Like the other handcrafted alternative photographic processes, printing in gum and casein bichromate uses a contact printing method. This involves placing a large negative directly into contact with the photographic paper and exposing them together to light. A printing frame is used to press the two together under a spring-tensioned piece of glass. Either digitally printed negatives or traditional film negatives can be used.
In Gum Printing and Other Amazing Contact Printing Processes, Anderson introduces gum’s alchemy this way: “Gum printing is a 19th century photographic process wherein a mixture of gum arabic, a photo-sensitive dichromate, and watercolor pigment is brushed on paper and exposed to light under a large negative. The light activates the dichromate to harden the gum proportionately to the amount of light received, and with a simple water development, unhardened parts of the gum layer wash away to reveal a positive image of the negative.”
During the water development, a little manipulation with a soft, fine-tipped paintbrush can encourage selected areas of the print to release the gum layer and its attendant color, giving the photographer greater control during the development stage than in most processes.
The tri-color gum bichromate print involves repeating the process for each of three colors—typically red, yellow and blue, or some variations upon those. Anderson’s prints often carry two or more applications of each color layer, which gives them their notable depth in tone and texture.