The Insides of Metals^ 



By Carl A, Zapffe 



Metallurgist, Baltimore, Md. 



[With 4 plates] 



Lost in antiquity are the origins of many methods used for examin- 

 ing metals, but the epochal discovery of the optical microscope a few 

 hundred years ago originated the study of metals at high magnifica- 

 tion. In its early stages, investigation with the microscope was limited 

 to an exploration of surfaces. This was uninstructive because the 

 principle service of metals lies in their strength, hence in their in- 

 ternal constitution. The French scientist de Reaumur in 1722 and 

 Sweden's Sw^edenborg in 1734 advanced the application of the micro- 

 scope somewhat by studying the surfaces of fractures of metals, which 

 disclosed some information regarding the manner in which metals 

 are constituted. However, the difficulty of bringing the microscope 

 lens close to the jagged surface of a fracture discouraged, for more 

 than two centuries, these and all later scientists from developing 

 such an application. 



Among students of minerals, the microscope became a tool — and a 

 great one — principally through the discovery in 1849 by Henry Clifton 

 Sorby that minerals could be examined by transmitted light if they 

 were sliced sufficiently thin. This introduced the thin-section tech- 

 nique, which is the bulwark of the science of petrography and min- 

 eralogy today. 



Metals, in considerable contrast to most minerals, are completely 

 opaque and do not submit to such thin-section study. Gold leaf has 

 been beaten so thin that it transmits some light, but this is an excep- 

 tion which contributes nothing to the problem. It was at the close 

 of the last century that metallurgists in Europe discovered the method 

 of polishing and etching the surface of sections cut through metals 

 to disclose their inner structure, and this has been the means of provid- 

 ing almost the entire body of technical information on the microscopic 

 constitution of metals to date. 



» Keprinted by pprmisslon from IMiyslcs Today, vol. 3, No. 9, September 1950. 



253 



