of Bronzes from Machu Picchu, Peru. 573 



be determined by metallographic examination whether this 

 reheating followed cold working of the metal or marked the 

 beginning of the mechanical treatment. It appears most 

 probable that reheating was solely for the purpose of softening 

 the metal to facilitate cold-working. In any event, the tem- 

 perature employed was relatively low, or the period at red heat 

 was very short as shown by the small size of the grains and 

 the general subversion of this secondary structure to the primary 

 cast structure. Furthermore, even if the working was started 

 hot, the metal was vigorously worked as the temperature fell 

 below red heat and finally left in the cold-worked condition. 



It is inconceivable that this piece was repeatedly hammered 

 and reheated to an annealing temperature, as was almost cer- 

 tainly the case with certain other specimens to be described 

 later. Such treatment results in complete refining of the 

 grain with little or no indication of the pre-existing cast 

 structure. 



As already suggested, the cast structure was modified 

 mechanically only in the parts a and c. The amount of reduc- 

 tion desired was small and, according to my interpretation 

 of the structures encountered, the shank was constricted towards 

 the center and the blade tapered to a blunt edge through a 

 process of hammering somewhat facilitated by mild reheating. 



The complete development of the structure of this specimen 

 was a matter of some difficulty. As already indicated, three 

 distinct, but superimposed, structures were observed; (1) the 

 cast structure, somewhat modified (partially homogenized) by 

 heat treatment, (2) the recrystallized structure, appearing as 

 an incipient refining of the cast structure, and (3) the inter- 

 granular deformational detail of (2) due to overstrain during 

 cold working. Since each of these structures possesses its 

 own etching characteristics, it is clear that all of them will not 

 be clearly indicated in a single etching. Thus, etching of the 

 character shown in fig. 37, in which maxinram heterogeneity 

 was developed by using ammonia and hydrogen peroxide, fol- 

 lowed by ferric chloride and hydrochloric acid, rather effectively 

 masks the (2) type of structure. This is clue to the fact that 



unable to produce twin crystals by annealing castings which have been 

 handled with extra care. Vigorous agitation, or stirring, during crystal- 

 lization does not lead to their formation in bronze. Whatever the facts 

 relative to the genesis of twin crystals in cast metal which has not been 

 intentionally strained, they cannot be mistaken for those developed after 

 vigorous working of the metal. In the former case, the twinning is 

 seen only in isolated regions, while in the latter case, it is everywhere 

 abundant in the case of copper and its alpha solutions. 



Am. Jour. Sci. — Fourth Series, Vol. XL, No. 240. — December, 1915. 

 39 



