A cyanotype is a photographic print in Prussian blue, made without a camera: water-soluble iron compounds, painted onto a surface and exposed to ultraviolet light, convert to a non-soluble salt wherever the light strikes. The unexposed areas — still soluble, still yellow — wash away in water; what remains oxidizes to a deep blue. It is among the oldest photographic processes, invented in 1842 by the English astronomer Sir John Herschel, and its logic is not far from the four-thousand-year-old magic of indigo, where a thing turns from yellow to blue in seconds.
The process was reprographic before it was photographic — a cheap, fast way to copy notes and diagrams, which is how architectural drawings became “blueprints.” Its first use as a picture-making rather than copying medium was Anna Atkins’s, who between 1843 and 1853 laid seaweed directly on sensitized paper and published three volumes of Photographs of British Algae: Cyanotype Impressions — arguably the first book illustrated with photographs.
Cyanotype — working formula
Chemistry
In a clean non-metal or amber-glass bottle, dissolve
25 g ferric ammonium citrate (green)
into lukewarm
100 mL distilled water.
In a second bottle, dissolve
10 g potassium ferricyanide
into lukewarm
100 mL distilled water.
Store the two solutions separately in a cool, dark place; let them mellow 24 hours before first use. Both contain cyanide compounds — work ventilated, wear gloves, avoid skin contact.
To print
Under low light, mix equal amounts:
1 part Solution A
1 part Solution B
With a foam brush, glass rod, or soft brush, lay a thin, even coat onto
paper or other absorbent surface.
Dry completely in a well-ventilated dark place — a cabinet or drawer works well. Keep in darkness until ready to expose.
To expose
Lay objects, plants, or a film negative in contact with the dry coat; a sheet of glass holds it flat. Expose to ultraviolet — direct sun or a UV / fluorescent source — until the coated areas turn dull grey-blue or bronze (a few minutes in strong sun; longer under lamps). The image looks faint and reversed; it comes up in the wash.
To develop
Rinse in cool running water until the unexposed yellow-green washes out and the water runs clear. Meeting the air, the blue deepens to Prussian blue. (To force it, a brief bath with a splash of hydrogen peroxide turns it blue at once.) Hang to dry in the dark.