Etching, Electro-

This mode of etching, which is in many respects superior to the ordinary mode, is based upon the destructive action of certain ‘anions’ during ‘electrolysis.’ [281] If two plates of copper be connected with the opposite ends of a voltaic battery, and placed in a ve el containing very dilute sulphuric acid, the plate connected with the copper of the battery will be attacked by the anion oxygen which is released during the decomposition of the acid. This destructive action can be localised at pleasure by covering certain parts of the plate with a protecting stratum of varnish, ordinary ‘etching ground’ for instance. In the practice of electro-etching, the drawing is ‘scratched in’ in the usual way through an ordinary ground; a stout wire is then soldered to the plate, and this, as well as the back of the plate, is coated with sealing-wax varnish. Thus prepared, the plate is placed in a suitable ‘decomposition cell’ opposite a plate of somewhat similar size, and the two are connected respectively with the copper and zinc of a ‘Daniell’s cell,’ or the silver and zinc of a ‘Smee’s cell.’ [282] After about ten minutes the plate is removed, washed, and dried; and when the ‘fine work’ has been stopped out with Brunswick black, it is returned for another space of ten minutes. By alternately exposing the plate to the action of the decomposing fluid, and ‘stopping out’ parts of the work, the required gradation in tints is obtained. The exact duration of the various exposures, as well as their number, must, of course, be regulated by circumstances. See Etching Fluids ( below ). [281] See Electrolysis and Electrotype , pages 428 and 429. [282] See Voltaic Electricity .