In planographic printing, as opposed to intaglio and relief processes, there is no difference in level between the inked surface and the non-inked surface. The following sections detail variations of lithographic prints.
The design is drawn or painted on the polished, or grained, flat surface of a stone, usually Bavarian limestone, with a greasy crayon or ink. The design is chemically fixed on the stone with a weak solution of acid and gum Arabic. In printing, the stone is flooded with water which is absorbed everywhere except where repelled by the greasy ink. Oil-based printer's ink is then rolled on the stone, which is repelled in turn by the water-soaked areas and accepted only by the drawn design. A piece of paper is laid on the stone and it is run through the press with light pressure, the final print showing neither a raised nor embossed quality but lying entirely on the surface of the paper. The design may be divided among several stones, properly registered, to produce, through multiple printings, a lithograph in more than one color. A transfer lithograph (French,autographie) employs the same technique, but the design is drawn on special transfer paper and is later mechanically transferred to the stone. A zincograph is the same technique, but employing a zinc plate rather than a stone.
Also known as Screenprinting
The principle of screenprinting, or silkscreening, consists in applying stencils to a screen (constructed of silk or of some synthetic or metallic material), in such a way that when ink is applied it is prevented from passing through some parts while penetrating the rest of the screen, thereby printing an image on paper placed underneath. The screen is stretched across a frame and attached to a base in such a manner that it can readily move up and down, so that paper can be easily placed and removed as required. For each impression, the paper is placed against registration tabs to ensure that the printing is done in the correct position. The ink is poured over the masking at one end of the screen and when this has been lowered into position, the ink is scraped across the screen with the aid of a squeegee.
The most important part of the process is the preparation of the screen. Stencils may be applied in a variety of ways, including the use of filling-in liquid, varnish or plastic film. A drawing can be made directly on the surface with a special ink which is removed in readiness for printing after the rest of the screen has been blocked out. A photographic stencil is made by initially sensitizing the screen.
Stencils are an essential part of screenprinting: they are attached to or incorporated with the screen to ensure that the ink passes through in the correct places. They can be made in many different forms, e. g. as a simple masking or covering stencil; as a "wash-out" stencil, which involves drawing the design on the screen in a greasy substance, then covering the whole screen with filler or gum, and finally dissolving the greasy image in turns, thereby forming a 1. positive stencil; or as a photo-stencil, whereby photographic images are incorporated into the screen. 2. Stencils are also used for coloring prints by hand. Stencils of the areas to be colored are cut out in zinc or aluminum; the colors are dabbed on with a large brush (known as a pompon in French); they may be juxtaposed or superimposed over each other. The method was much used in the coloring of maps, topographical prints and devotional woodcuts. It is still used today for book illustration and on greeting cards.