538 C. H. Mathewson — Metallo graphic Description 



It is far easier to believe that the Incas learned to recognize 

 certain characteristic copper and tin minerals which were care- 

 fully hand-picked and smelted independently, or perhaps in 

 association. Native copper, which occurs in small quantities 

 throughout the Cordilleras, was probably known to them, while 

 stream tin (cassiterite), also of likely occurrence in a limited 

 sense, would furnish a very pure source of tin. The idea that 

 native copper may, itself, contain percentages of tin equivalent 

 to those encountered in these bronzes cannot be entertained. 



The fact that the percentage of tin contained in the Inca 

 bronzes is not governed by the use for which they were intended 

 raises the question as to what has governed the percentage of 

 tin in case this was subject to some controlling influence. 

 Joyce's observation that tin is found in greatest quantity in 

 those objects which require it least is particularly interesting, 

 since it suggests that the objects which require the least tin 

 for proper service may require the most tin for some other 

 reason. While I have not found time to search carefully 

 through South American archaeological literature for analyses 

 pertaining especially to objects of known character, the present 

 collection offers certain evidence bearing upon this question. 



This evidence deals with the casting properties of bronze. 

 As far as the present collection goes, those objects which would 

 require the least tin in service are the more delicate, or orna- 

 mental pieces. As already pointed out, the two finest castings, 

 Nos. 2 and 5, contain maximum percentages of tin (13.45 and 

 9.39, resp.). A few rough experiments indicated that these 

 high percentages of tin yield the best impressions in casting. 

 Bronzes of this character expand in solidifying, whereby the 

 finer details of the mould are registered in the metal, even 

 though subsequent contraction, on cooling to ordinary tempera- 

 ture, determines a total shrinkage effect. Wust (7) determined 

 the value of this expansion (also the total shrinkage) in alloys 

 containing approximately 5, 10, and 20 per cent tin. The 

 percentage expansion was found to be 0.085, 0.122, and 0.01, 

 respectively. Later, Haughton and Turner (8) investigated 

 the same property, using a somewhat different method, and 

 located the maximum expansion at 10 per cent tin. Thus, we 

 see the advantage of choosing alloys containing in the neighbor- 

 hood of 10 per cent tin for casting purposes. It is also worthy 

 of note that the 10 per cent alloy begins to freeze at a tempera- 

 ture some 50° lower than the 5 per cent alloy, or some 80° 

 lower than pure copper. This means that, as the tin-content 



