Lithography

The technique of lithography is based on the principle that grease and water don’t mix. The artist creates an image using greasy materials in the form of pencils, crayons and tusche, a medium that can be mixed with water or solvent to form delicate washes. The image may be drawn onto a metal plate made of zinc or aluminium that has been ‘grained’ to give its surface a tooth. However, by far the most attractive surface to work on is the Bavarian limestone that is used in stone lithography.

The stone is prepared by regraining its surface with progressively finer grades of carborundum. This process is done wet and either a second stone or a flat metal weight called a levigator is used. The surface of the stone must be grained until it’s perfectly flat to ensure maximum print quality. Any dips or bumps will show up in the printed image.

Once suitably smooth, the surface is dried and the stone is ready to take the drawing. A wide range of marks can be made using pencil, crayon, tusche, or by scraping back into drawn areas to expose the surface of the stone. Care must be taken not to touch the surface during the drawing up process because any greasy marks, like fingerprints, will form part of the printed image.

When the drawing is complete, the exposed areas of the stone must be desensitized for the printing process. Firstly, the image is protected by dusting it with rosin and then talc.

Next an etching solution consisting of gum arabic and nitric acid is applied to the entire surface of the stone. The acidity of the etch is varied according to the delicacy of the drawing. The surface is buffed with cheesecloth until the gum forms a thin film, which is left to dry thoroughly.

Once dry, the image is washed-out using pure turpentine and the grease content of the drawing is reinforced by rubbing in liquid asphaltum. Throughout this process, the exposed areas of the stone are protected by the gum arabic solution.

The gum arabic etch is washed off and the image is inked-up with a non-drying ‘Process Black’ lithographic ink. The surface of the stone is kept damp during the inking-up to ensure that only the image surface accepts the ink. The image is then dried and the etching process repeated.

To print the image, the stone is washed-out and rolled-up with litho printing ink. Again this is done with the stone being kept sufficiently damp to ensure that the blank surface areas repel the greasy ink.

The printing paper is placed carefully over the image and covered with a backing sheet. The stone goes through the press and the image is transferred onto the paper.