Why are blueprints blue?
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The term "blueprint" was originally derived from the visual aspects of prints made using the contact printing process of cyanotype. It is cyanotype which produces the white lines on blue background which are characteristic of the traditional blueprint.
[edit] History
The blueprint process is essentially the cyanotype process developed by the British astronomer and photographer Sir John Herschel in 1842.[1] The photosensitive compound, a solution of ammonium ferric citrate and potassium ferricyanide, is coated onto 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.
A similar process was used to produce printing proofs for offset printing.
Various materials have been used for blueprints. Paper was a common choice, but for more durable prints linen was sometimes used, but with time, the linen prints would shrink slightly. To combat this problem, printing on vellum and, later, mylar was implemented.
[edit] Use
A blueprint had several advantages over other types of drawings; the contact printing method allowed an unlimited number of prints to be made without degradation and assured highly accurate dimensions on the copies. The resulting prints were resistant to fading, marking, and alteration. Additionally, the blueprint process was significantly less expensive than a silver-based photographic process.
Revision control was done in contrasting color on the blueprints, for example red markup of a blueprint copy by the engineer, then yellow markup on the copy by the draftsman who implemented the changes on the original drawing, then brown markup by the checker, on a check-print (a brown-line) or sepias. Finally, the architect or engineer, draftsman, checker and supervisor would sign the original drawing, thus making it a legal document. This final, approved drawing would become the original from which new blueprints were made.
[edit] Replacements for blueprints
Traditional blueprints have largely been replaced by more modern, less expensive printing methods and digital displays. In the early 1940s, cyanotype blueprint began to be supplanted by Diazo prints or whiteprints, which have blue lines on a white background; thus these drawings are also called blue-lines or bluelines. Other comparable dye-based prints are known as blacklines.
Diazo prints remain in use in some applications but in many cases have been replaced by Xerographic print processes similar to standard copy machine technology using toner on bond paper. More recently, designs created using Computer-Aided Design techniques may be transferred as a digital file directly to a printer; in some applications paper is avoided altogether and work and analysis is done directly from digital displays.
As print and display technology has advanced, the traditional term "blueprint" has continued to be used informally to refer to each type of image
Why is the grass green?
A special coated yellow paper is used, I fail to remeber the name but i do recall that it was very sensitive to light therefor kept in a dark cabinet. But anyways, the paper was laid beneath the media that was to be copied. Then both sheets were loaded into the blue print machine and exposed to a bright florecent bulb. The sheets are feed out of the blue print machine leaving vage yellow lines that were shadowed from the original print. The bright light has burned the coated yellow away. The sheet is then but back into the blue print machine were it is exposed to an amoania chemiacal vapor were the chemical reaction changes the yellow line into a permenant blue. So the best answer i can give you is simply by chemical reaction.
This is my real answer not a cut and paste from web site!
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