The Preparation of Illustrations for Reports of the United States Geological Survey / With Brief Descriptions of Processes of Reproduction

by John L. Ridgway

Before a drawing is made on stone a stone of the quality suited to the particular design in hand is selected. The stone is then ground and polished, and if the drawing is to be made with crayon it is "grained" according to the special requirements of the subject. If the drawing is to be made with a pen and is to consist of "line work" the stone is polished. The first step is to obtain on the stone an outline or "faint" of the design. There are several ways to do this. By one method a tracing of the design is made, a sheet of thin paper covered with red chalk is laid face downward on the stone, the tracing is laid face downward over it, and the design is again traced in red-chalk lines on the stone. The method described is simple, but there are others that are more complicated and that are particularly applicable to the reproduction of photographs and other illustrations. Crayon work is often used in combination with pen and ink, stipple, and brush work. This method of drawing on stone is used also for preparing color stones in the process of chromolithography, in which there are many added details of manipulation. After the drawing has been made on the stone or transferred to it the stone is "gummed"—that is, it is covered with a solution of gum arabic and nitric acid—and dried. The stone is then dampened with water and carefully rolled with lithographic ink, which adheres to the pen or crayon work and is repelled elsewhere. It is then "rubbed" over with powdered rosin and talcum, which adheres to the ink and further protects the drawing from the effects of the etching fluid, which is next to be applied to the stone. This fluid consists of a 10 per cent solution of gum arabic to which 2 to 7 per cent of nitric acid has been added, the degree of acidity being varied according to the subject and the hardness of the stone. The fluid is applied with a brush or sponge and is left on the stone just long enough to decompose slightly the carbonate of lime on its surface and, after washing, to leave the design or drawing in very slight relief. The stone is again gummed and dried, and the design is "washed out" or brought out by removing the surface gum with a wet sponge and applying to the stone a rag sprinkled with turpentine and charged with printing ink. These operations wash away the tusche and the crayon that have been decomposed by the acid and expose the design faintly in white at first, but it gradually grows darker as it becomes charged with printing ink from the rag. The stone is next "rolled up" or inked. The slightly moistened surface repels the ink and the design takes it up, so that when the stone is run through the press the design is carried to the paper.

Lithographic prints from stones prepared in this way are made on a flat-bed press. The stone is carried forward to print and on its return is dampened and inked, an operation slower than that of rotary printing.

Corrections and changes are made on the stone by carefully scraping or polishing away the parts to be corrected and making the changes with a crayon or pen, but the design can not ordinarily be corrected twice in the same place, as the scraping or polishing removes a part of the surface of the stone and thus lessens the pressure at that place, and the impression there may be imperfect or may completely fail.

This form of lithography is seldom used for Survey illustrations but was formerly much used and is well adapted to the reproduction of drawings of fossils, particularly of remains of dinosaurs and other types of large extinct animals. Examples may be seen in Monographs 8 and 10 and in other early reports of the Geological Survey. The drawings for these illustrations were made directly on stone.

A drawing made on one stone may be transferred in duplicate or in any desired number to another stone, or to a properly grained sheet of zinc and aluminum, from which impressions may be printed on a lithographic press. Both these metals are also used for lithographic printing on rotary presses, the zinc or aluminum plate being bent and secured around a cylinder which rotates continuously in one direction. As one impression is made at each revolution of the cylinder the printing is rapid; but the best printing from a metal plate is inferior to the best printing from a lithographic stone.

PHOTOLITHOGRAPHY.

Photolithography, like other lithographic processes, has been improved greatly during the last few years—not particularly in results but in methods—by the introduction of metal plates, the rubber blanket offset, the Ben Day films, and many mechanical and chemical devices, so that a brief description of it will not explain the process except in a most general way. As photolithography is a direct process and is relatively cheap it is the one most used for reproducing large maps and other line drawings that have been carefully prepared. Zinc and aluminum plates are now much used in photolithography, for a direct contact photographic print can be made on them, they can be printed flat or bent for use on a rotary press, and they can be stored for future use more economically than stones.

There are two somewhat distinct methods of producing photolithographs. In both the ordinary photographic methods are used, but it is often necessary to "cut" or trace parts of the negative in order to open up lines and other features that are not sharp or well defined, so that the negative will print them sharp and clear. If the copy to be reproduced shows three colors, three negatives are made, one for each color, and the parts to be shown by each are preserved by "opaquing" or painting out all other parts. By the older method the negative thus perfected is placed in a printing frame in contact, under pressure, with sensitized transfer paper and is exposed to light. The printing frame is then carried to the dark room and the paper is removed from the frame and its surface covered with transfer ink. The paper is then laid face upward on water and soaked for several minutes, after which it is placed in the same position upon a slab of stone or metal and thoroughly washed with water. This washing removes the ink and the sensitive film from the parts that were unaffected by the action of light (the parts corresponding to the white paper in the design), but the ink still adheres to the lines of the design in the precise sharpness and clearness of the negative. The design is now ready to be transferred to the printing stone or zinc plate. The sheet is again slightly dampened between moist blotters and laid face downward in its correct position on a prepared stone or zinc plate, which is then pulled through a press under heavy pressure. The paper is then removed from the stone or plate, to which it has carried the design. From this point the gumming, etching, and other operations are practically the same as those used in ordinary lithography.

The bichromate-gelatin transfer process described above has been replaced in the Survey by a more satisfactory one, which insures absolute scale and reproduces the finest line drawings perfectly without thickening the lines or without distortion. In this process, which is known as the planographic process, a photographic negative of the "copy" is placed in a vacuum printing frame in contact with a zinc or aluminum plate that has been sensitized with a bichromate-albumen solution and exposed in front of an arc lamp. After proper exposure the plate is removed from the frame, inked over, and placed under water. The parts not hardened by the action of light (the unexposed parts) are then rubbed away with cotton, and the plate is chemically etched, gummed over, and dried. The plate is then ready to be printed from in a lithographic press. If a large map is to be reproduced it is photographed in parts, and contact prints are made on zinc plates. From these plates transfers are pulled and the parts are assembled and laid down in proper position on a stone or an aluminum plate, which is then prepared for printing.