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.
The earliest engraved printing units were wood engravings, in which the nonimage areas of an illustration were removed by carving them from the surface of a flat wood block. The oldest known illustration printed from a wooden block was a Buddhist scroll discovered in 1866, in Korea. While the dating of the print is not exact, it is believed to have been prepared about 750 ce. The Chinese Diamond Sutra, dated 868, incorporates a woodcut title page and text that includes numerous woodcut images.
From these 8th- and 9th-century dates, it is clear that the use of woodcuts (images cut into a surface parallel to the wood grain) and wood-block engravings (images incised into the end grain of an assembled block) antedates the invention of movable type. The earliest extant example of a European print from a wood engraving to which a reliable date may be attributed is a print titled “St. Christopher,” dated 1423, discovered in the library of the Carthusian monastery in Buxheim, Germany. Another authenticated example of 15th-century wood-block printing is the “Apocalypse of St. John,” printed in 1450, after a 14th-century manuscript.
Plates engraved in wood continued to find use in printing application through the late-medieval and early-modern periods. Plates made of copper, pewter, and other metals were also produced, by a process in which an image in wax or bitumen was drawn on, or transferred to, the surface of the plate and nonimage areas removed by action of appropriate acids.
Preparation of intaglio printing plates by coating a metal plate with an etchant-resistant substance (ground) such as wax, bitumen, or shellac, scratching through this substance (ground) to expose the plate surface, then etching in acids is also a late-medieval European development. This process, however, developed as a medium of artistic expression, rather than a technique for the mass production of printed images.
The first experimental application of light-sensitive materials to the production of printing surfaces was made by Joseph Nicéphore Niepce, of France, an early researcher in lithography who began his experiments in about 1813. He is credited with having produced the first permanent photograph. In 1826 Niepce coated a pewter or copper plate with a photosensitive asphaltum and exposed the surface to bright sunlight through an etching of a portrait, which served as a positive image. Sunlight passing through the background of the etching hardened the asphaltum, while the protected areas, under the inked portion of the etching, were developed in oil of lavender and white petroleum to create an image in exposed metal. This image was then etched into the plate, and from the intaglio image, prints were made on a copperplate press.
Though this basic discovery was of historical importance, it did not bring about the immediate use of photoengraved images for printing, and many other attempts to produce engravings by exploitation of the photosensitivity of various natural compounds were made by experimenters in Europe and the United States. The origin of the modern photoengraving process rests, however, on the report (1839) by a Scottish scientist and inventor, Mungo Ponton, of the light-sensitive properties of certain chromium compounds. But Ponton, who demonstrated the chemical change that occurs when glue containing a compound of chromium is acted upon by light, was not concerned with preparation of printing plates, and it remained for William Henry Fox Talbot, an English pioneer in photography, to propose the use of chromium-treated colloids such as albumin as an etchant-resistant for preparation of intaglio printing surfaces.
Early 19th-century work on production of chemically etched letterpress printing plates antedated, in many instances, the invention of photography. A researcher in Paris developed a process for the preparation of engravings on zinc. His work involved transfer of an image to the zinc plate by mechanical means, using ink or wax, and the removal of the nonprinting areas in a series of etching operations, each of which involved applying a coating of ink to the sidewalls of the etched lines by means of resilient rollers. The ink served to protect the lines of the engraving from the action of the etching acid, so that the printing area was not reduced.
The introduction in 1851 of a so-called wet-collodion process for photography provided a means for producing a photographic negative as the basic element in the preparation of engravings. In this process, a glass plate is coated with an alcohol–ether solution of collodion (cellulose nitrate) containing potassium iodide. While still wet, the plate is immersed in a silver nitrate solution, producing light-sensitive silver iodide in the collodion layer. Without drying the film of collodion, the plate is placed in the camera and exposed, followed by development in ferrous sulfate solution and chemical “intensification” to produce an image of greater opacity. The image consists of deposits of metallic silver and other heavy metals imbedded in the collodion layer.
This photographic process also provided a method of stripping the photographic image from the glass plate, permitting assembly of a number of images for plate making, and also making possible the geometric reversal of the image needed in letterpress plate making to produce a right-reading print on paper. The wet-collodion process was used extensively in engraving until the 1930s, when it was gradually replaced by commercially coated stripping films.
