Lithography is a printing technique based on the repulsion of oil and water. The artist draws the image on a flat stone surface using a greasy litho crayon or a greasy black ink.
The drawing is fixed with an ‘etch’ (gum arabic and nitric acid) to prevent the grease from spreading. Because of the repulusion of grease and water the image attracts the oily ink but repels the water so when the surface is inked the ink sticks to the greasy drawing and not the wet stone, and so now can be transferrred to paper. Lithography is noted for its ability to capture fine detail and subtle differences in tonal shading. Photo-lithography is a more modern process of making a lithographic plate using photographic methods and the stone has been replace by metal plates. A drawing is made onto a transparent plastic film (Trugrain) which is then put on a exposure unit with a light-sensitive aluminium plate (Toray Plate) over it. The plate is exposed to UV light through the drawing.
The more the exposure the lighter the areas of the drawing become. The less exposure the more greys are retained. Once the plate is developed to the desired effect it is ready to be inked up and put through a printing press.