Cyanotype Cyanotype is a photographic printing process that produces distinctive blue and white images through a chemical reaction activated by ultraviolet light. Developed by Sir John Herschel in 1842, this technique - the original "blueprint" - uses a combination of two iron salts—ferric ammonium citrate and potassium ferricyanide—which are mixed together to create a light sensitive solution, and applied to paper or fabric. 

The light sensitised substrate is then contact-printed with negatives, transparencies, or objects, or painted directly with the light sensitive solution to produce gestural marks.  After exposure the image is washed in water to remove unreacted chemicals and a rich blue image is revealed.

Post-processing toning can be used to alter the characteristic blue color using various chemical baths—tannic acid, for example, creates brown tones.