Lithography is a printing process that uses chemical processes to create an image. It works because of the repulsion of oil and water. An image is drawn on the surface of a stone printing plate with an oil-based, or hydrophobic, medium, and then the rest of the plate is covered with an acid emulsion that seeps into the pores of the stone and creates a salt layer in all the negative space of the image. This process is called etching. Using lithographic turpentine, the oil-based medium is removed, leaving only the salt layer as a skeleton of the image's original form. When printing, the stone plate is kept wet with water, and the water gravitates to the layer of salt created by the acid wash. Ink that bears a high lipid content is then rolled over the surface. The water repels the grease in the ink and the only place for it to go is the cavity left by the original drawing material. When the cavity is sufficiently full, the stone and paper are run through a press which applies even pressure over the surface, transferring the ink to the paper and off the stone.
The basics of this process have remained unchanged since lithography was invented in 1797 by an Austrian actor and playwright. The process is used today to mass-produce books, posters, cd covers, and many other smooth printed items. And today, flexible aluminum or plastic printing plates are used in place of stone. The image is transferred from the plates to a drum covered with what is called a rubber blanket, and from there transferred to paper – this double transfer gives us the commonly used term "offset printing".