PHOTO-LITHOGRAPHY
Early Experiments--An Analysis--The Direct Process--
Transfer Process--Line and Half-tone--Some Difficulties--
A Natural Grain--Ink Photo-screen Effects--Essential
Features.
One of the most promising features of lithography is its co-partnership
with photography as a rapid and accurate method of reproduction. The
resources offered by this combination are very extensive. For facsimile
copying and proportionate enlargement or reduction photography stands
unrivalled, and, although in certain phases it may be somewhat
mechanical in its effects, its relation to lithography as a reproductive
art is nevertheless of an intensely practical nature, and far from
inartistic in character.
The first idea of inking up a photographic print so that it might be
transferred to the lithographic stone was suggested in the simplest
possible manner. A brief account of its inception will be instructive as
well as interesting, inasmuch as it will lead to a clearer conception of
the elementary principles involved.
During the early experiments in carbon printing it was discovered that a
gelatinous film sensitised with certain bichromates could be charged
with a coloured pigment, and a picture developed thereon. At first it
was not realised that images produced by the action of the light on such
a surface could be inked up with a greasy composition and afterwards
transferred to the lithographic stone, but it was not long before this
important point became apparent. It was found that after exposure under
the negative the transfer ink would only adhere to such portions of the
gelatinous surface as had been acted upon by the light.
Photo-lithography will best be considered under two sections, namely:--
1. The _direct_ process, in which the actual printing surface is
prepared and exposed under the negative.
2. The _transfer_ process, in which a gelatine-coated paper is
sensitised in a solution of bichromate of potassium and the photographic
print made upon it.
The _direct_ process in its application to the lithographic stone is
uncertain in its results. It is impossible to secure sufficiently close
contact between the negative and the stone, particularly when large
surfaces are under operation, and consequently the print is rarely if
ever an unqualified success. The erasure of defective work is also a
serious matter, and can only be effected by polishing and preparing the
stone again.
In the _transfer_ process absolutely close contact can be assured by the
use of the transfer paper; and should the print from any cause whatever
prove defective, another can be made immediately without any serious
loss of time or material.
The successful application of the _direct_ process to zinc and aluminium
plates is, however, an accomplished fact. The metal plate is
sufficiently elastic to adapt itself to any inequalities on the surface
of the negative. Under such conditions as these this process offers at
least one very important advantage. There is not the slightest
possibility of distortion such as might occur in the development of a
transfer. The metal plate also lends itself to easy manipulation.
_Photo-lithography_ in _line_ is simply the reproduction of line
drawings or prints in which the design is represented in black and white
with only such gradations as may be suggested by lines or dots.
_Half-tone_ photo-lithography is the reproduction of a design or copy
which has in its composition gradations of tone in the form of flat
tints.
It is sometimes described as the translation of the graduated light and
shade of the original copy into a surface which can be printed from by
mechanical means, for which purpose the ink-bearing surface is broken up
into the most minute sections, and thus forming an almost imperceptible
grain. The first attempts to reproduce the half-tones of a copy, in the
form of a grain consisting of minute dots of varying size and contiguity
according to the gradation of tone required, were made with a screen of
open textile fabric. This screen was placed between the lens and the
sensitive plate, but the results were crude and unsatisfactory.
The invention of cross-lined screens, in which the lines were cut on
glass and filled with black or other suitable colouring matter, was a
decided advancement in the half-tone photo processes.
The "screeny" effect produced by the "unvarying uniformity of grain" in
half-tone work is undoubtedly the chief drawback to its more extensive
adoption for photo-lithography. Fine etching cannot be resorted to as in
photo-engraving, neither is it possible, to emphasise effects by skilful
overlay and underlay; consequently half-tone impressions from a
lithographic stone are frequently disappointing. There are no
insurmountable obstacles to hinder the production of excellent
transfers, nor is it a difficult matter to transfer them to stone. The
trouble is, as already pointed out, the unvarying uniformity of the
grain.
This effect, or rather this lack of effect, has been to some extent
overcome by the use of a "four-line" screen in lieu of the usual
"crossed" screen, but even this is merely a _remedy_ and not a _cure_.
It has been confidently asserted that the highest degree of excellence
in photo-process work will be attained by the adoption of what may be
termed a natural grain. Several processes have been introduced which are
undoubtedly based upon collotype methods in which a reticulated grain is
produced more or less suitable for lithographic printing. Unlike the
mechanical screen grain the texture of these processes reproduces the
original copy with but little, if any, loss of expressive power. This is
indeed a feature of considerable importance, and suggests many
possibilities in the way of artistic reproduction.
To reproduce an old chalk drawing so that it might be successfully
transferred to stone and printed in the usual way, would be practically
impossible by any other process. In copying through a ruled screen many
of the delicate contrasts of light and shade would be so reduced as to
become almost valueless, consequently the print loses both in artistic
and expressive power. In contradistinction to this a natural grain
exhibits no harshness or indistinctness in the gradations of tone, and
retains its clearness and sharpness throughout the printing operation.
Reverting again to the half-tone ruled screens, it may be well to state
that small prints, being usually subjected to a closer inspection than
large ones, must be reproduced with great attention to the finer details
to ensure a certain amount of fidelity, and for this reason a screen
with fine rulings must be employed. Naturally, stronger and more
vigorous reproduction can be secured with the coarser rulings, but the
_screen_ effect will be too pronounced for close scrutiny.
There is still much to achieve in photo-lithography, and it is probably
owing to a full recognition of this fact that the progressive character
of the process is maintained. Its commercial value is undoubted, and its
successful application is chiefly a question of _how_ and _where_ it can
be most effectively introduced.
The essential features of photo-lithography are:--
1. A copy or original in which the modelling is well defined, and the
light and shade well emphasised, even to a point of slight exaggeration.
2. A _negative_ in which the whites of the original appear opaque, with
clear glass to represent the lines and solids.
3. A _print_ which can be developed or inked up with a pigment
sufficiently greasy in nature to transfer to the lithographic stone.