Since the letterpress printing process provides a uniform coating of ink on all printing elements, no provision can be made for reproducing tones intermediate between black and white by varying the thickness of the ink film laid down by the press. The production of shades of gray was then the role of the halftone process, in which the image is broken up into dots, and variations of gray tones are obtained by varying the size of the dots, thus controlling the amount of ink laid down in a given area.
The feasibility of the method was demonstrated in about 1850, when a halftone image was produced by photography through a screen of loosely woven fabric. The screen was placed some distance forward of the plane of the receiving photographic surface (film or plate) and had the effect of breaking the gray tones of the subject into dots of varying sizes, through a combination of geometric and diffraction effects involving the spacing of screen from the image surface, the size of the openings in the screen, distance from lens to image plane, and the size of the aperture in the lens. It was obvious that a screen designed for this use could consist of a pattern on a glass or other firm, transparent surface.
A French patent of 1857 described a screen with parallel lines scratched in a single direction in an opaque background. As early as 1869 an image with a crossline halftone was produced in the Canadian Illustrated News. Later, in 1882, a crossline halftone was produced using a single-direction screen, by making half the exposure with the screen in one position and half with the screen rotated a quarter turn. Two brothers, Max and Louis Levy, of Philadelphia, in 1890 produced the first commercial halftone screens. The Levy brothers coated selected plates of high-quality optical glass with a lacquer, in which parallel lines were cut. The ruled lines were then etched with hydrofluoric acid and filled with an opaque material. Two such plates were cemented face to face with the lines at 90°, the edges sealed, and the assembly bound in a metal frame.
There has been no significant change in the methods of making halftone screens since those developed by the Levy brothers. Other screen patterns, including triangular dot patterns and the grained (mezzograph) screen, have been proposed, but none has produced consistently satisfactory results. For special effects, screens having straight or wavy line patterns and screens that produce a pattern of circles, concentric about a point that is chosen to be the focal point of the readers’ interest, are in use. These screens are generally produced photographically from hand- or machine-drawn patterns and are used in the form of contact screens.
Halftone screens may be obtained with line frequencies of 50 to 400 lines per inch (one inch equals 25.4 millimetres). The coarser screens are used for reproductions printed on coarse papers, the fine screens for higher quality reproductions on highly finished and coated papers. Screens in the 50–85-line frequency range are used primarily in newspaper illustration, while 100-, 110-, and 120-line halftones are suitable for highly polished papers and for some magazines, where single-colour and some multicolor work is involved. The 120-, 133-, and 150-lines-per-inch screens are generally used for colour illustrations in magazines and books printed on coated papers, when picture detail is important. Screens of 175 and more lines per inch are seldom used in letterpress printing, since the inks tend to fill the screens, causing difficulties in the press run. Such screens, however, do have some use in printing by offset lithography. In general, where paper quality permits, the finer screens are used when reproduction of fine detail is important. But since the letterpress process requires that the diameter of the finest highlight dot should not be less than 0.0015–0.002 inch, the use of very fine screens will lead to loss of image contrast, since some 3 to 5 percent of the picture area, in highlights, will be ink covered.
An interesting development in glass screens was the “Altone Gradar Screen,” manufactured in Germany. These are glass screens, ruled and etched in the usual manner, but with the rulings of the two glass elements filled with a transparent magenta lacquer of two different optical densities. When the screens are assembled, lines in one direction exhibit a density different from that of lines in the perpendicular direction, and the intersections have a density equal to the sum of the densities of the two lacquers. The effect is to provide elongated halftone dots, with improved tonal reproduction in intermediate gray tones on coarse paper such as newspaper stock.
Perhaps the most significant recent advance in the halftone process has been the use of contact screens—films bearing a gray or magenta-dyed image of the light-distribution pattern behind a conventional halftone screen. The screen is placed in contact with the surface of a high-contrast film, in the image plane. The image, as recorded on the film surface, has the characteristic of a halftone exposed through a glass screen, with significant improvements in rendition of detail of the subject. Though first proposed in 1855 and developed by a number of later investigators, this technique was not fully exploited commercially until the 1940s. The contact screen eliminates certain diffraction effects inherent in glass screens, frees the operator from some of the lens-diaphragm restrictions imposed by the glass screen, and eliminates the necessity that lens opening, bellows extension, screen distance from the focal plane, and screen ruling all be in a particular relationship.
Contact screens are made with a silver (gray) screen pattern image and with a magenta dye image. The dyed screen gives some additional control over halftone negative quality through use of colour filters on the camera.
