Paper is then placed against the surface and the plate is run through a press. 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. 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.
Photochemical or transfer processes can also be used. The image is applied to a grained surface (traditionally stone but now usually aluminium) using a greasy medium: such as a special greasy ink – called tusche, crayon, pencils, lacquer, or synthetic materials. A printing process based on the fact that grease and water don’t mix.