To render the lithographic process intelligible, let it be supposed that the artist now completes a drawing with the chalk just described, upon a grained stone, much as he would make a drawing in pencil or chalk upon paper. if, while in this state, a sponge filled with water were passed over the face of the stone, the drawing 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 neutralises 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 atone, 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 removable, bemuse 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 process.
Having with a sponge filled with water, to which an extremely small quantity of acid has been added, wetted, or rather damped, the whole surface of the stone 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 tho antipathy of oil and water. But the parts occupied by the drawing being, as we have seen, dry and grmsy, have an affinity for the printing-ink, which there fore 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 orer it, and the whole being passed through a preen, 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 aimed unlimited number of impressions may be obtained.
Now, as we have said, the modes of lithography are 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, that is, in its power of imbibing either with equal avidity. But, as will be inferred, to ensure complete success, Freud, nicety is requisite in the preparation of all the agents employed in this art, and in the process of printing, as well as in waking the drawing on the stone, Imitations of etchings or pen and ink drawings, writings, &c., executed with the chemical ink upon a polished stone, are prepared and printed in precisely the same manner.
Transfer lethogrophy, or Autography, from the facility of its execution and its great utility, claims especial notice. A paper called transfer paper or "autographic paper," is used to receive the writing in this mode of lithography. This paper has previously had a liquid gummy preparation washed over one aide of it, which, when dry, constitutes its face, and the writing being performed with a modification of the chemical ink already described, reels upon this gummy surface, and does not penetrate to the paper beneath. When the writing or drawing done on the transfer-paper is dry, the back of it is wetted slightly but equally with a sponge and water ; and the paper being very thin, the gum preparation on the front of it becomes at once partially dissolved. In this state the paper is laid, with its face downwards, on a polislad atone, and being passed through a press, the transfer-paper is found strongly and closely adhering to it. The back of the transfer paper is now well wetted, and, one corner being first raised, it will readily peel off, leaving the gum preparation, and of course also the writing which was above it, attached to the stone. The gum is then washed off with water, and the preparation and printing proceed in the manner already explained. This mode of lithography being eminently calculated to facilitate the despatch of business, its great utility has been sensibly felt in the commercial departments of the country and in several of our government offices, as by its means one written despatch can be multiplied at pleasure, without delay or the risk of typographical errors.
Another style of lithography is commonly called "etching," or "en raring en atone," although it must by no means be inferred that in this process the stone is incised, but that the results correspond with the clean-cut lines of the graver or etching-needle. A coat of gum water, with some lamp-black or vermilion mixed with it, to give it colour and render the work visible, is thinly but evenly rubbed over a pofiaAed stone, and, when dry, effectually protects it from any applica tion of grease. On this gum ground the design is executed with an etching-needle, precisely the same as in etching upon copper ; and wherever the needle passes, of course the atone is laid bare, and it is best to cleanly remove the gum ground without cutting into the atone. After this some oil is rubbed over the whole surface, and is imbibed by the stone wherever the needle has passed and exposed its surface by removing the ground. The gum is then washed off, and the work may be at once rolled in and printed, without any previous acidulation.