Cyanotypes are produced using a photographic process developed by engineers to make copies of hand-drawn master plans – blueprints. Two chemicals – potassium ferricyanide and ferric ammonium citrate – are mixed to create a light-sensitive solution, which is applied to the paper and dried in a dark place. A digitally-prepared negative is placed on top of the coated paper, which is then exposed to sunlight or (in Scotland!) another a source of ultraviolet light. The result is a vivid, prussian-blue print.