550 ANNUAL REPORT SMITHSONIAN INSTITUTION, 19 63 



numerous investigators over many decades, and the results are sum- 

 marized in monographs on metal corrosion like those of Evans (1960), 

 Ulilig (1948) , La Que and Copson (1963) , and others. 



COPPER 



Of all the metals used in antiquity, copper forms the most colorful 

 and interesting group of mineral alteration products. 



In the older literature one occasionally encounters the word "aerugo" 

 (from Z; aes — brass or copper) which refers particularly to the "rust" 

 of bronze, copper, or brass. Although now an archaism, aerugo is 

 quite an appropriate term. Much less correct is the terra "verdigris" 

 which is sometunes used to describe the green on copper, but this name 

 refers specifically to the green product formed by the action of 

 acetic acid (vinegar and other organic products of fermentation) on 

 copper and its alloys and hence is not a proper mineralogical term. 



COPPER OXIDES 



Probably the most widely occurring alteration product of ancient 

 copper and its alloys is the red cuprous oxide, CuaO,^ called cwprite. 

 Usually most of the cuprite is concealed beneath overlying green and 

 blue basic salts of copper and it seems to be an intermediate compound 

 in the conversion of metal to those salts. A cuprite underlayer is 

 often revealed when the outer green oxidation crusts are removed 

 by mechanical or partial chemical cleaning. In cast bronzes cuprite 

 sometimes forms along grain boundaries or in seams that penetrate 

 deeply into the metal core, but more often it occurs in coarsely crystal- 

 line masses in which perfect crystals of cubic habit abound. A frac- 

 tured or scaled surface of cuprite on bronze occasionally has a sugary 

 or drusy appearance caused by reflection of light from numerous 

 small crystal faces. This crystalline cuprite is usually cochineal 

 red in color but sometimes fine-grained cuprite in inner or inter- 

 mingled layers is quite orange-red or even yellow. In primitively 

 smelted copper, cuprite is often disseminated in small globules among 

 the copper crystal grains. This is not a corrosion product but is 

 cuprite formed under inadequate reducing conditions at the time of 

 smelting. Collins (1934) concluded from evidence of X-ray diffrac- 

 tion studies that the "mirror black" patina of certain Chinese bronzes 

 consists of crystalline cuprite of unusual structure possibly having 

 a thin film of tenorite on the surface. 



Tenorite: Fink and Polushkin (1936), who studied the microstruc- 

 ture of patina, observed that cuprite and/or tenorite is always found 

 as an intermediate layer between copper or bronze and malachite. 



2 The chemical formulae used here for minerals are those employed by Palache, Berman, 

 and Frondel In their revision of Dana's "System of Mineralogy." 



