Lithography

Lithography

Lithography (from Ancient Greek lithos, meaning ‘stone’, and graphein, meaning ‘to write’) is a method of printing originally based on the immiscibility of oil and water. The printing is from a stone (lithographic limestone) or a metal plate with a smooth surface. It was invented in 1796 by German author and actor Alois Senefelder as a cheap method of publishing theatrical works. Lithography can be used to print text or artwork onto paper or other suitable material.

A printing process based on the fact that grease and water don’t mix.

The image is applied to a grained surface (traditionally stone but now usually aluminum) using a greasy medium: such as a special greasy ink – called tusche, crayon, pencils, lacquer, or synthetic materials. Photochemical or transfer processes can also be used. A solution of gum arabic and nitric acid is then applied over the surface, producing water-receptive non-printing areas and grease-receptive image areas. The printing surface is kept wet, so that a roller charged with oil-based ink can be rolled over the surface, and ink will only stick to the grease-receptive image area. Paper is then placed against the surface and the plate is run through a press.

Lithography on limestone
Lithography originally used an image drawn with oil, fat, or wax onto the surface of a smooth, level lithographic limestone plate. The stone was treated with a mixture of acid and gum arabic, etching the portions of the stone that were not protected by the grease-based image. When the stone was subsequently moistened, these etched areas retained water; an oil-based ink could then be applied and would be repelled by the water, sticking only to the original drawing. The ink would finally be transferred to a blank paper sheet, producing a printed page. This traditional technique is still used in some fine art printmaking applications.

Modern lithographic process
In modern lithography, the image is made of a polymer coating applied to a flexible plastic or metal plate. The image can be printed directly from the plate (the orientation of the image is reversed), or it can be offset, by transferring the image onto a flexible sheet (rubber) for printing and publication.

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. Hydrophobic ink, which is repelled by the water and only adheres to the emulsion of the image area, is then applied by the inking rollers.

If this image were transferred directly to paper, it would create a mirror-type 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 passes between the blanket cylinder and a counter-pressure or impression cylinder and the image is transferred to the paper. Because the image is first transferred, or offset to the rubber blanket cylinder, this reproduction method is known as offset lithography

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