The drawing intended for reproduction is pinned on a board and placed
squarely before a copying camera in a good, even light. The lens used
for this purpose must be capable of giving a perfectly sharp picture
right up to the edges, and must be of the class called rectilinear,
i.e., giving straight lines. The picture is then accurately focused
and brought to the required size. A plate is prepared in the dark room
by the collodion process, which is then exposed in the camera for the
proper time and developed in the ordinary way. After development, the
plate is fixed and strongly intensified, in order to render the white
portions of the drawings as opaque as possible. On looking through a
properly treated negative of this kind, it will be seen that the parts
representing the lines and black portions of the drawing are clear
glass, and the whites representing the paper a dense black.
The negative, after drying, is ready for the next operation, i.e.,
printing upon zinc. This is done in several ways. One method will,
however, be sufficient for the purpose here. I obtain a piece of the
bichromatized gelatine paper previously mentioned, and place it on the
face of the negative in a printing frame. This is exposed to sunlight
(if there is any) or daylight for a period varying from five to thirty
minutes, according to the strength of the light. This exposed piece of
paper is then covered all over with a thin coating of printing ink,
and wetted in a bath of cold water. In a few minutes the ink leaves
the white or protected parts of the paper, remaining only on the lines
where the light has passed through the negative and affected the
gelatine. We now have a transcript of the drawing in printing ink, on
a paper which, as soon as dry, is ready for laying down on a piece of
perfectly clean zinc, and passing through a press. The effect and
purpose of passing this cleaned sheet of zinc through the press in
contact with the picture on the gelatine paper is this: Owing to the
stronger attraction of the greasy ink for the clean metal than for the
gelatine, it leaves its original support, and attaches itself strongly
to the zinc, giving a beautifully sharp and clean impression of our
original drawing in greasy ink on the surface of the zinc. The zinc
plate is next damped and carefully rolled up with a roller charged
with more printing ink, and the image is thus made strong enough to
resist the first etching. This etching is done in a shallow bath,
which is so arranged that it can be rocked to and fro. For the first
etching, very weak solution of nitric acid and water is used. The
plate is placed with this acid solution in the bath, and steadily
rocked for five or ten minutes. The plate is then taken out, washed,
and again inked; then it is dusted over with powdered resin, which
sticks to the ink on the plate. After this the plate is heated until
the ink and resin on the lines melt together and form a strong
acid-resisting varnish over all the work. The plate is again put into
the acid etching bath and further etched. These operations are
repeated five or six times, until the zinc of the unprotected or white
part of the picture is etched deep enough to allow the lines to be
printed clean in a press, like ordinary type or an engraved wood
block. I ought perhaps to explain that between each etching the plate
is thoroughly inked, and that this ink is melted down the sides of the
line, so as to protect the sides as well as the top from the action of
the acid; were this neglected, the acid would soon eat out the lines
from below. The greatest skill and care is, therefore, necessary in
this work, especially so in the case of some of the exquisitely fine
blocks which are etched for some art publications.
There are many details which are necessary to successful etching, but
those now given will be sufficient to convey to you generally the
method of making the zinc plate for the typographic block. After
etching there only remains the trimming of the zinc, a little touching
up, and mounting it on a block of mahogany or cherry of exact
thickness to render it type high, and it is now ready for insertion
with type in the printer's form. From a properly etched plate hundreds
of thousands of prints may be obtained, or it may be electrotyped or
stereotyped and multiplied indefinitely.--_G.S. Waterlow, Brit. Jour.
Photo._
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