The Mordants Used in Etching.—The two mordants now generally used are the nitric and the hydrochloric acid baths. The nitric mor dant is composed of nitric acid equally diluted with water. The hydrochloric bath, known as the Dutch mordant, is composed of chlorate of potash, 2 parts; hydrochloric acid, 10 parts; and pure water, 88 parts. The nitric mordant is more liable to vary in its action than the hydro chloric; but it has the great advantage of being decidedly more rapid, thus affording the artist the opportunity of watching the process of bit ing and checking it when necessary. For this reason, it is preferred by many etchers.
The action of either mordant may be re tarded or accelerated by the varying conditions of the weather ; it is advisable, therefore, to keep it at a uniform temperature (about 60° Fahren heit) throughout the biting. This is best regu lated by performing the etching on a plate warmer—an iron box *th gas-jets beneath.
Method of Biting-in t 1 e Sketch.— There are two methods now used in etching the plate. One way is by the process of stopping-out. The plate is immersed in the acid bath for about five minutes, if in the nitric mordant and three times as long in the hydrochloric. This will suffice for the lightest tints to be bitten-in. After withdrawing the plate from the bath, it is washed, and dried between blotters, and the light lines are painted over with stopping-out varnish—a combination of resin dissolved in turpentine to which a little lamp-black had been added.
When dry, the plate is returned to the bath and kept there for about 10 minutes. As after the first biting, the plate is again taken out, washed, dried and the next deeper tones stopped-out. The process is repeated four or five times until the darkest tints have been pro duced. About 30 minutes in the nitric mordant is sufficient to bite-in the deepest lines.
A more satisfactory way of etching the plate consists in taking several different proofs dur ing the process of biting. This method is as follows: The etcher makes his drawing on the grounded plate N‘ t 11 the needle, putting in only the lines the middle and dark tones, and leaving the light lines for subsenvent treat ment. The plate is immersed in the mordant for about 15 minutes—just long enough to bite in the middle tint. After removing the ground with turpentine and thoroughly cleaning it, a proof is taken and the part* intended to be darker are rebitten in the same lines. This of course necessitates the regrounding of the plate without filling up the lines already bitten-in.
To accomplish this, a very thin film of ground is spread upon another heated plate, and the roller, thinly charged with it, is lightly passed over the etched plate, also warmed for the pur pose. When the plate has been rebitten, another proof is taken, and if the middle tint then proves to be satisfactory, successive bitings are made, and proofs taken to obtain the dark tones. The middle and dark tones settled, the etcher next completely covers the plate with transparent etching-ground or stopping-out varnish so as to fill up the lines and protect the spaces between them, and draws in the delicate lines intended to represent the pale tints. These are bitten-in and the final proofs taken.
Correcting Processes.—Light lines that have been over-bitten may be reduced by rubbing with a piece of charcoal moistened with olive oil. Deep erasures are made with the scraper —a kind of a knife, triangular in section, and coming to a point. The polished surface in an erased area is regained by the use of the bur nisher —a polished tool, made of steel and tapering to a point. The burnisher is also used to slightly reduce an over-bitten passage by rubbing it on the plate, pressing, thereby, the copper more together into the lines.
Auxiliary Processes.— Etched plates are frequently finished and enriched with what are known as dry-point and soft ground etching. Dry-point is the name given to a form of en graving in which the lines are cut directly into the dry plate by means of the needle, and with out the use of a mordant. It is remarkable for yielding rich and velvety proofs, which is due to the burr or rough edge of the copper, pro produced by the point as it cuts the plate. Dry point is employed to deepen foreground tones, and with the burr removed by means of the scraper) it is also useful for putting in the delicate markings in a composition. Prints are often made of plates produced entirely by dry point. In soft ground etching, the plate is covered with etching-ground mixed with tal low, to prevent it from hardening, and a piece of slightly rough paper is laid over it on which the artist makes his drawing with a lead pencil. When the paper is removed it brings off with it etching-ground, exposing the copper in such a manner that when bitten-in and printed, the plate will yield a proof having much of the quality of a pencil drawing.