An entirely mechanical procedure for production of a halftone image on a metal printing plate is the benday process (1879), named after its inventor, Benjamin Day, a New York newspaper engraver. This process utilizes a series of celluloid screens bearing raised images of dot and line patterns. The screen surface is covered with a waxy ink and the ink transferred, by pressure and rolling, to prepared portions of a metal plate. By selecting different screen patterns for transfer to different parts of the image, a mechanically produced halftone image is rendered. The ink image is reinforced with powdered resins and the plate etched. This process has been supplanted by completely photomechanical techniques.
Such techniques as dropping out the highlights from the halftone negative (i.e., eliminating the dots in these areas) in order to achieve increased contrast in illustrations were studied and introduced by several individuals. Such a method was patented in 1893, and in 1925 a camera attachment was introduced, making it possible to impart a slight motion to the image on the film and thus reduce exposure to the point at which small highlight halftone dots were not printed or developed.
The most successful of the highlighting methods were those employing fluorescence phenomena, in which an object produces visible light when exposed to ultraviolet radiation. In 1938, for example, the fluorographic process, in which fluorescing materials were incorporated in the artist’s pigments, was patented. Similar pigments, designed for colour correction in watercolour illustrations, were patented in 1935 and 1938. Another process introduced shortly thereafter utilized a fluorescing paperboard. All of these processes were based on the same procedure: making an exposure under normal lighting for overall reproduction and then making an additional correcting exposure under ultraviolet. The fluorescence produced by the ultraviolet illumination provided additional exposure in the affected areas that gave the necessary correction for highlighting or colour correction, by eliminating the screen pattern from “white” areas, in the case of monochrome, or reducing printing dot sizes, in critical areas of colour work.
The discovery of the halftone screen was primarily responsible for the development and growth of photoengraving; further growth was related to other developments in the printing and allied industries. The introduction in 1935 of the first practical colour film for amateur and professional use probably did more to accelerate printing developments than any single invention. By making bulky studio-type colour cameras obsolete and permitting the use of readily portable camera equipment for the production of colour images, on-the-spot colour photography became possible, greatly increasing the use of coloured illustrations.
At approximately the same time, the commercial production of coated paper and heat-drying printing inks for letterpress printing began. Many colour developments for films, printing processes, and materials followed.
Early methods of etching zinc and copper, methods that have persisted in some areas to the present day, were tedious and inexact and could be learned only through trial-and-error training. The principal difficulty stemmed from the fact that the chemical removal of metal from nonimage areas proceeds in all directions. Thus, etching of the plate surface proceeds not only in the desired direction, to achieve the depth required for satisfactory printing, but also sideways, causing reduction in width of lines and dots of the printing image and also undercutting halftone dots—producing a below-surface dimension smaller than the printing surface. The mechanical weakening of the dot may lead to its collapse under printing pressure.
Some success in overcoming this problem has been achieved by depositing an etchant-resistant material about the sidewalls of etched lines and dots, thus preventing lateral etching. The method of rolling a waxy ink onto sidewalls of lines and dots, called gillotage, has found wide use among European engravers. The “powdering” process, most widely used in the United States, involves brushing a resinous powder (dragons’ blood) against the sides of partially etched lines and dots and fusing, with heat, to provide an etchant-resistant coating. Several repetitions of the operation—etching, application of the protective material, and etching again—are needed before sufficient depth is attained. Results of this process are dependent upon the skill of the operator and on such ambient conditions as temperature and relative humidity, since these affect the performance of the powder. A major step toward solving the problem—in fact, the most important development in the field of etching since photoengraving was invented—came with introduction of a process of etching a magnesium plate without the use of powder. Experimenters found that by adding an oily material and a surfactant (wetting agent) to the nitric acid bath and controlling the conditions under which the plate was etched, they could produce characters in relief with adequate etching depth and virtually no printing-area loss during the etching. Later adapted to the etching of zinc, the process was quickly adopted by engravers in all parts of the world.
With this major hurdle in the etching of zinc and magnesium overcome, attention turned to copper, and in 1954 it was found that a powderless etching process for copper resulted from the addition of an organic compound (thiourea) to the iron chloride etching bath. Further refinements in the process and the introduction of new compounds to add to the etching bath followed.
