Lithography is a printing process based on the fact that grease and water don’t mix. The image is applied to the grained surface of a stone using a greasy medium like tusche (an ink), lithographic pencils, or crayons. A solution of gum Arabic and nitric acid is then applied over the surface, producing water-receptive non-printing areas and grease-receptive image areas. The printing surface is kept wet so that a roller charged with oil-based ink can be rolled over the surface, and ink will only stick to the grease-receptive image area. Paper is then placed against the surface of the stone and the plate is run through a press. Each color is applied in a separate process in successive layers.