632 rawdon: standard zinc-bronze 



the tensile bar. The crystals after slow cooling are relatively 

 large and instances were found where the fracture occurred at a 

 point where one crystal extended across the greater portion of a 

 cross-section of the bar and thus determined the mechanical 

 properties of the whole sample. Often, adjacent crystals are 

 very similarly oriented and, mechanically, are practically equiva- 

 lent to a single crystal. In ordinary sand castings, however, 

 such cases are apparently unavoidable. 



Of all possible factors affecting the tensile strength the pres- 

 ence of oxides in the form of pits and films is especially serious. 

 The following illustrates the delete'rious effect of such oxide 

 films: 



In test-bars showing a tensile strength above 35,000 pounds 

 per square inch, oxide films were rarely found and then only in 

 small isolated spots. 



The presence of oxides of tin and zinc in the form of pits and 

 films may be considered, then, as the predominating factor in the 

 cause of mechanical weakness of cast bronze. 



