Lithography, also known as planographic printing, was devised by Aloys Senefelder. Flat stones were the first lithographic plates and are still used, although a variety of thin metal, plastic, and paper plates are now also employed. A drawing is made on the plate with greasy ink or crayon, and water is then applied to the plate. When the plate is inked for printing, the greasy parts accept the ink and the wet parts do not. Preparing a printing surface so that ink will adhere only to parts of it is basic in all planographic printing.
Collotype, also called photogelatin, is a lithographic process that uses a gelatin-faced plate to achieve the tonal distribution obtained through screen dots in engraving. It is chiefly used in the reproduction of fine illustrations or of scientific subject matter requiring accuracy of detail.
Photolithography, offset, litho-offset, and offset lithography are synonyms in commercial printing for the most widely used form of planographic printing, based on a modification of the lithographic press featuring a rubber-covered cylinder between the printing cylinder and the impression cylinder. The plate cylinder transfers the image to the rubber blanket cylinder, which in turn offsets it on the paper carried by the impression cylinder. Offset and other forms of planographic printing, through many technical refinements, make possible increased production speeds, improved quality in the reproduction of fine tones, and a substantial reduction in the number of impressions required to reproduce full-color copy.
Screen Process:
The fourth traditional type of printing, screen process, includes silk screen and has special applications in the printing industry. Silk screen printing is a form of stencil printing, i.e., printing where the ink is applied to the back of the image carrier and pushed through porous or open areas. The image is on a piece of silk stretched on a frame and backed by a rubber squeegee containing ink. The nonprinting areas on the silk screen are blocked out, and the ink is pushed through the porous areas corresponding to the design; the process is widely used for posters and for printing on glass, plastics, and textured surfaces. Mimeographing is another commercial application of stencil printing.
