CORROSION OF METAL ANTIQUITIES — GETTENS 557 



COPPER PHOSPHATES 



Bone and horn materials buried in direct contact with copper and 

 bronze are often found stained bhie-green with copper phosphate salts 

 formed by action of copper salt solutions on calcium phosphate of 

 the bone. Geilmann and Meisel (1942) , on the basis of X-ray diffrac- 

 tion studies, identified as libethenite [Cuo(P04) (OH)], a green-blue 

 mass on a bronze spiral which had been in contact with bone in a 

 German grave. H. Otto cites (1959) three occurrences of copper 

 phosphate on artifacts. He does not call this alteration product libe- 

 thenite, but gives the formula as Cu3(P04)2'3H20, a compound which 

 he claims is not known to occur in nature. 



OTHER COPPER MINERALS 



No occurrence of the natural silicate of copper chrysocolla seems 

 to have been reported. Native or redejJosited cofpev sometimes occurs 

 among the imier layers of heavily chloride-encrusted bronzes. It is 

 probably formed by reduction of cuprous chloride (nantokite) by the 

 more electropositive high-tin phases of bronze alloys. Redeposited 

 copper can occur in isolated pockets or patches or as a continuous 

 inner layer. It is sometimes revealed in the form of little scales be- 

 neath the green corrosion crusts when they are stripped from the bronze 

 surface with formic or other mild organic acids. Often the flakes 

 are intermingled with cuprite. 



In spite of the considerable amount of research already done, much 

 remains to be learned about corrosion products and corrosion processes 

 of copper and its alloys. Investigators for industry have been much 

 concerned with corroding agencies and corrosion prevention, but they 

 have given little attention to corrosion products themselves. These, 

 unfortunately, are usually summarily discarded as worthless and un- 

 interesting end products of corrosion reactions. 



LEAD 



Among ancient metals, lead, next to copper, forms the widest variety 

 of corrosion products. It also forms some double salts with copper. 



LEAD CARBONATES 



The commonest alteration product on lead is cerussite^ or lead car- 

 bonate, PbCOa. This is the dense, adherent, warm-gray deposit usually 

 seen on old lead seals, sarcophagi, statuary, and on all sorts of buried 

 lead objects. Cerussite, fortunately, seems to form a protective layer 

 on lead and prevents its progressive and complete disintegration. In 

 1959, during the restoration of the east front of the U.S. Capitol in 

 Washington, sheet lead pads were found under the old marble columns. 



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