562 ANNUAL REPORT SMITHSONIAN INSTITUTION, 19 63 



The second reason mentioned above for the rarity of metallic tin 

 may be a physical change called "tin pest," whereby tetragonal metal- 

 lic white )8-tin is transformed to cubic powdery grey a-tin by allo- 

 tropic modification at equilibrium temperature about 13.2° C. (55.8° 

 F.). This behavior of tin is well described by Plenderleith and 

 Organ (1953) and by F. Lihl (1962) who admit the existence of allo- 

 tropic modification but minimize its importance in respect to the decay 

 and disappearance of ancient tin objects. They agree that often what 

 appears to be tin pest is nothing more than intergranular oxidation of 

 tin with formation of mixed stannous and stannic oxides. This view 

 is strongly supported by the X-ray diffraction analysis investigation 

 carried out by Lihl on old corroded tin sarcophagi in Vienna. 



SILVER 



SILVER SULFIDES 



Like tin, silver forms few alteration products. Black silver tar- 

 nish formed by action of hydrogen sulfide in the atmosphere is familiar 

 to every householder. Strangely, the occurrence of the well-known 

 silver mineral argentite, Ag2S, has not been noted in the literature. 

 Two occurrences of argentite, however, have been observed recently in 

 the Freer Gallery Laboratory in the soft black corrosion crusts formed 

 on silver coins found in wrecks of wooden Spanish treasure ships. 

 One is a silver "piece of eight" from the Spanish ship San Antonio, 

 which sank off Bermuda in 1621. Another is from a ship of the 

 Spanish treasure fleet lost off Key Largo, Fla., July 13, 1733. It has 

 been previously noted that copper objects retrieved from ships long 

 sunk are commonly altered to sulfides (see under Copper) . Source of 

 the sulfide ion again may be sulfate-reducing bacteria harbored by the 

 decayed wood. A. B. Albright of the Department of Armed Forces 

 History, Smithsonian Institution, says that old wooden fragments 

 brought to the surface from these marine wrecks often reek with 

 hydrogen sulfide. 



Perhaps the rare mineral, stromeyerite, CuAgS, should also occur 

 where copper-silver alloys are subjected to similar environments; 

 but, if so, it is one of those many interesting corrosion products that 

 go down the workshop drain unrecognized, unadmired for color, and 

 unappreciated for rarity. 



Acanthite, AgzS, an orthorhombic dimorph of argentite was ob- 

 served as shining faceted black crystals inside the hollow stem of a 

 medieval silver chalice excavated at Mellifont, Eire. Identification 

 was made by the Department of Mineralogy, British Museum (Natural 

 History). It was also found as a black inlay (niello?) on a Sasanian 

 dish made of base silver by the Research Laboratory of the British 

 Museum. 



