[CHAPTER III]
CONCERNING ACIDS AND OTHER MATERIALS
I
GENERAL PROPERTIES OF ACIDS
Probably most lithographers still believe, as I did once, that the etching with acids prepares the stone, and that the succeeding application of gum merely increases this preparation. Countless experiments have taught me that the exact reverse is true. Gum arabic and a few other similar bodies are the true factors in preparation, and the acids simply make the stone more receptive for them. Only sulphuric acid, which changes the surface of the stone into gypsum, prepares it without gum; but this is available only for a few intaglio methods.
The stone used for lithography consists mostly of limestone sated with carbonic acid. Most acids, and even the salts, possess more affinity for limestone than the carbonic acid, which latter is freed and escapes in gaseous form as soon as another acid touches the stone. If aquafortis, muriatic acid, vinegar, etc., is poured on the stone, there rise a number of air blisters, which are nothing except the escaping carbonic acid, and the applied fluid seems to boil, in degree according to its strength. The boiling and bubbling last till the fluid has sated itself with lime, after which it becomes still, and is impotent for further etching.
The direct effect is the solution and destruction of parts of the surface of the stone. If it has been coated in parts with a fatty substance that resists the etching fluid, the places so coated are left untouched, so that, when the stone is cleaned, all the fat-coated lines and dots are in relief.
If the stone is coated with fatty matter, but not so thickly that the acid is entirely resisted, it will pierce the covering and eat away more or less of the stone. If the etching is continued or if the acid is strong, the fatty coat will be destroyed entirely, the surface of the stone will be clean, and ready for the ensuing preparation. The preparation of the stone for pen drawings with oil or soap-water and several aquatint methods, is based on this principle, that a very thin coating of grease can be etched away partly or wholly, at will.
After eating away the surface of the stone the acids have the property of giving it a fine polish.
Therefore if the stone has been covered with a design, and then etched with an acid, it could be inked and printed many times, as long as it is kept properly dampened and not too much pressure is used in applying the ink. However, this could be done also with a thoroughly clean stone, using only water, though the polish obtained from etching makes it much easier. But this apparent preparation is not by any means sufficient to print with certainty; and it becomes perfect only if the stone is coated with a solution of gum arabic in water after being etched. If a plate that has been merely etched and not treated with gum becomes dry during printing, or even if too much pressure be used in applying ink or in cleaning with the more or less smutty cleaning rags, it generally takes color and smut which are extremely hard to remove.
We may assume, therefore, that the acids have the following effects on the stone:—