Gum over platinum
Encyclopedia
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.
Photography
Photography is the art, science and practice of creating durable images by recording light or other electromagnetic radiation, either electronically by means of an image sensor or chemically by means of a light-sensitive material such as photographic film...
process, which was commonly used in art photography. It is a very complex process, in which a specially-treated platinum print
Platinum print
Platinum prints, also called platinotypes, are photographic prints made by a monochrome printing process that provides the greatest tonal range of any printing method using chemical development....
photograph
Photograph
A photograph is an image created by light falling on a light-sensitive surface, usually photographic film or an electronic imager such as a CCD or a CMOS chip. Most photographs are created using a camera, which uses a lens to focus the scene's visible wavelengths of light into a reproduction of...
is coated with washes of gum arabic
Gum arabic
220px|thumb|right|Acacia gumGum arabic, also known as acacia gum, chaar gund, char goond, or meska, is a natural gum made of hardened sap taken from two species of the acacia tree; Acacia senegal and Acacia seyal...
, then re-exposed to the same photographic negative. The finished process results in a sepia toned print, and is said to impart added luminosity
Luminosity
Luminosity is a measurement of brightness.-In photometry and color imaging:In photometry, luminosity is sometimes incorrectly used to refer to luminance, which is the density of luminous intensity in a given direction. The SI unit for luminance is candela per square metre.The luminosity function...
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.