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:—
(a) In pen work, the pen will catch often, whenever it comes to such a place. This, however, is not so important: but
(b) In the crayon method there will be defects and lights in the shadings on the softer places, which are very hard to correct.
(c) In the etched or engraved methods, the needle will sink in much deeper when it passes over such softer spots, making a deeper and broader line which injures the clearness of the drawing. In etching, also, the softer places are more affected by the acid; and it is better, therefore, to use a soft stone whose entire surface is uniform, than to have a stone that is hard but uneven.
A very soft stone cracks easily in the press, unless it consists of several layers, the lower of which are hard. But it is easier to engrave, and as a rule gives blacker impressions, because it sucks more color in, and holds it because of its greater porosity. Printing, however, is somewhat more difficult, because these stones take dirt readily; nor is it possible to get so many impressions. They are not useful for crayon work because the finest shadings are too easily etched away; and pen work is difficult on them, because the steel pen easily cuts into the stone, fills its point with fine dust, and thus gives no ink flow. This softest stone in Solenhofen generally looks yellow, or is marbled with red and white or has many yellow veins.
Even those stones whose uniformity, thickness, and hardness make them best for all methods, often have defects, such as so-called glass spots or tiny, sometimes invisible holes, broad veins and cracks. All these must be avoided when selecting stone. Very small deep veins, which often are fine as hairs, yellowish and grayish spots, impressions of fossil plants and fishes, etc., are not harmful. It is rare to find a stone as large as a sheet of note-paper that is entirely free from these little defects.
(3) The form of the stones also is to be considered, and must be selected according to need. To be sure, a small design can be drawn on a large stone; but apart from the inconvenience, the construction of the press demands that the stone be not much larger than the drawing. However, at the end where the impression begins and stops, there must be at least an inch margin to give sufficient room for the roller to take hold, as will be explained more particularly later.
When one has to print small things like visiting-cards, etc., it will not be profitable to use large stones, especially if they are to be saved for future use. Small stones of the size of an octavo sheet are better. Therefore it will be wise to have stones cut to various sizes in the beginning. It would be well also that one of the printers, or the polisher, strive to attain skill in cutting stones to size. Sometimes polishing discloses defects in a stone, making it useless for a design of any size. But it is possible to cut it up into many small ones that are perfect. Sometimes a stone cracks under the press or breaks through accident. Skill in cutting will enable one to make small and good stones out of the pieces.
It is essential for good work in the press that the stones be cut very true. The stones that are used for flooring in churches, etc., usually are cut so that the upper face is larger than the lower. This is done to make them set better in the mortar and to enable the stone-cutters to fit them closely together on the top. But this must not be done with stones for printing, because such stones could not be tightened properly in the press and would lift during the printing. Printing-stones must be cut absolutely true vertically. Indeed, in work where several plates are to be used to make one complete impression, and where steel guide-points in the frame are used instead of laying the paper on the plate, it is beneficial to cut the stones conically, so that the base is one fourth inch greater than the top. The plate can be tightened better and is less likely to be moved from its place during the impressions.
Despite their hardness the stones are brittle, and a single light but sudden blow with any hard body, such as a steel tool, may cause a crack in the thickest stone. It is necessary to exercise great care to avoid all shocks.
This property of the stone is used in Solenhofen to cut the stones according to desire. A small hammer of hard steel, weighing scarcely two ounces, is used. Its end is somewhat like a stone-chisel, but not nearly so sharp. With this hammer, which is set on a thin handle two or three feet long, the workman strikes light but very swift blows along the line of desired cleavage, each tap being about an inch from the preceding one. The stone is so laid that its greater part is free, resting on nothing. This light operation is sufficient to cleave the largest stones.
The cleavage is not always uniform and true. Therefore the stone usually is finished with a sharp stone-chisel. It is possible also to divide a stone as desired by supporting it at both ends so that there is nothing under the part to be split, and then cutting along the line with a chisel of hardened steel, not too sharp, which is tapped lightly with a light hammer. The varying sound tells at once when the stone cracks, and then a few light taps with the hammer on the other side suffice to separate it. Before one attains the necessary skill, however, he will smash many a stone. Therefore it is not advisable to try this on a stone that has a design on it, for a single incorrect or over-heavy blow often will split the stone in the wrong direction. Blows that are too light, on the other hand, often make it almost impossible in the end to cause a cleavage along the desired lines.
II
POLISHING
The stone plates that come from Solenhofen, even if polished according to stipulations, rarely are available for printing, but must be specially polished by one who understands the work thoroughly.
