LITHOGRAPHY, is the art of throw: ing off impressions, upon paper, of fig ures and writing previously traced upon stone. It has been partly treated of un der the bead " engraving." The pro cesses of this art are founded : 1. Upon the adhesion to a smoothly-po lished limestone of an encaustic fat which forms the lines or traces.
2. Upon the power, acquired by the parts penetrated by this encaustic, of at tracting to themselves, and becoming co vered with a printer's ink, having linseed oil for its basis.
3. Upon the interposition of a film of water, which prevents the adhesion of the ink in all the parts of the surface of the stone not impregnated with the en caustic.
4. Lastly, upon a pressure applied by the stone, such as to transfer to paper the greater part of the ink which covers the greasy tracings of the encaustic.
The lithographic stones of the best quality are still procured from the quarry of Solenhofen, a village at no great dis tance from Munich, where this mode of printing had its birth. They resemble in their aspect the yellowish white Has of Bath, but their geological place is much higher than the lias. Abundant quarries of these fine-grained limestones occur in the county of Pappenheim, along the banks of the Danube, presenting slabs of every required degree of thickness, part ed by regular seams, and ready for remo val with very little violence. The good quality of a lithographic stone is generally denoted by, the following characters : its hue is of a yellowish gray, and uniform throughout ; it is free from veins, fibres, and spots ; a steel point makes an im pression on it with difficulty ; and the splinters broken off from it by the ham mer display a conchoidal fracture. A new locality affording fine stone is at Bel beze, Haute Garonne, in French Pyrenees : they are found in the chalk formation, which is a peculiarity. This Continent does not as yet appear to contain any li-' thographic stone of good quality.
The Munich stones are retailed on the spot in slabs or layers of equal thickness ; they are quarried with the aid of a saw, BO as to sacrifice as little as possible of the irregular edges of the rectangular ta bles or plates. One of the faces is
then dressed and coarsely smoothed. The thickness of these stones is nearly proportional to their other dimensions ; and varies from an inch and two-thirds to 8 inches.
In each lithographic establishment, the stones receive their finishing, dressing, and polishing ; which are performed like the grinding and polishing of mirror plate. The work is done by hand, by rubbing circularly a movable slab over i another cemented in a horizontal position, i with fine sifted sand and water nterpos ed between the two. The style of work that the stone is intended to produce de termines the kind of polish that it should get. For crayon drawing the stone should be merely grained more or less fine, ac cording to the fancy of the draughtsman.
The higher the finish of the surface, the softer are the drawings ; but the printing process becomes sooner pasty, and a smaller number of impressions can be taken. Works in ink require the stone to be more softened down, and finally po lished with pumice and a little water. The stones thus prepared are packed fbr use with white paper interposed between their faces.
Lithographic crayons.—Fine lithogra phic prints cannot be obtained unless the crayons possess every requisite quality. The ingredients composing them ought to be of such a nature as to adhere strong ly to the stone, both after the drawing has undergone the preparation of the acid, and during the press-work. They should be hard enough to admit of a fine point, and trace delicate lines without risk of breaking. The following compo sition has been successfully employed for crayons by M M. Bernard. and Delarue, at Paris :— Pare wax, (first quality) 4 Dry white tallow soap White tallow Gum lac 2 Lamp black, enough to give a dark tint 1 Occasionally copal varnish 1 A simpler ink is as follows : White soap 6 parts, white wax 6 parts, lamp black 1 part, well fused together.