Cyanotype – One of the Oldest Photographic Processes
Cyanotype is one of the oldest photographic processes and one of the earliest photographic techniques based on the light sensitivity of iron compounds. It was developed by John Herschel in 1842.
From the beginning, the process was particularly well suited to making photograms and copies. These applications played an important role in its early history. Anna Atkins used cyanotype to create her famous botanical photograms, published in successive parts of Photographs of British Algae — one of the earliest books illustrated with photographs.
The process was later widely adopted for copying technical and architectural drawings. The English word blueprint comes directly from these characteristic blue copies and eventually came to mean not only the copy itself, but also a plan or design.
How Does Cyanotype Work?
Unlike processes such as salted paper printing, cyanotype relies on the light sensitivity of iron compounds rather than silver.
In traditional cyanotype, the working sensitiser applied to the paper is prepared by combining two separate solutions: ferric ammonium citrate and potassium ferricyanide.
Other variations of the process also exist, including single-solution cyanotype formulas.
After coating and drying, the sensitised paper is ready for exposure. Once exposed to UV light, the print is washed in water, removing unreacted compounds and revealing the characteristic blue image.
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| Prints made by children during a cyanotype workshop |
The Characteristic Blue of Cyanotype
Cyanotype prints have their distinctive blue colour because of the formation of Prussian blue.
The colour of the final print can also be modified relatively easily through toning, making it possible to create images with a very different visual character from the familiar intense cyanotype blue.