IkHNERALS IN ART AND ARCHEOLOGY — GETTENS 567 



spersed with patches of red cuprous oxide or cuprite. Equally color- 

 ful, but less wanted, is the crystalline green atacamite or basic copper 

 chloride frequently seen on bronzes which have been in contact with 

 the saline soils of desert regions like Egypt and Mesopotamia. Ata- 

 camite-encrusted bronzes sometimes cause alarm in museums because 

 underlying the atacamite and cuprite and next to the metal core is 

 a layer of colorless nantokite or cuprous chloride; nantokite is un- 

 stable, and if the object is exposed for extended periods to humid 

 atmosphere, it is transformed by hydration and oxidation to the end 

 product, atacamite. This behavior is called "bronze disease" and is 

 sometimes troublesome for museum curators since bronzes afflicted 

 with it have to be kept in a dry atmosphere. Most other mineral 

 alteration products on ancient metal objects are ugly and undesirable, 

 including limonite and goethite which form on iron, cerargyrite on 

 silver, and cerussite on lead. Tin in ancient bronzes is transformed 

 by corrosion into a product similar to but not identical with cassiter- 

 ite. The smooth gray-green patina on ancient high-tin Chinese 

 bronzes is mainly this hydrous form of tin oxide stained green with 

 copper impurities. 



Occasionally on ancient bronzes one finds deposits of minerals that 

 are quite rare. Some years ago, Prof. Clifford Frondel of the Har- 

 vard Mineralogical Museum observed a peculiar blue-green mineral 

 alteration product on the inside of an Egyptian bronze figurine of 

 a cat-headed deity known as Bast. By X-ray diffraction methods 

 he identified the product as botallackite, a basic copper chloride which 

 many years ago was first found in the Botallack Mine m Cornwall, 

 England. The type specimen in the British Museum was the only 

 other specimen of the mineral known to exist. The present writer 

 and Professor Frondel found on other Egyptian bronze objects a 

 bluish-green double salt of copper and sodium carbonate hitherto 

 unobserved. Since it was formed by natural processes, they gave it 

 the name "chalconatronite" and published its properties and its chemi- 

 cal formula which is CuCOs • NagCOs ' SHaO [27] . Other rare copper 

 and lead minerals such as cumengite, Pb4Cu4Cl8(OH)8-H20, and 

 phosgenite, PbCL-PbCOs, have been reported as occurring on cor- 

 roded bronzes. The mineral alteration products in ancient metal 

 objects is an interesting field of study in which there is still much to 

 learn. 



We have recorded many examples of the importance of minerals 

 to art and have demonstrated that plastic and pictorial art requires 

 a great variety of earthy materials as media of expression. Tliis 

 leads to the thought that a new kind of art exhibit, "Minerals in Art 

 and Art in Minerals," would be a challenging and most interesting 

 innovation. 



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