Cyanotype Quiz – Alternative Process Photography Knowledge

Cyanotype sensitizer is prepared by mixing two solutions, one containing ferric ammonium citrate and the other containing potassium ferricyanide. When mixed and coated on a surface, the combined chemistry reacts to UV light to form the characteristic blue-cyan image.

Cyanotype chemistry is sensitive mainly to ultraviolet light. Sunlight or a UV lamp provides that light and exposes the print. Ordinary indoor lighting has little UV, which is why coated paper can be handled indoors for short periods.

UV exposure causes the ferric iron in the sensitizer to reduce to ferrous iron, which then reacts with the ferricyanide to form ferric ferrocyanide, a compound known as Prussian blue. This reaction is self-fixing: the wash step removes only the unreacted sensitizer, leaving the stable blue compound in the exposed areas.

Water dissolves the unexposed ferric ammonium citrate and potassium ferricyanide that did not form stable Prussian blue during UV exposure. The wash removes this yellow-green residue, revealing the bright white and blue image. No chemical fixing bath or stop bath is required.

Cyanotype sensitizer can be coated onto any surface that absorbs and holds liquid: watercolor paper, cotton fabric, linen, unfinished wood, and other porous materials. The versatility of the substrate is a major reason the process is used in craft, fine art, and alternative process photography.

A digital negative is an inkjet-printed transparency of the inverted image. When laid over sensitized paper and exposed to UV light, it blocks or transmits light proportionally to the image's tones, transferring a photographic tonal range rather than the simple shadows of a physical photogram.

A cyanotype photogram is made without a camera or negative. Objects such as leaves, lace, or flowers are laid on coated paper and exposed to UV light. Where light reaches the paper it turns blue, and where the objects block it the paper stays white, leaving their silhouettes.

Prussian blue oxidizes further when exposed to hydrogen peroxide or air, deepening the blue and increasing shadow density. Applying a dilute peroxide rinse or simply hanging the print to air-dry accelerates and completes this oxidation, boosting the maximum density and perceived contrast.

Prussian blue, the pigment in a cyanotype, is most stable in slightly acidic conditions, so a dilute vinegar or citric acid rinse deepens and intensifies the blue. Bleaching for toning uses an alkaline bath, such as washing soda, instead.

Sir John Herschel discovered the cyanotype process in 1842 while researching light-sensitive iron compounds. The process predates many better-known photographic methods and became widely known through Anna Atkins, who used it to create the first photographically illustrated book.