Cyanotype or "blue print" was discovered in 1842 by English scientist John Herschel. It is a hand-printed photographic technique, known for producing prussian blue prints on a wide range of materials, including paper, fabric and glass.
Two chemicals are required - ammonium ferric citrate and potassium ferricyanide. When printing on paper, the mixed solution is applied and then dried thoroughly. This light-sensitive carrier can then have photo negatives of any size, or even objects placed upon it. Once exposed to natural light or a UV unit, the chemical composition changes. When developed in water, the unexposed iron compounds are washed away from the photographs, while the newly-formed water insoluble iron ferricyanide, with its characteristic blue colour, remains.