HALF-TONE, LINE, AND COLOR PLATES By Emlyn M. Gill

Practically all book illustrations, as well as those in catalogues and periodicals of all kinds, are made by some method of photo-engraving. Wood engraving is almost a thing of the past, and many who are in a position to know predict that after the present generation of wood engravers has passed out of existence, artistic wood engraving will be a lost art. It is certain that there is now no younger school of wood engravers growing up to take the place of the engravers whose work in the leading magazines, up to a few years ago, made them famous.

The quickly made and comparatively inexpensive process plates have not only taken the place of wood engraving, but have increased the field of illustration to a very large extent. They have made possible hundreds and even thousands of publications which could not have existed in the old days of expensive wood engraving. The use of photo-engraved plates has increased enormously each year during the past twenty years, and with this increased use has come the inevitable decrease in cost, so that illustrations are no longer much of a luxury to the publisher.

Photography is the basis of all the mechanical processes that come under the general head of photo-engraving. These processes are generally called mechanical, yet, as in photography, great skill is required to produce the best results. The higher grades of half-tone work require much careful finishing, which is all done by hand, and which, moreover, must be done by a skilful, intelligent, and artistic engraver. Practically all things may be reproduced successfully by photo-engraving, but the vast majority of subjects that go to the photo-engraver are either photographs or drawings.

All methods of relief plate photo-engraving come under two general heads: "Half-tone" and "line engraving," the latter being very generally known as "zinc etching." Zinc etching is the simplest method of photo-engraving and should be thoroughly understood before one begins to inquire into the intricacies of the half-tone process. It is used to reproduce what is known as "black and white" work, or line drawings. Any drawing or print having black lines or dots on a white background, without any middle shades, may be engraved by this process. The old-fashioned "wet-plate" photography is used in making practically all process plates, either in line or half-tone.

I will describe briefly all the operations gone through in making a line plate, taking for a subject a map drawn in black ink on white paper or a head drawn by Charles Dana Gibson,—subjects wide apart in an artistic way, but of absolutely equal values so far as making the plate is concerned. The drawing is first put on a copy board in front of a camera made especially for this work, in whose holder the wet plate has already been placed by the operator. The subject may be enlarged or reduced to any desired size, nearly all drawings being made much larger than they are desired to be reproduced in the plates. The exposure is much longer than in ordinary dry plate work, generally lasting in the neighborhood of five minutes. The result is a black and white negative. That is, the lines that were black in the drawing are absolutely clear and transparent in the negative, but the rest of the negative is black. From the photographer, the negative goes to the "negative-turning" room. Here the negative is coated with solutions of collodion and rubber cement, which makes the film exceedingly tough—so tough that it is easily stripped from the glass on which it was made, and is "turned" with the positive side up on another sheet of glass. If this were not done, the plate would be reversed in printing—that is, a line of type would read from right to left, or backward. After the negative is "turned," it is ready for the etching room. Here the surface of a sheet of zinc about one-sixteenth of an inch thick, which has been polished until it is as smooth as plate glass and without a scratch or a flaw of any kind, is flowed with a sensitized solution, easily affected by light. The negative is placed in a printing frame over the sensitized zinc and a print is made. That is, it is exposed to the sunlight or to a powerful electric light, and the light shines through the transparent parts of the negative, and hardens the sensitized surface; while the black part of the negative protects the sensitized surface from the action of the light. The plate is next "rolled up" with a lithograph roller which distributes a thin coating of etching ink over the entire surface. The plate is then washed off carefully by the operator, but the ink adheres to all portions of the plate that have been acted upon by the light. We now have a fully developed print on the highly polished surface of the zinc that is an exact reproduction of the original drawing. It is now necessary to make this print acid proof, and this is done by covering the plate with a coating of very fine resinous powder, called "dragon's blood," which adheres to the printed portions of the plate. The plate is subjected to enough heat to melt this powder, and is then ready for the acid bath.

