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.
FIG. 43.—MAN IN A BOTTLE.
Blue prints may be given a black tone by plunging them into a solution of 4 parts of caustic potash in 100 parts of water; then, when the blue color has entirely disappeared under the action of the potash, and a yellowish color has taken its place, they are immersed in a solution of 4 parts of tannin in 100 parts of water; then washing them again, we obtain prints whose tone may be assimilated to that of pale writing ink.
In the process giving blue lines upon a white ground, it is necessary that the action of the light shall be to convert the iron compound into one that can be discharged from instead of being fixed on the paper, so that we obtain a positive from a positive. Abney describes the process as follows: Thirty volumes of gum solution (water 5 parts, gum 1 part) are mixed with 8 volumes of a citrate of iron and ammonia solution (water 2 parts, double citrate 1 part), and to this is added 5 volumes of a solution of ferric chloride (water 2 parts, ferric chloride 1 part). This solution thus formed is limpid at first, but will gradually become thicker, and should be used soon after mixing. It is then applied with a brush to the paper (which should be well sized) and dried in the dark. Exposure is accomplished in a few minutes, the paper being placed under the drawing in the printing frame. It is then developed with potassium ferrocyanide, 50 grains, water 1 ounce, applied with a brush until all the details appear of a dark-blue color. The print is then rapidly rinsed, and placed in a dish containing the clearing solution, made of 1 ounce of hydrochloric acid and 10 ounces of water.
The third process, which gives violet-black lines on a white ground, is the following: Make up the sensitive solution with water, 16 ounces; gelatine, 4 drachms; perchloride of iron (in a syrup condition), 1 ounce; tartaric acid, 1 ounce; sulphate of iron, 4 drachms. The paper is floated on or brushed over with this and dried. The exposure is about the same as with the last process. When sufficient, the greenish-yellow color will turn white, except the lines, which should be somewhat dark. The developing solution is composed of 1 part of gallic acid in 10 parts of alcohol and 50 of water. When immersed in this solution the lines will turn blacker. The finish is then made by thoroughly washing in water.
