CORROSION OF METAL ANTIQUITIES — GETTENS 551 



The black cupric oxide, CiiO, is seldom reported ; if present, it seems 

 to be consistently overlooked. Tenorite was recently reported, how- 

 ever, by Perinet (1961) among the products found among the inner 

 layers of a corroded copper nail recovered from a sunken second cen- 

 tury B.C. Greek vessel found in 1952 off the islet of Grand Congloiie 

 in the Mediterranean not far from Marseilles. 



COPPER CARBONATES 



Malachite and azurite, both basic carbonates of copper, are perhaps 

 the most familiar natural alteration products on copper artifacts, and 

 they are the most desirable constituents of bronze patina from the 

 point of view of the collector. 



Malachite^ Cuo (OH) 2CO3, sometimes occurs as a smooth dark green 

 compact layer on the surface of a bronze and gives it an enameled 

 appearance, but more often it is seen in scattered rounded masses 

 described as mammillary, or as botryoidal because of their resem- 

 blance to clusters of grapes. Occasionally, rounded nodules several 

 millimeters in diameter are found on the interior of bronze vessels 

 and also on objects of base silver. The banded structure characteristic 

 of large malachite masses is sometimes seen on ancient bronze pieces 

 that have been cleaned down and polished. Occasionally, malachite 

 is observed on bronzes in delicate fibrous aggregates, sheaflike in 

 form. Patches of bright green malachite can make an ordinary cop- 

 per object look interesting and precious. 



Azurite, Cu., (OH) 2 (€03)2, although similar in composition to 

 malachite, has about 3 percent less water of constitution, and its color 

 ranges from bright blue to dark indigo. Like malachite it sometimes 

 occurs on objects as a thin, compact, enamel-like layer; but it is ob- 

 served ordinarily in fine blue crystal aggregates scattered among 

 patches of malachite. In nature azurite is less abundant than mala- 

 chite and on artifacts the same is true. Azurite is most often encoun- 

 tered on the interior surfaces of hollow vessels where less humid 

 conditions may favor azurite formation. Occasionally, a bronze will 

 appear to be completely transformed to crystalline azurite. 



Presimiably malachite and azurite are formed by contact of the 

 object with soil water or even rain water charged with carbonic acid 

 gas in the presence of atmospheric oxygen. Cuprite, as mentioned 

 above, is sometimes observed in an intermediate zone between the 

 carbonate surface layer and the metal core of an object, but it is not yet 

 certain that cuprite formation is essential to the reaction. 



Glmlconatronite : A few years ago Gettens and Frondel (1955) dis- 

 covered a bluish-green chalky crust on the hollow interior of an 

 Egyptian bronze figurine of the deity Seklimet in the Fogg Museum 

 of Art. Tliis product did not have the properties of any of the copper 



