Wash drawings, photographs or copy containing color tints cannot usually be reproduced by the zinc etching process. Pen drawings in black India ink on white paper furnish the best copy. Comparatively cheap stock may be used in printing zinc etchings, particularly if the artist has been instructed to make the drawing "open," so that the cut will not blur on long runs on cheap paper. The cost is lower' than for half tones.
14. Half-tones.—Half-tones stand at the head of engravings for most purposes—for catalogs, booklets, circulars and advertisements in mediums of the better class. The half-tone can be made to print on any stock which has a fairly smooth surface and can be used for color-work. The cost is reasonable. A re lief-plate, photographically made on metal, in which the printing surface is made up of a regular series of small dots, or a grating of fine lines in white, is called a half-tone.
Half-tone plates are produced as follows: The copy is photographed thru a screen or glass, marked by cross-lines meeting at right angles. The lines are opaque; the squares transparent. The photograph, therefore, is taken thru the transparent squares. An enameled copper plate is printed as in the zinc etching process. The plate is placed in a bath of perchloride of iron which eats away the coating of the plate that is unaffected by the lines and dots produced by the screen. A proof is then taken, and further etching may be done on parts of the plate by using the per chloride again. Hand-work, somewhat after the manner of wood engraving, may be done if a par ticularly fine plate is desired. Mounting, commonly on a wood base, completes the process.
Most half-tones are made from retouched photo graphs. Wash drawings, pen, pencil, crayon or char coal drawings as well as paintings in color are repro duced accurately by this method as has been done in a number of instances for this volume.
15. Importance of the - screen is a clear plate of glass ruled accurately in two directions with lines at right angles. The light cannot pass thru the screen at the points where the lines cross, but passes freely between the lines and registers heavily on the plate. The result is a series of light and heavy dots, bringing out the shading in the photograph. Screens are named according to their number of lines per inch. The coarser the screen the lower the grade of stock that may be used in printing. If a half-tone is to be printed in newspapers or on a similar grade of stock, 65-line screen may be used. If the paper is of a better grade, screen; for the ordinary fic tion magazine a 120-line screen; for booklet work on fairly good, coated paper, 133-line screen; while for the best results on very fine grades of paper, 150 or even 175-line screen is employed.
The following plates show different screens from 65 to It should be noted that the finer the screen the greater the detail which may be se cured.
16. Lithography.—Lithography, printing from specially prepared stones, is useful to the advertiser chiefly in the production of letterheads, catalog and booklet covers, and in reproductions of pictures and designs in color. The unit cost is rather high on short runs, but low on long ones. The range of use fulness is narrow, but the process is capable of fine results. It comes into competition, in single color work, with engraving and embossing, and in mul ticolor work with the other and later color pro cesses.
Lithographic stone of the best quality is procured from Bavaria, tho the United States furnishes a large part of the supply. A grained stone is used to give a stipple effect; the ordinary sharp line of lithography requires a perfectly smooth surface. The printing surface is washed with a solution of nitric acid and water, which roughens the stone where there is no design.
17. Hand-made engravings.—To the experimental work of Finiguerra, an Italian goldsmith, the world owes the art of engraving on copper. The process was invented about 1460, and copper was used until the beginning of the Nineteenth century when steel was generally substituted.
The printing is made from sunken characters, cut into the copper or steel plate, usually by hand. In some cases, machines are used for cutting or tracing the characters; in others, chemical processes are used. In printing, ink is applied to the face of the plate.
The sunken parts become filled with ink, and retain enough to bring the characters level with the surface of the plate. Any ink adhering to the smooth surface of the plate is carefully removed. The plate and the stock on which it is to print are forced between two rollers, one solid, the other covered with woolen cloth or a rubber blanket. The stock-paper or card board is forced into the depressions of the plate under the pressure of the meeting rollers and comes from the press printed and embossed. In the best work the embossing is plainly marked, owing to the depth of the incisions and to the heavy pressure applied. In cheaper grades of work, the embossing is compara tively slight, but the ink has a peculiar raised effect which distinguishes it as "engraved." Steer and cop per plate engravings are of limited utility. The lat ter are employed chiefly for conventional announce ments and cards. Copper plate is frequently used to announce an offering of millinery, furs, jewelry or other high-grade goods. Stationery of high grade is made from steel engravings.