Gum Bichromate Prints
Like carbon prints, the gum bichromate print process is based on the light-sensitive properties of dichromated colloids. The gum process was patented in 1855 but did not gain widespread popularity until the 1890s. The paper is coated with a solution of gelatin or gum arabic, potassium dichromate, and pigment. Once dry, the sensitized paper is exposed to light through a negative to harden the light-sensitive dichromated gum in direct proportion to the amount of light it receives. After exposure, the print is washed with water, leaving behind the hardened, pigmented gum, which forms the positive image. The gum process can be manipulated during processing with brushwork, variations in temperature, or by controlling the force of the water. It is also possible to create “multiple” gum prints by resensitizing already processed gum prints with additional coatings of gum solution in different colors. Platinum prints can also be sensitized with a gum solution and printed again, creating “gum-platinum” photographs.
Although few are extant, Alfred Stieglitz made gum bichromate prints at the turn of the twentieth century. There are no examples in the Key Set.