ALTHOUGH some of the methods of preparing photo transfers for lithography and zincography were known sixty years ago, it is only within the last quarter of a century that photography has gained acceptance as a means of reproducing illustrations for the printing press. Once recognised, it speedily conquered the whole field, to the exclusion of all rivals. In no other industry has the hand-craftsman been so completely displaced, and this from the sheer merit of the new processes rather than for economic reasons. Wood engravers of genius were few in number; artists as well as photographers got tired of seeing their productions travestied under the graving tool. More exact and conscientious expedients were bound to win their way.
Etchings, steel engravings, diagrams, and, in fact, any kind of black-and-white drawings, comprised entirely of lines, are very easy either to translate into a printing block, or to transfer on to the lithographic stone, by photographic means. Such drawings reproduce most clearly if executed in some perfectly black medium, such as process ink or ebony water stain. It facilitates matters if the paper is of pure white colour, and not too rough in grain, though pencil drawings on various patterns of hard stamped or grained paper reproduce as line work very effectively. The drawing is copied to the size required in the camera, with the reversing mirror if to be printed straight on to the zinc plate, exposure and development being so carried out as to produce a negative of great contrast. The wet-plate process or photo-mechanical plates are essential. All the lines of the original have to be represented in the negative by clear white glass on a dense black ground.
For the direct method, a zinc plate is carefully cleaned and polished with fine emery powder, finishing off with rouge and a bath of weak nitric acid, when it is ready for coating with the sensitive surface, which is to form the foundation of the acid resist. A machine called a whirler is usually employed for this purpose. The plate receives a pool of the liquid and is then whirled round at a great speed; meanwhile the material is spread homogeneously over it. Bichromated albumen is most commonly employed, though for the finest work sulphurised bitumen, dissolved in benzole, is still occasionally in use. After exposure under the negative, the unchanged albumen is washed off in water, leaving the image on the zinc for etching, a process outside the scope of this book. The bitumen image is developed either by turpentine or chloroform.
There are many ways of making photo-transfers, analogous in character to the oil printing process. Sometimes a paper, coated with gelatine and sensitised in a bichromate bath, is exposed under the negative, subjected to a prolonged soaking, and then an image in oily transfer ink printed upon it with the aid of a gelatine ink roller. Other transfer papers consist of a gelatine substratum covered over with a layer of albumen. The transfer ink is thinned with turpentine, and distributed as smoothly as possible over the surface of the print. A quarter of an hour in a warm temperature allows the turpentine to evaporate, when the paper is floated on cold water for a time, and sponged with a soft sponge, or pledget of cotton wool dipped in water. This removes the superfluous ink with the albumen, leaving the lines of the image clear for transference to the lithographic stone or zinc, as the case may be. With this paper the sensitiser is also a bichromate salt.
When translating an ordinary photograph into a form suitable to meet the, exigencies of the ordinary printing press, some mechanical device is required, corresponding to the various hatchings and cross-lines of the old-fashioned wood block. The metal surface must be so grained that the various gradations between the deep shadows and the high lights will be rendered in approximately their true value. For the lithographic stone we can use transfers, wherein, by introducing such chemicals as sodium chloride and potassium ferricyanide, a graining is imparted to the gelatine closely resembling the effect of lithographic chalk. Many attempts have been made to apply this principle to the typographic block, but without avail. The result invariably has been attended with smears and blurs; a grain of this nature is too rounded for the composition roller, and experimenters have therefore resorted to a screen of cross-lines through which the photograph or wash-drawing is copied in the camera. For the earliest screens fine netting was employed, after the manner of Fox Talbot's original method of photogravure; later on, indeed up to about ten years ago, the screens consisted of negatives obtained by photographing huge white cards engraved with a series of diagonal lines in one direction only. When the exposure was half completed the plates were reversed, so as to produce a crossline effect. Such screens were placed in contact with, and in front of, the plate used in photographing the original. In the blocks prepared in this manner, especially in ordinary commercial work, the pattern of the screen was generally very obtrusive, and the whole effect obviously mechanical.
The Levy screen, now almost universally used, consists of two sheets of crystal glass, ruled diagonally with black lines by a patent process and cemented together with Canada balsam. The ruling in different screens may vary in fineness, from 50 to 300 lines per inch, according to requirements, and the relation of black to white may be either as 1 : 1 or 5:4. Instead of being placed in contact with the sensitive plate, the screen is advanced a certain calculated distance in front of it, so that in place of the conspicuous pattern of the old-fashioned half-tone block, a series of dots appears of various sizes according to the gradations of the original. In practice, each aperture in the ruled screen acts as a pinhole lens, photographing a little picture, varying in shape and size according to the shape of the diaphragm aperture of the lens. A round aperture gives a round dot, a square aperture a square one, and so on. The object being to get several distinct kinds of dots, and especially to prevent vacant areas of transparency - above all in the high lights - the ordinary round lens-diaphragm is of little service, as compared with the square shape. Various shaped diaphragms are employed in practice, such as lozenge shape, with extended corners; or the Ray multiple diaphragms, pierced with three or more openings instead of one large opening, triangular, square, or round.