PHOTOENGRAVING

any of several processes for producing printing plates by photographic means. In general, a plate coated with a photosensitive substance is exposed to an image, usually on film; the plate is then treated in various ways, depending upon whether it is to be used in a relief ( letterpress) or an intaglio (gravure) printing process. Engraving is the broad term for the procedure used in making plates, in which printing and nonprinting areas are distinguished by their height with respect to the general plane of the surface, the artistic decoration created by mechanically incising a design into a surface, and the creation of original works of art by tooling or etching an image into the metal (or plastic) surface and transferring the resultant image to paper. For detailed information on these last two subjects, see printmaking. This article is limited to consideration of the procedures whereby a printing surface useful in the production of multiple ink-on-paper images is produced. The term photoengraving is correctly applied to the procedures discussed here, since the use of light energy, as involved in photographic processes, is essential. A distinction must be made between a relief printing plate, in which the ink-carrying (or image-bearing) surface coincides with the general level of the plate surface, with nonimage portions cut below the surface, and intaglio printing surfaces, in which the ink-carrying image elements are incised into the plate surface. In the first type of printing, a uniform film of ink is distributed over the surface of the plate and transferred from the individual image elements to the receiving paper surface. In the second, the plate is flooded with a low-viscosity (thin) ink, then wiped with a blade (doctor blade) to remove any ink adhering to the surface. The doctoring action leaves the incised intaglio image filled with ink; later, as paper is brought into contact with this image and pressure is applied, surface-tension and capillary-action forces cause the ink to transfer from the plate to the paper. any of several processes for producing printing plates by photographic means. In general, a plate coated with a photosensitive substance is exposed to an image, usually on film; the plate is then treated in various ways, depending upon whether it is to be used in a relief (letterpress) or an intaglio (gravure) printing process. In relief printing the printing areas of the plate are defined by cutting away the nonprinting areas to a lower depth; ink is uniformly distributed (usually by a roller) on the raised surface only and is then transferred to paper. With intaglio plates, the nonprinting areas are raised; the printing areas are cut or etched away with acid. The whole plate is washed with ink, which is then wiped off the top surface, leaving the incised areas filled with ink to be transferred, under pressure, to paper. Although early 19th-century experiments with photographically prepared printing plates and chemically etched letterpress printing surfaces were contemporary with the first developments in photography, the commercial development of photoengraving did not start until the 1850s, after the introduction of the wet-collodion process. This process, still widespread until the 1930s, used a plate coated with a light-sensitive solution of collodion to capture a negative image for transfer to the printing plate. A major difficulty was that of producing tones intermediate between black and white by letterpress printing. In the second half of the 19th century various kinds of halftone screens were invented that solved the problem and ensured the rapid growth of photoengraving. Light falling on a surface coated with a photosensitive substance was made to pass first through a patterned screen (usually glass ruled with a uniform grid of lines, either the lines themselves or the squares between them being opaque), as well as through a transparent film carrying the image (e.g., a photographic transparency). The screen broke the image into a pattern of dots of various sizes, the size of any given dot being dependent on the amount of light transmitted by the image at that point. Thus, when the plate was developed-usually by washing away the unexposed coating-the surface was left covered with dots of varying size corresponding to the varying tones of the image. The exposed, hardened coating acted as a resist during the subsequent etching of the plate by acid. This process can be applied whether a plate is being prepared for relief or intaglio printing. For relief printing, a halftone screen with opaque lines and a negative transparency are used. The areas of large dots (which take more ink than small ones) will print as the dark areas of the image. Intaglio printing using this process, known as photogravure, was invented in 1878. A transparent positive is used with a screen of opaque squares, producing a plate of tiny cells etched to varying depths, the deepest (which hold the most ink) representing the dark areas on the image. Because the cell pattern breaks up fine lines, photogravure is not ideal for printing type, but it is very useful for continuous-tone copy like photographs and drawings and is used in printing periodicals and other commercial work needing very long press runs. It is generally done by rotary printing (known therefore as rotogravure) from copper cylinders, which are often plated with chromium for longer wear. A related process, line intaglio, or copperplate gravure, in which the images are composed exclusively of lines of varying width and depth, is used extensively in the printing of bank notes, stamps, securities, and the like. The characteristic sharpness and variety of the lines make it hard to reproduce them by any photographic or other means. An important technical difficulty in photoengraving can occur at the etching stage, from the tendency of acid to act equally in all directions and not merely downward, thus broadening lines and undercutting and weakening halftone dots. This was overcome initially by coating the sidewalls of the etched areas with an acid-resist, sometimes repeatedly, but later by the discovery of various compounds that control the lateral action of the chemicals used to etch magnesium, copper, and zinc. Additional reading J.S. Mertle and G.L. Monsen, Photomechanics and Printing (1957), a good basic text, emphasizing equipment and techniques of use, now obsolete in many technical details; W.J. Smith, E.L. Turner, and C.D. Hallam, Photo-engraving in Relief, 3rd ed. (1951), basic information, should be supplemented with current texts; R.W.G. Hunt, Reproduction of Colour, 2nd ed. (1967), an authoritative presentation of fundamentals of colour printing and platemaking; F.G. Wallis and R.V. Cannon, Letterpress Platemaking (1969), a general work on the platemaking process, recommended as both comprehensive and current; H.M. Cartwright, Ilford Graphic Arts Manual, vol. 1, Photo-engraving (1962), an excellent basic text, combining historical data and practical information, with bibliographies for further study, and Photogravure, 2nd ed. (1939), a good presentation of principles and basic technology; H.M. Cartwright and R. MacKay, Rotogravure (1956), on the technology of gravure; H. Denison, A Treatise on Photogravure (1894), a classic volume dealing with this process.