To print from a flat surface is also possible. Planographic and lithographic are both terms that describe methods of printing from a flat surface.
Planographic printing is based on the principle that grease and water do not mix. The process works this way. First, a greasy image is placed on a flat plate. The image may be drawn directly on the plate with a grease pencil. It may also be placed on the plate photographically instead.
Next, water is applied to the plate. The water will cover the nonimage area of the plate. Water will be repelled from the image area because WATER AND GREASE DO NOT MIXI!
The entire plate is then coated with ink. Ink is a greasy substance and adheres to the greasy image. Ink is repelled from the wet areas of the plate because WATER AND GREASE DO NOT MIX!
Paper is then pressed against the surface of the plate and the inked image is transferred to the paper. The process of printing from a flat surface is shown in Figure 1-12.
Planographic or lithographic printing prints from a plane or flat surface, one neither raised nor depressed. The printing image is formed chemically by making some areas of the plate grease receptive and water repellent, while others remain water receptive and grease repellent.
OFFSET PRINTING. Lithographic printing is often called offset printing. Unlike plates for letterpress and gravure (which have their printing surfaces shaped to form the desired image). the printing image on a lithographic plate simply rests upon the plate's surface. The image can rapidly wear away when paper rubs against it during the printing process. This is especially true when the plates are used on high-speed printing presses. To minimize wear the image on the plate is first offset (transferred) to a rubber blanket, Figure 1-13(A). Note that the right-reading image is reversed on the blanket. Figure 1-13(B) shows how the paper receives the image from the blanket. The paper does not make contact with the printing plate. The entire offset lithographic process is diagrammed in Figure 1-13(C).
Alois Senefelder of Germany discovered the lithographic printing method in 1796. He printed directly on paper from a grease image applied to a heavy piece of limestone. Stone lithography is slow and tedious.
Today a wide variety of lithographic plates is used on high-speed offset presses and duplicators to print move products than any other printing method.