LITHOGRAPHY, the art by which impressions or prints are obtained by a chemical process from designs made with a greasy mate rial upon stone. It has therefore been properly termed chemical printing, to distinguish it from all other modes of obtaining impres sions, which are mechanical. In printing from an engraving on a copper or steel plate, the ink is delivered from the incisions made therein with the graver or etching-needle. An engraving on wood, on the contrary, gives its results from the projecting surface of the block, or those parts which are not cut away by the graver. The lithographic process differs from both these modes, the impressions being obtained (by strict attention to chemical affinity) from a level surface. The art of lithography was first practised by the inventor Alois Senefelder, in 1798, the peculiar property on which it depends having being dis covered by him a few years earlier. [SENEFELDER, ALOIS, in BIOG. Div.] There are various styles of lithography, as will presently be seen ; but the principle of the art is uniformly the same, being, as we have said, based upon that of chemical affinity.
The stone best calculated for lithographic purposes is a sort of cal careous slate, found in large quantities on the banks of the Danube in Bavaria, the finest being that found at Solenbofen, near Munich. Stones much resembling the German have been found in seine parts of Devonshire and Somersetshire, and also in Ireland ; but the stones found in this kingdom have been proved to want some of the essential qualities of those brought from Germany, which are therefore, at least for artistic works, exclusively used. Even the Bavarian stones vary much in quality, all the strata not being equally good : some are too soft, and others are rendered unfit for use by the presence of chalk, flaws and veins, and fossil remains. A good stone is porous yet brittle, of a pale yellowish-drab, and sometimes of a gray neutral tint. The stones are split or sawn into slabs varying from to 3 inches in thick ness, which are then cut or squared into the different sizes necessary for use, and the face or upper surface of each is made level. In this
state are the stones sent from the quarry ; but further preparation is yet necessary to fit them for the immediate use of the artist, and they are either grained or polished, according to the nature of the work they are intended to receive. The mode of preparing a grained stone, as it is called, is this :—A stone, being laid flat on a table, has its sur face wetted, and some sand sifted over it through a very fine wire sieve. Another stone is laid with its face downwards upon this, and the two are rubbed together with a circular motion, to produce the requisite granulation, which is made finer or coarser, to suit the taste or intention of the artist. The stones thus prepared are used for drawings in the chalk manner, or for imitations of those produced with the black-lead pencil. Great care is requisite in this mode of preparation, to keep the granulation uniform and the surface free from scratches, the presence of which would otherwise much disfigure the future work. Writings, imitations of coloured drawings, etchings, pen and ink sketches, &e., require the face of the stone to be polished, which is effected by it with pumice-stone and water ,or pumice stone dust and water applied with rags.
The two principal agents used for making designs, writings, &c., on stone, are called lithographic chalk and lithographic ink. They are composed of tallow, virgin-wax, hard tallow soap, shell-lac, sometimes a little mastic or copal, and enough lampblack to impart a colour to the mass. These are incorporated by a peculiar process of burning in a closely-covered saucepan over a fire, and the whole is ultimately cast into a mould, and receives the form calculated to fit it for use. The ingredients are the same in the chalk and the ink, but the proportions are varied, and a little Venice turpentine is often added in the latter. The chalk is used as it comes from the mould in a dry state, but the ink is dissolved by rubbing, like Indian ink, in water, and is used in a pen or camel-hair pencil. It will be perceived that it is the presence of the soap in this greasy material which renders it soluble in water.