ETCHING (from etch, front Dutch risen, from Ger..iitzen, to etch, from MIIG. rizen, CZ:"Cll, to give to eat, from ezan, Ger. t'SSra, Goth.
AS. etan, Eng. rut; connected with Ir. ith, ()Church Slay. yam), I eat, Lat. edere, Gk. gaerOaL, eflesihai, Skt. ad, to eat). The art and the proc ess of engraving by means of acid which cats lines in the surface. Etching may be on glass, in which ease the line is hard and invariable, and this process is used chiefly by artists who seek character drawing or book illustration in which but little light and shade is desired. It may be done on zinc, which is thought to give a peculiarly rich 'color'—that is to say, a black and white effect of unusual brilliancy—and for this purpose it is preferred by some modern etch ers of landscape subjects. Etchings are known to have been made by Albert Diirer and others on iron, and in modern times on steel, hut by far the greater number of plates etched for printing are of copper.
In order that the acid may attack only the parts desired, something which resists the action of the acid must be spread over the plate at the beginning. This is called the ground ; it is usu ally varnish of some kind. laid on in a coat thick enough to guarantee its uniformity, so that no small openings will allow a little of the acid to pass through and permit a dot or small blur on the surface. Many special grounds have been used, and one recommended by Hamerton is male of wax, gum mastic, and asphaltum. It is cus tomary then to smoke the surface of this ground, but this is unessential, as its purpose is merely to aid the etcher by allowing him to see his lines as he cuts them, by the contrast of the brilliant metal against the dead black ground of the smoked varnish. The tool by which the lines are drawn may be anything with a reasonably sharp point. It has been said that an old steel fork, of which all the tines but one have been broken off, is as good a tool as any. By far the most usual form of etehing-needle, howeter, is a steel bar weighing from one to three ounces• of which the point is made sharp; sometimes both ends are sharpened to points of different fineness. It is to be noted, however, that the needle does not cut the metal at all, but merely scratches through the surface of the varnish so as to expose the metal.
The drawing once made in this manner, the plate is plunged into the acid bath. usually made
with nitric add, diluted by about its own volume of water. The action of this acid is very rapid: it eats the copper away on either side of the line drawn through the ground by the needle, and even hollows out the metal below the surface, leaving sharp, thin edges which break down with great facility. To prevent this and to keep the lines of the width desired, what is called the Dutch mordant was introduced about 1870 and strongly advocated by Hamerton and others. This mordant is composed of chlorate of potash 20 grains, hydrochloric acid 100 grams, water 180 grains. The universal testimony of practitioners is that the bath should be large and deep, and contain a considerable quantity of the mordant. Before the plate is put into the mordant it should be hrnshed with a feather or something of the kind, to clear away from the lines little scraps of the varnish which may have collected there. When it has been laid in the bath it must still be watched, as bubbles arise that must be re moved by a feather or similar means, because they may prevent the free access of the acid to the metal. If, now, it is desired to have a line bitten much deeper than others, it must he ex posed for a greater length of time to the acid. For this purpose. the process of stopping out is employed. The plate is withdrawn from the bath and washed. Varnish is then applied with a brush, filling up ('stopping') those lines which have been bitten sufficiently deep, while the others are once more exposed to the acid. In this way a single plate may be withdrawn several times, more and more of the lines stopped out, and those that remain bitten more deeply. It may also be necessary to rebite the whole plate. as when it is thought, or found on actual trial, that the plate is feeble in effect. For this purpose it is necessary to clean the plate thoroughly and then to put the resistant ground on the plate afresh. This must be done with great care, so as not to fill up the lines already cut by the etching, and then careful examination must be made to see that those thin and shallow lines which have re ceived some part of the ground are cleaned before the revarnished plate is put into the acid bath. Small parts of the plate may, however, be re bitten by the simple means of covering the rest of the plate completely with the ground.