Gum Bichromate Printing
The gum bichromate printing method was one of the early techniques developed in the 1800's. It is a labor intensive method which was never widely used. Gum bichromate was not used as an “in camera” process as were many of the silver based processes due to the very low light sensitivity of bichromates and the fact that bichromates are almost exclusively sensitive to ultraviolet light. Printing with gum bichromate is significantly more complex than traditional silver gelatin printing as many more steps are required. Unlike silver gelatin printing which uses an enlarger to project an negative onto commercial photographic paper, gum bichromate is a contact printing method which requires a negative the same size as the image. The size of the image is fixed by the size of the negative. Gum printing uses a mixture of gum arabic as a binder, potassium bichromate as the light sensitizer, water color pigment as the colorant. Each time an image is printed, the mixture is prepared immediately before hand, coated onto heavy cotton watercolor paper, and then allowed to dry. Once coated, the paper must be used immediately as it has a very limited time before it begins to darken spontaneously. The coated paper is then placed in a pin registration system in a printing frame in contact with the negative, and exposed to ultraviolet light. The sun was the traditional source of ultraviolet light, but today ultraviolet light sources are used to allow better repeatability.
One disadvantage of the gum bichromate process is a narrow tonal range, requiring repeated printing to produce a full tonal range. By varying the amount of bichromate and exposure time, highlights, midtones and shadows can be printed in successive stages. Each printing step requires the paper to be coated, dried, printed and developed. To develop a gum print, the paper is floated face down in trays of room temperature water. In areas not exposed to ultraviolet light, the gum remains soluble and the gum and pigment slowly dissolve off of the paper. In areas exposed to ultraviolet, the gum is hardened and the gum and pigment remain attached to the paper. Developing each layer typically takes one to two hours.
Gum printing uses watercolor pigment as the colorant, the final image is highly archival. In most photographic processes, the images is formed by metals (silver, platinum, iron) which even with archival processing can be subject to degradation over time. The use of watercolor pigment also allows use of color.
Gum bichromate printing is a low resolution process which produces soft images, contributing to the painterly, pictorial quality. Pictorialists photographers produced most of the gum prints in the 19th and 20th century. Demachy, Kühn, Käsebier and others used gum printing as a primary medium. Gum printing also allowed color to be applied to monochrome prints, as with the blue layer Steichen applied to his platinum print of the Flatiron Building.
My images were originally photographed on film, and the resulting negative scanned and converted into a large negative for contact printing. The images are printed onto 100% cotton watercolor paper, principally Arches Platine. The final step in the process is to treat the image with metabisulfite to remove residual bichromate. The print then undergoes a archival wash.