Photo-engravings are produced by means of photography. It is a fact
worthy of note that experiments in photographic engraving gave rise
to photography itself. The aim of Nicéphore Niepce, when he began his
researches in 1813, was not only to fix the image obtained by the camera
obscura on a plate of metal, but to convert this plate into an engraving
which could be used on a printing-press. His early death prevented his
perfecting the process to which he had devoted much time and study.
Three distinct methods of photo-engraving are employed in the United
States, viz.: swelled gelatine, photo-etching, and wash-out. The latter
is known as photo-electrotyping.
The first steps to produce a plate by any of these processes are
exactly alike, _i. e._ a perfectly sharp negative, either in line or
stipple, must be produced. If the copy furnished is a wood-cut, steel
or lithographic print, in which the lines are absolutely black on white
paper or card, the negative is made direct and no drawing is necessary,
unless a very great reduction is required, when it becomes necessary to
make a drawing, the lines of which are made open enough to stand the
necessary reduction. Where the copy furnished is a photograph, or wash
drawing, it is first photographed one half larger, or, where fine work is
desired, twice the size the plate required. In cases where exceptionally
fine work is required it is even made three times the size. The
photograph thus obtained is technically termed a silver print, and is an
untoned print on plain paper. On this silver print the artist makes his
drawing, using the best India ink, which must be so black that the finest
hair-line, when examined through a magnifying glass, appears absolutely
jet black. After the drawing is made, an alcoholic solution of bichloride
of mercury is poured over it, and quickly washed under the tap, leaving
the drawing on perfectly white paper. The artist then does whatever
retouching may be necessary, and the drawing is ready to be photographed.
The advantages of this method of drawing are apparent. The artist, being
able to work directly on the enlarged photograph of the object, obtains
absolutely correct outlines and detail. The drawing, when finished, is
sent to the gallery, where it is photographed to the required size of the
plate. The focus of the camera is carefully adjusted with the aid of a
focusing glass, so that the negative resulting will be perfectly sharp.
This must be carefully done, for unless the negative be sharp a perfect
plate cannot be obtained by any process. The sensitized collodion plate
is exposed in the camera from one to six minutes, after which it is taken
again to the dark room, developed, and fixed. It is then intensified
until the portions representing the whites of the picture are perfectly
opaque. Up to this point all the processes are alike, and the differences
from here will be noted.
If the plate is to be produced by the _swelled gelatine_ process, the
negative is varnished and sent to the gelatine room. Here the gelatine is
dissolved and the sensitized solution of bichromate of potash is added,
and it is flowed on plate glass, then placed in a drying box, where a
current of air is continually passed over it. When the gelatine is dry it
is placed in a printing frame, in close contact with the negative, and
exposed to the light. On removing the negative the picture is plainly
seen on the gelatine, the action of the light having changed the color
of the exposed portions of the gelatine, besides rendering those parts
insoluble, while the parts protected from the action of the light, by the
opacity of the negative, remain soluble, and are swelled up by immersion
in cold water. The gelatine is then an exact opposite of the plate, the
_whites_ being represented by the raised portions. From this mould, or
relief, a cast is made in a preparation of wax or plaster, when it is
ready for the stereotyper.
For the _wash-out_ or photo-electrotype process, the negative, when dry,
is not varnished, but is first coated with a rubber solution and then
with plain collodion, after which it is immersed in a dish of acetic
acid for about five minutes, when it is stripped from the glass and
turned over. It is then what is termed a reversed negative. The method
of preparing the gelatine is very much the same as for the _swelled_
process, with the exception that it is _cooked_ for about forty-eight
hours, and with the addition of several preparations which are introduced
at the time of sensitizing, to make it easily washed out. The negative is
exposed in the same manner as described before, but the time of exposure
is generally less. After being taken from the printing frame the gelatine
is gently scrubbed with a line brush, and kept in tepid water until a
very slight relief is obtained. It is then immersed in alcohol for a few
seconds, and dried with a cloth, when it is covered with a preparation
of lamp black and glycerine, which is allowed to remain about five
minutes, when the surface of the plate is carefully rubbed with clean
muslin, exposing the surface of the lines,—all this being done in a dark
room before an orange light, which is non-actinic. The gelatine is now
placed in a frame and exposed to the light from five to twenty minutes,
the lampblack protecting the spaces between the lines from the action of
the light, so that those portions remain soluble. The gelatine is again
scrubbed until the proper relief is obtained, after which it is allowed
to dry for about twelve hours, when it becomes hard and is ready to be
electrotyped.
