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.
Various base materials have been used for blueprints. Paper was a common choice; for more durable prints linen was sometimes used, but with time, the linen prints would shrink slightly. To combat this problem, printing on imitation vellum and, later, mylar was implemented.
The coated material ready for use has a shelf life of two days. Every industrial area had one or two small independent suppliers who made blueprint coated materials to order. These suppliers also provided a copying service for small users.
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 India 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 washed in running water to remove the unexposed coating, then dried. It gave a clearly legible copy of the drawing with a white line on 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.
Diazo prints remain in use in some applications but in many cases have been replaced by Xerographic print processes similar to standard copy machine technology using toner on bond paper.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.
As print and display technology has advanced, the traditional term "blueprint" has continued to be used informally to refer to each type of image.
With the advent in large, conglomerate, corporate entities, it is becoming increasingly difficult to find the traditional, family-owned blueprint shop. Most small to medium-size "blueprinters" are now owned by much larger corporations like American Reprographics Company (they alone own over 235 print shops in the US and Canada).
A similar network has been built up as a connection of independent reprographers, as members of ReproMAX. As a result, ReproMAX has grown to be the largest association of reprographics companies, with over 230 current network partners in North America and Western Europe. The electronic planroom and document management solution for ReproMAX is available as ReproMAX/DFS.