Lithography

Lithography is a printing process based on the principle of repulsion between oil and water. Traditionally, lithograph images are drawn with a greasy crayon onto a finely sanded slab of limestone. The oil-based printing ink sticks to the greasy areas, where the image is drawn, and water repels the ink away from the rest of the surface. These 'plates' could be reused numerous times, since washing did not remove the greasy image from the stone. Today, limestone is expensive, heavy, and in depletion, so modern practice has replaced the slab with a thin metal plate. The same greasy crayon is used to draw the image, and a chemical process removes the crayon wax itself, but leaves a water-repellent finish where the crayon was drawn. During printing, the plate is kept wet and ink attracts only to the areas where the image once was.

Photo lithography uses the same printing methods, but the image on the plate is created differently. The litho plate is covered with a photo-sensitive coating, and an image-positive is exposed to the plate under strong UV light. The image is printed or drawn onto a clear material, usual mylar or frosted mylar. It is drawn in positive, so where the light shines through the mylar (the whites) the plate is burned and repels ink.

Sources: The Met Museum