Invented by Sir John Herschel in 1841, an astronomer trying to find a way of copying his notes. This easy process which is composed of white lines on a blue background is created by using a sensitising solution of Ammonium Ferric Citrate and Potassium Ferricyanide. When these substances are exposed to any light, they're reduced to their ferrous state, producing a high contrast blue image when oxidised. Oxidation is accelerated by being washed in running water.
This process became popular with pictorialists and was also used for copying architectural plans until it was recently made obsolete by photocopying and computer printers
Anna Atkins Polypodium crenatum, Norway, 1854 cyanotype