Preparation of the stone for printing.—The drawing being finished on the stone, it is sent to the lithographic printer, on whose knowledge of his art depends the success of the impressions. The first process is to etch the draw ing as it is called. This is done by placing the atone obliquely on one edge, over a trough, and pouring over it very dilute nitric acid. It is poured on the upper part of the stone, and runs down all over the surface. The stone is then turned, and placed on the opposite edge, and the etching water being collected from the trough, is again poured over it, in the same manner. The degree of strength, which is usually about one per cent. of acid, should be such as to pro duce a very slight effervescence ; and it is desirable to pass the etching water two or three times over the darkest parts of the drawing, as they require more etching than the lighter tints. Experience alone can, however, guide the litho grapher in this department of the art, as different stones, and different compo sitions of chalk, will be differently acted upon by the acid; and chalk drawings i require a weaker acid than the ink. The stone is next to be carefully washed, by pouring clean rain water over it, and afterwards with gum water; and when not too wet, the roller charged with printing ink is rolled over it in both direc tions—sideways, and from top to bottom—till the drawing takes the ink. It is than well covered over with a solution of gum Arabic in water, of about the consistency of oil. This is allowed to dry, and preserves the drawing from any alteration, as the lines cannot spread, in consequence of the pores of the stone being filled with the gum. After the etching, it is desirable to leave the stone for a day, and not more than a week, before it is printed from. The effect of the etching is first to take away the alkali mixed with the chalk or ink, which would make the drawing liable to be affected by the water • and secondly, to make the stone refuse more decidedly to take any grease. The gum assists in this latter purpose, and is quite essential to the perfect preparation of the sur face of the stone.
Printing.—When the intention is to print from the stone, it is placed upon the platten or bed of the and a proper sized scraper is adjusted to the surface of the stone. Rain water is then sprinkled over the gum on the stone, which, being dissolved gradually, and a wet sponge passed lightly over all, the printer works the ink, which is on the colour table placed beside him, with the roller, in all directions, until it is equally and thinly spread on the roller. The roller is then passed over the whole stone, care being taken that the whole draw ing receives a due portion of ink ; and this must be done, by giving the roller an equal motion and pressure, which will of course require to be increased, if the drawing does not receive the ink readily. When the drawing is first used, it will not receive the ink so readily as it will afterwards ; and it is frequently necessary to wet the stone, and roll it several times, before it will take the in easily. After this takes place, care must be taken not to wet the stone too much; the dampness should not be more than is necessary to prevent the ink adhering to the stone where there is no drawing. After the drawing is thus rolled on, the sheet of paper is placed on the stone, and the impression taken.
Upon taking the paper off the stone, the latter appears to be quite dry, owing to the paper having absorbed the moisture on the surface • it must therefore be wetted with a sponge, and again rolled with ink, the roller having been well worked on the colour table before being applied. During the printing, some gum must always remain on the stone, although it will not be visible, other wise the ink will be received on the stone as well as on the drawing, by which the latter would be spoiled ; so that if by too much wetting, or by rubbing too hard with the sponge, the gum is entirely removed, some fresh gum water must be laid on. If the stone has in the first instance been laid by with too small a quantity of gum, and the ink stains the stone on being first applied to it, gum water must be used to damp the stone, instead of pure water. Some times, however, this may arise from the printing ink being too thin, as will afterwards appear. If some spots on the stone take the printing ink, notwith standing the above precautions, some strong acid must be applied to them with a brush, and after this is washed off, a little gum water is dropped in the place. A steel point is here frequently necessary to take off the spots of ink. The edges of the stone are very apt to get 'soiled, and generally require to be washed with an old sponge after rolling in ; they must also frequently have an application of acid and gum, and sometimes must be rubbed with pumice stone. If an ink is too thin, and formed of a varnish not sufficiently burned, it will soil the stone, notwithstanding the proper precautions are taken of wet ting the stone, and preparing it properly with acid and gum ; and if, on the other band, the ink is too thick, it will tear the lighter tints of the chalk from the stone, and thus destroy the drawing. The consideration of these circumstances leads at once to the— Principles of the Printing.—The accidents just mentioned aria' e at the extreme points of the scale at which the printing inks can be used, for it is evident that the only inks that can be used are those which are between these points; that is, thicker than that which soils the stone, and, at the same time, thinner than that which takes up the drawing. Lithographers are sometimes unable to print in very hot weather, the reason of which may be deduced from the foregoing. Any increase of temperature will diminish the consistency of the printing ink ; the atone will therefore soil with an ink which could be safely used at a lower temperature ; hence a stiffer ink must be used. Now, if the temperature an much that the stone will soil with any ink at all less thick than that which will take up the drawing, it is evident that the print ing must cease till a cooler temperature can be obtained ; for as the drawing chalk is effected equally with the printing ink, the same ink will tear up the drawing at the different degrees of temperature. This, though it sometimes occurs, is a rare case; but it shows that it D3 desirable to draw with a chalk or ink of less fatness in summer than in winter; and also, that if the printing room is in winter artificially heated, pains should be taken to regulate the heat as equally as possible.