Lithography

Lithography (from Greek lithos, 'stone' + graphein, 'to write') is a method for printing using a stone (lithographic limestone) or a metal plate with a completely smooth surface. 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 any another suitable material.

Lithography has also been used for various filmography and other visual things. 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.

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