Cyanotype

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Cyanotypie, Dictyota dichotoma, by Anna Atkins
Sir John F.W. Herschel: Lady with a harp, 1842

Cyanotype is a photographic printing process, which produces prints that are blue. John Herschel discovered the process in 1842. Engineers used the process well into the 20th century as a simple and low-cost process to produce copies of drawings, referred to as blueprints. The process uses two chemicals: ammonium iron(III) citrate and potassium ferricyanide.