Cyanotype is a non-silver photographic printing process discovered in the mid 19th century. A specific blend of chemicals is applied to the receptive surface (paper, canvas, linen, wood, etc...) which makes it sensitive to ultraviolet lights. The negativ (film) is then placed on this light sensitive surface, and exposed to the sun. The resulting image is developed by water.
I am toning my cyanotype prints to change the Prussian blue of the original prints into a tinted brownish tone. Some of them I also color using aquarel paint.