A Treatise on Etching

by Maxime Lalanne

40. Strong Acid and Weak Acid.—It is, nevertheless, possible to put such strong acid to good service. A fine gray tint may, for instance, be imparted to a well-worked sky by passing a broad brush over it, charged with acid at 40°. But the operation must be performed with lightning speed, and the plate must instantly be plunged into pure water.

As a corollary of the fourth cause, it is well to know that an acid overcharged with copper loses much of its force, although it remains at the same degree. Thus an acid taken at 20°, but heavily charged with copper from having been used, will be found to be materially enfeebled, and to bite more slowly than fresh acid at 15° to 18°. To continue to use it in this condition would be dangerous, because there is no longer any affinity between the liquid and the copper, and if, under such circumstances, you were to trust to the appearance of biting (which would be interminable, besides), you would find, on removing the varnish, that the plate had merely lost its polish where the lines ought to be, without having been bitten. It is best, therefore, always to do your biting with fresh acid, constantly renewed, as the results will be more equal, and you will become habituated to certain fixed conditions.

Some engravers, of impetuous spirit and impatient of results, do their biting with acid of a high degree, while others, more prudent, prefer slow biting, which eats into the copper uniformly and regularly, and hence they employ a lower degree. In this way the varnish remains intact, and there is not that risk of losing the purity of line which always attends the employment of a stronger acid.

41. Strength of Acid in relation to certain Kinds of Work.—Experience has also shown that, with the same proportion in the time employed, the values are accentuated more quickly and more completely by a strong than by a mild acid; this manifests itself at the confluence of the lines, where the acid would play mischief if the limit of time were overstepped.

Another effect of biting which follows from the preceding, is noticeable in lines drawn far apart. Of isolated lines the acid takes hold very slowly, and they may therefore be executed with a cutting point and bitten in with tolerably strong acid.

The reverse takes place when the lines are drawn very closely together; the biting is very lively. Work of this kind, therefore, demands a needle of moderate sharpness and a mild acid.

Hence, interweaving lines and very close lines are bitten more deeply by the same acid than lines drawn parallel to each other, and widely spaced, although they may all have been executed with the same needle. If, in an architectural subject, you have drawn the lines with the same instrument, but far apart on one side, and closely and crossing each other on the other, you must not let them all bite the same length of time, if you wish them to hold the same distance. It will be necessary to stop out the latter before the former, otherwise you will have a discordant difference in tone. There will be inequality in the biting, but it will not be perceptible to the eye, as the general harmony has been preserved. (See [Pl. IV.] Fig. 1.)

In short, strong acid rather widens than deepens the lines; mild acid, on the contrary, eats into the depth of the copper, and produces lines which are shown in relief on the paper, and are astonishingly powerful in color. This is especially noticeable in the etchings of Piranesi, who used hard varnish.

42. Last Stages of Biting.—But let us return to our operation. You noticed that I allowed your plate to bite quite a while; this was necessary to detach your foreground and middle-ground vigorously from the sky and the background. You may now stop out the trees, the tower, and the tent in the middle-ground, and the vertical part of the bridge, which is in half-tint, and then proceed. Note that the number of bitings is not fixed, but depends on the effect to be reached.

“In that case it is to be hoped, for the sake of my apprentice hands, that I shall never have many bitings to do. Just look at my fingers! They are in a nice state. The prettiest yellow skin you ever saw!”