Cyanotype

The Cyanotype process was discovered by Sir John Herschel in 1842. Herschel also helped William Talbot in makes his photogenic drawings permanent.The process depends on the photochemical reduction of ferric salts into ferrous salts leading to the formation of Prussian blue, an iron-based pigment. The process was used sporadically throughout the 19th century and more frequently in the twentieth century for the reproduction of architectural plans and technical drawings, called “blueprints."The process uses two chemicals: ferric ammonium citrate and potassium ferricyanide.