Invented by Sir John Herschel between 1792 – 1871, this simple process basically, this is a process in which we mix an emulsion of ammonium ferric citrate (which is used for cyanotype) and silver nitrate (which is used in the salt print process) with a small amount of acid. When exposed to natural or artificial ultraviolet light, these iron salts are reduced to their ferrous state, producing a high contrast deep brown image when oxidized. The Vandyke process gets its name from its similarity in color to the deep brown pigment used by the Flemish painter Van Dyck.