Cyanotype

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The cyanotype (from Ancient Greek: κυάνεος, kyáneos 'dark blue' and τύπος, týpos 'mark, impression, type') 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. 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.

Read on Wikipedia · Image Atkins, Anna, 1799-1871, Public domain