Gum printing

A heavy weight cotton watercolor or printmaking paper that can withstand repeated and extended soakings is best.

A simple duotone separation combining orange watercolor pigment and a cyanotype can yield surprisingly beautiful results.

William Henry Fox Talbot later found that sensitized dichromated colloids such as gelatin and gum arabic became insoluble in water after exposure to sunlight.

When mixtures of mucilaginous, protein-containing materials together with soluble salts of dichromate are exposed to ultraviolet light, the protein content becomes tanned and resistant to solution in water.

Exceeding these concentrations results in deposits of chromic acid in the dried film which ruins any attempts at printing.

A contact negative the same size of the finished print is then placed on top of the dried coating and exposed to an ultraviolet light source, typically bright sunshine.

William Henry Fox Talbot later found that sensitized dichromated colloids such as gelatin and gum arabic became insoluble in water after exposure to sunlight.

The print is then floated face down in a bath of room-temperature water to allow the soluble gum, excess dichromate, and pigment to wash away.

When all layers are complete and dry, a clearing bath of sodium metabisulfite is used to extract any remaining dichromate so the print will be archival.