Gum Arabic or Gum bichromate has been used for centuries as a component of paints. Initially soluble in water, it is oxidized over time by the oxygen in the air to an insoluble form. Photographers' Formulary Gum bichromate simply uses dichromate as an oxidizing agent to speed up this "hardening" process. Since the dichromate ion is brightly colored, it is therefore known that it absorbs light. In fact, its oxidizing power is increased by exposure to light.
Gum bichromate prints make use of the light sensitivity of dichromates (previously called bichromates). Light changes the dichromate into a material that hardens gum arabic (and other colloidal materials). Paper - usually a thick watercolor paper - is coated with a mixture of gum arabic, and watercolor pigment.
Gum bichromate printing is a technique that relies on a coating of pigmented gum arabic and potassium or ammonium bichromate, which allows the photographer to manipulate the printed image during development.