528 0. H. Mathewson — Metallographic Description 



as a whole. Most of the bronzes contain sulphur in small 

 amount which, from the chemical relationships involved, must 

 occur in the form of cuprous sulphide. In the thin, flat bronze 

 knives of the collection, the particles of cuprous sulphide are 

 elongated to a degree attainable only through several successive 

 drawing and annealing operations. A copper knife of similar 

 form contains in the neighborhood of one per cent of cuprous 

 oxide which, in the broader and thicker parts of the specimen, 

 occurs in normal eutectic form as small globules characteristi- 

 cally grouped throughout the copper matrix. In the intensively 

 worked parts of the specimen, notably the blade, each group of 

 oxide particles has been dragged out into a continuous train 

 which gives the appearance of a dotted line in the photo- 

 micrograph (fig. 71). 



6. Positive identification of hot- worked metal is not always 

 possible, since, in hot-working at effective temperatures, the 

 deformation is followed by recrystallization as in the anneal of 

 cold-worked metal. Variables of the same character affect the 

 result. In hot-working, however, recrystallization always takes 

 place at temperatures* which would produce relatively coarse 

 grain if time enough were allowed. Thus, the grain size is 

 determined mainly by the time available for recrystallization 

 between the blows, or after working has ceased. With small 

 specimens, this is almost invariably brief, since the small mass 

 of metal cools rapidly. Upon re-heating, a coarser grain will 

 develop a relatively coarse grain while other portions are 

 work the whole mass of metal simultaneously, attention will 

 first be directed to some particular portion which will later 

 develop. Since, with primitive facilities, it is impossible to 

 being worked. This results in a highly non-uniform conglom- 

 erate. Hot working is, therefore, indicated where coarse and 

 fine grains are found closely associated and where neighboring 

 parts of the specimen exhibit wide variation in grain size. 



The foregoing statements will indicate in a general way what 

 manner of conclusions may be drawn from metallographic 

 examination as applied to metal of sensibly uniform composi- 

 tion throughout, viz., in the case of pure metals or solid solu- 

 tions. Some of these statements will be elaborated further on 



* Where both hot-working and cold-working are permissible, as in the 

 case of these alpha bronzes, the former is undertaken in order to avoid 

 the necessity of annealing between stages of intensive working, i. e., the 

 metal stays soft, owing to rapid recrystallization and relief of strain, if the 

 temperature is high enough. A full red-heat is required for continued 

 softness under a rapid succession of blows. 



