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.
The blueprint process is essentially the cyanotype process developed by the British astronomer John Herschel in 1842. The photosensitive compound, a solution of ferric ammonium 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 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, polyester film (Mylar) was implemented.
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 the manufacture of railway locomotives and rolling stock.
The coated material ready for use has a shelf life of two days. Industrial areas would often have many independent suppliers that made blueprint coated materials to order and provided a copying service.
The typical practice 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, the 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.
"Man And His Machines: Electric Blue Printing Machine". The World's Work: A History of Our Time XXXI: 113. November 1915. https://books.google.com/books?id=09_Sr9emceQC&pg=PA113. Retrieved 2009-08-04.
- The-Blueprints.com - The largest free blueprint/3-view/template collection on the internet, with over 36000 images online.