_REPRODUCTION BY HALF-TONE PROCESS._
From the processes with which I propose to engage my reader's attention,
wood and steel engraving, and kindred methods, stand apart.
Were we dependent upon these alone, not one-thousandth part of the
illustration matter of to-day could ever have been produced, encumbered
as the older methods are with the two things which, in the production of
anything "for the million," are serious drawbacks, namely, time and
expense of production.
Whilst these older methods undoubtedly possess characteristics which
will always give them a peculiar value, and secure for them immunity
from extermination (and make them, indeed, essential for certain
purposes), there was long ago felt a need for a method of rapid
reproduction unattended with individual artistic skill on the part of
each worker employed, and, above all, free from heavy expense. Such a
need has been more than met by what we now know as the mechanical
processes. I say more than met, because, gradual improvement in the
processes themselves, and an increased knowledge of the particular
requirements of these processes on the part of the draughtsmen or
artists, has resulted in giving us a process which is not only rapid and
cheap, but which produces prints of high quality and merit.
It will of course be at once apparent that in order to get our
illustration into the printed pages of book or newspaper we must first
transfer the original to a plate, or block, and then so manipulate the
surface that, like a printer's type, it shall take the printing ink in
such manner as shall leave an exact imprint upon the paper, or other
surface, upon which it is pressed.
The transfer of the original is accomplished by photography. The
preparation of the surface is effected by a chemical or mechanical
process. Correctly defined, these processes are "photo-mechanical," and
such are generally understood by the literally ambiguous title
"process," which is colloquially applied to all such methods.
It is not the intention of the present book to give instructions whereby
to work the processes, beyond a general outline which shall make the
illustrator acquainted with the method in which his drawing or photogram
is utilised. This knowledge will enable him, to some extent, to adapt
himself and his work to its special requirements.
Photo-mechanical processes are of two kinds: those by which the image is
_ingraved_, known as _intaglio_, and those in which the image is
produced in _relief_, or _relievo_. In the first of these the result is
more or less similar to an ordinary engraved copper-plate, which, being
wiped after inking, retains the ink in the engraved or indented
portions, and prints accordingly. The intaglio processes are confined to
what is known as Photogravure, or Photo-etching, and modifications
thereof. This will not engage our attention, for, beautiful as are its
results, it is comparatively expensive, and lacks that characteristic of
speed which has made the _relief_ processes so useful. The engraved
plate must be printed _separately_; it cannot be set up with type and
printed as letterpress. At the same time, in order to clear my reader's
mind, and my own course, before proceeding further, mention may be made
of other separate-printing processes, such as Collotype.[1] (also
known under many fancy titles), in which the image is printed on a
machine or press from an inked gelatine surface. These can easily be
distinguished by the image having a perfectly even appearance as though
produced by a water-colour wash, without grain, reticulation, or lines.
Photo-lithography, in which, as the name implies, the picture is
transferred on to stone, from which it is then printed as in ordinary
lithography, may also be mentioned here.
[Illustration: HALF-TONE FROM PHOTOGRAM; HIGH LIGHTS STRENGTHENED WITH
CHINESE WHITE.]
Another beautiful process of reproduction is the Woodbury-type, named
after its inventor, Walter B. Woodbury, in which a bichromated gelatine
film is exposed under a negative, and the soluble portions afterwards
removed by hot water. The resulting gelatine relief, which contains a
facsimile picture, is allowed to dry, when it becomes as hard as stone.
It is next forced by hydraulic pressure into a sheet of polished lead,
leaving therein an exact counterpart of its every elevation and
depression. The lead plate (_intaglio_) is next placed in a handpress,
and flowed with a pool of hot gelatinous colour; a piece of paper is
next placed on top and pressure applied, when all surplus colour oozes
from the sides. After a few moments, when the gelatine becomes
sufficiently cool, the paper bearing its delicate gelatine print is
pulled off and dried.
[Illustration: LINE REPRODUCTION FROM PEN AND INK.
(_Original 14 x 11 inches._)]
Beautiful as many of these are, they can only be applied where the
expense attending them and the slower printing is not an objection, and
where letterpress is not required on the same sheet at the same
printing. Printing matter can of course be afterwards introduced, but
this must be by a second operation.
We have now left for consideration the relief processes, in which the
design to be printed is produced similar in character and appearance to
that of the movable letterpress type used by printers.
[Illustration: FEEDING THE CHICKENS.
