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XL VI I. On the Cause of a Pink Colour in White-Lead Corrosions. 

 By William Baker, F.C.S., Associate of the Royal School 

 of Mines, London*. 



IN some contributions to the metallurgy of lead, published in 

 the Philosophical Magazine in 1862, I attributed a certain 

 pink tint, occasionally seen in white-lead corrosions, to the pre- 

 sence of small quantities of copper. As the results of any expe- 

 riments upon the corrosion of lead by the combined action of 

 fermenting bark, acetic-acid vapour, and atmospheric oxygen 

 can only be arrived at after the expiration of ten weeks or three 

 months, the progress of further investigation in this matter has 

 been necessarily slow. I have been unable to isolate the colour- 

 iag-matter; but I wish to correct the 'statement that the pink 

 colour is due to copper, and to detail some conclusive proofs that 

 it is caused by finely divided silver. 



Having obtained many tons of lead which contained only 

 traces of copper, I found in several instances the pink colour 

 still quite evident in the corrosions. By the method which had 

 been employed for refining the metal, there could be only silver 

 left as an impurity in any perceptible amount. I therefore 

 sought for evidence that this substance could produce such a 

 result. Upon analyzing 5000 grains of a perfectly white corro- 

 sion, and one which was distinctly and uniformly pink, the 

 result showed that the composition of the two samples differed 

 mainly in the amount of silver : — 





CuO. 



FeO. 



NiO. 



Ag. 



White corrosion 



•0050 



•0022 



trace 



•0005 per cent. 



Pink corrosion . 



•0060 



•0022 



•0013 



•0058 „ 



A small quantity of silver was then added to a portion of the 

 lead which had produced the white corrosions, and this was again 

 submitted to the corroding action. The result was a decided 

 pink carbonate. This synthetical experiment was repeated many 

 times with a like result upon various samples of lead which had 

 before produced a white carbonate; and I find that the pink 

 colour begins to show at the edge of the metallic portion left 

 uncorroded when the silver amounts to more than -J oz. per ton 

 of lead. A decided colour, which is uniform throughout the 

 mass of the corrosion, is obtained when the silver amounts to 

 about 1^ oz. per ton. A fracture cf a dense corrosion often 

 shows the crystalline character of the metallic lead, which is de- 

 fined to some extent by the pink colour — as if the silver had 



* Conimimcated by the Author. 



