Cyanotypes – also known as sunprints – are created using a photographic process developed by Sir John Herschel in 1842.
Two chemicals – potassium ferricyanide and ferric ammonium citrate – are mixed to create a light-sensitive solution. This solution is applied to paper in a dark room. A negative is then placed in contact with the dried coated paper, which is then exposed to sunlight or a UV lamp. After exposure, unreacted solution is rinsed off with water. The result is a vivid, prussian-blue print.
Since cyanotype is a contact printing process, the final image is the same size as the negative.