Lithography

Lithography (Greek, lithos, “stone”; graphein, “to write”), process of printing developed in 1798 by a German map inspector, Aloys Senefelder. Lithography involves a flat, planographic surface on which the printing area is no higher than the non-printing area; it depends for its action on the mutual repulsion of grease and water. Senefelder found that if a drawing were made on a flat piece of limestone with a greasy crayon, the lines would attract and hold an oily or greasy ink when the stone was wet, whereas other portions of the stone would take no ink. The drawing could then be reproduced on a piece of suitable paper rolled into contact with the stone.

Among prints and printmaking methods in the 19th century, lithography became the chief means of reproducing works of art and illustrating books and magazines; the French artist Honoré Daumier is particularly known for his many lithographs that gently satirize the social foibles of his day.

When the photographic preparation of plates was introduced, photolithography came into existence, as did offset lithography, in which an extra rubber-surfaced cylinder was used. They are both now generally considered offset printing techniques. The rubber-surfaced blanket cylinder runs in contact with the printing-plate cylinder, which has replaced the flat stone, and receives the inked image from it; this image is then offset on to the paper. The rubber blanket keeps the delicate printing plate from coming into contact with the printed substrate. Very long printing runs are thereby made possible, and the faithful reproduction, fine lines, and shading unique to lithography are preserved.

In the second half of the 20th century, offset lithography became the dominant commercial printing process, used for almost every type of printed product. It eclipsed other methods of printing because it could produce high-quality reproductions at lower cost than other processes. Although it requires a higher level of skill to operate the press and achieve the correct balance between ink and water, the relative ease of platemaking, the speed with which a press can be prepared for printing, and the wide range of substrates that can be printed make lithography an attractive choice.

Most lithographic presses today print multiple colours in a single pass through the press. A sheet-fed press may have up to 8 or even 12 printing units capable of printing both sides in full colour. For longer runs, reel-fed presses are used. Here the paper is printed in a continuous web and folded before leaving the press, reducing the cost and time involved in post-press operations.

In the 21st century, the competition with lithography is intensifying. In the very longest runs, gravure has always been the most cost-effective process, while for short runs customers are increasingly turning to digital printing. Flexography is also taking some market share from lithography, especially in the packaging sector.

The term lithography is also used to describe techniques for producing printed circuits and other semiconductor devices. This form of lithography has little in common with the process described above. A photoresist is prepared and coated on to a suitable substrate, and then exposed to light or to some other form of electromagnetic radiation. The exposed areas are hardened and thus resistant to a subsequent wash-out stage, which removes the coating in the unexposed areas. By using short-wavelength exposure sources, features as small as 100 nanometres or less can be produced.