(1) They will not attack the parts coated with grease.
(2) They will penetrate more or less if the fatty coating is only thin.
(3) Where they touch the stone they dissolve it and eat it away.
(4) They give it a polish that facilitates printing. This polish disappears after a time on account of the cleaning with sponge or rag, but is replaced by a new polish produced by this very means.
(5) They do not prevent the adherence of fatty material later, as soon as the stone is dry, for which reason the parts prepared in the beginning with acid and gum arabic must be prepared again by renewed etching, to take the ink.
(6) Finally the acids have the property of giving to prepared stones that have been used for impressions, a rough surface instead of a polish when they are applied again, because they attack some parts more than others, producing little pores with sharp edges which catch the ink. This fact, as I will show more clearly later, makes necessary extraordinary care if one wishes to clean prepared plates or correct defects with new etching, because unskilled handling will often make them worse.
II
THE ACIDS SPECIFICALLY
Nitric acid or aquafortis, muriatic acid, vinegar, tartaric acid, and acid of wood sorrel, all have nearly similar effects, but aquafortis and muriatic acid are used because of their greater cheapness.
Oil of vitriol or sulphuric acid, very much diluted with water, is available for light but not for extensive etching, because it transforms the surface of the stone into gypsum and deposits it again, so that after that the acid cannot penetrate at all, or only partially. If a part of vitriol, say diluted with twelve parts of water, is poured on a cleanly ground stone, there ensues a violent action which, however, is only brief. It might be supposed that the acid is sated with lime when it ceases to act, but if it is moved to another part of the stone it etches anew.
If the acid is washed from the stone and a woolen rag be used to rub it after it is dry, it takes on a mirror-like polish. In this dry condition it can be cleansed of color as easily as a copper plate, and if a stone thus polished is engraved with a steel tool, it is possible to make several impressions from it just as from copper. The polish is not lasting, however, because the skin of gypsum is very thin. But it is a useful method if it is desired to engrave the stone and ink it frequently to see the effect.