Definition:  A blueprint is a type of paper-based reproduction usually of a technical drawing documenting an architecture or an engineering design. More generally, the term "blueprint" has come to be used to refer to any detailed plan.

History

As print and display technology has advanced, the traditional term "blueprint" has continued to be used informally to refer to each type of image – here’s how it all started and where it is today.

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.

Various base 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.

The term "blueprint" was originally derived from the visual aspects of prints made using the contact printing process of cyanotype.  Cyanotype produces the white lines on blue background which are characteristic of the traditional blueprint.

Use

For almost a century blueprint was the only low cost process available for copying drawings. Once invented no technical development was required, the process was put to widespread use immediately, notably in shipbuilding and manufacture of locomotives and rolling stock for railways. Civil engineering providing steel framed buildings and steel bridges adopted blueprint and all engineering adopted the technology.

The normal use was to have a wooden frame with a spring loaded back, similar to a picture frame with a glass front. The drawing would be traced in indian ink on tracing paper or tracing cloth. Indoors, coated paper and tracing would be loaded into the frame which was then brought out to sunlight. Exposure time varied from less than a minute to about an hour under an overcast sky. The operator could see the blue image appear through the tracing, when ready the frame was brought indoors. The material was then washed in running water to remove the unexposed coating, then dried. It gave a clearly legible copy of the drawing with a white line and dark blue background. This copy possessed unlimited resistance to light and resistance to water that was as good as the substrate.

The diazo document copying process progressively took over from blueprint during the period 1935 to 1950.

Blueprints Today

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 computer printer or plotter; in some applications paper is avoided altogether and work and analysis is done directly from digital displays

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