782 HANDBOOK OF PHOTOGRAPHY 



It is obvious that the serrations in the knife edge produce scratches in the face of 

 the specimen and that any section cut from the specimen will have scratches on the 

 upper face due to the previous cut of the knife and on the lower face due to the cut just 

 completed. By decreasing the thickness of the sections, a point will be reached when 

 a complete section is no longer secured but onl}'^ fragments of a section, i.e., the 

 scratches on top and bottom have so little material in between that the section 

 crumbles. It has been found that, when the microtome cuts complete sections 2ix 

 in thickness, the surface of the specimen will be satisfactory to etch for metallographic 

 examination. If it is necessar}^ to set the microtome at 4 n to cut sections, it indicates 

 that the knife is not well honed or stropped. These specimens will not etch free of 

 scratches when viewed under the microscope. Even a well cut and etched specimen 

 often presents to the unaided eye the appearance of slight furrows or undulations in 

 the prepared surface, but these imperfections disappear and are not troublesome under 

 the microscope. From the standpoint of what can be seen with the microscope there 

 is no comparison between the old polishing technique as used for lead specimens, for 

 example, and the newer method of microtome preparation. Moreover a specimen 

 may be prepared in 2 or 3 min. by the sectioning method while the old method not 

 uncommonly required several hours work to prepare a single specimen. The improved 

 means of preparation makes it possible to study the structural effects of small quan- 

 tities of alloying elements, incipient fusion, cold-working, aging phenomena, etc. 



Hard specimens such as iron and steel are gradually brought to a plane smooth 

 surface free from scratches, pits, and other polishing imperfections by a sequence of 

 grinding and polishing operations. 



Large specimens are more difficult to prepare and require a much longer time in 

 preparation than small specimens. The ideal specimen is one about 94 or 1 in. in 

 diameter and not more than j-z in. in height. If the specimen is much higher than 

 this, difficulty will be encountered in maintaining a flat surface. For smaller speci- 

 mens the height should be proportionately less. Specimens much smaller than }-^ in. 

 in diameter are difficult to hold in the fingers, and it will be found advantageous to 

 mount small specimens and irregularly shaped ones in a plastic molding medium such 

 as bakelite. If the edge of the specimen is to be examined, it is always advisable to 

 mount the specimen either by molding it in a compound or clamping it between metal 

 blocks. 



Surface Preparation. — The first step in the preparation of the surface is to grind 

 one face of the specimen flat. A bench grinder may be used for the purpose, and it is 

 desirable in many cases to keep the specimen cool by directing a stream of cold water 

 against the wheel. For general work an Aloxite Brand — 60 grit — Grade J Bond 

 3-ft. wheel measuring 8 by 1 in. and revolving at about 3400 r.p.m. in a vertical plane 

 is recommended. Grinding wheels should not be used if their surfaces are clogged 

 or scored. The edges of the specimen should be beveled slightly on the grinding 

 wheel. If this is not done, the paper work to follow will produce on the specimen a 

 sharp knifelike edge which is liable to catch and tear the lap cloths used in subsequent 

 polishing operations. 



After grinding on the wheel the specimen is ground by hand on various abrasive 

 papers. Aloxite papers are recommended for the purpose, starting with No. 240 and 

 grinding in turn on Nos. 280, 320, and 400. The papers are placed on a bench plate 

 or other smooth hard surface such as a piece of plate glass. The specimen is then 

 lield firmly in the fingers and pushed back and forth slowly under pressure the entire 

 length of the paper. The direction of grinding on the No. 240 paper should be across 

 the previous wheel scratches, and this grinding operation should be continued until 

 the wheel scratches are completely removed. On each succeeding paper a new set of 

 uniform scratches is established across the preceding scratches imtil the latter are 



