Gum Bichromate

Gum Bichromate

The gum print works on the principle that an organic colloid (in this case gum Arabic) when combined with a dichromate becomes light sensitive. Exposure of this dichromated colloid to light causes the organic colloid (gum arabic) to harden in proportion to the light striking it. Adding watercolour pigment to the gum provides the colour. Development is achieved by floating the exposed print in water for 30 minutes or more. The unhardened gum is washed away, leaving the hardened, exposed gum to form the image on the paper.

 

The gum print, unlike most traditional photographic prints, can be physically manipulated to a considerable extent. During development, when the print is wet and fragile, details can be rubbed out with the use of a brush or water jet. Images can be re-sensitised and re-exposed several times, either to deepen tone or to achieve definite colour shifts. Tone, texture and colour can all be altered and manipulated.

 

Most photographic processes are essentially the product of chemical interaction, but gum printing is a balance between chemical and physical interaction.

 

http://www.gumphoto.co.uk/about_gum.html

History

In 1839, Mungo Ponton discovered that dichromates are light sensitive. William Henry Fox Talbot later found that colloids such as gelatin and gum arabic became insoluble in water after exposure to light. Alphonse Poitevin added carbon pigment to the colloids in 1855, creating the first carbon print. In 1858, John Pouncy used colored pigment with gum arabic to create the first color images.

Gum prints tend to be multi-layered images sometimes combined with other alternative process printing methods such as cyanotype and platinotype. A heavy weight cotton watercolor or printmaking paper that can withstand repeated and extended soakings is best. Each layer of pigment is individually coated, registered, exposed and washed. Separation negatives of cyan, magenta, and yellow or red, green, and blue are used for a full-color image.

when in a bath of room-temperature water to allow the soluble gum, excess dichromate, and pigment to wash away. Several changes of water bath are necessary to clear the print. Afterwards, the print is hung to dry. When all layers are complete and dry, a clearing bath of sodium metabisulfite is used to extract any remaining dichromate so the print will be archival.

To read more in depth

http://en.wikipedia.org/wiki/Gum_bichromate