press runs, and produces a superior grade of printing. The process
started as hand drawings on a flat limestone with greasy crayons. The
area covered by the greasy crayon rejected the water when flushed, and
retained only the oily ink. Pressing the paper against the inked
surface created the printed copy. You can still identify modern
lithography by its characteristic flat plane with the image on the
surface of the plane.
From drawings on stone, this process has moved to photoprinting on
sensitized metal. A lithographic plate is a thin, flat, chemically
treated zinc or aluminum metal surface. Because its surface has a
photosensitive grease chemical treatment, the image remains in a
chemical form on the plate, after photographically exposing a negative
to the plate. The chemical areas exposed to the light (the clear areas
of the negative) remain after developing and washing. The chemical
areas not exposed to the light (the black areas of the negative) wash
away exposing the bare metal surface. As the principle of lithography
states, the photosensitive chemical image repels the water and accepts
the ink. The bare metal surface accepts the water and repels the ink
(figure 225).
Figure 225. Producing a photolithographic printing plate
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