Cyanotype
An iron-based printing process producing images in Prussian blue, developed simply by washing in water.
In depth
Invented by Herschel in 1842 and used by Anna Atkins for the first photographically illustrated book, the cyanotype needs no silver, no darkroom chemistry and no toning — just coated paper, ultraviolet light and water. Its simplicity and low cost make it the standard introduction to alternative processes, and the same chemistry gave engineering its blueprints.
Examples in practice
A negative laid on coated paper and left in sunlight for fifteen minutes, then rinsed, yields a deep blue print.