While these developments in chemical etching were taking place, other experiments were being conducted to assess the feasibility of replacing traditional methods with the techniques of electronics, optics, and mechanics. The first successful result of these efforts was a device, introduced in 1947, that optically scanned a picture and simultaneously reproduced it as a relief printing plate on a plastic sheet. This device found wide application, particularly in newspaper plants, where the slowness of photoengraving procedures was particularly objectionable. Within a short time, machines were developed that were capable of making etched plates in metals.
Meanwhile, investigators in the United States discovered about 1950 that some methacrylate compounds could be quickly polymerized (converted to products of high molecular weight and low solubility) by exposure to light. Nylon was also found to be photosensitive, and by 1958 both materials were being offered for use in printing plates. Another plate-making system, reportedly based on light-sensitive polyurethane resins, was introduced in 1968.
Paralleling the development of the electromechanical engraving machine, experimenters in the United States and Europe independently devised a number of electromechanical devices that automatically produce, from a colour-transparency image, corrected film negatives from which the four printing plates used in full-colour reproduction can be prepared.
In one of these, a photographic transparency, wrapped around a glass cylinder, is scanned by a narrow beam of light. After passing through the transparency, the light continues through a colour splitter, and the blue, green, and red components are directed onto the sensitive surfaces of photocells. The electronic signals thus generated are modified and amplified in a computer that functions as an electronic analogue of photographic colour-separation processes. The computer activates a series of lamps, which expose the colour-corrected images onto photographic films mounted on another drum, attached to the same shaft as the transparency holder. This development was based on initial experimentation in commercial laboratories in the late 1930s and early 1940s. Other units, based on similar principles but differing in some details of structure and operating procedures, have been manufactured in the United States, West Germany, and Great Britain.
In terms of cost, engraving methods range in ascending order as follows: line engravings; halftone engravings; combination line-and-halftone engravings; single-colour, two-colour, and duotone engravings; and process colourplates. Each of the types may be produced in any of the customary metals or plastics. Process colourplates are usually made of copper in the United States and United Kingdom and of zinc elsewhere.
The essential operations for the production of all types of photoengravings are similar. They include photography, photomechanical operations, etching, finishing, routing, blocking, and proofing.
The engravers’ camera, called a process camera, is a rigidly built machine designed to allow precise positioning of the lens and copyboard so as to provide control over the enlargement or reduction in size of the copy. It has a colour-corrected lens designed to give the sharpest possible overall image when focussed on a plane surface, without the distortions common (though usually unnoticed) in the average portrait or amateur camera lens. Process cameras are designated as gallery or darkroom types. The gallery camera is freestanding and may be installed in any convenient location, but film must be removed in a light-tight cassette and processed in a separate darkroom. The darkroom camera is installed with its film holder as an integral part of the darkroom wall, giving easy access to the darkroom facilities.
The material to be reproduced, called copy, is mounted on a board or glass-covered copyholder, carried on the bed of the camera. Illumination for exposure is provided by arc lamps or high-intensity gas-discharge lamps. The most common camera lamp systems in late years have involved pulsed xenon lamps, in which a high-voltage alternating current, passing through a glass tube containing the rare gas xenon, causes the emission of a light rich in the ultraviolet wavelengths.
Virtually all photographic work is done on film coated with high-contrast emulsions especially developed for graphic arts work. The introduction of dimensionally stable film bases has nearly eliminated the use of glass plates. Film emulsions used for halftones yield the extremely high contrast needed for halftone or line reproduction. Stripping film, a laminated film with a soft adhesive layer between the base and the emulsion layer, is widely used to permit images to be removed from the base and properly oriented on the glass or film flat through which the metal plate will be exposed.
In the early days of photoengraving, with wet-plate images on a glass support, it was impossible to process photographic images by any means other than immersion in solutions contained in a shallow pan or tray or by dipping into a tank of solution. Such tank and tray processing remains important but is now being supplanted by the use of automatic film-processing machines. Derived from equipment originally designed for processing of motion-picture film or photostat prints, these consist of belt- or roller-driven apparatus that carries the film through developer, fixing, and washing solutions, and, in most cases, through a drier, permitting delivery of a processed, dried film within three to five minutes after insertion into the machine. Such machines, with different processing solutions, may be used for continuous-tone or lith-type films.
