With offset lithography, the image is reversed twice, and appears on the final sheet the same way round as on the stone or plate. The process is ‘offset’ because the plate does not come in direct contact with the paper, which preserves the quality of the plate. lithography, planographic printing process that makes use of the immiscibility of grease and water. Offset lithography involves printing the image onto an intermediate surface before the final sheet. It also made colour printing easier: areas of different colours can be applied to separate stones and overprinted onto the same sheet. Its invention made it possible to print a much wider range of marks and areas of tone than possible with earlier printmaking relief or intaglio methods. Lithography was invented in the late eighteenth century, initially using Bavarian limestone as the printing surface. 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.
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