When a mechanic in the shop receives a working drawing, it is in the form of a blue-print, a blue paper on which the lines of the drawing appear in white.

A specially prepared paper, known as blue-print paper, is used for making blue-prints. This paper is prepared by the application of a chemical solution of red prussiate of potash, water, citrate of iron, and ammonia. The solution is applied with a camel's-hair brush and is then allowed to dry. After drying, the paper assumes a greenish yellow color.

The blue-print itself is made in the following manner. The tracing of the drawing is placed over a piece of blue-print paper. The two are then put into a frame constructed similarly to an ordinary picture frame. The frame is then exposed to the direct sunlight. The rays of the sun pass through every portion of the tracing paper except the black lines of the drawing, and act upon the chemical solution of the blue-print paper in such a way as to turn to a yellow color the entire paper, with the exception of that portion beneath the black lines. The blue-print paper is then dipped into water, which changes it, with the exception of the lines, to a blue color. The lines become white and are, of course, an exact reproduction of the tracing.