Mezzotint Engraving.—This method differs from all other processes of metal engraving in that, while other engravers work from light to shade, and each line which they draw prints as a dark, the mezzotinter works from dark to light, and each touch which he adds to his plate prints as a light. Mezzotint plates are prepared by the action of a kind of chisel, termed a "cradle" or "rocking tool," which passing over the surface roughens it, raising a "bur" of innu merable small metal points, so that if the plate were then inked and printed it would yield an impression of a uniform black. The engraver, having traced his subject on the plate, proceeds to smooth the surface by removing the "bur" with a scraper, in proportion as he wishes to introduce light into his design; the bur being left untouched in the darkest shad ows, partially removed in the half lights, and wholly cleaned away in the high lights, in which the surface is per fectly smooth, and brought to a high polish by means of the "burnisher." The process of mezzotint was invented by an amateur, Ludwig von Siegen (1609 1680). Among the mezzotint engravers of England may be mentioned Simon, Pelham, Beard, Miller, Houston, Frye, and Purcell. Noted in America for this art were Thomas B. Welch (1814-1874) and John Sartain (1808-1898).
Aquatint Engraving.—In this process the polished metal plate is covered with a solution of resinous gum dissolved in spirits of wine. The spirit evaporates, leaving the resin deposited in minute granulations on the metal surface. The •design is then transferred to the metal and the plate is bitten in a bath of dilut ed nitrous acid, which corrodes the por tions left exposed between the grains of resin. The darkest parts of the design are longest exposed to the action of the mordant, the lighter parts being succes sively protected by a series of "stoppings out," consisting of oxide of bismuth and turpentine varnish applied with a brush in a manner similar to that employed in the "stopping-out" of an ordinary etch ing. The impressions produced resem ble those yielded by mezzotint, both proc esses working by spaces and not by lines. This method is believed to have
been invented by Jean-Claude-Richard de Saint-Non (1730-1804).
Chalk or Stipple Engraving. — The metal plate is coated with an ordinary etching ground, and the subject is drawn upon it by means of a succession of small dots produced by the point of the etching-needle. The plate is then bitten in the usual way with the acid, which corrodes the metal at the points uncov ered by the needle; and it is afterward finished by dots, applied with the point of the etching-needle or burin on the bare metal. Jean Charles Frangois (1717-1769) is said to have been the first engraver to employ this process.
Mechanical and Photographic Process. —Engraving in recent times has been generally superseded by photographic and mechanical substitutes. The most important of these is known as photo gravure or heliogravure. The beauty of the work produced by means of this proc ess in the reproduction of paintings, of drawings in monochrome and of photo graphs direct from nature, has raised it to a position of great importance. A photo-mechanical process which is much used in the reproduction of the plates of the older engravers and etchers, and in the production of intaglio etched plate reproductions from pen drawings has been carried to great perfection, some of the work produced by Amand-Durand of Paris being almost equal to the finest original etchings. A positive photo graph is taken of the drawing or en graving to be reproduced (i. e., the lines are black, the whites clear glass) ; this is placed over a copper plate coated with a bituminous varnish, and exposed to the light. Where the lines of the photo graph have protected the varnish from the light it remains soluble, but where the light has affected it through the glass it becomes insoluble. The varnish may then be dissolved from the lines and the copper exposed exactly as if the etch ing point had been used to make the drawing on an etching ground. The plate is then bitten in the usual manner, and finally touched up with the graver.