Cyanotype is a photographic printing process that produces a distinctive, cyan-blue image using the light sensitivity of iron compounds rather than the silver compounds used in most other early photographic techniques. Paper or fabric is coated with a solution of ferric ammonium citrate and potassium ferricyanide, dried in the dark, and then exposed to ultraviolet light, typically sunlight, with an object or a photographic negative placed directly on top; exposed areas turn blue upon washing in water, while areas shielded from light wash away, leaving white. Invented by the astronomer and chemist John Herschel in 1842, the process was adopted almost immediately by the botanist Anna Atkins, whose cyanotype photograms of algae specimens, published from 1843, are considered the first book illustrated entirely with photographic images. Cheap, simple to prepare, and requiring no darkroom, cyanotype was long used for technical document reproduction, giving the everyday word blueprint its name, and it remains popular today among artists and educators for contact-printing photograms.
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Cyanotype emerged as a simpler contemporaneous alternative process alongside wet-plate collodion photography.
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The cyanotype is a slow-reacting, photographic printing formulation sensitive to a limited near-ultraviolet and blue light spectrum, the range of 300 nm to 400 nm, known as UVA radiation. It produces a monochrome, blue-coloured print on a range of supports, and is often used for art and reprography in the form of blueprints.
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Why is the word blueprint connected to the cyanotype process?
Cyanotype produces a distinctive cyan-blue image using light-sensitive iron compounds, and because it was cheap, simple to prepare, and needed no darkroom, it was long used for technical document reproduction. That everyday use for reproducing documents is where the word blueprint comes from.
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