Thus the coat of color is changed into an etching varnish of excellent resistance and adhesion, and the plate is ready for etching.
THE ETCHING AND ITS INTERMEDIATE TREATMENT BEFORE MOUNTING THE CLICHÉ.
Introductory remarks.—If we examine a wood cut or an electrotype with regard to the depth of its narrow and wide-line complexes, we can at once determine a plane for the closest hatchings; in a uniformly deep layer the lines lie at a moderate distance apart, and those of the highest lights are very deep. These three planes are situated higher or lower according to the very narrow or very wide series of lines. This gives a natural division of the different stages of etching, and, according to the three depths, we distinguish the sharp etching or etching on in which the engraving must advance far enough to place the narrowest portions sufficiently low; middle etching, which has to accomplish the same for the moderately narrow portions, and deep etching, which must attain sufficient depth for the highest lights.
On account of the lateral action of the acid it is impossible to etch down perpendicularly, but a wider covering is taken for each stage. Of these wider coverings there remain after etching all around the relief two terrace like grades, which must also be removed.
We have, then, in addition to the above, three sorts of etching, a round etching, in which the grade remaining from the deep etching is to be removed, and a clean etching, or a removal of the wire-edge left after middle etching.
This natural subdivision is found in the old French method as well as in the {80} new method, and both coating and the acids are of a strength proportionate to the different stages of the work.
We have already alluded to the lateral action of the acid; it is a factor that taxes the care of the etcher in the highest degree. In the beginning of the action of the acid, the edge of the metal under the coating is exposed the more, the longer the action lasts; here, of course, the dissolving power becomes apparent. The deeper we go, the more the metal under the line is exposed, and the action on the lateral edge continually increases. In the effect of the acid we distinguish thence, a perpendicular and a lateral action. Since the latter cannot be removed by changing the character of the acid, we must give it a part in this process of dissolving metals, but at the same time must invent a method which will prevent the lines from growing thin.
But in order to do this we must know the relation of the perpendicular to side action. Both are in proportion, a result both of theoretical and practical consideration. In order then to attain a certain depth of etching, a proportional lateral protection must take place, which is by degrees destroyed by the lateral action. Since this lateral action sets in at the beginning of the work, the dimension of the line must be secured from the start. This is done in the Vienna method by the already mentioned formation of the etching varnish, by means of dusting with resin and burning in, by which an almost imperceptible growing of the width of line is attained. The French method endeavors to correct this by using a very weak acid and a correspondingly thin coating, but coating oftener and etching at every new coating, as we shall see below. Also the coating for middle and deep etching must be arranged in accordance with the principles above elucidated. As deep as the middle or deep etching is desired, so wide must the lines be surrounded by etching varnish.
ETCHING BY THE FRENCH METHOD.
We have already seen (p. [76]) how a copy is made by the French method for etching, and we now continue the development of this process.