A blueprint is a photographic print composed of white lines on a blue background. Blueprints were commonly used in the past for copying architectural plans and engineering drawings.
The blueprint process was developed by the British astronomer Sir John Herschel in 1840. The photosensitive compound, a solution of ammonium ferric citrate and potassium ferricyanide, is coated onto blueprint paper. Areas of the compound exposed to strong light are converted to insoluble blue ferric ferrocyanide, or Prussian blue. The soluble chemicals are washed off with water leaving a light-stable print.
Blueprints have mostly been replaced by Diazo prints or whiteprints, which have blue lines on a white background. These are sometimes also known, incorrectly, as blueprints.
