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Reference Library: Lithography
'Lithography' is a method for printing on a smooth surface. It can be used to print text or artwork onto paper or another suitable material. It can also refer to a method of manufacturing semiconductor and MEMS devices.

Printing


The principle


In lithography the entire print block comes in contact with the paper sheet, and a chemical process confines the ink to the desired image on the block. This contrasts with relief printing where the ink is carried on a raised image, and intaglio, where it lies in the grooves of an engraved image. Because the print block is flat rather than relief, lithography is described as a 'planographic' print process.

The chemical process


Lithography works because of the repulsion of oil and water. The image is drawn on the surface of the print block with an oil-based medium. The range of oil-based mediums is endless but the dexterity of the image relies on the lipid content of the material being used--its ability to withstand water and acid. Following the placement of the image is the application of an acid emulsified with gum arabic. The function of this emulsion is to create a salt layer directly around the image area. The salt layer seeps into the pores of the stone, completely enveloping the original image. This process is called etching.
Using lithographic turpentine, the printer then removes the greasy drawing material, leaving only the salt layer; it is this salt layer which holds the skeleton of the image's original form.
When printing, the stone or plate is kept wet with water. Naturally the water is attracted to the layer of salt created by the acid wash. Ink that bears a high lipid content is then rolled over the surface. The water repels the grease in the ink and the only place for it to go is the cavity left by the original drawing material. When the cavity is sufficiently full, the stone and paper are run through a press which applies even pressure over the surface, transferring the ink to the paper and off the stone.

The early process


Lithography was invented by Alois Senefelder in Bohemia 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-color printed images, a process known by the middle of the 19th Century as Chromolithography. A separate stone was used for each colour, and a print went through the press separately for each stone. The main challenge was of course to keep the images aligned (in register). This method lent itself to images consisting of large areas of flat colour, and led to the characteristic poster designs of this period. Many fine works of chromolithographic printing were produced in America and Europe.

The modern process


Modern high-volume lithography is used to produce posters, books, newspapers, packaging, credit cards, decorated CDs – just about any smooth, mass-produced item with print on it.
In this form of lithography, which depends on photographic processes, flexible aluminum or plastic printing plates are used in place of stone tablets. Modern printing plates have a brushed or roughened texture and are covered with a photosensitive emulsion. A photographic negative of the desired image is placed in contact with the emulsion and the plate is exposed to light. After development, the emulsion shows a reverse of the negative image, which is thus a duplicate of the original (positive) image. The image on the plate emulsion can also be created through direct laser imaging in a CTP (Computer-To-Plate) device called a platesetter. The positive image is the emulsion that remains after imaging. For many years, chemicals have been used to remove the non-image emulsion, but now plates are available that do not require chemical processing.
The plate is affixed to a drum on a printing press. Rollers apply water, which covers the blank portions of the plate but is repelled by the emulsion of the image area. Ink, applied by other rollers, is repelled by the water and only adheres to the emulsion of the image area--such as the type and photographs on a newspaper page.
If this image were directly transferred to paper, it would create a positive image, but the paper would become too wet. Instead, the plate rolls against a drum covered with a rubber blanket, which squeezes away the water and picks up the ink. The paper rolls across the blanket drum and the image is transferred to the paper. Because the image is first transferred, or offset to the rubber drum, this reproduction method is known as offset lithography or offset printing.
Many innovations and technical refinements have been made in printing process and presses over the years, including the development of presses with multiple units (each containing one printing plate) that can print multi-color images in one pass on both sides of the sheet, and presses that accommodate continuous rolls (
webs) of paper, known as web presses. Another innovation was the continuous dampening system first introduced by Dahlgren. This increased control over the water flow to the plate and allowed for better ink and water balance. Current dampening systems include a "delta effect" which slows the roller in contact with the plate, thus creating a sweeping movement over the ink image to clean impurities known as "hickies".
The advent of desktop publishing made it possible for type and images to be manipulated easily on personal computers for eventual printing on desktop or commercial presses. The development of digital imagesetters enabled print shops to produce negatives for platemaking directly from digital input, skipping the intermediate step of photographing an actual page layout. The development of the digital platesetter in the late Twentieth century eliminated film negatives altogether by exposing printing plates directly from digital input, a process known as computer to plate printing.

Semiconductor lithography


Main article: photolithography.''
Semiconductor lithography was developed for use in manufacturing microchips. It is also used in MEMS applications, as it is one of the best methods currently in use for manufacturing devices on scales much smaller than a micrometer. Although silicon lithographic technology is most advanced, other materials are also used. The emerging technology of a maskless lithography process and nanoimprint lithography for the semiconductor is also being used.

Lithography as an artistic medium


During the first twenty-five years of the nineteenth century, the practice of lithography was predominantly restricted to cheap reproductions of paintings and drawings. However, around 1825 the French artists Ingres, Géricault, Delacroix, and Rodolphe Bresdin embraced the process as a way to avoid the problems inherent in wood-block and copper engraving, namely, the near necessity of middlemen like draughtsmen (who transferred the image to the wood or copper plate) and engravers (who carved the image out of the plate). The advantage to lithography (for an artist's point of view) was that he or she could draw or paint directly onto the lithographic material and avoid entirely the intermediate steps and craftsmen involved in engraving. Therefore, an artist's drawing and a lithographic print made from it were nearly identical — no reworking or transfer to another medium was necessary. It also afforded, at the time, the most complete range of line color from white to black.
Goya's lithographs The Bulls of Bordeaux (1828) and Delacroix's illustrations to Goethe's Faust were the groundbreaking "artist's lithographs" that sparked a flood of (mostly French) artists who dabbled in lithography, including Prud'hon, Cezanne, Manet, Toulouse-Lautrec, and, of course, its greatest practitioner, Daumier, whose prints began to appear in the 1830s.
For the first time in history, an artist was able to send out into the world his or her own drawing, not in unique specimen but in editions. Each impression had all their personality, skill, and genius, with no recourse to intermediary persons and technological steps.
See: [http://www.wesleyan.edu/dac/coll/grps/dela/faust_01-10.html Delacroix's Faust lithographs at the Davison Art Center, Wesleyan University]
This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Lithography".