I
The stone that has been used exclusively hitherto in Munich for printing is a stratified limestone, found in the territory from Dietfurt to Pappenheim, and along the Danube down to Kellheim; hence the name Kellheimer plates, presumably because in past times the stone was quarried there first, or else found in its best quality. Now the Kellheimer quarry is exhausted, and the trade in the stones has transferred itself to Solenhofen, a village in the judicial district of Mannheim, three hours distant from Neuberg-on-the-Danube. All the inhabitants of Solenhofen are quarrymen, and the entire surrounding country seems to have a surplus of the stone, so that even with the greatest demand no scarcity is to be feared for centuries.
When the upper layer of earth is removed to the depth of six to ten feet in Solenhofen, the stones are found in strata lying horizontally on each other. First come strata of brittle stone, which often are composed of hundreds of plates as thin as paper. With proper care, each plate can be loosened and lifted whole. These layers are useless, being too brittle, and yet being too firm and not white enough to permit their possible use as chalk.
The Solenhofen stone consists chemically mostly of lime earth and carbonate. It is almost wholly soluble in nitric and other acids, the carbonate being liberated in gaseous form and disappearing. Since the various kinds of marble have almost the same component parts, one might suppose that marble should be available for lithography. But the many dark, uneven colors of marble and chiefly the many cracks and veins make considerable difficulty. However, I have found many evenly colored greenish, gray, bluish, and brownish Bavarian and Tyrolean marbles very useful for some methods, especially because of their superior hardness. Still, the Solenhofen stone will retain the advantage because of its light color and its greater cheapness.
The white Parian or Carrara marble is still lighter in color, to be sure, and really is rather useful for pen and crayon work. But though in part it is harder, on the whole it is much more porous and not so finely grained as the Solenhofen stone, and therefore not at all available for the intaglio method.
Since lithography began to arouse general interest, there have been attempts to find a stone similar to the Solenhofen, and there has been some fair success in France, Italy, England, and lately in the Kingdom of Prussia. With the enormous masses of limestone which cover the surface of the earth, it is not unlikely that this stone will be found in many places, either in layers of plates one, two, or more inches thick, or in great blocks which can be cut into plates.
In the Solenhofen stones one layer is not as good as another, and even in the same layer there may be a decided difference. Therefore, if one would produce perfectly beautiful work, it is necessary to obtain selected and perfect stones. This should be stipulated beforehand with the quarrymen, who now know pretty well how the best stones should be constituted.
A good stone must have the following properties:—
(1) The proper thickness. Thickness must be proportionate to the size. Smaller plates will resist the pressure of printing even if they are not so thick as the larger ones must be. But it is best to buy no stone less than one and one half or more than three and one half inches thick, because the thinner ones will not bear frequent grinding and the thicker ones are too heavy and inconvenient, besides taking up too much room. The best thickness of a stone is two to two and one half inches.
(2) Good mass. There are soft and hard stones. Sometimes the same stone is hard above and soft underneath, or the reverse. Often, also, a stone may consist of several thin and unequal layers. In the latter case, if the union is good and the layers are not easily separated, it will make no difference, so long as the stone is good in other respects. On the whole, however, it may be assumed that the harder stones are the best for all methods, so long as their mass is entirely uniform and they are not marred, as is the case with many, with white dots and patches. Then, to be sure, they are not worth much for any process, and at best can be used only for pen designs or for such of the intaglio processes where the lines need no particular sharpness. Such stones, generally gray, very hard, with softer, somewhat lighter patches or specks, are very hard to grind evenly because the softer parts are most powerfully attacked by the grinding material and become depressed. This produces the following defects:—