540 



BRONZING 



Vanadate of copper has recently been recommended as a new bronze. Dr. B. W. Ger- 

 land has shown that a solution of vanadate of copper in aqueous sulphurous acid deposits 

 brilliant yellow crystals, after part of the sulphurous acid has been removed by boiling. 

 These crystals are quite uniform in appearance, and contain cupric oxide, vanadic acid, 

 and sulphurous acid. They change rapidly, under the influence of air, their beautiful 

 metallic lustre quickly disappearing, and the yellow colour changing to a dark groen. 

 If, however, these crystals are removed from the mother-liquor, and treated with fresh 

 sulphurous acid, although in the first instance formed in sulphurous acid solution, they 

 now decompose, and two fresh kinds of crystals, the one brown the other orange 

 yellow, are formed. Continuing to add sulphurous aqd, the brown crystals are redis- 

 solved, while the orange yellow are left intact. 



After filtration, washing, and drying, a residue is left of microscopic scales of a deep 

 orange-yellow colour, possessing a most beautiful lustre. They are free from copper 

 and sulphur, and are perfectly unalterable in the air. 



The composition of this substance previously dried over vitriol is, according to 

 analysis, as follows : 



Water (loss by heating) . . , . . 8'73 

 Pentoxide of vanadium . , . . .91-06 

 Impurities 0-21 



10-000 

 These numbers correspond to the formula of metavanadic acid, which requires 



Water . 8-97 



Pentoxide of vanadium . . . , .91-03 



100-00 



In some instances, Dr. Gerland obtained the same bronze or gold-like substance 

 by treating vanadate of copper suspended in water with sulphurous acid gas ; but in 

 many others the effect of the gas was the formation of vanadic oxide in solution. 



The vanadate of copper used in these experiments was prepared by precipitation 

 from vanadate of ammonia by sulphate of copper. The mother-liquor contained 

 both copper and vanadic acid. After evaporation the latter is found in the residue as 

 metavanadic acid, with the same metallic appearance as that above described, and 

 can be obtained by washing with water. The crystal, however, obtained in this 

 manner most obstinately retains copper, sometimes to the extent of 12 per cent. 

 Repeated treatment with sulphurous acid is the best method of purification. A 

 sample of the bronze as thus prepared was analysed by Professor Roscoe, who found 



Water 9-12 



Pentoxide of vanadium . . < . . 90*88 



100-00 



Samples of vanadium bronze obtained by these throe different methods have the 

 same composition, the same appearance, and the same chemical properties. Vanadium 

 bronze is essentially distinguished from the amorphous brick-red hydrated vanadic acid 

 by its indifference to reagents. Sulphurous acid scarcely acts on it, neither does 

 ammonia, and even a solution of carbonate of soda only dissolves it after long- 

 continued boiling. In the air it is perfectly permanent. It is probable that 

 this metavanadic acid will become a favourite bronze, and bo rated at even a higher 

 value than gold bronze. 



BRONZING. The process for giving to metals, plaster, wood, or any other body, 

 a bronze-like surface. Various processes have been adopted for producing this effect. 



