GUM BICHROMATE PRINT
The gum bichromate print patent was filed in 1858 by the English photographer John Pouncy. A little earlier, in 1850, the French scientist Alphonse Poitevin had demonstrated the property that potassium bichromate had to make gum arabic insoluble after exposure to light. At this time the process only met with limited success and was soon forgotten.
It resurfaced in the late nineteenth century in a publication by Albert Rust-Ladevèze, who had a more artistic rather than scientific approach.
Gum bichromate printing was then very popular for several years before declining from 1914 onwards.
The photographer uses a brush to coat a thick paper sheet, capable of taking several immersions, with a mixture of gum arabic, potassium bichromate or ammonium and a pigment to give the print its colour. Several pigments can be applied in layers, enabling a three-colour or four-colour print to be obtained. Most often, however, gum bichromate prints have warm colours ranging from orange to dark brown. The technique therefore requires know-how and a good command of the gestures involved to avoid excess thickness, or conversely areas without a sensitive layer. The original appearance of the paper, mostly watercolour paper; gives a grainy surface to the image. After drying, the now photosensitive paper is brought into contact with a negative in a printing frame. Gum bichromate has the property of hardening when exposed to ultraviolet rays. Thus, after exposure, the paper is placed in a basin of water and "stripped" by using a brush. The exposed parts will remain while the still soluble parts will be eliminated with the water, taking the pigment with them and again showing the paper. The sheet is then allowed to dry.
Depending on the desired effect several passages are often necessary. The steps must therefore be repeated as necessary.
Once the final rendering is obtained, the sheet is immersed in a sodium bisulfite bath which removes bichromate residues and it is again rinsed with water and dried.
Old prints are mostly stable, unlike some recent work due to the very nature of the pigments used which sometimes cause the image to be weakened.
Following the impressionist movement, the process attracted artists due to its lack of sharpness in the contours. Due to its random nature, it also leaves much room for personal interpretation, explaining the success of this technique with pictorialist photographers such as Pierre-Antoine Demachy or Edward Steichen.
More recently, there is renewed interest in the process among contemporary photographers who are seeking alternative techniques such as Nancy Wilson-Pajic or Sally Mann.