The English scientist and astronomer Sir John Herschel discovered this procedure in 1842. In a typical procedure, equal volumes of an 8% solution of Potassium Ferricyanide and a 20% solution of Ferric Ammonium Citrate are mixed. This photosensitive solution is then applied to a receptive surface and allowed to dry in a dark place. Cyanotypes can be printed on any surface capable of soaking up the iron solution.
I take my digital photographs, and turn them into a negative via photoshop, then print the negative on an ink jet printer on clear film, and wallah! A negative now turned into a positive print by exposing the print in sunlight. Various chemical processes can turn the monochromatic blue prints into other colors, or hand painting them with water color paints adds a nice warm touch. Take a look at some of the '34-'35 Ford Hot Rods I have photographed and recently printed.