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.