Lithography, the art of producing pictures or writing on stone in such a way that impressions may be taken from it in ink by a method similar to that employed in ordinary printing. Lithography was invented at Munich by Aloys Senefelder in 1796. The physical facts which render it possible are the adhesive character of greasy matter when brought into contact with calcareous stone, the mutual affinity of greasy substances and their antipathy to water, and the readiness with which water is absorbed by calcareous stone. The best stone for the purpose is composed of clay, lime, and siliceous earth, which is quarried at Solenhofen, in Bavaria. A clear grey colour is preferable to any other. The most important processes by which the design is produced are the following: (1) By direct drawing. The design may be executed either with a crayon or by applying ink with a pen or brush. The ingredients used in the preparation of the inks are lard, hard soap, shelllac, white wax, carbonate of soda, Venetian turpentine, and Paris black. Chalk-drawing is the most artistic kind of lithography, but it is now almost entirely superseded by more convenient processes. The first step is to grind the stone with a little fine gravel sand and water, and to pumice it. A chalk drawing is first traced in outline with the crayon, and then tinted or shaded. When the drawing is complete, the stone is etched, the same means being employed in both processes. It consists in the application of a weak solution of nitric acid in gum-water, which removes the grease-oils from the stone, brings out the lines of the drawing, and lays bare the pores in the unmarked portions, so that they may more easily imbibe a solution of gum-arabic in water with which the stone is now flooded. When this has been removed, the printer "washes out" the picture with turpentine, leaving only a faint white impression, but, though the ink has disappeared, the grease remains in the stone. A damp towel is now applied to the stone, and the inking-roller is passed over it, the greasy lines absorbing the ink, whilst the wetted and gummed surface remains free from it. An impression of the design is then taken on a piece of paper, which is passed through the printing-press.
(2) Transfer from paper, or from another stone. The design is executed in lithographic transfer ink on paper which has received a coating of isinglass, flake-white, and gamboge. The paper is then damped and pulled through a press with the coated side towards a heated stone. After this the paper is removed, whilst the ink remains adhering to the stone. The subsequent treatment is identical with that in the preceding process. Chalk drawings also are now usually executed in this manner, the design having been first of all drawn with lithographic chalks on paper to which a grain has been given by mechanical means.
(3) Engraving on stone. A thin coating of gum is placed on the surface of a polished stone, and coloured by rubbing in Paris black or some other pigment. The design is then cut through the guui on the stone by means of steel needles fixed in cane handles. The stone is next covered with oil or some other greasy matter, which passes into the incisions formed by the needles. The stone is afterwards treated in the same manner as in the processes already described, except that in place of a roller, an instrument called a "dabber" or "dauber" is used to rub the ink into the lines.
(4) Chromo-lithography. By this process coloured pictures are produced, a separate stone being used for each colour and tint required. Usually the whole outline is first drawn on, or transferred to a stone called the keystone, and from this again it is transferred to as many other stones as may be required, thus enabling the artist to determine the outline of each of the colours, and prevent them overlapping. The unnecessary lines can be easily removed in each case by the use of water. In one of the impressions taken from the key-stone the entire outline is retained, it is given a neutral grey tint, and is made the basis of the intended picture. There are various methods of "registering," i.e. securing the exact correspondence of the outline on each of the stones.
(5) Photo-lithography. By this process it is possible to obtain copies of drawings executed in clear lines or dots. The drawing is photographed on glass, specially-constructed lenses being employed. The negative is inserted in a photographic printing-frame, and an impression is taken from it on a piece of sensitive transfer-paper, and thence transferred to a stone, after which the printing is effected in the manner already described.