Stone printing, the original form of lithography introduced in 1798 by Aloys Senefelder, is a form of planography, the process of printing from a flat plate. The process is based on the principle that water and grease do not mix. Generally with stone printing, an image is drawn using a greasy medium that will adhere to the surface of the stone, such as a crayon. All blank areas on the stone are prevented from absorbing grease by the application of a solution of gum arabic and nitric acid.
At this point the stone is ready for inking. The stone is sponged wet and ink is applied with a large hand roller. The moistened areas resist the ink, but the drawing accepts it. This step is repeated until the ink buildup is sufficient for printing. Stone printing is perhaps the most sensuous of all printing media because of its unique response to the artist's hand.
In Petersen's lithographs, the entire stone is a possible field of action. At least three elements merge in the exploration for patterns, forms and images in that area: the lines of the drawing, the combinations and variations of colors, and the results from the process of printing itself, i.e., the distribution of color. Like Noh, classical Japanese performance that combines drama, music and poetry, there is a central square of activity, but interesting things often happen around the edges or in other spaces as well. These spaces define that central area of activity more than might be obvious at first.
For some people, including Petersen, stone prints represent more than a lithographic function, and bring to mind a line from Gary Snyder's Myths and Texts:
The thin edge of nature rising fragile
And helpless with its love and sentient stone.
Petersen's concept of the stone is more primal, with the stone becoming a Mesolithic monument testifying to a prehistoric age of greater spiritual intensity and purity.

This information is excerpted from the catalog of the exhibition "Stone-Prints 1963-1976" at Swarthmore College, Swarthmore, Pennsylvania, in 1976.