Cyanotype Process
British · Cyanotype 1842–present
The blueprint. A photographic process so simple, cheap and stable that it was used for a century to reproduce engineering drawings, and it produced the first book ever illustrated with photographs.
Origin
Invented by the astronomer John Herschel in 1842, and taken up almost immediately by the botanist Anna Atkins, whose Photographs of British Algae, issued from 1843, is the first photographically illustrated book — she made contact images of specimens directly on sensitised paper, without a camera. From the 1870s the process was adopted industrially for copying technical drawings, which is where the word blueprint comes from, and it remained standard until diazo and then photocopying replaced it in the mid-twentieth century.
Technique
Paper is coated with a solution of ferric ammonium citrate and potassium ferricyanide, dried in the dark, and exposed to ultraviolet light under a negative or an object placed directly on the surface. Light reduces the iron, which then reacts to form insoluble Prussian blue. Development is simply a wash in water — no developer, no fixer, no silver — which makes it the cheapest and safest of all photographic processes and the one most commonly taught to beginners.
The palette
One colour: Prussian blue, the same compound accidentally discovered in a Berlin laboratory in 1706. The image is white where light was blocked and deep blue where it was not, with a full range of intermediate blues. Toning with tannin, tea or alkaline baths can shift it toward violet, brown or black, and bleaching then redeveloping produces further variations, but the unmodified cyanotype is unmistakable.
Legacy
Chemically it is extremely stable in the dark, though it fades in prolonged light and recovers in storage — the same reversible behaviour as the pigment. Atkins's algae remain among the most beautiful photographic objects ever made, and the process has had a substantial revival in contemporary practice for its simplicity, its tolerance of hand-application, and the fact that it needs nothing but sunlight and water.
Palette
The colours reconstructed for this artist, with the reasoning behind each.
Pigments used: Prussian blue