Lithography is a flat surface printing process in which an artist creates an image with a tusche, or oil-based crayon, on a porous limestone or sheet of metal. After being fixed in an acid wash, the surface of the stone is moistened. Printing ink is then applied to the stone. A sheet of paper is laid over the stone and light pressure is applied, producing a print.
Because oil and water do not mix, the oily areas drawn on the limestone when wet, repel the ink, leaving blank or clear spaces on the stone. Ink does not appear in these areas of the print.
Early lithographs used more than one stone to create the print area. Color tints were applied to each stone and printed separately. Originally, most lithographs were produced to create landscapes or topographical illustrations. As portraits, lithographs were hand-colored by the artist once the main color was printed, creating a colorized portrait with a watercolor wash effect.
Portraits could now be printed in color using several stones, one for each color. The ability to reproduce portraits and images led the way to modern offset printing, which uses metal plates to make an impression on the paper.