Cyanotype
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![Cyanotype postcard, Racine, Wis., c. 1910](/Images/godic/202501/07/Cyanotype_postcard,_Racine,_Wis.,_ca._1910,_front2221.jpg")
Cyanotype is a photographic printing process that produces a cyan-blue print. 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.