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Blueprint, a method for copying
architectural drawings. The blueprint process was developed by the British
astronomer Sir 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
onto the paper. The portions of the blueprint paper protected from light by
opaque (including red, black, green and yellow but not BLUE) 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The use of blueprints has largely been
replaced by an alternative process for reproducing diagrams that creates a Diazo
print, whiteprint, which has blue 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.
"Blueprint," Microsoft® Encarta® Online Encyclopedia 2000
I bet you always wanted to know this but where to afraid to ask! :) s.
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