- Basic production processes
- Colourplate production
Modern photoengraving techniques
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.
Basic production processes
The essential operations for the production of all types of photoengravings are similar. They include photography, photomechanical operations, etching, finishing, routing, blocking, and proofing.
Camera and darkroom equipment
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.
Plate coating and printing
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.