By far the most important and versatile printing[printing?:printing cutting finishing solution] process today is offset lithography. The underlying principles were established at the end of the 18th century. It was discovered that a wet limestone surface would repel an oil-based printing ink but if an image was drawn on the surface with a grease pencil it would repel water and attract ink. Thus, any drawing on the stone surface could be reproduced by bringing a damp sheet of paper[paper?:paper sample maker cutting machine] into contact with the freshly inked image. This cycle could be repeated several hundred times before the drawing could no longer be faithfully reproduced. Then, in the early part of the 20th century, it was discovered that ink could be transferred from the lithographic surface to an intermediate rubber surface and then to paper. The rubber intermediate, called a blanket, can transfer ink to paper and to a wide variety of materials that cannot be printed directly, including plastics and metals. Because the soft blanket conforms to the texture of the surface to be printed, lithographic image quality is unrivalled.
Today, thin aluminium plates and other materials, such as stainless steel and plastic[plastic?:Plastic sample cutting machine], are used. The plates are wrapped around the circumference of the printing cylinder and make direct contact with the rubber blanket cylinder. Rubber rollers carry ink and water to the plate surface. The printing plate is treated with an oil-based substance so that water will not adhere to the design. When the dampened printing plate roller is run by the inking rollers, only the design accepts ink. The ink is transferred first to the blanket cylinder and then to the paper, which is why it is called 'offset'.