Photomechanical operations include cleaning the metal plate surfaces, coating with a light-sensitive solution, drying the coating (known as the top or enamel), and making the exposure on this coating through the negative prepared in the photographic step. Throughout these operations care is required to prevent imperfections such as bubbles, dirt, or scratches in the light-sensitive coating. The zinc, magnesium, or copper is prepared by careful cleaning with pumice and water. The light-sensitive coatings are usually poured over the surface, and the plate, held flat, is whirled to ensure uniform coverage by the solution.
Light-sensitive coatings are usually a dichromated colloid material, but light-sensitive resins are also used. “Cold top” enamels are used on zinc and magnesium, which cannot be heated; these are usually slightly alkaline solutions of shellac or polyvinyl alcohol to which a dichromate is added. “Hot top” enamels nearly always contain fish glue as well as some egg albumin, to which is added a dichromate sensitizer. Mixtures of glue and albumin are used when it is necessary to control the etch resistance and the ease with which the edges of the enamel break away during the etching process. Hot top enamels must be set at temperatures of 550–650 °F (285–345 °C) and are used mainly on copper, the crystal structure of which is not altered at these temperatures. Polyvinyl alcohol and shellac resistants are set at temperatures of 350 and 220 °F (175 and 105 °C) respectively; therefore they are used on zinc and magnesium.
The tops are high-contrast materials that, when exposed to strong ultraviolet light, harden where the light has struck them and lose their solubility in water. Development in water then removes the coating from the unwanted areas of metal, exposing the metal for the etching process. Photosensitive resinous materials find wide application in electronic circuit printing, an operation analogous to photoengraving. They have more limited applications in the making of photoengraved letterpress plates, where they are used especially on zinc and magnesium and where their excellent storage properties permit their application in the metal-finishing plant, obviating the necessity for coating of the resist onto the metal in the photoengraving shop. These resinous materials are developed in organic solvents.
Nitric acid is commonly used in etching zinc and magnesium, the strength varying from 6 to 15 percent, depending on the metal. Copper is more readily attacked by ferric chloride (iron chloride), which is commonly used in concentrations of 28–45 percent. The etching may be done in an open tub or tray, though this method does not give the control needed for economical operation and is employed only where control is not critical. Most quality work is carried out in etching machines provided with impellers that break up the etchant into a spray and force it against the plate.
In the conventional etching processes, the acid or iron chloride is used without modification, although great care is needed to prevent overetching. In many cases, especially when making line plates, etchers powder to protect the upper printing areas from attack while continuing to etch in depth. The powderless etching processes, described earlier, have made the powdering technique obsolete and are now almost universally in use. Line plates are usually etched to depths of 0.010 to 0.045 inch. Halftones may be etched to depths of 0.0023 to 0.009 inch, depending on the fineness of the screen. Coarser screens are etched deeper.
Photosensitive plastic plates are not etched in the ordinary sense. Unexposed resins, from nonprinting areas, are washed out with either dilute alkali or alcohol. Overetching is not a problem with this type of plate.
Finishing includes hand operations with engravers’ tools, to remove imperfections in the image area of the plate and to improve its appearance. In colourplates, finishing also includes colour correction, a process of further etching or burnishing selected areas to improve the fidelity of reproduction. Finally, unwanted metal in the nonprinting areas of the plate is removed by a mechanical routing machine.
Blocking consists of attaching the plates to cherry wood, plywood, or metal blocks to bring the printing surface to type height, which is 0.918 inch. Until the development of thermoplastic adhesives in the 1940s, blocking was always done by nailing the plates to wooden blocks. This tedious and costly operation has been largely replaced by hot mounting, in which process the plate is placed on a block of wood precoated with adhesive and this sandwich is subjected to heat and pressure. Upon cooling, the plate adheres firmly to the block.
Proofing consists in placing the plates on a precision press and taking sample impressions, or proofs, that show how the plates will print during a regular press run.
The first printed colour work was produced manually; artists painted in the necessary colours on black-and-white printed sheets. Later, stencils were used to speed this work, and in a further development, colours were printed, either as solids or tints, from hand-engraved plates. All of the work was crude by modern standards, however, and nothing approaching four-colour process printing was possible.
Modern colour printing, done with either three or four plates, each using a different colour of ink and overprinting the others, is based on a subtractive system of colours in which intermediate hues are obtained by some combination of two or more of the subtractive, or secondary, colours. The best colour printing is usually done with four process colours: yellow, magenta (blue-red), cyan (blue-green), and black.
