In lithography, a picture is drawn on a smooth flat stone with a special type of oily crayon. Because the printing surface is flat, lithography is an example of planographic or surface printing. Then the lithographer passes a water-soaked roller over the stone. The water adheres to the bare stone surface, but does not stick to the oily crayon marks. Another roller soaked with printer's ink is passed over the stone. Since the ink will not mix with water, it cannot stick to the wet stone, but does stick to the oily crayon marks. When a sheet of paper is pressed against the inked stone, the paper takes up ink only from the places where the crayon lines are. This produces a print of the original drawing on paper.
Photolithography is a variation of lithography performed by machine and using a camera. In this case, a zinc plate is used instead of the stone. The picture is placed on the plate by photographic means rather than by hand. Characters and words can also be printed on the plate. The zinc plate is then curved around the printing cylinder. As the cylinder turns, the plate first presses against a wet roller, and then against an ink roller. This has the effect of covering the blackened portions of the plate with ink. The inked plate next rolls against a rubber-blanketed cylinder so that the image is picked up. The blanketed cylinder then transfers the image to the paper. This kind of printing is known as offset printing.
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