2 Litho Stone
is dense limestone of the mountain limestone group, and is found in the Tertiary deposits at a depth of two to three metres. Its constituents are lime, clay, and silicious earths, combined with carbonic acid, but lime is in the preponderance; it only contains a very small proportion of silica. In the stronger acids litho stone dissolves completely. The varieties of very fine stalactitic chalk of tolerable hardness, with flat shell-like fracture, without rust spots, glassy crystals or impurities, and those with very even grain, are the most useful for photo-lithography. Marble, which consists of pure chalk, cannot be used for lithography because, on account of its greater density, the greasy matter cannot penetrate so deeply, and therefore such stones would give only a few pulls.
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 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.