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.

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.

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.

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.

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.

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.

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.

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.

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.