The cyanotype was the first simple and successfully realized practical non-silver iron process. Discovered by Sir John Herschel (1792-1871) in 1842, a mere three years after the “official” announcement of the discovery of photography, the cyanotype provided permanent images in an elegant assortment of blue values. Herschel is the same gentleman who coined the words “positive and negative,” “photograph,” and “snapshot.” He is also credited, in 1819, with discovering that a solution of sodium thiosulfate (which he referred to as hyposulfite of soda) had the ability to dissolve silver chloride and what that particular chemical’s role might be in permanently fixing a photographic image. This is an important bit of information that he passed along to Talbot twenty years later.
The Cyanotype was popular for a short time and the first commercial use of the process was initiated in 1876 at the Philadelphia Centennial Exposition, introducing the adoption of the process for schematic blueprint drawings that would be used by architects, engineers and builders. One other odd historical bit concerning the cyanotype involved Lt. Col. Baden-Powell, founder of the Boy Scouts, who used the cyanotype process to make stamps and money during the siege of Mafeking in the Boer War (1899 – 1902) between Great Britain and the Transvaal.
Making a cyanotype is amazingly simple. There are two chemicals, and water, in the sensitizer. Part A, is ferric (iron) ammonium citrate and it is a light-sensitive compound that changes from its ferric to a ferrous state when subjected to ultraviolet (UV) light. Part B, potassium ferricyanide is the other half of the formula. When parts A & B are mixed in equal proportion, applied to paper or fabric, and exposed to UV light, the ferrous salt reacts with the potassium ferricyanide to form an insoluble ferric ferricyanide creating the familiar Prussian Blue cyanotype print. The fully exposed image is then “developed” in running water until the highlights are clear. If desired, a hydrogen peroxide bath can be used to speed up the darkening (normal oxidation) of the print. The print is then rinsed and hung to dry. Once dry, it can be re-soaked and toned in a vast menu of chemical toners to create color alternatives to blue and complex, combination, tonal splits.