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The Technique of Lithography
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Thomas Hart Benton (1889-1975)
'Cradling Wheat ' 1939


Henri de Toulouse Lautrec (1864–1901)
'Jane Avril' 1899
Lithography - Invented in 1796 by Bavarian author Alois Senefelder as a low-cost method of publishing theatrical works, lithography can be used to print text or artwork onto paper or another suitable material.

Lithography originally used an image drawn in wax or other oily substance applied to a lithographic stone as the medium to transfer ink to the printed sheet. In modern times, the image is often made of polymer applied to a flexible aluminum plate. The flat surface of the plate or stone is slightly roughened, or etched, and divided into hydrophilic regions that accept a film of water and thereby repel the greasy ink, and hydrophobic regions that repel water and accept ink because the surface tension is higher on the greasier image area which remains dry. The image may be printed directly from the stone or plate (in which case it is reversed from the original image) or may be offset by transfer to a flexible sheet, usually rubber, for transfer to the printed article.
This process is different from gravure or intaglio printing where a plate is engraved, etched or stippled to make cavities to contain the printing ink, and in woodblock printing and letterpress where ink is applied to the raised surfaces of letters or images.

Most books, indeed all types of high-volume text, are now printed using offset lithography, the most common form of printing production. The word "lithography" also refers to photolithography, a microfabrication technique used to make integrated circuits and microelectromechanical systems, although those techniques have more in common with etching than with lithography.

The principle of lithography
Lithography uses simple chemical processes to create an image. For instance, the positive part of an image is a hydrophobic, or "water hating" substance, while the negative image would be hydrophilic or "water loving". Thus, when the plate is introduced to a compatible printing ink and water mixture, the ink will adhere to the positive image and the water will clean the negative image. This allows a flat print plate to be used, enabling much longer and more detailed print runs than the older physical methods of printing (e.g., intaglio printing, Letterpress printing).
Lithography was invented by Alois Senefelder in Bohemia in 1796. 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, a solution of gum arabic in water was applied, the gum sticking only to the non-oily surface. During printing, water adhered to the gum arabic surfaces and avoided the oily parts, while the oily ink used for printing did the opposite.

Lithography on limestone
Lithography works because of the mutual repulsion of oil and water. The image is drawn on the surface of the print plate with a fat or oil-based medium (hydrophobic) such as a wax crayon, which may be pigmented to make the drawing visible. A wide range of oil-based media is available, but the durability of the image on the stone depends on the lipid content of the material being used, and its ability to withstand water and acid. Following the drawing of the image, an aqueous solution of gum arabic, weakly acidified with nitric acid HNO3 is applied to the stone. The function of this solution is to create a hydrophilic layer of calcium nitrate salt, Ca(NO3)2, and gum arabic on all non-image surfaces. The gum solution penetrates into the pores of the stone, completely surrounding the original image with a hydrophilic layer that will not accept the printing ink. Using lithographic turpentine, the printer then removes any excess of the greasy drawing material, but a hydrophobic molecular film of it remains tightly bonded to the surface of the stone, rejecting the gum arabic and water, but ready to accept the oily ink.
When printing, the stone is kept wet with water. Naturally the water is attracted to the layer of gum and salt created by the acid wash. Printing ink based on drying oils such as linseed oil and varnish loaded with pigment is then rolled over the surface. The water repels the greasy ink but the hydrophobic areas left by the original drawing material accept it. When the hydrophobic image is loaded with ink, 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.
Senefelder had experimented in the early 1800s with multicolor lithography; in his 1819 book, he predicted that the process would eventually be perfected and used to reproduce paintings. Multi-color printing was introduced through a new process developed by Godefroy Engelmann (France) in 1837 known 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 color, and led to the characteristic poster designs of this period.

The modern lithographic process
The earliest regular use of lithography for text was in countries using Arabic, Turkish and similar scripts, where books, especially the Qur'an, were sometimes printed by lithography in the nineteenth century, as the links between the characters require compromises when movable type is used which were considered inappropriate for sacred texts.

High-volume lithography is used today to produce posters, maps, books, newspapers, and packaging - just about any smooth, mass-produced item with print and graphics on it. Most books, indeed all types of high-volume text, are now printed using offset lithography.
In offset lithography, which depends on photographic processes, flexible aluminum, polyester, mylar or paper 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 ultraviolet 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 cylinder on a printing press. Dampening rollers apply water, which covers the blank portions of the plate but is repelled by the emulsion of the image area. Ink, which is hydrophobic, is then applied by the inking rollers, which 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 mirror image and the paper would become too wet. Instead, the plate rolls against a cylinder covered with a rubber blanket, which squeezes away the water, picks up the ink and transfers it to the paper with uniform pressure. 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.

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.

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