Engraving on copper is performed by cutting lines representing the subject on a copper plate by means of a steel instrument, called a graver, or burin, ending in an unequal-sided pyramidal point. Besides the graver, the other instruments 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 small 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 inches long. The scraper, like the burnisher, is of steel, with three sharp edges to it; it is about 6 inches long, tapering towards the end. Its use is to scrape off the burr raised by the action of the graver. To show the appearance of the work during its progress, and to polish off the burr, engravers use a roll of woollen or felt, called a rubber, which is used with a little olive-oil. The cushion, which is a leather bag about 9 inches diameter filled with sand, for laying the plate upon, is now rarely used except by writing engravers.
For architectural subjects, or for skies, where a series of parallel lines are 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 in the ordinary way.
The plate must be perfectly polished, very level, and free from any 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 sufficiently hot to melt white wax, a piece of which is then rubbed over it and allowed to spread, so 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 tracing having been taken of the original design with a black-lead pencil on a piece of thin tracing paper, it is spread over the face of the prepared plate, with the lead lines downwards, and,being secured from slipping, a strong pressure is applied, by which operation the lead lines 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 outline will be found on the copper, on which the engraver proceeds to execute his work.
(of) The engraving is first exposed to the vapour of iodine, which deposits upon the black parts only. The iodised engraving is then applied, with slight pressure, to a plate of silver, or silvered copper, polished in the same manner as daguerreotype plates. The black parts of the engraving which have taken up the iodine part with it to the silver, which is converted into an iodide at those parts opposite to the black parts of the design. The plate is then put in communication with the negative pole of a small battery, and immersed in a saturated solution of sulphate of copper, connected with the positive pole by means of a rod of platinum. Copper will be deposited on the non-iodised parts, corresponding to the white parts of the engraving, of which a perfect representation will thus be obtained; the copper representing the white parts, and the iodised silver the black parts. The plate must be allowed to remain in the bath for only a very short time, for, if left too long, the whole plate would become covered with copper.
The plate, after having received the deposit of copper, must be carefully washed, and afterwards immersed in a solution of hyposulphite of soda to dissolve the iodide of silver, which represents the black parts; it is then well washed in distilled water, and dried.
(6) Heat a silver plate, previously coated with copper, to a temperature sufficient to oxidise the surface on the copper, which successively assumes different tints, the heating being stopped when a dark-brown colour is obtained It is then allowed to cool, and the exposed silver is amalgamated - the plate being slightly heated, to facilitate the operation. As the mercury will not combine with the oxide of copper, a design is produced, of which the amalgamated parts represent the black, and the parts of the plate covered with oxide of copper represent the white parts. The amalgamation being complete, the plate is to be covered with three or four thicknesses of gold leaf, and the mercury is evaporated by heat, the gold only adhering to the black parts. The superfluous gold must then be cleared off with the scratch-brush; after which the oxide of copper is. dissolved by a solution of nitrate of silver; and the silver and copper under neath are attacked with dilute nitric acid. Those parts of the design which are protected by the goll, not being attacked, correspond to the black parts of the plate; the other parts, corresponding to the white parts of the engraving, may be sunk to any required depth.
When this operation is completed, the plate is finished, and may be printed from in the ordinary method of printing from woodcuts.
(c) To obtain from the same prints plates with sunk lines, similar to the ordinary engraved copper-plates, a plate of copper, covered with gold, is operated upon. On immersion in the sulphate of copper solution, the parts corresponding to the white parts of the engraving will become covered with copper. The iodine, or compound of iodine, formed, is then removed by the hyposulphite; the layer of deposited copper is oxidised, and the gold is amalgamated, which may be removed by means of nitric acid, the oxide of copper being dissolved at the same time. In this instance the original surface of the plate corresponds to the white parts of the print, and the sunk or engraved portions to the black parts, as in ordinary copper-plate engravings.