Blueprint - The Blueprint Process

The Blueprint Process

In 1861 Alphonse Louis Poitevin, a French chemist, found that ferro-gallate in gum is light sensitive. Light turns this to an insoluble permanent blue. A coating of this chemical on a paper or other base may be used to reproduce an image from a translucent document.

The ferro-gallate is coated onto a paper from aqueous solution and dried. The coating is yellow. In darkness it is stable for up to three days. It is clamped under glass and a light transmitting document in a daylight exposure frame, which is similar to a picture frame. The frame is put out into daylight requiring a minute or two under a bright sun or about ten times this under an overcast sky. Where ultra-violet light is transmitted the coating converts to a stable blue or black dye. The image can be seen forming, when a strong image is seen the frame is brought indoors and the unconverted coating, under the original image, is washed away. The paper is then dried.

The result is a copy of the original image with the clear background area rendered dark blue and the image reproduced as a white line. The image is stable. The contact printing process has the advantage that no large-field optical system is required. A further advantage is that the reproduced document will have the same scale as the original. A drawback is that the dark blue background makes it difficult to add new information to the print (such as recording as-built changes); a blueprint cannot easily be altered. Since the paper is soaked in liquid during processing, some change of scale can occur,and the paper becomes brittle. Engineering drawings often are marked to remind users not to rely on the scale of reproductions.

Later other blueprint processes based on photosensitive ferric compounds were used. The best known is probably a process using ammonium ferric citrate and potassium ferricyanide. In this procedure a distinctly blue compound is formed and the process is also known as cyanotype. The paper is impregnated with a solution of ammonium ferric citrate and dried. When the paper is illuminated a photoreaction turns the trivalent (ferric) iron into divalent (ferrous) iron. The image is then developed using a solution of potassium ferricyanide forming insoluble ferroferricyanide (Turnball's blue identical to Prussian blue) with the divalent iron. Excess ammonium ferric citrate and potassium ferricyanide are then washed away.

This is a simple process for the reproduction of any light transmitting document. Engineers and architects drew their designs on cartridge paper; these were then traced on to tracing paper using Indian ink for reproduction whenever needed.

Introduction of the blueprint process eliminated the expense of photolithographic reproduction or of hand-tracing of original drawings. By the latter 1890s in American architectural offices, a blueprint was one-tenth the cost of a hand-traced reproduction. The blueprint process is still used for special artistic and photographic effects, on paper and fabrics.