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Lithography

From Wikipedia, the free encyclopedia.

The Principle

Lithography as a manual process is based on the repulsion of oil and water. The image is placed on the surface with an oil-based medium; acid is then used to 'burn' the oil into the surface. When printing, the surface is covered in water, which remains on the non-oily surface and avoids the oily parts; a roller can then apply an oil-based ink that adheres only to the oily portion of the surface.

The Early Process

Lithography was invented by Alois Senefelder in Germany in 1798, and it was the first new printing process since the invention of relief printing in the fifteenth century. In the early days of lithography, a smooth piece of limestone was used (hence the name "lithography"—"lithos" is the ancient Greek word for stone). After the oil-based image was put on the surface, acid burned the image onto the surface; gum arabic, a water soluble solution, was then applied, sticking only to the non-oily surface and sealing it. During printing, water adhered to the gum arabic surfaces and avoided the oily parts, while the oily ink used for printing did the opposite.

Within a few years of its invention, the lithographic process was used to create multi-colour printed images, a process known by the middle of the 19th Century as Chromolithography, and many fine examples of chromolithographic colour printing and publishing were achieved in America and Europe during this period. A separate stone was used for each colour, and the print went through the press separately for each stone. The main challenge was of course to keep the images in register.

The Modern Process

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).

The above article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Lithography".

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