blueprint

blueprint, white-on-blue photographic print, commonly of a working drawing used during building or manufacturing. The plan is first drawn to scale on a special paper or tracing cloth through which light can penetrate. The drawing is then placed over blueprint paper, prepared with a mixture of potassium ferricyanide and ammonium ferric citrate. When the attached drawing and the blueprint paper are exposed to a strong light, the unprotected ferric salt not lying beneath the lines of the drawing is changed to a ferrous salt that reacts with the ferricyanide to form Turnbull's blue. This blue is the background of the finished print. The ferric salt under the lines of the drawing, protected from the light, remains and is dissolved during the washing in water that follows exposure. As a result, the lines of the original drawing appear white in the finished blueprint.