(_Half-tone from photogram. Original 14 x 11 inches._)]
These processes are roughly divisible into two sections, "tone" and
"line," to understand which I will refer the reader to the accompanying
illustrations.
In these the image appears to be respectively composed of _lines_ of
varying strength and proximity in the one, and of _tints_ ranging from
grey to black in the other. If the latter be closely examined, however,
or looked at through a magnifying glass, it will be found that what at
first appears to be a flat even tint is composed of an infinite number
of dots arranged in a reticulated or geometrical pattern. We will now
see how this effect is produced, and what are the especial uses of this
so-called "tone" or "half-tone" process.
[Illustration: THE HERON HOUSE.
_Half-tone from photogram._ (_Original 14 x 11 inches._)]
HALF-TONE PROCESS.
As has been already said, it is of course necessary to produce from the
original a printing surface of such a kind as shall take ink and print
an image therefrom. Now it will of course be obvious that with ink and
white paper we can only produce two things--black and white--and that
therefore all the intermediate shades must be produced by a greater or
less number of black dots. The process under consideration, in common
with some others, is based upon the fact that gelatine or albumen
sensitised with bichromate of ammonium or potassium, becomes _insoluble_
after being acted upon by light. A solution of bitumen in benzole also
forms a light sensitive coating which is frequently used in these
processes. If we were to expose such a sensitive film to light under
(for example) a photographic negative of a figure taken against a light
background and then washed it in a suitable solvent, those parts which
had been protected from light by the opaque portions of the negative,
such as the background, the face, hands, and white portions of the
dress, would dissolve away, leaving the insoluble or light affected
portions standing.
[Illustration: LINE REPRODUCTION FROM PEN AND INK.
(_Original 14 x 11 inches._)]
If we were now to ink these portions, we could print a black image which
would represent the general form of the portrait like the old-fashioned
silhouettes, or shadow pictures; but the "half-tone" process provides
for the breaking up of the solid black image, substituting for it black
dots closer and more numerous where the darker greys are, and less so
where the shading is lighter. A very considerable variation in apparent
tint may thus be effected.
The actual method is briefly thus:--What is known as a "screen" is first
made by taking a photographic negative of a print from a copper-plate,
on which has been ruled about 120 fine parallel lines to the inch. This
is done with extreme care and exactness, so that the negative gives a
clear transparent copy of the ruled lines of microscopic sharpness. In
many cases the ruled copy is moved round a quarter of a circle during
the exposure, thus resulting in the effect of double ruling, the lines
intersecting each other at right angles. Or two printings may be made
from the copper-plate to produce the same result and the negative then
taken. We shall thus have a negative having the appearance of a very
fine lattice of transparent glass on an opaque ground.
With this "screen" placed nearly in contact with an ordinary
photographic plate, film to film, we proceed to make a copy negative, in
the camera, of the original picture. This, when developed, will give the
picture _and_ an image of the screen, which has been interposed,
together on one plate.
If this is now clear to my readers, they will probably at once see what
will result when such a plate is used on a sensitised sheet of zinc or
copper, and the soluble parts of the film washed away as before
suggested.
The parts of the film made insoluble from exposure to light will be
reticulated all over by minute soluble dots or lines where the image of
the screen has interrupted the light, and more so or less in proportion
to the lightness or deeper shade of the original, and by this means what
would have been unbroken blacks are sprinkled over, so to speak, with
tiny white interstices, the ink when applied remaining on the
alternating projections of undissolved film.
Suppose the film to have been of bitumen and spread upon a sheet of
zinc, we should have a reticulated image in insoluble bitumen with
interstices of plain zinc. This bitumen forms a protective coating, so
that when immersed in a weak solution of nitric acid the acid only eats
a way into the bare metal. Gradually, and by subsequent acid baths, the
parts covered by the film are left in strong relief, and in a fit
condition to print from. The film which has thus resisted the acid is
then washed away, leaving the zinc relief.
To carry out the above process many details, which I have not thought it
the office of this book to enter into, will be required. Thus the
solvent used for developing, or, in other words, washing away the
soluble portions of a bitumen film, is turpentine; but water is used in
the case of bichromatised albumen. An acid resisting preparation is
finally applied to the plain zinc relief, and the whole block re-etched
or "re-bitten," so as to strengthen the image; certain precautions,
moreover, are taken to prevent the acid "under etching" the image--and a
great deal more which, of course, would have to be clearly described
were it intended to teach the process of block-making.