The black plate is used to provide added uniformity of colour reproduction, since it will overcome changes in hue of critical neutral tones that could occur with random or cyclic variations in the amount of ink being transferred to the plate from the press inking system. Further, the use of a black plate aids in maintaining sharpness of picture detail.
In theory, black should result whenever the three subtractive colours are superimposed. Thus, it should be possible to produce black wherever all three of the secondary colours are present without affecting reproduction. Further, any colour that is within the range of colours reproducible with inks on paper can theoretically be obtained by using only black plus the proper pair of the secondary colours. But this has not been found practical because of the nature of printing ink pigments and the lack of total precision in the printing operation. Consequently, it is common practice to use the black plate to supplement the colourplates, portions of which are allowed to print in all except pure white areas of an illustration. The colourplates and the black plate must all be printed in register; i.e., they must be superimposed so that identical portions of the image in each plate colour overprint each other.
In manufacture, the production of an individual colourplate involves the same steps used in producing an ordinary black-and-white engraving, once the etchant-resisting image has been printed on the metal. Prior to this, the only differences lie in the use of colour filters on the engraver’s camera and in steps to reduce the range of colour contrast of the copy. Negatives representing the images to be printed with each of the coloured inks are obtained by photographing the colour copy through colour filters. These filters, usually used in the form of thin sheets of dyed gelatin inserted into the lens, are complementary in colour to the coloured printing inks used.
Masking is the use of positive or negative images, taken from one or more of the set of colour-separation negatives and used in register with a given negative, to correct for the deficiencies in printing inks and colour of the copy. Common colour errors corrected by masking include the removal of excessive yellow values and magenta values from the blue (yellow printer) and green (magenta printer) negatives.
Colourplates may be made by the use of two general photographic methods—one indirect and one direct. The indirect method produces either continuous-tone negative images, from which halftone negatives are made, or continuous-tone negatives, from which continuous-tone positives are prepared. In the direct method, screen negatives are prepared directly from the copy through the colour-separation filters and a halftone screen onto a high-contrast panchromatic film or plate to produce a negative ready for transfer to the metal plate.
The proofing of halftone colourplates for wet printing on high-speed presses (when one colour does not have time to dry before the next is laid down) is a critical operation, for the proofing must be carried out under conditions simulating as closely as possible those that will be encountered on the production press. Specially built proof presses make this possible. In appearance they resemble four conventional press units placed end-to-end, and the sheet of paper is passed in turn over the four plates. However, because the production press employs not the original flat plates but curved duplicates made from them, and because ink and paper specifications are highly variable, exact duplication of production results in a proofing operation is difficult.
A serious problem in colour reproduction is the occurrence of an interference pattern, or moiré, caused by the overprinting of the screens in the colourplates (a similar effect can be obtained by superimposing two pieces of window screening or fine net cloth). Because it is impossible to maintain printing register within the degree necessary to avoid such an effect, it is common practice to rotate the halftone screen when making the negatives so that each of the four plates has its screen pattern in a different position.
Reference has been made to devices for the electromechanical production of relief printing plates. The first of these utilized a heated pyramidal stylus, the motion of which was controlled by an electrical signal from a scanning photocell, to penetrate a plastic plate to a distance inversely proportional to the optical density of copy, thus burning out varying areas from the plate surface. In another machine of the same general type, an oscillating gouge cuts a halftone pattern in a flat plastic or metal plate, under control of a signal from a scanning photocell; in yet another a spiral groove, of varying width, is cut into the surface of a plastic plate wrapped on a rotating cylinder.
The colour scanner has been described elsewhere in this article. The first such devices were capable only of producing colour-separation negatives of the same size as the copy that was scanned. In later developments, circuits were provided to produce positive images, and mechanical or electronic devices were developed to allow enlargement or reduction of the size of the final image as compared with size of the original copy. When scanners were first made available, it was believed that their cost would limit their use to a few large plate-making establishments, but their acceptance exceeded expectations.
These include specifications for line plates, halftone specifications, and combination plates.
In line illustrations all of the image areas are either black or white, and hence no halftone screen is required to copy them for use in making a printing plate. Suitable copy consists of line drawings, etchings, etc. The negative as it comes from the process camera is suitable to transfer the line image onto the metal.
