A cyanotype print with the fern that cast the shadow seen in the print. In this process an object is placed on the ultraviolet sensitive chemically treated paper and exposed to a strong UV light source -- in this case sunlight. The object is then removed and the print washed in cool water to remove the unreacted chemicals. Cyanotype is a photographic printing process that produces a cyan-blue print. Engineers used the process well into the 20th century as a simple and low-cost process to produce copies of drawings, referred to as blueprints. The process uses two chemicals: ammonium iron(III) citrate and potassium ferricyanide. The English scientist and astronomer Sir John Herschel discovered the procedure in 1842.