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All planographic printing is based on chemical action, and lithography is based on the mutual antipathy of oil and water. As the name [Gr.,=writing on stone] implies, a lithograph is printed from a stone (except in commercial processes, where grained metal or plastic plates are employed). The process was invented c.1796 by the playwright Aloys Senefelder, and the Bavarian limestone that he employed is still considered the best material for art lithography.
The slab of stone is ground to a level surface, which may be of coarse or fine texture as desired. The drawing is made in reverse directly on the stone with a lithographic crayon or ink that contains soap or grease. The fatty acid of this material interacts with the lime of the stone to form an insoluble lime soap on the surface, which will accept the greasy printing ink and reject water. Accordingly, those parts of the stone that have been drawn upon have an affinity for ink.
Sometimes the drawing is made on paper and transferred to a heated stone by pressure. This is known as a transfer lithograph and does not require the artist to reverse his or her drawing. Next, the surface of the stone untouched by grease is desensitized to it, and the portions drawn upon are fixed against spreading by treatment with a gum arabic and nitric acid solution. The grease has now penetrated the stone, and the drawing is washed off with turpentine and water. The stone is ready to be inked with a roller and printed, but it must be kept moist. The printing requires a special lithographic press with a sliding bed passing under a scraper.
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