of Bronzes from Machu Picchu, Peru. 585 



left at rest at high temperature after working (ordinary 

 annealing after cold- working or a sojourn at the forging tem- 

 perature while work was confined to another portion) and 

 the same ultimate grain characteristics are developed in either 

 case. The observed grain count of 13 indicates that this 

 temperature was in the neighborhood of 800.° 



The condition of the metal near the edge, however, throws 

 much light upon this question. Passing towards the edge 

 from the point at which photo-micrograph, fig. 58, was taken, 

 the grains very soon begin to show lines of deformation and 

 elongation in the direction of the principal axis. Moreover, 

 smaller recrystallized units begin to appear among the distorted 

 parent grains. This structure is shown in fig. 57, after ordi- 

 nary etching with ammonia and hydrogen peroxide. (This 

 method of etching permits a clearer photographic reproduction 

 of the finer detail than is ordinarily the case with ferric 

 chloride, which gives pleasing contrasty effects with coarser 

 structures, as in fig. 58.) As the edge of the chisel is 

 approached, fig. 56, the grains become finer, the lines of defor- 

 mation more abundant, and the elongation more pronounced. 

 In other words, the proportion of strain-hardened metal is 

 greater. This was tested by the scleroscope which gave average 

 values of 15, one half inch from the edge; 24, five sixteenths 

 of an inch from the edge; and 30, three sixteenths of an inch 

 from the edge. 



The structures shown in figs. 56 and 57 are characteristic 

 of hot-working at a temperature in the vicinity of 800° with 

 continuation of working as the temperature falls to a low value. 

 As a grain is deformed under the blow of the hammer, it 

 spontaneously recrystallizes at those points where the strain 

 was greatest. In tapering an object, as in the present case, 

 deformation is greatest in the thinner portions, hence there will 

 be more points of recrystallization in these regions and more 

 abundant lines of deformation if the cold-work is confined 

 largely to these regions, as is likely to be the case. The same 

 succession of structures was obtained in the laboratory by hot- 

 and cold-working a piece of metal from the shank of the chisel 

 to a tapered edge.* The structure at one point is shown in 

 fig. 62, with diagram of position at the left, fig. 61. It will 

 be observed that this structure corresponds closely with that 

 shown in fig. 57. Cold-working has been somewhat more 



* Metal from this chisel was used in order to obtain the best possible 

 comparison. Precisely the same effects were obtained with alloys prepared 

 in the laboratory. 



