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.