cyanotype

Cyanotype is a contact printing process that yields images in a characteristic blue. It uses two water-based chemicals, ferric ammonium citrate and potassium ferricyanide. When coated onto a sheet of paper or fabric and dried in the dark, the surface becomes light sensitive. Under ultraviolet light, the ferric ions are reduced and react with ferrocyanide to form ferric ferrocyanide, the insoluble Prussian blue pigment, which stays on the support after washing away unexposed material.

To make a cyanotype, a sensitized sheet is prepared by brushing on the mixed solution and allowing it to dry in a dark area. A transparency, negative, or found objects are then placed in contact with the coated surface and exposed to sunlight or another UV source for several minutes to a quarter of an hour, depending on light intensity. After exposure, the print is washed in water to remove the unfixed salts, revealing a blue image that dries flat and stable.

History and use: The process was invented by Sir John Herschel in 1842 and quickly became known as the blue blueprint for architectural drawings. It gained prominence in botanical illustration through the work of Anna Atkins and has returned as a popular, low-cost method for artists, educators, and hobbyists. Modern practice often involves watercolor paper, cotton rag, or fabric, and may employ simple home kits or more sophisticated studio setups.

Overview: Cyanotype is valued for its simplicity, safety, and inexpensive materials. It produces a distinctive blue tone and can be used for educational demonstrations, art prints, and reproduction of botanical and historical subjects. Variations and toning techniques exist to alter hue or contrast, but the classic cyanotype is defined by its Prussian blue image.