A strong solution of nitric acid is used for etching zinc plates. This acid is placed in trays, which are rocked constantly, either by power or by hand, while the plate is being etched. The melted dragon's blood makes a perfect acid resistant and the acid, therefore, does not affect the print (or picture itself), but eats away the bare surfaces of the metal between the black lines and the dots. When this etching has proceeded far enough to make a plate that may be used in printing, the lines and dots of the picture stand up in bold relief, while the metal around these lines and dots has been eaten away to a considerable depth.

There are many details that cannot be described in a short article, but these are the principal operations gone through in etching the plate. One very important detail in etching is to prevent "undercutting." It is obvious that if the acid will eat down, it will also eat sidewise. The acid resistant is only on the surface. If means were not taken to prevent it, as soon as the acid got below the surface, it would begin to eat in under the print and the lines and dots of the picture would disappear; therefore, as soon as the plate has had its first "bite," it is taken from the acid, dried, and dragon's blood is brushed against the sides of the lines. This powder is then melted and the plate given another etching. While the plate is being etched down, it is removed from the acid several times, and the sides of the dots and lines are again protected. After leaving the etching room the plate goes to the "router," an ingenious machine, with a cutting tool revolving at a speed of fourteen thousand revolutions a minute, which quickly removes the waste metal in the large open places between the lines and dots. The zinc plates are carefully looked over by a finisher, defects are removed, and the metal plates are then nailed on wooden blocks, so that they will be "type-high," that is, of exactly the same height as the metal type-forms used in printing. Hand presses are a necessity in all photo-engraving shops, and with these several "proofs" of each plate are printed in order that the customer may judge of the quality of the plate.

While the line, or zinc etching process is immensely useful, in reproducing pen-and-ink drawings, maps, wood-cut prints, etc., yet the half-tone process is the one that practically revolutionized all known methods of illustration, after it had become perfected. While zinc etching is limited in its capabilities to the reproduction of black and white subjects, practically everything in art or nature may be reproduced by the half-tone process. The half-tone "screen" makes it possible to take a photograph or wash drawing and break the flat surface of the picture up into lines and dots, with the white spaces between that are an absolute essential in relief plate printing. If a half-tone print taken from any magazine or periodical is examined closely, either with the naked eye or a magnifying glass, it will be seen that the entire picture is a perfect network of lines and dots, and that there are two sets of lines running diagonally across the plate at right angles to each other. In the darker portions of the picture it will be seen that the lines are very heavy, with a small white dot in the centre of each square, made by the intersecting lines. In the lighter portions of the picture, these lines will be found to be very fine, while in the lightest parts, or in the "high lights," as they are called, the lines disappear and in their places are a mass of fine dots, not much larger than a pin point.

To make a half-tone plate of a photograph or other subject, it is necessary to break the negative up into lines and dots. It is for this purpose that the half-tone "screen" is used. The screen consists of two thin pieces of plate-glass, on the surface of which a series of very delicate parallel black lines have been ruled running diagonally across the glass. When these pieces of glass are placed together, face to face, the parallel lines ruled on them intersect each other at right angles, giving a very fine "mosquito-netting" effect. The method of making the negative is very similar to that described in making line negatives, excepting that in making a half-tone negative the screen is placed in the plate-holder directly in front of the negative. The subject is then photographed, and the result is a negative completely covered with a mass of fine transparent lines and dots.

Copper is generally used instead of zinc in making half-tone plates. In making a print on copper the light shines through the transparent lines and dots of the negative and hardens the sensitized surface of the plate. The black parts of the negative between the transparent lines and dots protect the sensitized surface. When the plate, after printing, is placed under a water tap, the parts of the sensitized surface that have not been acted upon by light wash away, leaving a print that becomes acid proof after being subjected to an intense heat.

The method of etching a copper plate is similar to that already described for etching zinc plates, excepting that sesquichloride of iron is used instead of nitric acid. In a half-tone the dots and lines are so close together that great depth is neither desirable nor possible, and no steps are taken to prevent undercutting.

