Cyanotype

The cyanotype process has existed nearly since the inception of photography itself, and cyanotypes are immediately recognizable by their vibrant cyan blue tones. The process was invented in 1842 by the Englishman Sir John F. W. Herschel (1792–1871), who intended it for reproducing mathematical tables. It was also used for reproducing technical drawings, known as blueprints.

The cyanotype process is quite simple. It uses light-sensitive iron salts—potassium ferricyanide and ferric ammonium citrate—rather than light-sensitive silver salts. The salts are sensitive to ultraviolet light and must be applied either in the dark or under weak light with little ultraviolet content. The image is produced through contact printing, by pressing a negative or object directly to the surface and exposing the paper to light (historically sunlight).

To develop the cyanotype, the paper is simply washed in plain water. The areas exposed to light become blue shadows and middle tones; the excess, unexposed iron salts are washed away, creating highlights.