BLUE PRINT, a photo-print on a paper sensitized with ferro-prussiate, according to a method first invented by Herschel in 184o. Used in the early days for the printing of photographic negatives, blue prints have come more recently to be used chiefly for making negative copies of drawings and documents. The cheapness of the materials, the permanence of the prints, and the ease and simplicity of the method—only a water bath is necessary for fixing and toning them—have united to make blue printing almost the standard way of reproducing all sorts of draw ings for architectural and engineering use, especially in the United States.
In making blue prints, the drawing is executed on transparent paper or specially treated linen (tracing paper or tracing cloth). The drawing is placed in a frame or machine in contact with the sensitized paper and between it and an adequate source of light, and after a suitable exposure, the print is removed and washed in clean water. The result is a clear negative print with the dark lines of the drawing appearing as light lines on a dark blue ground. Formerly most of the blue prints were produced individually in printing frames by exposure to the sun or daylight, but at the present time almost all are made in machines using electric light. These machines are based generally upon a rotating glass drum within which the lights are placed, and around which the drawings and the sensitized paper are fed, held tightly in place by rollers. The blue print paper is in a continuous band; after it has rolled around the light drum it is carried by machinery through the water bath and around heated rollers which dry it. The speed of the rotation of the light drum naturally controls the amount of exposure and the depth of the print; it is therefore made adjustable to suit the differences in the quality of the drawings. Documents, typewritten on thin paper, may be blue printed as successfully as drawings, and many architectural specifications are thus copied for the contractor's use.