7 Etching the Stone
There are two principal etchings, the simple ordinary or first etching, which obviously has the purpose of giving the drawing the necessary stability and protecting the blank places from taking ink, and the deep or sharp etching gives for combination printing a still greater lasting power, and for large runs keeps the drawing clean and sharp.
The ordinary etching chiefly causes the fatty substances to assume a new chemical condition, since in combination with the stone they form insoluble fatty salts of lime, which represent the printing surfaces. It has, however, also the property of cleaning the blank stone surfaces from any adherent grease, and makes the blank parts of the stone incapable of taking ink; also it makes the drawing clearer and more precise.
The strength of the first etching may differ, according to the following circumstances:—
(a.) The stone; for hard stones stand a stronger, and soft stones a weaker etching.
(b.) The printing image; fine drawings must at first be etched with a weaker solution; coarser drawings, on the other hand, will stand a fairly strong etcher.
(c.) The material with which the print or drawing has been prepared; very fatty developing inks and asphalt stand well even with strong etching, whereas with lean or hard inks only a weak etching should be used.
An etching solution which is at the limit of strength, but still suitable for a hard stone or a very coarse drawing, would irretrievably ruin a very fine drawing on a soft stone.
Commercial acids differ considerably in strength, and it is, therefore, better to use a hydrometer than a measure. It is also always more satisfactory not to use the acid by itself, but in conjunction with gum. A solution of gum of eight to ten degrees strength is diluted with nitric acid till it measures 10 to 12 degrees on the hydrometer. This solution may be safely used for every case, and it is only necessary to allow it to act longer for coarser drawings; it should be evenly distributed over the surface of the stone with a clean soft pad. In very hot weather it acts more energetically, and then it should be used weaker for fine work. In any case it is advisable to let the stone or plate stand for some hours before etching, so that the ink may thoroughly combine. The etching of zinc plates will be more fully described later on; in other respects the treatment is the same as for stone.