648 THE ART OF CASTING BRONZE IN JAPAN. 



Another important point is the prevention of the formation of sub- 

 oxide of <'0j)per (Cn/)) during- the nieltiuii' of the copper before the 

 addition of the tin and other metals, as, wlienever this is formed, it dis- 

 solves in the copper, and when the tin and zinc are added it reacts with 

 them, forming difficultly fusible oxides, which diminish very greatly the 

 fluidity of the bronze.' 



It is prevented, to a certain extent, by keeping an excess of charcoal 

 on the melting hearth, but too often this is an insufficient precantion, 

 and I would strongly recommend in all cases that the copper should be 

 treated by the process known as "poling" before the addition of the 

 tin and other metals. This is performed by placing the end of a pole 

 of green wood below the surface of the copper; much gas and vapor 

 are given off, the metal is violently agitated, so that every partic^le is 

 brought into contact with the charcoal on its surface, and all traces of 

 suboxide are removed. 



Phosphorus, aluminium, and manganese have each the effect of 

 removing this oxide. They should not, however, be added in excess, 

 but only in sufficient quantities to insure this result. The kind of cop- 

 per used is also of great importance. 



The purest metal obtainable should alone be employed in the prepa- 

 ration of bronze for valuable "cera perduta" castings, electro-deposited 

 copper if possible, or at least Avhat is known as "best selected" copper, 

 and this should have an electric conductivity of not less than 98 per 

 cent on Matthiessen's scale. 



5.— STAINS AND PATINAS. 



The rich patinas which Japanese bronzes acquire by age or special 

 treatment, and which enhance so much the beauty of the artists' work, 

 are worthy of a brief consideration, although perhaps they do not 

 strictly come within the scope of the title of this paper. In many bronzes 

 the beautiful color is due merely to a "stain," i. e., a colored film of 

 infinitesimal thinness. In others the surface of the metal is altered to a 

 considerable depth, and in these only have we true patinas. Frequently 

 both a " stain " and a patina are produced by similar treatment, but the 

 operations required for the latter are of a more jirolonged character than 

 for the former, and are accompanied by s|>ecial nmnipulations, in addi 

 tion to the apiilication of what are termed pickling solutions. For the 

 production of true patinas of the richest and darkest shades of brown 

 by Japanese methods it is essential that lead should form one of the 

 constituents of the bronze, and that zinc should either be absent alto- 

 gether or be present only in small proportions. On the other hand, 

 "stains" of any color can be given to metal of any eom])Osition and 

 even to unalloyed copper. 



The substances used in the operations are copper sulphate, basic 



'This can be easily demonstrated by melting a little cojiper iu a crucible, allowing 

 it to oxidize, and then adding zinc, when the resulting alloy will be found to be so 

 pasty that it can hardly be poured out of the crucible. 



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