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.
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A liquified pale yellow to golden orange gum exuded in lumps often the size of a walnut by the Acacia tree found in Egypt and the Middle East.
Gum Bichromate printing
Not apllicable to mixtures
Gum Arabic is used to slow down the absorption of the ink into the paper, so that the ink dries more on the surface, giving a more intense color. It also imparts a glossiness to the dried ink.
Gum Arabic was used for a similar purpose in some photographic formulae and was also the basis of the gum bichromate process popular among pictorialist photographers around 1900. It is also enjoying a revival in recent years among those interested in handmade photographic papers.