Gum printing is a different process from silver gelatin (standard black and white printing) - it is a non-silver
process relying on the hardening of a "glue" by ultraviolet light. The most expensive photographic print sold in
auction was a gum print in combination with other alternative processes.
Gum printing is based on a process invented in 1839 by Scotsman Mungo Ponton
and improved by Frenchman Victor Artigue among others. It was popular among the "Linked Ring" (a UK based group
including Alfred Maskell and Robert Demachy) and the "photo-secessionists" (a US based group including Edward
Steichen and Alfred Stieglitz) from about 1890 to 1920. It is a very flexible procedure and can produce prints
which are between watercolour paintings, woodblock prints and photographs.
How are they made? Briefly a mixture of pigment, gum arabic or another "glue" and a dichromate salt is painted on
watercolour paper and left to dry, then exposed under a black and white negative by contact printing in sunlight.
Prints are necessarily the same size as the negative. The print is then washed in water to reveal an image.
Usually several coats of pigment are required - entailing repeated painting, exposure and drying typically
taking between a week and two weeks.
The image below shows the corner of a gum print. The brush marks from the application of several coats of different layers of pigment can be seen. Prints can be mounted to hide or to reveal the brushed edges.