560 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1963 



has perished, perhaps for two reasons : First, it has been transformed 

 by direct intercrystalline oxidation to mixed stannous and stannic 

 oxides. Plenderleith and Organ (1953) have described tin oxidation 

 in some detail, and Caley (1941) and Mantell (1949) have emphasized 

 the important role of halides, especially of sodium chloride, in stimu- 

 lating corrosion attack on tin. Secondly, it is sometimes transformed 

 by allotropic modification to powdery gray tin {vide infra) . 



Although we have few objects of pure tin from remote antiquity to 

 examine, an impressive bit of evidence of early commerce in tin was 

 found preserved in the Bronze Age shipwreck found off Cape 

 Gelidonya on the southwest coast of Turkey in 1960 by a University 

 of Pennsylvania expedition lead by George F. Bass (1961). The 

 wreck yielded the largest hoard of pre- Classical metal tools and "ox- 

 hide" ingots of copper ever found. In addition there were lumps of 

 tin oxide, all that remained, it is presumed, of ingots of tin. Sea 

 water had promoted the complete oxidation of the tin, but it had acted 

 only superficially on the surface of the copper ingots. Samples of 

 the powdery white tin residues from these ingots were supplied to the 

 author by Mr. Bass. The powder gives the X-ray diffraction pattern 

 of Gcissiterite, Sn02, and the lines of the pattern are as sharp as those 

 given by cassiterite ore specimens. 



In one of the earliest published articles on applications of chemistry 

 to archeology. Dr. Otto Olshausen (1884) of Berlin described three tin 

 objects from early German graves which had been converted almost 

 completely to tin oxide (Zinnsdure) . These are further examples that 

 show why so many early tin artifacts have completely disappeared. 



Tin oxide, however, is an important alteration product on the 

 surface of ancient high-tin bronze objects. Gettens (1949) found that 

 stannic oxide is a major constituent of the smooth gray-green patina 

 on Chinese bronze mirrors and ceremonial vessels sometimes called 

 "water patina" by collectors. It also occurs on high-tin Etruscan 

 and other European bronzes. This alteration product, which may 

 penetrate into the bronze for a distance of 1 to 2 millimeters, may be 

 a hydrous form of a stannic oxide. The pale greenish color is a 

 stain caused by a small impurity of copper. It appears that, under 

 certain conditions of soil contact, the copper is dissolved away from 

 the surface of the bronze and replaced, atom by atom without volume 

 change, by stannic oxide. This phenomenon is identical with 

 pseudomorphic substitution of elements in mineralogy. The tin 

 oxide encasing corroded high-tin bronzes is sometimes compact and 

 translucent like ceramic glaze. Geilmann (1950, 1956) has published 

 the analyses of a dozen Bronze Age artifacts from Europe in which 

 tin oxide resulting from alteration is now the principal constituent. 

 These were all originally bronze objects in which copper has been 

 pseudomorphically replaced by a brownish colored hydrous tin oxide. 



