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Concept
Cyanotype
Summary
The cyanotype (from Ancient Greek κυάνεος - kuáneos, “dark blue” + τύπος - túpos, “mark, impression, type”) is a slow-reacting, economical photographic printing formulation sensitive to a limited near ultraviolet and blue light spectrum, the range 300 nm to 400 nm known as UVA radiation. It produces a cyan-blue print used for art as monochrome imagery applicable on a range of supports, and for reprography in the form of blueprints. For any purpose, the process usually uses two chemicals: ferric ammonium citrate or ferric ammonium oxalate, and potassium ferricyanide, and only water to develop and fix. Announced in 1842, it is still in use.
The cyanotype was discovered, and named thus, by Sir John Herschel who in 1842 published his investigation of light on iron compounds, expecting that photochemical reactions would reveal, in form visible to the human eye, the infrared extreme of the electromagnetic spectrum detected by his father and the ultra-violet or ‘actinic’ rays that had been discovered in 1801 by Johann Ritter. Though Döbereiner had published in 1831 in German on the light-sensitivity of ferric oxalate, of which Herschel became aware during his visit to Hamburg, it is too lightly toned to form a satisfactory image and would require a second reaction to make a permanent print.
Alfred Smee had in 1840 used electrochemistry to isolate a pure form of potassium ferricyanide, which he sent to Herschel whose innovation was to use the ammonium iron(III) citrate or tartrate, then commercially available as an iron tonic and also introduced to him by Smee, for photographic purpose. He mixed the ammonium ferric citrate in a 20% aqueous solution, with 16% of the potassium ferricyanide, to make the sensitizer for coating plain paper. Exposed to sunlight, the ferric salt is reduced then combines with the ferricyanide to yield ferric ferrocyanide; Prussian blue (also known as Turnbull’s blue, or Berlin Blue in Germany). Intensifying and fixing is achieved simply by rinsing the print in water in which unexposed sensitizer and reaction products are readily soluble.
Official source
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