Page 47 - Expanded Media & the MediaPlex

Expanded Media - and the MediaPlex 47/206
John Herschel: Cyanotype 1842 (portrait of Herschel by Julia Margaret Cameron 1867)
The astronomer, chemist, inventor and polymath John Herschel, had come to William Henry Fox Talbot’s aid in 1839 with suggestions for ‘fixing’ the latent image on Talbot’s paper negative. It was based upon an earlier paper he had written (in 1820 for the Edinburgh Philosophical Review) and suggested that hyposulphite of soda and silver was the fixative to stop the exposed paper from turning completely black as it was exposed to daylight. A few years later Herschel was experimenting in the capture of colour in photographs, and a spin-off was his discovery of the cyanotype or ‘blue- print’. He had written to Fox Talbot about capturing colour in 1839: “By using the prism first to separate all but the pure prismatic tint of given refrangibility and then re-analyzing this by media I conceive it possible to obtain rays totally exempt from any colour but the elementary one wanted”. The cyanotype process used a solution of ferric ammonium citrate and potassium ferricyanide (aka Prussian Blue) on paper, allowing it to dry in the darkroom, then placing an object or line-drawing on top of the sensitised paper, topped with a sheet of plate glass - the sandwich could be exposed in the sun for a few minutes - the result is a blue photogram. The cyanotype process was deployed by the botanist Anne Atkins (see Anne Atkins: Photographs of British Algae: Cyanotype Impressions 1853). After Fox Talbot’s Pencil of Nature (1844), this was one of the first books illustrated with photographs.
“In 1861 Alphonse Louis Poitevin, a French chemist, found that ferro-gallate in gum is light sensitive. Light turns this to an insoluble permanent blue. A coating of this chemical on a paper or other base may be used to reproduce an image from a translucent document.” (https://www.dominionblue.com/pages/blog/ item/374-history-of-the-blueprint)