gelatineachtige stof (in dit geval Arabische Gom), gemengd met ammonium bichromaat wordt lichtgevoelig. Als deze stof belicht wordt zal de Arabische Gom uitharden in de mate waarin het wordt belicht.
Door pigment aan de gom toe te voegen kan met kleur worden gewerkt. Het ontwikkelen wordt gedaan door de de belichte afdruk 30 minuten in water onder te dompelen. De ongeharde gom lost op, waardoor de geharde overblijvende gom de afbeelding op het papier vormt. De "gom afdruk" kan in grote mate technisch worden aangepast. Tijdens het "ontwikkelen" in het waterbad, kunnen details worden weggeveegd met een bordstel, of een straal water. Afbeeldingen kunnen opnieuw lichtgevoelig gemaakt worden, en opnieuw belicht. Hiermee kan de toon verdiept worden of kleurveranderingen worden aangebracht. Toon, struktuur en kleur kan allemaal worden veranderd en aangepast. De meeste fotografische processen zijn gebaseerd op chemische wisselwerkingen, maar gom bichromaat drukken is een vorm die zich verhoudt tussen chemische (lichtgevoeligheid) en fysieke (aanpassen prent) wisselwerking.

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