INDUSTRIAL RESEARCH 



sive papers, and the metallographic polishing metallographic procedure (20, 21). Even 

 performed on wheels covered with cloth textiles have been etched. However, when 

 bearing different grades of abrasives. In reflectivity is low, image contrast has been 

 cutting, grinding, and polishing, flow or cold improved by metallization, 

 work is usually produced, but may be essen- Metallization technicjue (7) consists of the 

 tially eliminated in most specimens by re- vacuum evaporation of metal onto the speci- 

 peated etching and repolishing with the finer men. The metal is evaporated from a heated 

 grades of abrasives. The pulling out of struc- filament, and during the evaporation travels 

 tures is another common difficulty in grind- in straight paths from the filament to the 

 ing and polishing. When this takes place, the surface of the specimen where it deposits 

 holes left in the surface may be misconstrued only on the exposed portions. If the specimen 

 as inherent porosity in the sample. These is inclined to the direction of evaporating 

 are but a few cases, but they serve to exem- metal, a shadow effect is produced. The as- 

 plify a typical artifact incurred by metal- perities receive a heavier metal coating and 

 lographic preparation. the pits less or none. The difference in re- 

 Even small quantities of finely particulate flectivity and resulting image contrast is 

 minerals (21), fibers, textiles, and papers can expressed in terms of surface topography, 

 be prepared by metallographic methods. It is Even when metallizing is done at normal 

 often expedient to study a polished mount of incidence, and no shadow effect is produced, 

 a material rather than to attempt to cut the a more contrasty image will be seen by 

 sample into thin sections. The plane of sec- vertical illumination. Surface relief is present 

 tion is not difficult to control, and if a trans- in some specimens in the polished condition 

 parent mount is made, it is sometimes due to differences in rates of abrasion among 

 convenient, by looking into it, to view the specimen constituents. Metallization has 

 surface of the specimen and relate it to the been used extensively in electron microscopy 

 internal structure visible at the plane of of surface replicas (23). 

 section. The epoxy resins, in particular, Replication plays an important role in the 

 Epon 828* catalyzed with triethylene tetra- microscopical examination of surfaces in a 

 mine, have been found quite satisfactory in wide variety of industrial problems. Replicas 

 mounting soft materials prior to sectioning, may be made from plastic, most often cellu- 

 grinding, and polishing. Frequently these lose acetate in the authors' laboratory, and 

 samples must be impregnated with the they are employed to reproduce surface 

 medium before mountiiig and sometimes this structure. An impression or imprint is made 

 can be done by vacuum impregnation. Im- by pressing the surface of a plastic sheet, 

 pregnation is also possible by an adaptation softened by solvent, onto the surface of a 

 of embedding techniques used in the prepara- sample. The plastic is allowed to dry while 

 tion of biological specimens for electron mi- in contact with the sample. When they are 

 eroscopy (22). By this method, the sample is separated, the plastic retains a negative 

 soaked in a liquid which is a solvent for the imprint of the sample; that is, the topog- 

 epoxy monomer, then in a solution of mono- raphy of this replica is a mirror image of the 

 mer in this solvent, and later in catalyzed sample. Not always is it necessary to metal- 

 monomer which is subsequently hardened by hze replicas to achieve adeciuate contrast, 

 polj^merization. Enough light is sometimes reflected, by sur- 

 Most metals and some minerals are sufh- faces bearing no metal, to give instructive 

 ciently reflective to produce images of high images. Artifacts most commonly found in 

 contrast when etched, and this is a common replicas are bubbles resulting from incom- 

 * Available from Shell Chemical Company. plete contact while taking the imprint. It is 



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