Plate preparation, coating, burning in, etching, and finishing are essentially the same as for halftone plates. Certain specifications must be met, however. The nonprinting areas must be etched sufficiently deep to prevent the ink rollers from touching them on the press, and to prevent them from rubbing on the surface of the paper during wet colour printing. For presses with accurately adjustable ink rollers, the etch depth may be as little as 0.01 inch. The same depth is permissible in thin, wraparound press plates. For conventional printing presses, the minimum etch depth is about twice this. Plates that are to be duplicated by electrotype or stereotype processes may require slightly greater depths, although normal etching ordinarily is sufficient to produce good duplicates. Plates from which rubber duplicates are to be made will require etch depths as great as 0.045 inch.
Etch depths in halftone plates need not be as great as those in line plates, but the contour of the halftone dot and the depth of the etched areas are very important. Etch depth in highlight areas, the most critical portions of halftones, varies from 0.006 inch in a 65-line halftone to 0.002 inch in a 133-line.
These plates must be prepared by assembling, in the negative form, the halftone and the line portions of the illustration and then, after transferring them onto the metal, etching them in two operations, so as to attain the best results for both portions. The powderless etching processes, however, permit easier etching of coarse-screen combination plates for use in newspapers. Combination line-and-halftone plates may also be produced by making two plates in separate operations and mounting them on a single block in proper position with respect to each other.
Procedures similar to those described for production of letterpress printing surfaces are applicable to the production of intaglio printing surfaces. In intaglio, or gravure, printing, the image to be transferred to paper is etched or incised into the surface of the printing plate or cylinder. The entire surface is covered with ink, and by means of a doctoring, or wiping, operation, excess ink is removed from the surface, leaving only that which is retained in the image areas. Paper is brought into contact with the surface, and, under high mechanical pressures, the ink transfers from plate to paper.
Intaglio printing surfaces are of two general types: line intaglio (sometimes referred to as copperplate gravure), in which the ink-retaining image consists of discrete lines that may vary in width and depth; and gravure (also known as rotogravure), in which both continuous-tone and line copy are reproduced as a series of tiny cells etched into the printing surface. These cells commonly vary in depth, and hence in the volume of ink they will retain. Variations in density are produced on paper by the different amounts of ink that the cells transfer to paper.
In the rotogravure printing process, the walls surrounding each cell act as a support for the doctor blade that removes ink from the printing cylinder or plate surface.
This process is widely used in the production of bank notes, securities, stamps, and engraved documents. The distinctive sharpness of fine lines and readily discernible differences in ink thickness that the process produces make it a preferred technique for production of bank notes and securities. These appearance characteristics cannot be readily counterfeited by photomechanical processes.
The printing surface is created either by mechanically scratching an acid-resistant ground from the plate surface, as described above, or by use of a photographic positive of the desired line pattern to prepare a photoresist image on the metal. The image is etched into the plate, using the techniques of letterpress line etching, with maximum depth of etch usually less than 0.007 inch. Metals commonly used include steel, brass, and copper.
When a mechanical engraver is used to expose the metal for etching, a pointer or stylus is used to follow a usually enlarged pattern in a metal or plastic master stencil, causing a diamond stylus, which is in contact with the lacquer-covered plate surface, to remove the lacquer in a sharply defined pattern. The intaglio image is then prepared by etching the exposed metal with the appropriate chemicals.
In printing from intaglio forms, the plate is flooded with an ink of medium viscosity and the surface of the plate wiped clean with either a metal doctor blade or a piece of hard-surfaced paper. To minimize wear of the plate from the abrasion of the wiping mechanism, the surface is ordinarily protected by an electroplated chromium layer.
Wiped free of excess ink, the plate is brought into contact with the paper surface. A roughly outlined relief image (counter) of the printing pattern is often used to provide high local pressures, forcing the paper into the ink-filled intaglio image. As the paper is pulled from the plate, capillary-attraction and surface-tension forces act to pull the ink from the plate. After drying, the image has a distinctive appearance in which the ink has appreciable thickness, and thin lines have less thickness than wider lines.
The gravure printing process is one of the three major processes that are used for catalogs, magazines, newspaper supplements, cartons, floor and wall coverings, textiles, and plastics. The gravure printing process is done with flat plates or, more commonly, with cylindrical surfaces. A screen pattern is superimposed over all image areas; thus the edges of type or lines printed by gravure will have a rough, or sawtoothed, appearance. This does not detract from readability.
