FIG. 40.—THE HEAD UPON A PLATE.
[HOW TO COPY DRAWINGS.]
There are three principal methods of copying mechanical drawings, tracings, sketches, etc. These are: (1) A process to obtain white lines upon a blue ground; (2) a process by which blue lines upon a white ground are obtained; and (3) a process giving black or violet-black lines upon a white ground.
FIG. 41.—THE SAWED-OFF HEAD.
The first process is undoubtedly the simplest, as after printing upon the paper it is developed and fixed by simple immersion in cold water; but, at the same time, the white lines on the blue ground are not so clear and effective as the other processes. The cyanotype paper, as it is called, can be obtained ready for use at any draughtsman's stores, but if you prefer to make it yourself, here is the recipe: Two solutions are made—20 parts of red prussiate of potash are dissolved in 100 parts of water, and 10 parts of ammonio-citrate of iron in 60 parts of water. These two solutions should be mixed together immediately before using, and the operation must be performed in the dark. Paper is floated on this solution, or applied with a broad camels-hair brush, and hung up to dry. If it is well dried and carefully preserved from light, moisture and air, this paper will keep for some time. After printing—which, when sufficient, should show the lines copied of a yellow color upon a blue ground—the prints should be washed in several waters, and if a few drops of chlorine water or dilute hydrochloric acid be added to the washing water, the blue ground will appear much darker and the lines rendered clearer and whiter. The commercial paper sold is generally prepared by this method.
FIG. 42.—THE REDUCTION.