Lithography using limestone slabs
Lithography exploits the fact that oil and water do not mix. It is a planographic printing technique i.e., the picture and the blank ‘undrawn’ area of the lithographic stone are on the same level.
Preparing the stone: The previous drawing has to be removed and the stone has to be given the required grain. This is done manually by grinding the surfaces of two stones against one another. The lower stone is coated with a sludge of coarse silicon carbide (carborundum) and the upper stone is moved on top of it in a regular motion over the whole surface. It is essential that the surface of the lower stone remains flat. A less course carborundum is used in the later stages, i.e., once the picture has been removed, to give the stone the required grain.
Drawing the picture: The mirror image of the desired picture is drawn using special lithographic ink or crayons. These contain grease or soap (fatty acids). The upper surface of the stone should not be touched by hand as a finger print could show up on the final print.
Processing the stone: When the picture is finished the whole stone is coated with a thin film of gum arabic acidified with nitric acid (2%) and left for at least 8 hours. A hydrophillic layer of calcium nitrate and gum arabic is formed on all blank areas of the stone. The stone is then wiped with a cotton rag soaked in terpentine. This removes most, but not all, of the greasy drawing materials; a thin hydrophobic film remains tightly bound to the limestone surface.
Printing with a hand press: The stone is transferred to the steel bed of the press and dampened with water. Oily printing ink is then rolled out evenly over the whole surface of the wet stone. The blank hydrophilic areas repel the ink, hydrophobic regions where the drawing was accept it. A sheet of paper is placed over the stone and covered by several sheets of soft backing paper, and finally by a sheet of perspex with a lubricated upper surface. The teflon scraper of the press is lowered onto the perspex and applies an even pressure while the stone and its covering paper and perspex layers are manually drawn through the press with an even motion.
The first few proofs vary, becoming darker as more ink is retained by the stone. Optimum printing conditions are then attained and retained until the stone suddenly starts to accept too much ink.
A new stone has to be prepared for each colour of a lithograph.
Further information can be found in Wikipedia under Lithography and Lithographic_limestone