The early work as described above formed the foundation for modern gravure engraving and printing. Karl Klič (also spelled Klietsch) of Bohemia, who was instrumental in making photogravure a practical commercial process, in 1878 exposed a positive transparency over carbon tissue, a film that was made of coloured gelatin sensitized with potassium dichromate and backed by a sheet of paper. The exposed film was pressed down on a copper plate that was coated with an even layer of resin or asphalt powder. The carbon tissue was developed in water, making the gelatin swell in inverse proportion to the exposure it had received. The plate was etched with ferric chloride in successive baths of varying strengths. Penetration of the tissue by the etchant, and hence the resulting depth of etch in the metal surface, was controlled by the degree of swelling of the gelatin. Klič’s process produced sure and predictable results and became the preferred method for later workers.
In later developments, the irregular grain pattern, which is produced by use of resinous powders, was replaced by a regular overall pattern of intersecting lines, which is produced by exposing the carbon tissue to a glass screen bearing an overall pattern of clear lines, intersecting at right angles.
In rotogravure, separate negatives of type matter, other line copy, and continuous-tone copy are assembled and positioned according to a prepared layout. After the negatives are retouched, a continuous-tone positive is made and retouched to ensure desired density values.
A sheet of carbon tissue is next exposed under a gravure screen. This screen is a film or sheet of glass on which fine transparent lines, usually 150–175 per inch, cross at right angles to form opaque squares. The lines of the screen are positioned at an angle of 45° to the axis of the printing cylinder. Their purpose is to provide a support for the doctor blade, which wipes ink from nonprinting areas. The lines of the screen allow the light to penetrate to the film and harden the gelatin. The square “islands” remain soft.
Next, the continuous-tone positive is placed in contact with the carbon tissue and is exposed under an arc light. The soft squares are hardened in proportion to the amount of light that penetrates the varying grays of the positive. The carbon tissue is pressed with a rubber roller to a cylinder that has a polished electroplated copper surface. After adhering the exposed carbon tissue to the cylinder surface, it is “developed” with warm water, which has the effect of swelling and removing unhardened gelatin. The result is an image in hardened gelatin, which varies inversely in thickness according to the density of the photographic positive. The cylinder is rotated in a tray of ferric chloride, which produces an etched image in the copper surface. The squares are etched to varying depths, depending on the degree to which they were hardened. The crosslines of the screen, which were entirely light-hardened, are not etched at all. In this way, pits or wells of different depths are etched into the copper. For very long press runs, the cylinder can be strengthened by plating with nickel or chromium.
In rotogravure printing, the cylinder usually is arranged so that during its rotary movement it passes through a trough filled with a thin solution of fast-drying ink. A thin steel doctor blade moves across the cylinder with a slight oscillating action and removes the ink from the surface, but not from the wells beneath. The cylinder then comes in contact with the paper, and the paper draws the ink out of the wells in the plate. After being printed, the paper shows through thin deposits of translucent ink, thus creating pale grays; heavier ink deposits from the more deeply etched wells appear correspondingly opaque. Thus a full range of tonal values can be printed. Since the ink used in rotogravure printing is quite fluid, it penetrates through the pores of the paper surface, obliterating the screen pattern. In reproducing illustrations, gravure comes closest to simulating continuous-tone copy. In colour printing, a separate cylinder is prepared for each colour.
In the so-called Dultgen halftone intaglio process, which is widely used in colour work, two positives are made from the continuous-tone copy, one through a halftone screen or a special contact screen and the other without a screen. The carbon tissue is first exposed to the screened positive, which produces an image of dots of varying sizes, then to the continuous-tone positive, which produces differing degrees of hardening of the dot image. When etched, the dots are of differing sizes and of differing depths. This method thus uses two methods for controlling tonal values.
The Henderson process, sometimes referred to as “direct transfer,” or “inverse halftone,” gravure, has won some acceptance in the printing of packaging materials. Retouched continuous-tone positives are used in preparation of halftone negatives and, by a contact-printing operation, halftone positives. These positives show dot size variations proportional to the desired print density. The cylinder is coated with a cold-top photoresist, as in letterpress engraving. This resist is then exposed to ultraviolet light, through the positives, and the image developed. The cylinder may then be etched either in ferric chloride solution or in a powderless etching bath, similar to that used for letterpress photoengraving. Tonal variation in the resulting print on paper is caused almost entirely by variation in the area of dots.