In the variety of half-tone blocks, known as Typogravure, a different
method of breaking up the surface is adopted; no intervening screen is
used, but the surface of the metal has imparted to it a preliminary
roughness or grain, and the image is printed and etched on this rough
surface. These blocks, when carefully printed from, yield exceedingly
nice results, the grain having something of an "aquatint" character,
which appears to be more discriminating than that derived through the
use of the ruled screen. The softness of outline and freedom from
anything like a mechanical texture is well seen in blocks made by this
method. The remarkable difference obtained from the same block by
different printers will at once suggest that a very great deal depends
upon the printing quite irrespective of the quality of the block itself.
Many letterpress printers make a specialty of block-printing, the chief
art being in the "making-ready" and "underlaying," by which terms is
understood the careful adjusting of the block, so that its surface be at
exactly the proper elevation to secure the proper amount of pressure,
neither more nor less, when on the printing machine. Some further
remarks on this subject will be found in Chapter X.
[Illustration]
I trust, however, enough has been said to give a general idea to the
uninitiated of how we arrive at the dotted ink print, which we recognise
as a reproduction from a photogram, or wash-drawing, or indeed anything
which is similarly made up of flat tints.
[Illustration: UNTOUCHED HALF-TONE FROM PHOTOGRAM.
(_Original 4 x 3._)]
[Illustration: HALF-TONE FROM PHOTOGRAM--THE BLOCK ENGRAVED ON BY HAND.
(_Original 4 x 3._)]
It will, of course, have been understood that the ruled "screen," which
is interposed between the picture to be copied and the plate on which it
is copied, will appear over _the whole_ of the copy negative, whether
the image extend so far or not, so that, in the reproduction, even what
should be blank whites will be covered with the fine black dots or
grain, though more widely separated by little white spaces.
If the accompanying reproductions be examined, this will be found to be
the case.
To any one who has given pictorial matters much thought, the
disadvantage of this will be at once apparent.
[Illustration: ON SLAPTON LEY, SOUTH DEVON.
_Photogram touched up by hand--block untouched._
(_Original 4 x 3._)]
In black-and-white pictures, white is the highest expression of light,
and yet how far the whitest paper is from sunlight, and how much shorter
the whole gamut of tones, from blackest ink to whitest paper, is when
compared with the scale of Nature, have often been pointed out and are
now generally understood. But our half-tone process makes the range of
tones still shorter by curtailing it at the top of the scale and cutting
off the white: the pervading "tint," or "grain," reducing white to a
light grey, and not even the deepest blacks and intermediate tones are
nicely rendered except by very careful printing. The printing of
half-tone process blocks has received great attention amongst
better-class printers of late, with the result that marked improvement
has taken place, and it is clearly seen that be a half-tone block ever
so well made it is only admirable when special ink and special paper
(notably a fine clay surface paper) are used, and more than ordinary
knowledge and care expended in the machining.
While the ideal process block is one in which, when the process is
completed, the block is ready for the press (and many process houses
pride themselves upon turning out "untouched" blocks), yet there are few
houses who do not employ some hands who are constantly working with
engraver's tools to "improve" the blocks after the last etching is done.
Much brilliancy of contrast and effect may be accomplished if the
engraver cut away the grain altogether on that part of the surface of
the block where it should print white; but this must necessarily be done
by men of instinctive taste and good judgment, for immediately hand
carving be admitted the essential character of an _automatic facsimile_
process is lost. The illustrator or artist will, therefore, unless he
give careful and precise instruction as to what parts are to be cut
away, or can superintend the work himself, feel considerable hesitation
in entrusting such a delicate task to a stranger. One little touch in
the wrong place, one bit of plain white too many, and the harmony of the
whole illustration will be upset; so that many will reasonably prefer a
weak flat print to the uncertainty which must attend the leaving of a
block to a mechanic's mercies.
In the accompanying three illustrations we have, first, an untouched
"half-tone" block from an ordinary photogram; secondly, a block from
the same original, "fine etched" or with the etching controlled so as to
brighten the effect; and thirdly, a block made from the same photogram
which, in order to compensate for any failings of the process, has been
worked on _by the artist_, strengthening the shadows and brightening the
lights.
[Illustration]
This brings us to consider the subject of working on photograms by hand,
and the preparation of illustrations generally, which is dealt with in a
separate chapter.
[Illustration]