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From George Eastman House : Notes On Photographs

The gum bichromate process (also known as gum dichromate process) is one of the techniques based on the selective solubility of bichromated colloid resulting from the action of light. The gum bichromate process, also described as the photoaquatint process, was first demonstrated in 1894 by A. Rouillé- Ladevèze in Paris and London. The process enjoyed immense popularity among pictorial photographers and was adopted as the process of choice in the fine art photography movement between 1890 and 1900.

The gum bichromate technique is valued for the high degree of artistic control in the printmaking process. A print is made by brushing a sensitizing solution of gum arabic, pigment (usually in the form of watercolor pigment), and potassium bichromate onto a sheet of heavily sized paper. Upon drying, it is exposed to ultraviolet radiation in direct contact with a negative the desired size of the final image.

After exposure, the print is placed in a tray of water, and the gum solution that was not exposed to light and therefore has remained soluble is slowly dissolved and washed away. The process lends itself to a great deal of hand alteration and manipulation by brushing selected areas of the surface of the print to dissolve greater amounts of the pigmented gum, thereby controlling the density of the deposits of pigment.

Once dry, the print can either be recoated with the same color gum solution, for a richer print, or other color gum solutions can be built up layer upon layer with careful registration of the negative image to alter, deepen, or enrich the print overall or in selective areas. It was not uncommon to combine the gum process with other processes such as the platinum process. The gum process does not contain silver, and the image is permanent. ^[1]

1. ↑ Osterman, Mark. 2007. Gum Biochromate Process. In The Focal Encyclopedia of Photography: Digital Imaging, Theory and Applications, History, and Science, ed. Michael R. Peres, 84, Focal Press.