cyanotypes

Cyanotype is a photographic printing process that yields a distinctive blue image. It is a contact printing method based on iron salts and light, invented by Sir John Herschel in 1842. The process became widely known as a “blueprint” because it was used to reproduce architectural or engineering drawings, as well as botanical studies in the 19th century.

The sensitizer for cyanotypes is made by mixing ammonium iron(III) citrate with potassium ferricyanide and applying the solution to paper or fabric. After coating, the sheet is dried in the dark. Exposing the coated surface under ultraviolet light, with an object or negative placed on top, reduces iron(III) to iron(II). The iron(II) then reacts with ferricyanide to form insoluble Prussian blue. Unexposed areas remain white, and washing the print removes residual chemicals, leaving a blue image.

Historically, cyanotypes were employed by scientists for botanical illustration and by engineers for technical drawings. Anna Atkins used the process extensively in her cyanotype volumes of algae and ferns, making it one of the earliest photographic books. In modern practice, cyanotypes are valued as a simple, non-toxic alternative to silver-based processes and are widely used by artists and educators for photograms and experimental prints.

Characteristics and variations: cyanotypes produce a spectrum of blue tones depending on exposure and chemical preparation, from pale to deep Prussian blue. The method is contact-based, so the final image is a direct reproduction of whatever is placed on the coated surface. Some artists experiment with toning or alternative substrates to achieve warmer or different hues.

Preservation notes: cyanotypes are relatively stable when stored away from prolonged direct sunlight and environmental fluctuations. Proper handling and archival storage help preserve their color and detail over time.