Printing

Printing, reproduction of words and pictures in ink on paper or other suitable media. Despite the advent of information retrieval systems, the storage and dissemination of knowledge are still based primarily on the printed word. Modern printing began with the work of Johann Gutenberg, who invented movable type and type metal in the 15th century. Individual characters could be used several times. The process was little changed for 400 years, until the invention of machines that could cast type as it was required.

Letterpress and lithography are today the two most used printing techniques. Letterpress uses raised type that is a mirror image of the printed impression. The type is inked and the paper pressed to it. Lithography depends on the mutual repulsion of water and oil or grease. In fine art a design is drawn with a grease crayon on the surface of a flat, porous stone, which is then wetted. Water is repelled by the greasy areas; but ink is repelled by the damp and adheres to the greasy areas. Modern mechanized processes use the same principle. Commonest is photo-offset, where the copy to be printed is photographed and the image transferred to a plate such that the part to be printed is oleophilic (oil-loving), the rest hydro-philic (water-loving). Gravure is another major printing technique. The plate is covered with a pattern of recessed cells in which the ink is held, greater depth of cell increasing printing intensity. Little-used for books, it is used extensively in packaging.