APPENDIX
Gowland: "When bars of debased silver (i.e. silver containing undue proportions of copper) were cast, a practice which unfortunately was not seldom followed, even in the old mints—especially for commercial bars—if the military rulers of the country were in need of money, a special mode of procedure was adopted. The silver was poured into canvas moulds, which were set in troughs of hot water, the reason for this being that the alloy contained so much copper that, if cast in the ordinary way, the bars would be coated with a black layer of oxide from the action of oxygen of the air on the copper, and this was difficult to remove. By placing the moulds under water this oxidation was prevented, and castings with a clear metallic surface were obtained. The bars were, however, of a coppery hue, and this required removal. They were therefore heated to redness over a charcoal fire, and then plunged into vinegar—made from the juice of unripe plums—containing common salt in solution. After digestion in this for some hours, they were washed with water and then boiled in plum vinegar without salt for one or more hours, when they were washed with boiling water and dried. By these operations the copper in the alloy was removed from the surface layers and a coating of pure silver left."
Note 34.—Professor Rein, in his great work "The Industries of Japan," describes the method adopted by the celebrated artist Gorosaburo of Kyōtō to produce a dark coffee-brown patina on copper and bronze: "Equal weights of green vitriol, copper vitriol, and sulphur are mixed with water. The copper article is then dipped in this bath, which must be often stirred on account of the finely distributed sulphur, and then rinsed in a second bath prepared in the same way but very much thinner. This process is repeated until the necessary corrosion is recognised by long practice. The vessel is then brought to the brazier and heated on an iron grate, whose bars are from eight to twelve centimeters distant from each other, and with frequent turning. In order not to endanger the soldering, these bars are sprinkled from time to time with water in which kariyasu (Calamagrostis hakonesis) has been boiled. The vessel is now rubbed with a cloth; then painted lightly with lacquer, rubbed again with the cloth, painted once more, and now heated until the sprinkled kariyasu water, rolling away in balls, indicates the amount of heat. The copper article is then taken from the grate with a pair of tongs and coated with a mixture of raw lac and lamp-black. It is then heated again up to the point where the water rolls away in balls, brushed over and painted anew with the lac mixture, and so on, till colour and lustre have the desired shade, whereupon the work is finished and the article is set aside for a second cooling."
372
