572 C. H. Mathewson — Metallographic Description 



throughout the whole casting operation is very much greater 

 than in the present case. The absorption of the a + $ 

 complex was, therefore, brought about by reheating. Since 

 reheating at bright red heat causes practically complete homo- 

 genization in a very few minutes,* it is, on this basis alone, 

 almost certain that the reheating was moderate, i. e., at a low 

 red heat, in order that the present heterogeneity of the specimen 

 may be accounted for. Other reasons for the same conclusion 

 will be stated presently. 



The normal site of the a -\- h complex, viz., the centers 

 of the rounded tin-rich excrescences already described, is fre- 

 quently occupied by very small patches which are faintly dis- 

 tinguishable from the surrounding somewhat lighter material. 

 The particles of cuprous sulphide sometimes appear as kernels 

 within these faint patches. These patches were not observed 

 in any other specimen, either Peruvian or synthetic, and their 

 identity is undetermined. They may represent a stage in the 

 absorption of the a -f- 8 'complex under the influence of the 

 0.3 per cent of iron which is present in this specimen. The 

 only other specimen containing an appreciable amount of iron, 

 viz., the axe numbered 8, is entirely homogeneous aside from 

 the considerable quantity of sulphide which it contains and 

 cannot be used for comparison in this respect. 



The whole knife was undoubtedly cast in one piece. The 

 structure shown in fig. 37 is continuous across the specimen 

 in the region where welding was suspected. ISTot the least 

 trace of a weld or other type of joint can be found. This 

 structure is, however, modified in the regions, a and c, which 

 are differentiated from b and d in the diagrammatic sketch. 



This modification has been effected mainly by cold-working. 

 In these pages, we shall use the term, cold working, with 

 reference to the temperature range within which work upon 

 the metal produces rather permanent distortion visible as lines 

 of deformation within the crystalline grains. The upper limit 

 of this range approximates incipient red heat. That the piece 

 was heated to a temperature of recrystallization either during 

 or after working is shown by the occurrence of small polyhedral 

 grains, largely twinned, as a secondary structure. f It cannot 



* Compare Diffusional Characteristics, pp. 562 to 569. 



f The possible occurrence of congenital twins does not invalidate the 

 conclusion drawn above. In common with others (cf. Desch, Metallog- 

 raphy, London, 1913, p. 183) we have, in this laboratory, observed twin 

 formation in annealed castings (bronze, german silver, cupro-nickel). In 

 no case, however, was it certain that the metal had been rigorously 

 guarded from strain incidental to handling, etc. Thus far, we have been 



