540 C. H. Mathewson — Metallographic Description 



treatment, the significance of which will be pointed out later 

 on (p. 543). The four axes and chisels of the present collection 

 (Nos. 6, 8, 15 and 16) contain from 3.71 to 5.53 per cent 

 tin. Shepherd and Upton (12) have shown that the ductility 

 of cast specimens decreases rapidly beyond 5 per cent tin. 

 The latter alloy gave an ultimate elongation of about 20 per 

 cent, scarcely inferior to that obtained from pure copper, while, 

 at 10 per cent tin, the elongation dropped to half this value. 

 In the words of the author, "by suitable heat treatment, it is 

 possible to vary the ultimate elongation of a bronze containing 

 90 per cent copper from 10 per cent to 37 per cent without 

 affecting the tensile strength materially." 



As will be explained later on, in dealing with the constitution 

 of these alloys, the heat treatment indicated is adapted to bronzes 

 containing the $ constituent and permits the use of higher 

 percentages of tin than any found in the present collection. 

 It is probable that the early Incas, at least, were unfamiliar 

 with refined methods of heat treatment and were compelled to 

 sacrifice the extra hardness and strength obtainable by increas- 

 ing the tin-content in favor of very free working properties. 

 We would hardly expect the many different objects to show the 

 same composition within narrow limits, since, aside from 

 certain marked variations quite likely to arise in foundry prac- 

 tise which is not subject to analytical control, a greater or 

 lesser degree of softness (measured by the tin-content) would 

 be desired to favor particular requirements in shaping the 

 object. We may add, at this point, that cold-working was 

 invariably depended upon to produce the final stiffness and 

 hardness of an object. 



Constitutional Relationships. 



From a metallographic standpoint, the most noteworthy 

 conclusion to be drawn from the table of analyses given on 

 p. 531 may be expressed as follows: The full range of con- 

 centration covered by this set of alloys corresponds almost 

 exactly with the natural range of stability in the form of 

 so-called alpha solutions of tin in copper. As shown in the 

 equilibrium diagram, fig. 1, the alpha solutions reach from 

 to 13 per cent tin at moderately low temperatures. At higher 

 temperatures, the tin concentration of the saturated alpha 

 solution is lower and amounts to some 9 per cent at 800°. 

 Of the bronzes analyzed, No. 9 (Table I) contains no tin and 

 No. 2 contains the maximum percentage of tin, 13.45. All 



