Cyanotype
History and Technique
Invented by Sir John Herschel in 1841, this simple process produces a continuous tone image of Prussian Blue using a sensitizing solution of ferric ammonium citrate and potassium ferricyanide.These iron salts, when exposed to natural or artificial ultraviolet light, are reduced to their ferrous state, producing a high contrast blue image when oxidised. Oxidation is hastened by immersion in running water, which also washes away the unused iron salts.
The process was eminently suited to its traditional role in reproducing technical drawings, its most common use in engineering and architecture until the advent of modern photocopiers. However, it was a versatile process, and was used throughout the 19th century from Anna Atkins’ photograms of plants and seaweed for her books on botany (1843–55) to Henri LeSecq’s still life studies of the 1850s. Photographers at the end of the century used cyanotype paper for proofing negatives.
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