A printing process that gives a typical blue image. Paper is coated with a solution of iron salts and dried. An object, a drawing or a negative is placed on the paper, which is then exposed to sunlight or UV light. The exposed areas turn blue. Washing the paper in water removes the unexposed chemicals and fixes the image.
Sir John Herschel invented the process in 1842. Anna Atkins used it to make the first book illustrated with photographic images. Later, engineers and architects used it to copy plans, which is where the word "blueprint" comes from. Today it is a popular alternative process.
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