The fatty substances which are transferred to the stone by writing, drawing, or transfer, are autographic ink, lithographic ink, and chalk, the so-called transfer and photo-lithographic colours. There are also other substances, which are used especially in photo-lithography, which do not evidently penetrate the stone and which cannot be considered as greasy substances, but which, as we shall see later on, have exactly the same action.
The above-mentioned fatty substances actually penetrate the porous stone and make it in places suitable to take more printing ink. By the so-called etching—dilute nitric acid mixed with gum water—two effects are produced. On the one hand the fatty bodies are altered in chemical composition, being decomposed into the fatty acids which combine intimately with the material of the stone—calcium carbonate. On the other hand, the surface of the stone not covered with the fatty substances is chemically changed, since the carbonate of lime is converted into nitrate, which has now the property of keeping moderately damp when moistened with water, and thus repelling grease. If such a transfer of greasy bodies on to the stone is allowed to act for some hours, and the fatty substances are removed with turpentine, it will be seen that these places have a lighter colour than the other parts of the surface of the stone, and if the stone be rolled up with a leather roller, charged with a greasy ink, after the stone {4} has been damped, only these places will take the greasy ink. By means of a litho steam or hand press prints can then be prepared from this.
It is frequently necessary to make corrections or additions to the drawing on the stone; if the stone has been already etched some means must be used which will remove the action of the etching, or else the stone will not take the greasy matter, or at least not so readily as is desirable. Very weak acetic acid, about 1–2 per cent., will do this. If the stone is flowed over with this dilute acetic acid or dilute table vinegar, the action of the stronger nitric acid and the gum is stopped, and the surface of the stone will again take fatty matter.
Before the penetration of the grease the stone is prepared by polishing with a concentrated solution of oxalic acid. Oxalic acid poured on to the stone would exert an etching action, but make the stone to a certain degree capable of taking the ink. If this liquid is rubbed on the stone with a cloth or linen pad with moderate pressure, the stone receives a high polish—especially the hard kinds—and in this condition the stone takes the ink well, like any flat object, but will not retain it. If a transfer is made on to such a stone, and the same etched, the image will completely disappear the first time the roller passes over it. This polish can be removed with 1 or 2 per cent. acetic acid, and the stone then has its primitive qualities. This method is used to reverse the image, which will be described later on.
2.—Litho stone
The commercial litho stones are of different colours, from whitish yellow to greenish shining grey. The latter has a finer and more even grain, and is considerably harder than the other kind. These two kinds are briefly designated in the trade as “yellow” and “blue,” and the brighter kinds are, as a rule, softer and cheaper than the darker.
The faults which the stones sometimes show differ considerably, and these produce certain inconveniences in continuous work. Many stones have dark and light spots, and as these spots have, as a rule, different densities, the etching solution cannot exert the {5} same action over the whole of the stone, and consequently in fine drawings, autotypes, etc., there are formed bright and dark spots. This also occurs with calcareous, speckled, and marbly stones. The very bright stones have less density, and therefore suck up the damping solution very quickly, and begin with repeated printing to become tinted. Stones with calcareous, rusty or other veins split easily, especially in the large sizes. Lime spots or lime veins, which are easily recognized by their brighter colour, take in graining a very unequal grain. The drawing is darker on these places, since the grain is greater according to the softness of the stone. Such stones are unsuitable for those photo-lithographic processes in which graining is used, such as the processes of Orell and Füssli and Bartös. The so-called glass veins and glass spots are crystallizations consisting of silicon, and are harder than the other parts of the stone. On these places the fatty inks do not penetrate deep enough, and the drawing does not adhere well, and can be easily removed by the rollers.
The hard stones receive a much finer surface than the soft when polished with pumice stone; they also reproduce every point, line, and detail of the drawing much more delicately, finer, and sharper; and, lastly, they have the advantage that they etch much more evenly, and re-etching is easy if moderate care be taken.
For the finer photo-lithographic work, if quality and continuous working is desired, only the good “blue” stones should be used; for cheaper line drawings “yellow” stones free from faults may be used.