Blueprints

A blueprint is a reproduction of a technical drawing, documenting an architecture or an engineering design, using a contact print process on light-sensitive sheets. Introduced in the 19th century, the process allowed rapid and accurate reproduction of documents used in construction and industry. The blue-print process was characterized by light-colored lines on a blue background, a negative of the original. The process was unable to reproduce color or shades of grey.

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.

The process has been largely displaced by the diazo whiteprint process and by large-format xerographic photocopiers, so reproduced drawings are usually called "prints" or just "drawings".

The term is also used less formally to refer to any floor plan (and even more informally, ).

In 1861, Alphonse Louis Poitevin, a French chemist, found that ferro-gallate in gum is light sensitive. Light turns this to an insoluble permanent blue. A coating of this chemical on a paper or other base may be used to reproduce an image from a translucent document.

The ferro-gallate is coated onto a paper from aqueous solution and dried. The coating is yellow. In darkness, it is stable for up to three days. It is clamped under glass and a light transmitting document in a daylight exposure frame which is similar to a picture frame. The frame is put out into daylight requiring a minute or two under a bright sun or about ten times this under an overcast sky. Where ultra-violet light is transmitted the coating converts to a stable blue or black dye. The image can be seen forming when a strong image is seen the frame is brought indoors and the unconverted coating, under the original image, is washed away. The paper is then dried.

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Wikipedia