Cyanotype

Cyanotype is an iron-based photographic printing process that produces monochromatic Prussian blue images. Its name derives from Greek roots meaning dark blue and mark or impression. Formulated and named by Sir John Herschel in 1842, it emerged from his experiments with the photochemical action of light on iron compounds. Herschel initially saw it as a practical method for copying engravings, notes, and line-based documents, but its low cost, simplicity, and durability also made it a more accessible alternative to early silver-based processes. Anna Atkins gave the process its first major photographic application in 1843 with her botanical volume Photographs of British Algae: Cyanotype Impressions, widely recognized as the first photographically illustrated book.

In practice, cyanotype depends on a light-sensitive mixture usually made from ferric ammonium citrate and potassium ferricyanide, brushed or otherwise coated onto an absorbent support. Exposure to sunlight or artificial ultraviolet light reduces the iron salts and forms insoluble ferric ferrocyanide, or Prussian blue. The print is developed by washing in cold water, which removes unexposed sensitizer, without requiring a separate fixer. Cyanotypes are often made as contact prints, with a negative or object held against the prepared surface. This method lends itself to botanical photograms, architectural blueprints, maps, technical drawings, and experimental work on paper, cotton, silk, wood, glass, or ceramics. Toning can shift the blue toward violet, green, brown, red, or black, while later formulas such as Mike Ware's new cyanotype increased sensitivity and addressed problems such as mold growth.

Historically, cyanotype occupied an ambiguous position between photography, science, and reproduction. It was valued by botanists, engineers, amateurs, and document makers, yet often dismissed by art photographers and critics because of its fixed blue palette and association with utility. In contrast to calotype, salt print, and other silver processes, it was inexpensive and relatively permanent, but its color was frequently treated as a limitation. Later artistic uses have emphasized the same qualities once seen as defects: its matte surface, intense blue field, contact-based clarity, and associations with water, sky, and technical drawing. As a siderotype, it belongs to the broader family of iron-based photographic processes and sometimes overlaps with combination printing when used beneath gum bichromate or platinum layers.