Basic Chemistry of Cyanotyping (2)
The Cyanotype is part of a group of processes that include the palladium print, the platinotype and the kallitype.
Iron-salt processes basically work like this: All ferric (iron III) salts, when combined with organic substances, become sensitive to light. A commonly used mixture is ferric ammonium citrate and potassium ferricyanide. Exposure to ultraviolet light breaks down the iron compound by oxidation, thereby releasing carbon in the form of carbonic acid. The exposed print is then immersed in water, causing a reaction between the new compound (peroxide iron salt) and the potassium ferricyanide. A deep-blue compound, ferric ferrocyanide or Iron (III) Hexacyanoferrate (II), is formed.
For a detailed explanation of the chemistry of Cyanotyping see Mike Ware's book: Cyanotype The history, science and art of photographic printing in Prussian blue ISBN 1 900747 07 3 published by the Science Museum, London, for the Nation Museum of Photography, Film and Television.