Other Dilj'iculties in Printing, not referable to the foregoing general Principle.— If the pressure of the scraper be too weak, the ink will not be given off to the paper in the impression, although the drawing has been properly charged with it Defects will also appear from the scraper being notched, or not correctly adjusted, or from any unevenness in the leather or paper. After printing a considerable number of impressions, it sometimes happens that the drawing takes the ink in dark spots in different parts. This arises from the printing ink becoming too strongly united with the chalk or ink of the drawing, and if the printing be continued, the drawing will be spoiled. The reason of this is easily ascertained. The printing ink readily unites with the drawing, and being of a thinner consistency, it will, by repeated applications, accumulate on the hoes of the drawing, soften them, and make them spread. In this case, it is necessary to atop the printing, and let the atone rest for a day or two, for the drawing to recover its proper degree of hardness. If the drawing should run smutty from any of the causes before enumerated, the following— Mixture for cleaning the Drawing while printing must be used.—Take equal parts of water, spirits of turpentine, and oil of olives, and shake them well together in a glass phial, until the mixture froths ; wet the atone, and throw this froth upon it, and rub it gently with a soft sponge. The printing ink will be dissolved, and the whole drawing will also disappear, -though, on a close examination, it can be distinguished in faint white lines. On rolling it again with printing ink, the drawing will gradually re-appear, as clear as at first.
Bleached Paper unfit for Lithographic Printing.—Aecidents sometimes occur in the printing from the qualities of the paper. If the paper have been made from rags which have been bleached with oxy-muriatic acid, the draw ing will be incurably spoiled after thirty impressions. Chinese paper has some times a strong taste of alum ; this is so fatal, as sometimes to spoil the drawing after the first impression. When the stone is to be laid by after printing, in order that it may be used again at a future period, the drawing should be rolled in with a— Preserving Ink; as the printing inks would, when dry, become so hard, that the drawings would not take fresh printing ink freely. The following is the composition of the printing ink :—Two parts of thick varnish of linseed oil, four parts of tallow, one part of Venetian turpentine, and one part of wax, These must be melted together, then four parts of lamp black, very carefully and gradually mixed with it, and it must be preserved for use in a close tin box.
afukgropkic Ink, or that which is suitable for transferring on to the stone the writings or drawings which have been executed on paper prepared for that purpose, should possess the following properties. The ink ought to be mellow, and somewhat thicker than that used immediately on stone ; so that when it is dry on the paper, it may still be sufficiently viscous to cause adherence to the stone by simple pressure. The following is the com position. Dry soap, and white ' wax free from tallow, each 100 drachms, mutton suet, 90 drachms, shellac and mastic, each 50 drachms, lamp black, 30 to 35 drachms ; these materials are to be melted in the way described for litho graphic ink. (See INK, LITHOGRAPHIC.) "telegraphic Paper.—The operation by which a writing or drawing is trans
ferred from paper to stone, not only affords the means of abridging labour, but also of producing the writings or drawings in the same directions in which they have been traced ; whereas, when they are executed immediately on stone, they must be performed in a direction opposite to that which they are eventually to have. Thus it is necessary to draw those objects on the left, which, in the impression, are to be on the right hand. To acquire the art of reversing subjects when writing or drawing, is both difficult and tedious ; while, by the aid of transparent, and of autographic paper, impressions may be readily obtained in the same direction as that in which the writing or the drawing has been made. In order to make a transfer on to stone of a writing, a drawing in lithe graphic ink, or in crayons, or an impression from a copper plate, it is necessary 1st, that the drawing or transcript should be on a thin and flexible substance, such as common paper ; 2d, that it should be capable of being easily detached from this substance, and transferred entirely on to the stone, by means of pres sure. But as the ink with which a drawing is traced penetrates the paper to a certain depth, and adheres to it with considerable tenacity, it would be difficult to detach them perfectly from each other, if, between the paper and the draw ing, some substance was not interposed, which, by the portion of water which it is capable of imbibing, should so far lessen their adhesion to each other, that they may be completely separated in every point. It is to effect this that the paper is prepared, by covering it with a size, which may be written on with facility, and on which the finest lines may be traced without blotting the paper. Various means may be found of communicating this property to paper. The fol lowing preparation has always been found to succeed, and which, when the opera tion is performed with the necessary precautions, admits of the finest and most delicate lines being perfectly transferred, without leaving the faintest trace on the paper. For this purpose, it is necessary to take a strong, unsized paper, and to spread over it a size prepared of the following materials : starch, 120, gum arabic, 40, and alum, 21 drachms. A moderately thick paste is made with the starch, by means of heat ; into this paste is thrown the gum Arabic and the alum, which have been previously dissolved in water, and in separate vessels. The whole is mixed well together, and it is applied warm to the sheets of paper, by means of a brush, or a large flat hair pencil. The paper may be coloured by adding to the size a decoction of French berries, in the proportion of ten drachms. After having dried this autographic paper, it is put into a press, to flatten the sheets, and they are made smooth by placing them, two at a time, on a stone, and passing them under the scraper of the lithographic press. If, on trying this paper, it is found to have a tendency to blot, this inconvenience may be remedied by rubbing it with finely-powdered sandarac. Annexed is another recipe, which will be found equally useful, and which has the advantage of being applicable to thin paper, which has been sized. It requires only that the paper be of a firm texture ; namely, gum tragacanth, 4 drachms ; glue, 4 ; Spanish white, 8 ; and starch, 4 drachms.