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: ferric ammonium citrate and potassium ferricyanide.
This is a photo of the Artis Ateliers group I was in for a long time. I used this picture to discover the cyanotype process.