Cyanotypes are easily identified by their cyan image color. The cyanotype process is based on the light sensitivity of iron salts, specifically the interaction between iron(III) salts and potassium ferricyanide to form the pigment Prussian blue (ferric ferrocyanide). Prussian blue is responsible for the cyanotype’s characteristic cyan/blue tone.

The cyanotype process was first officially recorded by Sir John Herschel on April 23, 1842. Simplistic in nature, very few changes have been made to the process described in Herschel’s original experimental notes. Varying formulas exist (including fifteen devised by Herschel himself) many of which are based on his second cyanotype (working-negative) recipe, recorded May 10, 1842 which called for the combination of ammonium ferric citrate and potassium ferricyanide to yield Prussian blue.

This process provides a relatively simple and inexpensive means of producing contact prints. Two stock solutions, ammonium ferric citrate and potassium ferricyanide are mixed then promptly used to coat paper. Upon drying, papers are exposed to ultraviolet light (sunlight) beneath a negative or object for roughly 20-30 minutes. Because the cyanotype is a non-silver process, the only fixation required is a 20 minute bath in running water.

 In the cyanotype process, the formation of Prussian blue occurs in two steps. Upon exposure to light, ferric or iron (III) ions in ferric ammonium citrate are reduced to ferrous or iron(II) ions. The ferrous ions then react with ferricyanide ions to yield an unstable compound, ferrous ferricyanide. Air gradually oxidizes ferrous ferricyanide into ferric ferrocyanide (Prussian blue). For rapid oxidation, prints may be placed in a dilute bath of an oxidizing agent such as hydrogen peroxide.

During the immediate years following Herschel’s discovery of the process, the cyanotype fell into obscurity. A notable exception is the work of Anna Atkins who, between 1843 and 1853, used the process to create her botanical study, British Algae: Cyanotype Impressions, an extensive collection of photograms illustrating seaweeds. Atkins’ book was the first to be photographically printed and illustrated. In 1864 Henry Fox Talbot commented favorably on the permanence of Atkins’ cyanotypes in The British Journal of Photography, highlighting one of the great advantages of the process. Permanence aside, the cyanotype was not viewed favorably by the artistic community being regarded unsophisticated and, due to its blue tone, unsuitable for rendering photographic imagery. Despite these sentiments, cyanotypes were defended as a suitable medium for cloudscapes and water related scenery and a simple means of proofing negatives. Additionally, they experienced resurgence during the early hand camera era (1888-1920) as a process by which amateurs could create multiple prints from their negatives.

Commercial blueprint paper for copying architectural plans was released by Marion and Company of Paris in 1872. Papers for photographic use soon followed with the 1885 release of J.C. Millen’s French Satin Jr. By the turn of the century, a variety of cyanotype photographic materials were on the market including Eastman’s Ferro-Prussiate paper and postcard paper, E.A. Anthony Ferroprussiate paper and Silkdown cyanotype fabric.

Beyond 1920, the cyanotype process was adapted and utilized almost exclusively for line drawing. As a photographic medium it received little attention until the alternative process movement towards the end of the 20th century.