The Cyanotype (4 of 12)

Sir John Herschel invented the cyanotype in 1842. 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 nineteenth century and more frequently in the twentieth century for the reproduction of architectural plans and technical drawings, called “blueprints.”