Cyanotype

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.

Cyanotypie, Dictyota dichotoma, by Anna Atkins

Sir John F.W. Herschel: Lady with a harp, 1842

Cyanotype photogram on linen by Kate Cordsen. 2016 at Florence Griswold Museum