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.
[Illustration: 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.
[Illustration: FIG. 42.--THE REDUCTION.]
[Illustration: 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.