Cyanotype Printing Process

Cyanotype is one of the earliest photographic printing processes, invented by Sir John Herschel in 1842. The process uses a light-sensitive solution of two iron salts, ferric ammonium citrate and potassium ferricyanide, which are mixed together with water and painted onto paper, fabric, or other absorbent surfaces. When exposed to ultraviolet light, typically sunlight, the iron compounds undergo a chemical reaction that produces the process's signature Prussian blue pigment. Unexposed areas are then washed away with water, revealing the final image.

Cyanotypes are most commonly made as contact prints, meaning a negative or object is placed directly on the coated surface and exposed to UV light for anywhere from a few minutes to half an hour depending on conditions and light source.

The process is loved for its accessibility, its longevity, and the striking, deep blue tones it produces. When properly processed and stored, cyanotypes are among the most stable of all photographic processes, with some 19th-century examples remaining vivid to this day.