Gum bichromate printing originated in the 19th century. The print is made by coating cotton rag paper with a mixture of gum arabic, watercolor, and photo sensitizer. Like most other alternative processes, the paper is exposed to ultraviolet (UV) light. Development is done with water only.
"What makes it different from other processes is that it allows multiple layers of color to be built up by multiple printings, using a different color for each pass. The image is very fragile during printing and is very unpredictable, but that result is very beautiful and subtle. I've always loved unpredictable photo processes because they allow for surprises and new discoveries. Some photographers use the color layering of gum bichromate to get true color separations, making full color photographs, but I prefer to make monotone images with subtle shadings of color."