The Cyanotype process also has ties to Sir John Herchel. It is one of the first permanent photo processes. Potassium ferricyanide and Ferric ammonium citrate are mixed and applied the sameway as Van Dyke Brown sensitizer. The resulting prints have a distinctive Prussian blue tone.  I tone my cyanotypes in black tea. The toning leaves the darker tones blue and shifts the mid to light tones warm. This creates an added feeling of depth. I feel that this process lends itself to a limited amount of imagery, but is very effective for the right type. For this reason I don't use this process often.