442 ART INDUSTRY AND RELATED OCCUPATIONS. 



This patina formation is due partly to the composition of the 

 bronze and partly to the atmosphere. Precious copper bronze 

 consisting of copper and tin only, is marked by it more than other 

 kinds. A large amount of dampness in the atmosphere, and salt, 

 together with rain and frequent washing, favour its production, 

 while coal-dust, sulphide of hydrogen and sewer gases hinder it. 



The black coating of many bronze monuments, which so often 

 takes the place of the beautiful colour of the fresh casting, is not 

 due usually to sulphide of copper, but to particles of coal and dust, 

 with a small mixture of oxides. A watery solution of carbonate 

 of ammonia put on with a brush, is excellent for removing this,^ 

 while the artistic production of the patina is best done by means 

 of acetic ammonia of copper-potassium. Zinc alloys, especially 

 brass, blacken easier than those without zinc. Copper containing 

 arsenic also shows greater inclination to blacken. 



Now-a-days patina is understood to include every accidental or 

 intentional colouring of a metal or an alloy which differs from 

 the original. Chemical analysis has shown that the beautiful dead- 

 black colouring of many of the Japanese bronzes, which sets off so 

 finely its decoration of inlaid work, incrustation and other orna- 

 mentation, is due to the lead in the alloy, which usually amounts to 

 something over lo per cent, and in single instances sometimes to 

 20 per cent, as shown in table B at the end of this chapter. Oi 

 the old bronzes only the small Egyptian idols, of which table A 5 

 gives an analysis, exhibit as high and still higher percentage 

 of lead. When the alloy contains so large a mixture of lead 

 it becomes very brittle, while the Japanese bronzes with 9 to 14 

 per cent, of lead, 7 to 2 per cent, of tin, and a corresponding 

 amount of zinc, satisfy all claims, as they are easily cast, form a 

 homogeneous mass, and by this means, as well as by their even 

 hardness, are easy to work, which is not possible when the zinc 

 is left out. The fine dead-black patina is produced by simple 

 heating in a close furnace, and is caused in part by the formation 

 of a sub-oxide of lead. Christofle and Bouilhet have shown, how- 

 ever, by their investigations, that a fine black patina may be 

 obtained without lead. Their process amounted to the same thing 

 as forming a sulphide of copper on the surface of the bronze. 

 Brown, red and orange-yellow tints were also produced, which 

 answered every purpose. 



The Japanese have an expedient for shading according to taste 

 the colour obtained in the tempering of the bronze, which has not 

 been known nor tried elsewhere. This is by a kind of grass, called 

 Kari-yasu {Calamagrostis Hakonensis, Fr. and Sav.), a corrosive sub- 

 stance of astonishing effectiveness. By boiling its roots and apply- 

 ing the liquid to the bronze, they obtain the said effect. An exact 

 chemical analysis of this substance has not yet been mad'e, but very 

 possibly it may have some importance for our bronze industry. 

 ^ According to Briihl in Diugl. Polyt. Journ. 1882, p. 256. 



