LITHOGRAPHY. 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 engra ving on wood gives its results from the pro jecting 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.
There are various styles of lithography, but the principle of the art is uniformly the same.
The stone best calculated for lithographic purposes is a sort of calcareous slate, found in large quantities on the banks of the Danube in Bavaria. Stones much resembling the German have been found in some parts of Devonshire and Somersetshire, and also in Ireland ; but they want some of the most essential qualities of those brought from Germany, which are therefore almost exclu sively used. Even these vary much in quality, all the strata not being equally good. A good stone is porous yet brittle, of a pale yellowish drab, and sometimes of a gray neutral tint. The stones split into slabs varying from 14 to 2i inches in thickness, 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 the stones are 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. Grained stones are used for drawings in the chalk manner, or for imitations of those produced with the black-lead pencil. Writings, imita tions of etchings, pen and ink sketches, &c., require the face of the stone to be polished, which is effected by rubbing 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, soap, shell-lac, and enough lamp-black 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 ingredi ents are the same in the chalk and the ink, but the proportions are varied. 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 is the presence of the soap in this greasy material which renders it soluble in water.
To render the lithographic process intelligi ble, let it be supposed that the artist completes a drawing with the chemical chalk just de scribed, upon a grained stone. If, while in this state, a sponge filled with water were passed over the face of the stone, the &awing would wash out, the chalk with which it is made being, as we have seen, soluble in water, by reason of the soap which it contains.
Before, therefore, it is capable of yielding impressions, a weak solution of nitrous acid is poured over it, which unites with and neu tralises the alkali or soap contained in the chalk, and renders it insoluble in water. After this, the usual course is to float a solution of gum over the whole face of the stone; and when this is removed, if a sponge and water be applied to its surface, as before supposed, the drawing is found to be no longer removes ble, because the chalk with which it is executed is now no longer soluble in water. In this state the work is ready for the printer, who obtains impressions by the following pro cess.
Having thrown with the ends of his fingers a few drops of water on the stone, and spread them with a sponge, so as to wet, or rather damp, the whole surface equally, the printer finds that the water has been imbibed by the stone only on those parts not occupied by the drawing, which, being greasy, repels the water and remains dry. A roller properly covered with printing ink is now passed over the whole stone, which will not even be soiled where it is wet, from the antipathy of oil and water. But the parts occupied by the drawing, being, as we have seen, dry and greasy, have an affinity for the printing ink, which therefore passes from the roller, and attaches itself to the drawing. In this state it is said to be charged, or rolled in. Damped paper is then put over it, and the whole being passed through a press, the printing ink is transferred from the stone to the paper, and this constitutes the impression. By repeating in this manner the operations of damping the stone and rolling in the drawing, an almost unlimited number of impressions may be obtained.
The modes of lithography are, as we have said, various, but the illustration just given will explain the principle of them all. It con sists in the mutual antipathy of oil and water, and the affinity which the stone has for both, i. e. in its power of imbibing either with equal avidity. Some of the coloured litho graphs now produced are exquisite produc tions.
To insure complete success, great nicety is requisite in the preparation of all the agents employed in this art. Those who wish to study or practice the art in its full extent will do well to consult A Complete Course, of Lithography,' by its discoverer, M. Senefelder, or 'A Manual of Lithography,' by 31. Raucourt, both translated into English.
Plates of zinc have lately been much used as substitutes for the German stones, in che mical printing, and the practice is then called zincography : but, excepting the difference of the material on which the work is per formed, it is precisely the same art as litho graphy.