The negatives for photo-etching are stripped and reversed in the same
manner as for the _wash-out_ process. The metal generally employed for
this process is zinc, though copper is sometimes used for very fine work.
The zinc is very highly polished, and a thin sensitizing solution is
flowed over it and dried, after which it is exposed under the negative
to the action of the light. It is then rolled up with lithographic
ink, placed in a dish of cold water, and gently rubbed with absorbent
cotton, the ink readily leaving the unexposed parts, but remaining on
the exposed lines or dots. It is then quickly dried and dusted over
with dragon’s-blood powder, which adheres only to the remaining inked
portions. The plate is then heated and cooled, and is ready for the
etching bath, which consists of a small portion of nitric acid and water.
After the first _bite_ the plate is again powdered, heated and cooled,
and more acid added to the bath. This is repeated several times, after
which the plate is ready for the press. The sides of the lines are
protected by the manner in which the powder is applied after the first
_bite_. The relief obtained in this way is greater than can be obtained
by any other process.
In reviewing the three processes above described, it is readily seen that
the photo-etching process is the shorter method, no moulding or casting
being necessary, and the sharpness of the finest lines is preserved in
a manner impossible by the other methods. The plates are much deeper,
and are equally suitable for the finest art work, down to the roughest
newspaper work. This process, of which there are several modes of
operating, has become very popular in the last few years, owing to the
many improvements introduced by the _process inventors_, who have turned
their attention to it. It has many advantages, among which is the fact
that a plate can be put on the press within two hours from the time the
copy is ready; and the wearing capacity of the plates is greater. By this
process more than 300,000 impressions have been taken from plates of
fine work, while swelled gelatine plates of the same character of work
would not stand over 5000, and electrotypes 50,000 impressions. The main
advantage of photo-etchings to the printer is, that the plates do not
require constant washing up, as is the case with plates made by the other
processes.
A photograph, brush drawing, or any copy that is not made up of line
or stipple, can be produced without the necessity of a line drawing by
the aid of the half-tone process. There are several of these processes
in operation in this country; and, although originally introduced in
Germany by Miesenbach, it has been so improved by American inventors that
the European work is far below the standard of the United States. The
principal methods of _half-tone_ in this country are worked secretly by
the inventors, each having modifications and improvements of his own.
It is impossible to give a thorough description here, as none of the
inventors are willing to risk patenting their processes, and a complete
publication is not desirable. The copy is first photographed, giving a
negative with all the details of the original. This negative is then
exposed to the camera, and the result is a _positive_, or, as commonly
called, a transparency. This positive is then placed in contact with a
glass plate covered with ruled lines. This plate is termed a _grating_.
Being placed in contact, they are then photographed together, giving a
negative of the object made up of lines and dots, representing the lights
and shades of the picture. Here the half-tone process ends, the resulting
plate being produced by any of the photo-engraving methods; but the most
satisfactory results are obtained by photo-etching. The main objection
to the half-tone plates is their lack of relief. No great depth can be
obtained without sacrificing the effect. In printing these plates the
greatest care in _making ready_ must be exercised, and a smooth surface
paper must be used. It is also necessary that a fine grade of ink be used
in small quantity, and that it should be properly distributed.
[Illustration: METAL HOUSE.]