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John Hershel invents the blueprint
The blueprint process is the cyanotype process developed by british photographer and astronomer John Herschel in 1842. The exact date, however, is not specified. It is a solution of ferric ammonium citrate and potassium ferricyanide which is then coated onto paper. Areas exposed to strong light are changed to insoluble blue ferric ferrocyanide, or Prussian blue. The chemicals are washed off with water and leave a light stable print.
