Cyanotype

Invented in 1842 by John Herschel, this early photographic process uses iron salts (ferric ammonium citrate and potassium ferricyanide). When exposed to UV light, it creates an insoluble Prussian blue image after rinsing with water. Simple and economical, it was used for technical drawings ("blueprints") before becoming an artistic process. Under the effect of UV light, ferric ammonium citrate (Fe³⁺) is reduced to ferrous ions (Fe²⁺), while potassium ferricyanide is converted to ferrocyanide.