Photolithography is a lithographic reproduction process by means of which exposures are made on a working material. A half-tone negative is exposed for several hours on a stone plate coated with light-sensitive asphalt. The effect of the light is to harden the asphalt. The unexposed parts are removed with oil of turpentine. Depending on the half-tone value, a grainy surface is created. Thus the original print quality characteristic of this printing technique is achieved. Robert Rauschenberg was particularly fond of using photolithography in his work.