Gum over platinum is a historical chemical

photographic Photography is the art, application, and practice of creating images by recording light, either electronically by means of an image sensor, or chemically by means of a light-sensitive material such as photographic film. It is employed in many ...

process, which was commonly used in

art photography Fine-art photography is photography created in line with the vision of the photographer as artist, using photography as a medium for creative expression. The goal of fine-art photography is to express an idea, a message, or an emotion. This stand ...

. It is a complex process, in which a specially treated

platinum print Platinum prints, also called ''platinotypes'', are photographic prints made by a monochrome photographic printing, printing process involving platinum. Platinum tones range from warm black, to reddish brown, to expanded mid-tone grays that are ...

photograph A photograph (also known as a photo, or more generically referred to as an ''image'' or ''picture'') is an image created by light falling on a photosensitivity, photosensitive surface, usually photographic film or an electronic image sensor. Th ...

is coated with washes of

gum arabic Gum arabic (gum acacia, gum sudani, Senegal gum and by other names) () is a tree gum exuded by two species of '' Acacia sensu lato:'' '' Senegalia senegal,'' and '' Vachellia seyal.'' However, the term "gum arabic" does not indicate a partic ...

, then re-exposed to the same

photographic negative In photography, a negative is an Photograph, image, usually on a strip or sheet of transparent plastic film, in which the lightest areas of the photographed subject appear darkest and the darkest areas appear lightest. This reversed order occurs ...

. The finished process results in a

sepia tone In photography, toning is a method of altering the color of black-and-white photographs. In analog photography, it is a chemical process carried out on metal salt-based prints, such as silver prints, iron-based prints ( cyanotype or Van Dyke ...

d print, and is said to impart added

luminosity Luminosity is an absolute measure of radiated electromagnetic radiation, electromagnetic energy per unit time, and is synonymous with the radiant power emitted by a light-emitting object. In astronomy, luminosity is the total amount of electroma ...

and depth. It is sometimes called "pigment over platinum". To sensitize the gum arabic it must first be placed in contact with ammonium or potassium dichromate. Gum arabic is not photosensitive by itself. To clear the chromic acid, the print is washed in 1% potassium metabisulfite after proper development in water. Interested individuals should read up on the process before attempting, as the chromic acids are very dangerous to work with. The mechanics of the gum portion is not entirely known; what occurs is the exposed gum is hardened and becomes water-insoluble. Upon washing, the unexposed portions wash away, leaving the white paper exposed.

References

Platinum Photographic processes {{photo-stub