Cyanotypes

Cyanotyping is the photo process by which blueprints were made.  You mix ferric ammonium citrate and potassium ferricyanide with water, apply it to a substrate (in this case, silk), block out areas you wish to remain unexposed (with leaves, negatives, etc.) and then expose it to the sun.  After full exposure, the print is rinsed and dried. The result is varying shades of the beautiful Prussian blue.