The half-tone plate, after it has been carried as far as possible by mechanical processes, is capable of great improvement in the hands of skilful engravers. The plate as it comes from the etching bath may be termed a mechanical product. Though great skill is necessary in making the negative, the print, and the etching, the hand-finishing gives the plate many of its artistic qualities. The unfinished plate is apt to be more or less "flat" in appearance; the high lights may not be light enough, while the dark portions of the plate are apt, in cases, to be too light. The most common methods of finishing are reëtching and burnishing. The finisher dips a camel's-hair brush in acid and applies it to the high-light portions of the plate, or other places that are too dark, and allows it to act on the metal until these parts of the plate are lightened sufficiently. The parts of the plate that are too light are made darker by rubbing down the surface of the plate with a tool called the burnisher. The skilful, artistic finisher has other methods at his command of making the plate reproduce as accurately and as artistically as possible the original drawing or photograph. High lights are sometimes cut out entirely, or a fine engraver's tool may be "run" between the lines; while a "wood-engraved" finish is produced by cutting, in certain portions of the plate, lines similar to those used in wood engraving.

In the price-cutting that has been going on as a result of the fierce competition that has existed among photo-engravers during the past few years, the artistic possibilities of the half-tone have been lost sight of to a certain extent. The product of the engravers is sold by the square inch, regardless of the fact that the cost of one plate may be double the cost of another plate of the same size, but from a different subject.

A point also worth remembering is that until the plate reaches the finishers' hands, it has been more or less of a mechanical product; and that the plate is made an artistic creation by the skill, care, and brains of an intelligent class of men earning from $25 to $50 a week. Those expecting "the best" at "the lowest price" can easily guess about how much of this high-priced finishing they will get when the price paid barely covers the cost of the mechanical product. Then, engravers striving for high quality in the product pay from twenty-five to fifty per cent higher wages, as a rule, than the cheap, commercial shops. But the idea of square-inch price has so generally permeated the buying public, that the larger and better shops have been compelled, to a greater or less extent, to meet the prices of their less skilful competitors. They are enabled to do this and give their customers much greater value for their money, only through better business methods, more modern facilities, and by conducting the business on a very large scale.

The screens used in making half-tones represent an enormous outlay in the large shops. A comparatively small screen costs in the neighborhood of $100. A screen 18 × 20, ruled 120 or 133 lines to the inch, costs about $500. Screens are made with different numbers of lines to the inch, from 65, for coarse, newspaper work, up to 400. The screens in general use are 65, 85, 100, 110, 120, 133, 150, 166, 175, and 200; but intermediate sizes are also used, such as 125 and 140. A screen containing 200 lines to the inch is about the finest ever used for ordinary printing purposes, though a few screens with 250, 300, and 400 lines to the inch have been made. A well-equipped photo-engraving establishment must have all these screens, and all of them in many different sizes. In the writer's shop there are fifteen cameras, all of them in constant use in the daytime and five or six of them are always in use all night. Some days the bulk of the work in the place will be a fine grade of magazine engraving calling for a 175 screen. In order to keep all the cameras at work all the time, a thing that is very important in a well-regulated place, it is necessary to have a number of 175 screens almost equal to the number of cameras. The same is true of most of the other screens in general use. Fortunately for the engraver and the consumer these screens practically last forever if carefully handled.

The greatest developments in process work during the past few years have been in the making of color plates. Beautiful results are obtained in two colors by the "duograph" or "duotone" processes, the plates being made for two printings. The three-color process aims to reproduce all colors in three printings, by using inks of red, yellow, and blue. This process is very interesting, but somewhat intricate. Primarily, the results are made possible by color separations. The aim is to take a colored subject—an oil painting, for instance—and by photographing it three times, each time through a different colored piece of glass, to divide all the colors into what are called the three primary colors—red, yellow, and blue. From each of these color separations a half-tone plate is made, and when these plates are put on the printing-press, and the impressions are printed over each other in yellow, red, and blue inks, respectively, the result is a printed picture reproducing correctly all the colors of the original subject.

While many subjects may be reproduced accurately by this process, yet the three-color process seems inadequate to give perfectly satisfactory results in all cases. Nearly all three-color process houses are now prepared to add a fourth, or key, plate, to be printed in black, in case the subject seems to need it. The three-color process has enabled many of the leading magazines to use illustrations in colors, and there is not the slightest doubt but that there is a great future for this class of work.[Back to Contents]