526 0. H. Mathewson — Metallographic Description 



statements may be of service to those who are "unfamiliar with 

 this class of work :* 



1. An unaltered cast structure can be recognized at once and 

 a qualitative distinction between rapidly cooled metal, e. g., 

 metal cast in a small unit, and slowly cooled metal, e. g., metal 

 from a casting of considerable size, can be made. 



2. An annealed casting can be distinguished from an 

 unannealed casting, but such annealing would serve no useful 

 purpose in small bronzes of low tin-content and, in any case, 

 would hardly be undertaken except as a preliminary to other 

 operations resulting in more or less complete obliteration of the 

 casting structure. 



3. Mechanical alteration of a cast piece, or of a piece with 

 more extended but not necessarily known past history, by 

 rolling, hammering, etc., can be detected with certainty pro- 

 vided the piece has been worked with considerable intensity. 

 A few gentle blows with the hammer, a bend, or a twist, would 

 not alter the visible structure of the metal, but mechanical 

 treatment resulting in pronounced change of form gives rise 

 to characteristic structural changes as well. These effects do 

 not, however, serve to differentiate at all sharply between dif- 

 ferent forms of mechanical treatment, whether by rolling, 

 hammering, etc. It seems necessary to rely mainly upon sur- 

 face observations for evidence of this character. Under the 

 influence of mechanical treatment, the metal stiffens and gradu- 

 ally loses its ductility until, eventually, further change of 

 form cannot be brought about without fracture. The precise 

 nature of these changes cannot be detected by observations under 

 the microscope. Certain coincident effects, such as elongation 

 and etching peculiarities of the crystalline grains, serve, 

 together with physical tests, to establish the worked condition 

 of the metal. 



4. Annealing restores the ductility by producing an entirely 

 new growth of grain in which certain unfailing characteristics 

 may be recognized. Thus, a recrystallized bronze can be recog- 

 nized at once and the evidence of previous mechanical treatment, 

 with following anneal, is thereupon complete. The nature of the 

 recrystallization, whether fine- or coarse-grained, localized or 

 evenly distributed, incipient or sufficiently developed to entirely 

 obliterate the pre-existing structure, depends upon the compo- 

 sition of the metal, the nature, intensity and distribution of 

 the mechanical treatment, as well as the temperature and dura- 



* Ample discussion and illustration of these statements will be given 

 in ensuing portions of this paper. 



