On Copp) —Engraving on copper is performed by cutting lines representing the subject on a plate of copper by means of a steel graver or "burin." The Plate.—The plate must be perfectly polished, quite level, and free from imperfection ; to this, must be transferred an exact copy of the outlines of the drawing. To do this, the plate is uniformly heated in an oven or otherwise, till it is eufficiently hot to melt white wax, a piece of whirl is then rubbed over it and allowed to spread, se as to form a thin coat over the whole surface, after which, it is left in a horizontal position till the wax and plate are cold. A tmcing having been taken of the original design with a graphite pencil on a piece of thin tracing-paper, it is spread over the face of the prepared plate, with the lines downwards, and, being eecured from slipping, a strong pressure is applied, by which operation the line,s are nearly removed from the paper, being transferred to the white wax on the plate. The pencil-marks on the wax are now traced with a fine steel point, so as just to touch the copper ; the wax is then melted off, and a perfect ou6ine will be found on the copper, on which the engraver proceeds to execute his work.
The Tools.—Besides the graver, ending in an unequal-sided pyramidal point, the other instru ments used in the process are a scraper, a burnisher, an oil-stone, and a cushion for supporting the plate. In cutting the lines on the copper, the graver is pushed forward in the direction required, being held at a slight inclination to the plane of the copper. The use of the burnisher is to soften down the lines that are cut too deeply, and for burnishing out scratches in the copper ; it is about 3 in. long. The scraper, like the burnisher, is of sted, with three sharp edges to it ; it is about 9 in. long, tapering towards the end. Its use is to scrape off the burr raised by the action of the gmver. To show the appearance of the work during its progress, and to pnlish off the burr, engravers use a roll of woollen, or felt, ealkd a rubber, which is used with a little olive-oil. The cushion, which is a leather bag about 9 in. in diameter, filled with sand, for la) ing the plate upon, ie now rarely used exeept by writing engravers. For architectural subjLets, or for skiea, where a series of parallel lines is wanted, a ruling-machine is used, which is exceedingly accurate. This is made to act on an etching ground by a point or knife connected with the apparatus, and bit-in with aquafortis (commercial nitric acid) in the ordinary way.
Facing with 1ron.—The relative hardness of iron and copper furnishes a means of greatly increasing the number of impressions that may be taken from a copper plate. Ordinary copper plates will not afford more than 800 good impressions without re-touching. This may he success fully increased to 2000 by electra-depositing a surface of iron upon the plate, and this surface may bo renewed indefinitely, The necessary apparatus consists of a Bunsen's battery (20 elements) and a guttapercha-lined trough (45 in. long, 22 in. wide, 32 in. deep) filled with a solution of 100 lb.
sal ammoniac to 1000 lb. water ; to the positive pale of the battery, is attached a sheet of iron of the dimensions of the trough, and immersed in the liquid, while another plate of half the size is simi larly fixed to the negative pole. After several days, the bath should be fit for use, the battery having meantime received necessary attention. The iron sheet at the negative pole is now replaced by the copper sheet to be coated, aud momentary immersion should suffice to cover it with an iron deposit ; if not, the bath is not yet ready. The copper plate must not remain in the bath after the bright iron coating appears blackish at the edges. Immediately the coated copper plate is removed from the bath, it is carefully washed under a water-jet of some force ; when dry, it is again washed with spirit of turpentine, and is ready for printing from. Before re-coating the plate, the residue from the former ooat must be removed by washing in nitric acid diluted with 8 parts of water, taking care to cleanse the plate from this acid liquid the moment the iron has disappeared, or the copper will begin to undergo similar destruction.
071 Steel.—Engraving on steel is the same as copper-plate engraving, except in certain modifi e,ations in the use of the acids ; therefore, so far as the process itself is concerned, no particular description is necessary ; but the means employed for first decarbonizing and recarbonizing the steel plate, so as to reduce it to a, proper state for being acted upon by the graving tool, must be explained. In order te decarbonate the surfaces of cast-steel plates, by which they are rendered much softer and fitter for receiving either transferred or engraved designs, fine iron-filings, divested of all foreign matters, are used. The stratum of decarbonated steel should not be too thick for transferring fine and delicate engravings ; for instance, not more than three times the depth of the engmving; but for other purposes, the surface of the steel may be decarbonated to any required thickness. To decarbonate it to a proper thickness for a fine engraving, it is exposed for four hours to 9. white beat, enclosed in a cast-iron box with a tight lid. The sides of the box must be at least I in. thick, and at least a thickness of in. of pure iroa-filings should cover or surround the cast-steel surface to be decarbonated. The box is allowed to cool very slowly, by shutting off all access of air to the furnace, and covering it with a layer of 6-7 in. of fine cinders.