A blueprint is a paper-based reproduction of a drawing, usually a technical drawing. Blueprints use the cyano-type process whereby the paper (or plastic) is coated with a solution of two soluble iron(III) salts, potassium hexacyanoferrate(III) and iron(III) ammonium citrate.
The two iron salts do not react with each other in the dark, but when they are exposed to ultraviolet light the iron(III) ammonium citrate becomes an iron(II) salt. The iron(II) ion reacts with the potassium ferricyanide to form an insoluble blue compound – ferric ferrocyanide, also known as Prussian blue.
A blueprint starts out as a black ink sketch on clear plastic or translucent tracing paper, the ink sketch is laid on top of a sheet of blueprint paper and exposed to ultraviolet light or placed in sunlight. When the light strikes the paper, it turns blue.The black ink prevents the area under the drawing from turning blue.
After exposure to UV light, the water-soluble chemicals are washed off the blueprint, leaving a white (or whatever color the paper is) drawing on a blue background. The resulting print is light-stable and as permanent as the substrate upon which it is printed.