of Bronzes from Machu Picclm, Peru. 557 



31, represents the same surface, re-polished and etched, after 

 the specimen had received a homogenizing anneal of one and 

 one-half hours at 775°. Here, the contrasts are greater owing 

 to the homogeneous character of the grains. Practically no 

 change in the individuality of the grains has occurred, i. e., 

 there has been no growth of grain. The large individuals of 

 fig. 30 can be recognized in fig. 31 and the small grain previ- 

 ously mentioned has retained its orientation during the heat 

 treatment in spite of the directive influence exerted by the large 

 grain which totally surrounds it. In repolishing the specimen, 

 about 0.003 in. was removed from the surface. Consequently, 

 the small grains, some of which are only a few thousandths of 

 an inch in diameter, do not wholly correspond in the two photo- 

 micrographs. The general effect of re-polishing and etching 

 may be seen by comparing fig. 31 with fig. 32, the latter repre- 

 senting a re-treated surface, which, although subsequently 

 strained, shows how the grain-contours of fig. 31 are altered by 

 a second preparation of the surface. 



Any ordinary annealing treatment of cast bronze of this 

 character does not bring about growth of grain. Even the 

 small grains of fig. 30 have not coalesced upon annealing. This 

 may be due to the fact that the grains, while small, are still 

 larger than the normal grains produced by annealing after 

 strain at this temperature. Moreover, the surface conditions 

 in the case of cast grains are less favorable to coalescence than 

 in the case of deformed grains, since, here, a film of impurity 

 may separate the grains, while, after deformation, recrystal- 

 lization starts within the original grains, at the internal 

 surfaces of slip, where there are no impurities to prevent coales- 

 cence or destruction of free surfaces under the influence of 

 surface tension. 



Drilling of the specimen has produced a condition of strain 

 which has caused recrystallization upon anneal, as shown in 

 fig. 31. The metal surrounding the aperture now shows the 

 usual fine-grained recrystallized structure in which twins are 

 abundant. A few small twinned grains are also seen at other 

 points upon the surface. This shows that not enough of the 

 surface was removed by polishing after anneal to entirely 

 remove the layer which was strain-hardened in the preceding 

 polishing operation and which recrystallized during the anneal. 

 After the second preparation of the surface (fig. 32), no 

 twinned grains of this sort can be seen. 



The small section from which these photo-micrographs were 

 obtained was rectangular with deeply rounded edges at the 



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

 38 



