The Cyanotype process

This is an early process first introduced by John Herschel in 1842. It is still in use today, and is more commonly known as the blue-print process. In the early days paper was impregnated with iron salts and then used in contact printing. The paper was then washed in water, and the image would come out as a white image on deep blue. Long expsoure times were required, but one advantage was that since the chemicals were mainly sensitive to Ultra-Violet rays, the solutions can be prepared in subdued light, rather than in a darkroom. One of the earliest users of the process was Anna Atkins, who produced the first photographically illustrated book. By Dr. Robert Leggat

Information provided by: http://www.rleggat.com/photohistory/history/cyanotyp.htm