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Offset prints are produced by indirect planography. In indirect printing the ink is not directly passed from the printing forme onto the printing material (paper), but onto a rubber blanket, which in turn, after contact with the printing material, offsets the picture on it. In this method the paper does not touch the printing forme.
Lithography
Lithography is a planographic method of printing, which was invented by the German Aloys Senefelder in 1798. The name derives from the Greek words for lithos (= stone) and graphein (= to write). The litho stone, a fine pored limestone, which comes from Solnhofen in Bavaria, naturally is water- and grease-friendly. With the aid of grindstones of different grains the wet stone is cut completely plane. Afterwards the design is put or sprayed on with lithocrayon and lithographic drawing ink. Then, the stone is etched. The aim of etching is to make those parts of the stone, which are covered by design, able to absorb grease, whereas those parts which are not covered by design are prepared to absorb water and to repel grease. In order to protect the design it is then covered with talcum. Afterwards a mixture of dissolved gum arabic and nitric acid is applied, which has to dry up. In this way the pores of the stone close at those parts which are not covered by design, whereas at those parts which are covered by design, the acid saponifies with the lithocrayon. Thus, crayon and ink become insoluble to water. Then, the etching is washed off and the design is again supplied with grease by applying a washing-out solution, consisting of asphalt, dissolved in oil of turpentine, virgin wax, wood tar and lavender wax. Now the design is perfectly visible on the stone. Onto the stone, which is always kept wet, the desired printing ink is now applied with a rough leather or rubber roller. Printing is carried out with the stone press. The strong pressure exerted by the scraper transmits the printing ink from the stone onto the paper. For every new ink a new stone is prepared. The number of inks to be used is unlimited, still, it is also a question of cost.
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