ETCHING (Fr., Morsure ; Ger., Aetzung) The incision of metals by means of acids or other corrosive fluids, as distinguished from engraving, which implies incision by cutting with a graver. Etching may be of two kinds : (I) The older form consists in spreading an acid resisting coating, or " ground," on the metal plate and scratching through it by means of sharp points so as to lay bare the metal. This is the process used by artist etchers, from Rembrandt down to the present-day workers. (z) The other and more modern method, in which photographic processes play so large a part, consists in forming an image on the plate in ink, varnish, or other suitable acid-resisting medium, and then etching away the bare parts of the metal. The image may be applied by draw ing direct on the plate, by transfer from a greasy ink image on paper, or by photography with a sensitive film. The last-mentioned method, now by far the most common, is called photo-etching. The methods of forming the image on the metal are treated under various headings—for example, " Albumen Process," " Fish-glue Process," " En ameline," " Bitumen," etc.—and the etching
inks, solutions, metals, etc., are also separately described. Etching may be either in relief or in taglio, the former being necessary for typographic printing and the latter for copper and steel plate printing. There are two divisions—line etching, which reproduces lines, stipples, and solid patches of colour ; and half-tone etching, which interprets the tones of a photograph or painting by means of a dot system. Colour etching is merely an extension of one or other of these processes.
Photogravure etching (which see) is different in principle. In ordinary etching the sunk lines, or spaces between the lines, are practically of uniform depth, but in photogravure etching the depth is proportionate to the tones, the shadows being sunk the deepest into the plate.