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Last edited 20 Oct 2020
Blueprints were first introduced in the 19th century, to allow the reproduction of documents, and in particular drawings used in industries such as construction. Blueprints are generally recognisable from their blue background and light-coloured lines. They create an accurate negative reproduction of the original using a contact print process on light-sensitive sheets. Before this, reproductions were created using a photolithographic process, or by hand-tracing, both of which were expensive and time consuming.
Blueprints were developed in 1861 by French chemist Alphonse Louis Poitevin, who discovered that ferro-gallate becomes an insoluble, permanent blue when exposed to light. If it is coated onto paper (or a similar sheet material such as imitation vellum or polyester film) in an aqueous solution and then dried, (at which point it is yellow) it can be used to accurately reproduce large-scale translucent documents such as drawings.
Reproductions are made by placing the original, translucent document above a ferro-gallate coated sheet in a frame and then exposing them to light. The unconverted coating is then washed away, leaving a stable negative reproduction of the original.
Other photo-sensitive coatings have been developed, along with the similar diazotype or ‘whiteprint’ process (also known as the blue-line process as it produces blue lines on a white background), but it has now been made largely redundant by the development of large format black and white and colour printers. It is still sometimes used to for artistic reasons.
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