The first requisite for this is a straightedge of iron or brass, as true as possible. This ruler must be laid on the stone in various directions, and the lithographer must note all parts where there is space between the straightedge and the stone. The greater the space, the greater the unevenness of the stone; and those that show especial unevenness should be set aside from those that have little.
When this has been done, the very uneven stones must be ground with a coarse sandstone and plenty of water applied to the elevated places till the straightedge can be applied in all directions without showing any material interstices. Then these ground stones may be placed with the others that were fairly even in the beginning.
Now we take one of these stones, and lay it on a strong, firm table, the best being one to be described later. Finely grained sand is sprinkled over its surface. In the absence of sand, a substitute can be made by powdering a common sandstone of the kind used for coarse grindstones. A spoonful of water is poured over this. A little soap may be mixed with the sand. It facilitates the grinding and makes the sand take hold of the stone better. Now another stone is laid on the first one, and is moved back and forth continually in all directions. The sand and water must be renewed often. Thus both stones, the upper and lower, will be ground simultaneously, and very evenly and true, if the work is done right.
One must take care never to draw the upper stone far beyond the lower one, because that would throw the centre of gravity of the upper plate too near its ends, as a result of which the upper plate would become concave and the lower plate convex. To avoid this defect, the upper plate should be moved around only in small circles. It is good also to change the plates around frequently, so that the upper shall be the lower. Another good plan is not to use two stones of equal size, but to take for the upper stone one only half as large as the lower. It is necessary also that the straightedge be applied frequently. The stone must always be cleansed thoroughly before this test.
Once one has the proper experience, it is possible to tell by mere touch if the plates have been sufficiently ground. So long as they still have uneven spots, a certain resistance is noticeable, so strongly sometimes that it is impossible to move the upper plate further without lifting it and sprinkling new sand. Sometimes this friction is so great that manual strength does not suffice to separate the stones, especially if they happen to dry. If tools are used to separate them, it happens often that pieces are torn from the stones, because they adhere so mightily. In this case a very simple and convenient remedy is the best. An ordinary table-knife is inserted gently and then tapped very lightly, when the stones will separate at once.
Whenever sand is applied, water must be applied also, but not too much, as in that case it would only wash away the sand. Here, too, practice must teach the exact proportions.
From the stone-cutter's work, as well as from the primary grinding with sandstone, the plates will have visible furrows and scratches made by the coarser grains of sand. Under the polishing all these disappear bit by bit, and there appears a fine grain, consisting entirely of fine dots; and this is the finer in proportion as the sand is crushed by the process of polishing and also according as less fresh sand has been used.
When the marks of the sand have vanished completely, it is fairly certain that the stones are polished sufficiently. To make sure, the straightedge can be applied again. It must not be imagined, however, that it is necessary or possible to polish a plate so perfectly that there will be absolutely no spaces between any part of its surface and the straightedge. A perfect and mathematically level plane surface is hardly likely ever to be produced. If the stone is almost level, and the unevennesses do not exceed the thickness of letter-paper, it is quite sufficient.
Although this sort of polishing, with two plates at once, is not used in all lithographies, some preferring to polish with small pieces of sandstone, I give it here as the best, because it demands little skill and is quicker, so that one can grind off four stones in the time required for one under other methods.
In this matter of smoothness of the stone it is impossible to be too careful. The beauty of the imprint depends upon it. Errors in the polishing cause great trouble afterward. Therefore the manager of a lithography must pay close attention to this work. In the Lithographic Institute in Berlin the rule has been adopted that no engraver shall accept a plate that he has not found thoroughly good, under penalty of reimbursing the printers for all extra trouble and work.
This first polishing, however, is only the general preparation of the stone. Afterward they must be polished and prepared especially for each particular method, as will be explained in the proper place.
III
SORTING AND STORING
When the plates have been polished, they are cleansed with water and sorted for their various uses. Now it is easier to see just what quality the stones have, their defects, and consequently, what work they are best for. Those not uniform are best for coarse pen work. Those of uneven coloring, but hard and thick mass, can be used for the finer pen drawings, for etching and engraving, or for transfer work. For crayon work the clearest and most evenly colored stones of extreme hardness are to be selected.
They can be stored anywhere that is not too damp and not too much exposed to winter cold. Dry cold does not hurt them; but if they are wet through and through and then freeze, they will crack. In constant dampness, too, saltpetre and other salts enter them and they crumble. In clean water they do not undergo any changes.
I will describe the storage of etched and designed stones later.
If the stones are to be used after being stored in any damp place, they should be kept for several days in a temperate and dry place till they have dried thoroughly, as otherwise they are not easy to work in any style. This is not necessary if their place of storage has been perfectly dry.
