Lithography

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Lithography is a printing method. It involves the microscopic "roughening" of portions of a smooth surface . When water is applied to the surface, it adheres to the areas that have been roughened, and beads off the areas that are smooth. An oil-based ink is then rolled over the surface, which will stick to the smooth areas, but not the moistened areas. If paper is then rolled over the surface, the ink will transfer to the paper in a reverse impression.

In the early days of lithography, a smooth piece of limestone was used (hence the name "lithography" - "lithos" is the Latin word for stone). Portions of the stone were masked, and acid was used to etch the exposed areas, roughening them.

Today, however, aluminum plates are used. The plates already have a brushed, or "roughened" texture, but they are covered with a smooth photosensitive emulsion. A photographic negative of the desired image is laid on top of the plate, and exposed to light, transferring a positive image to the emulsion. The emulsion is then chemically treated to remove the unexposed portions of the emulsion. The plate is affixed to a drum on a printing press, and water is rolled over the plate, which adheres to the rough, or negative portions of the image. A roller coated with ink is then rolled over the plate, which adheres to the smooth, or positive portions of the image. If this image were directly transferred to paper, it would create a positive image, but the paper would be moistened. Instead, a drum covered with a rubber surface is rolled over the plate, which squeezes away the water, and picks up the ink. The drum is then rolled over the paper, transferring the ink. Because the image is first transferred to the rubber drum, the process is called "offset lithography," due to the fact that the image is offset to the drum before being applied to the paper.

Many innovations and technical changes have occurred to this process over the years, including the development of presses that utilize several plates to build up a multi-color image in one pass through the press, and the Dahlgren inking system, which eliminates the separate moistening step (instead combining it in the inking step).