Potassium Ferricyanide, as well as Ferric Ammonium Citrate, is used to create the sensitising solution for Cyanotype printing.
The cyanotype is a slow-reacting, economical photographic printing formulation sensitive to a limited near ultraviolet and blue light spectrum, the range 300 nm to 400 nm known as UVA radiation.
Cyanotype is a 170 year old photographic printing process that produces prints in a distinctive dark greenish-blue. The word cyan comes from the Greek, meaning “dark blue substance.”
The process was invented by Sir John Herschel, a brilliant astronomer and scientist, in 1842.