Halftone screens are obtainable in various "gratings" or rul ings, ranging from 4o to 400 lines to the inch, though for com mercial use they rarely exceed r so lines. The different rulings are used to suit the grade of work to be reproduced and the class of paper on which the illustration is to be printed. When printing on cheap, coarse paper, such as "news," a 6o-line screen is usually employed.
Plates made with fine screens require printing on a highly fin ished or coated paper. For ordinary "black and white" commer cial work, screens from 1 20 to 15o are used, and for fine colour reproductions, screens from 15o to zoo.
The screen is secured in a holder and placed in the camera im mediately in front of the photographic plate. The distance—an important factor—is governed by the particular ruling of the screen, the particular subject and the effect desired. When the negative is developed, the tones in the original are represented by a number of dots varying in area according to the tones of the original but of more or less uniform opacity.
The next operation is to make a print from the negative on to a metal plate, usually copper. The solution for coating plates is composed of water, fishglue and ammonium bichromate. The copper plate is polished to remove grease and foreign matter. The sensitive solution is poured on the plate which is then placed in a whirler to distribute the coating evenly.
After printing, the exposed halftone image is developed in wa ter, after which the plate is sometimes placed in a bath of aniline dye, rendering it more visible. The margins and back of the plate are painted with asphalt or shellac to protect them from the acid.
The fishglue or enamel image, after being burned in, forms the acid resist. The plate is then placed in an etching bath of iron perchloride.
The plate is etched until the fine dots in the higher lights are as small as it is possible to etch them without undercutting the "dots." The "re-etcher" or "finisher" then continues a series of fine etching by stopping out with an acid-resisting varnish those parts which have been rough-etched, and applying a series of local etchings for the purpose of recording the highest lights.
A proof is then pulled from the plate, compared with the origi nal copy, and still further necessary alterations made. When
approved, the plate is "finished" and mounted on wood or metal ready for the printer.
When a vignetted plate is required, the design and close sur rounds are protected with varnish and the extreme edges are treated with acid so that the "dots" on the latter are reduced to the smallest point; gradually results are obtained until the dots merge into the white paper without showing a hard edge.
When pencil drawings are to be reproduced in halftone, it is usual to make a "high light" negative in which the high lights of the drawing and the paper on which it is made do not show a "screen." One of several methods may be employed: (a) direct, when an over-exposed negative is made and certain stops used during the exposure, which tends to make the "dots" in the high lights overlap and become solid, the other tones being brought back to their correct relation by manipulative reduction of dot size; (b) the Bassani Apparatus, which, attached to the camera, causes a slight circular movement of the halftone screen suffi cient to veil the highlight dots and makes them disappear in the subsequent intensification of the negative; (c) a patented diaphragm of the Waterhouse or Groesbeck type, used at one portion of the exposure to transpose the posi tions of the highlight dots and the spaces between them thus veil ing the dots which disappear when intensified; and (d) the indirect method, entailing the use of three photographic plates : first, a continuous tone negative is made of the original, which can be retouched; from this a positive is made through a screen; and from the positive a negative is obtained with the "dots" eliminated from the highest lights, which include the paper surface on which the drawing has been made.
Combination line and halftone plates are frequently made for commercial purposes, either by making two plates, and mounting finished line and halftone plates together; or by making two negatives, one in line and the other in halftone, stripping the films and combining them in their respective positions on one piece of glass which then forms the negative for printing on metal. In this way the combined design is etched on one plate and in one operation.