of Bronzes from Machu Piccliu, Peru. 539 



increases, alloys from the same initial heat will remain longer 

 in the fluid condition, whereby the casting operation is facili- 

 tated, particularly in the case of small objects which tend to 

 chill rapidly. 



Modern bronze compositions used in casting objects of art 

 (statuary, etc.) usually contain fair percentages of zinc. This 

 lowers the freezing point, increases fluidity, soundness, and 

 adaptability to hammering, chipping, etc. (lead is also an 

 important addition agent where the latter property is particu- 

 larly important), decreases cost and produces a pleasing color 

 effect. Thurston (9) specifies several suitable compositions, 

 as follows: (1) Copper— 92, Tin— 2, Zinc— 6; (2) Copper— 

 85, Tin— 5, Zinc— 11; (3) Copper— 65, Tin— 3, Zinc— 32. 

 The expansion during solidification and total shrinkage are 

 generally favorable in well chosen ternary allovs of this type. 

 Cf. Miller (10). 



Zinc occurs only as an incidental impurity in these bronzes 

 and the Inca metallurgists were unable to avail themselves of 

 its useful properties in this connection. ISTor were they 

 acquainted with other addition agents (e. g., phosphorus) 

 which, by reducing the heavy, mechanically entangled tin oxide, 

 itself formed during alloying by the reducing action of tin 

 on the copper oxide commonly present in molten copper, slags 

 out the oxide and cleans up the metal. Thus, their castings 

 leave much to be desired by way of soundness and strength. 



Having accounted for the high percentage of tin in orna- 

 mentally cast objects, some attention will now be devoted to 

 the objects which require strength and hardness for general 

 industrial uses. ~No unusual characteristics were observed in 

 any of these bronzes. They are simple alloys of tin and copper 

 in which the physical properties were modified by the ordinary 

 operations of forging, annealing, and cold-working. There is 

 no evidence that any special heat treatment was adopted in 

 order to facilitate the working of alloys high in tin. The 

 metallographic tests indicate that cast pieces were usually 

 hammered in their original condition and annealed as occasion 

 demanded. Only moderate percentages of tin, preferably 

 below 1 or 8, are safe when free working of the cast metal is 

 a primary consideration. Foote and Buell (11), in a recent 

 examination of three Peruvian bronze axes of uncertain origin, 

 found 12.03, 5.58, and 3.36 per cent tin, respectively. They 

 were unable to cold-work cast metal similar in composition to the 

 first of these axes without first applying a special form of heat- 



