Gum over platinum is a historical chemical photographic process, which was commonly used in art photography. It is a very complex process, in which a specially-treated platinum print photograph is coated with washes of gum arabic, then re-exposed to the same photographic negative. The finished process results in a sepia toned print, and is said to impart added luminosity 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 photo sensitive 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 isn't 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.
The technique is related to platinum printing.
Famous quotes containing the words gum and/or platinum:
“Two wooden tubs of blue hydrangeas stand at the foot of the stone steps.
The sky is a blue gum streaked with rose. The trees are black.
The grackles crack their throats of bone in the smooth air.
Moisture and heat have swollen the garden into a slum of bloom.
Pardie! Summer is like a fat beast, sleepy in mildew....”
—Wallace Stevens (1879–1955)
“Flouncing your skirts, you blueness of joy, you flirt of
politeness,
You leap, you intelligence, essence of wheelness with silvery nose,
And your platinum clocks of excitement stir like the hairs of a
fern.”
—Karl Shapiro (b. 1913)