of Bronzes from Machu Picchu, Peru. 565 



concentration between such points or the actual rate of cooling 

 which gave rise to the structure in question. It is probable 

 that very similar distribution of material is encountered when- 

 ever the average distance between branches (in different 

 specimens) is the same. 



The Peruvian bronze objects are, for the most part, small 

 and they were cooled rapidly enough during casting to produce 

 grain characteristics quite comparable with those obtained in 

 the laboratory by chill-casting 60 gram ingots in an open iron 

 mold from a pouring temperature just high enough to hold 

 the metal fluid during the filling of the mold. The laboratory 

 specimens measured 0.015 to 0.04 mm. between branches and 

 a large number of them was used in homogenization experi- 

 ments with wholly consistent results. In these homogenization 

 experiments, the effect of time and temperature was investigated 

 with respect to this single type of casting structure, which, as 

 already explained, is approximately comparable to some of the 

 structures observed among the Peruvian bronzes. One bronze 

 only (92 per cent copper) of distinctly coarser structure was 

 homogenized at one temperature only (775°) in order to obtain 

 a rough idea of the effect of the third variable on the rate of. 

 homogenization at a given temperature. 



There is no standard test which may be applied to determine 

 the degree of homogenization after a given treatment. We 

 must rely upon the appearance of the specimen after etching 

 and seek to characterize this in some way. Different etching 

 agents produce different effects and the same etching agent 

 may also produce different effects on the same specimen unless 

 used in wholly similar manner. In all of the experiments, a 

 mixture of ammonia and hydrogen peroxide was used under 

 uniform conditions as far as possible. Six stages in the appear- 

 ance of a specimen were distinguished and designated as fol- 

 lows: (1) VD, very distinct zones, or cores; (2) D, distinct 

 zones; (3) F, faint zones, clearly visible when the principal 

 detail of the specimen is in focus ; (4) VF, very faint zones, 

 visible only as a shadowy effect by manipulating the fine 

 focussing adjustment; (5) WF, less distinct than the preced- 

 ing; and (6) II, homogenized, no traces of zones. The final 

 condition does not imply complete homogenization, since these 

 specimens commonly tarnish on long standing in such manner 

 as to indicate some residual inhomogeneity. It does, however, 

 represent a distinct stage approximating the end-point and 

 clearly comparable in different specimens. 



