Cyanotype is a photographic process that creates deep blue prints. It uses light-sensitive iron salts—potassium ferricyanide and ferric ammonium citrate—rather than silver salts. When exposed to UV light and oxidized, these salts form ferric ferrocyanide, or Prussian blue. Images are made by contact printing: placing a negative or object on coated paper and exposing it to UV light. Washing in plain water develops the image, revealing blue shadows and white highlights.