Blueprint

Blueprint, a method for copying architectural drawings. The blueprint process was developed by the British astronomer John Herschel in 1840.

To create a blueprint, blueprint paper, which begins white, is first sensitized by coating it with a mixed solution of ammonium ferric citrate and potassium ferricyanide. Next, a translucent sheet upon which the architectural diagrams have been drawn is placed over the blueprint paper; a strong light is then directed through the translucent sheet on to the paper. The portions of the blueprint paper protected from light by black lines on the translucent sheet are unchanged, but the coating of the rest of the paper is reduced by the light to insoluble blue ferroferricyanide, or Prussian blue. When the paper is washed in water, the copy image emerges as a white-line print on blue paper that is stable in light.

The use of blueprints has largely been replaced by an alternative process for reproducing diagrams that creates a whiteprint, which has coloured or black lines on a white background. In contrast to the blueprint method, preparing a whiteprint does not require liquid chemicals. To create a whiteprint, a translucent sheet upon which the architectural diagrams have been drawn is placed on top of the whiteprint paper. The light-sensitive whiteprint paper is exposed to light, then developed using ammonia gas.