Photoengraving
a term applied to producing printing blocks or plates by photography. The most commonly employed process is to coat a metal plate with a thin film of asphaltum, and expose it to light under a reversed positive. The picture is next developed by dissolving away the parts of the asphaltum not acted upon by the light, and the plate is subsequently etched in the usual way.
This process is sometimes called photoaquatint. The second method is more elaborate. A film of bichromatized gelatine, on a sheet of glass or a copper plate, is exposed under a photographic negative, and the unprinted portions which are soluble in water washed away, leaving the printed parts in relief. The plate with the relief is next coated with a film of silver by electro-deposition, and placed in an ordinary electrotyping bath, in which it is allowed to remain till a shell of copper from one-sixteenth to oneeighth of an inch thick (according to size of plate) is formed. This, after the rough excrescences have been removed by filing, becomes the printing plate. It can be worked upon by an engraver, if necessary, to remove photographic defects, and is printed on a copper-plate press. When a relief block is required, a reversed negative is used to print from, and the etching is carried to a much greater extent. These processes answer for su subjects in black and white, as well as in colors. This is also the system by which conversion of the half-tones of a photograph into an ordinary printing PHOTOGRAPHY block or plate has become so eminently successful. See PHOTOELECTRO- TYPE; PHOTOGRAVURE.