The characteristic color of cyanotypes arises from a specific chemical reaction involving iron compounds. The process relies on the light sensitivity of ferric ammonium citrate, which, when exposed to ultraviolet light, is reduced to ferrous ions. These ferrous ions then react with potassium ferricyanide present in the cyanotype solution to form ferric ferrocyanide, also known as Prussian blue. This insoluble pigment is what imparts the distinctive coloration to the final image.
This unique hue is significant because of the relative simplicity and low cost of the cyanotype process. Historically, it offered an accessible method for reproducing documents and creating botanical blueprints, also known as “blueprints.” Its permanence and archival qualities further contributed to its widespread adoption in various fields.
