An author should carefully examine and approve the finished drawings, which can, of course, be greatly altered, if necessary, before they are engraved; but similar corrections can not be made on proof sheets of zinc cuts, which should not be marked for alterations except by eliminating parts. Minor changes can be made in such a cut by an expert "finisher," but if the cut is small it is generally cheaper to correct the drawing and have a new cut made.
Zinc etchings cost about 10 to 25 cents a square inch, the cost being varied according to a standard scale which is based upon the ascertained cost of reproduction. The minimum charge for a single cut is $2.
COPPER ETCHING IN RELIEF.
Copper etching, which produces a line cut in relief, requires the same kind of copy that is most often marked for zinc etching and is used to obtain deeper etching and a more permanent cut. It is said to produce better printing plates than those etched on zinc and is used largely for reproducing script lettering and other fine work. As copper plates will hold up longer in printing than zinc, a cut etched on copper may not need to be electrotyped.
The chemical part of the process is practically the same as that employed for etching half-tone plates, described under the next heading.
The cost of etching on copper is considerably greater than the cost of etching on zinc. This process is not often used in reproducing illustrations for publications of the Geological Survey.
HALF-TONE ENGRAVING.
The half-tone process is, in name at least, familiar to almost everyone who has had any connection with the making of books, whether as author, editor, illustrator, or printer. The invention of a photomechanical process of reproducing a line drawing to make a metal plate that could be printed along with type on an ordinary printing press naturally led to attempts to reproduce similarly a photograph. It was known that the intermediate shades between white and black in a photograph—the half tones—can be reproduced on an ordinary printing press only by breaking them up into dots or lines that will form a good printing surface and that by their variation in size or density will give for each shade the effect of a uniform tone. In the half-tone process this effect is produced by photographing the picture or object through a screen.
The half-tone screen consists of two plates of glass, on each of which lines running generally at an angle of 45° to the sides of the plate have been engraved, cemented together so that the lines cross at right angles. The lines, which are minute grooves filled with an opaque black pigment, thus appear as a series of black crossed lines on a white ground. The screen is placed in the camera in front of the negative. Screens are made that show from 60 lines to an inch for the coarser newspaper illustrations to 250 lines or more to the inch for fine book work. The screens used for magazine illustrations generally show 120 to 150 lines. Those used for Survey publications show 150 to 175 lines, and for reproducing delicate drawings and photographs of fossils screens bearing 200 lines to the inch are sometimes specified; but these finer screens require the use of highly super-coated papers, some of them made of cheap fiber and not known to be permanent. For a half tone that is to be printed in the text a 100-line or a 120-line screen is specified. (See [Pl. VI, p. 56.])
The method of etching a half-tone plate does not differ greatly from that used in zinc etching, and there are several kinds of half-tone plates, though most of them are etched on copper, not on zinc, those etched on zinc being used principally for newspaper illustrations. The half-tone screen is used also in other processes to obtain a negative.