· Invented by Sir John Herschel in the year 1842, a blueprint is a reproduction of a technical drawing using a contact print process on light-sensitive paper. · Original process produced white lines on a blue background.
· In early 1940, supplanted by diazo prints which have blue lines on a white background. Aqueous ammonia and black light are used to expose the drawing to the diazo paper.
· In both cases, an original drawing is produced on vellum or other medium that allows light to pass through it. The light-sensitive receiving paper turns blue (chemical reaction) leaving the corresponding drawing as white lines.
· The machines used to produce blueprints are relatively simple with few complicated parts, maintenance is simple, and reproduction costs are low. Aqueous ammonia is environmentally safe.
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