of Bronzes from Machu Picchu, Peru. 559 



The two ensuing photo-micrographs, figs, 34 and 35, repre- 

 sent further stages in the treatment of this specimen, viz., an 

 anneal of one hour at 775° and one of 15 minutes at 850°, 

 respectively. In both cases, there is very little change in the 

 appearance of the regions which were originally subjected to 

 moderate strain. In the severely strained regions, however, 

 the grains have grown in accordance with the temperature-time 

 requirements previously set forth (see earlier discussion under 

 the present heading). 



Theories oe Deformation. 



The finest grain which can be produced within a metal results 

 from reorganization of the material either by spontaneous 

 transformation according to the principles of heterogeneous 

 equilibrium whereupon new and initially minute crystalline 

 individuals are formed, or by disperse readjustment of the 

 component crystalline particles without change of phase. Since 

 no transformations (phase changes) are involved in the heat 

 treatment of the alpha bronzes, only the latter condition need 

 be considered here. 



An extremely fine grain is produced when highly worked 

 metal recrystallizes at the lowest effective temperatures. The 

 growth of grain by coalescence has been described. We have 

 still to account for the condition which causes the initial 

 development of extremely fine grain from a primitive coarse 

 grain. Deformation produces a condition in the primitive 

 grain which causes numerous new grains to develop on anneal- 

 ing. The starting point for each new grain is obviously a 

 fragment of the primitive grain, dislocated, or forced out of 

 alignment with neighboring particles during the process of 

 deformation. 



The first changes which occur in an individual grain when 

 it is strained to the point of permanent deformation are well 

 understood, at least from a proximate standpoint; sliding 

 movements occur along planes which correspond to the cleavage 

 planes of minerals so that a set of cleavage elements is formed. 

 These elements do not fall apart, however, but are held in 

 place by adhesive forces of some description. As deformation 

 proceeds, the planes of slip become more numerous and the 

 cleavage elements become smaller. The process may be fol- 

 lowed to a certain extent under the microscope by observing 

 the polished surface while under strain, but the precise nature 

 of the movements under severe deformation, leading finally to 

 rupture, cannot be clearly recognized. 



