GENERAL MICKOSCOPV 



methods are often followed by specific ones the decision concerning specimen prepara- 



unique to the problem under investigation. tion as has the transparency. 



Once a representative sample has been There is no hard-and-fast rule which must 

 decided upon, it is advisable, even before be observed stating that all opa(iue samples 

 preliminary preparation, to obtain from it are best studied by reflected light, or con- 

 as much information as is possible. This versely that transparent samples must 

 should be done to decide upon a preparative be studied by transmitted illumination. In 

 procedure, as well as to ascertain the ap- many instances, it is advantageous to make 

 pearance of the unaltered structure, and specimens of opacjue materials to be studied 

 thereby assist in the recognition of spurious by transmitted illumination (e.g., opaque 

 phenomena (artifacts) W'hich may be created particles to be studied in profile) and vice 

 by the specimen preparation. versa; nevertheless, in general, the examina- 



The data collected from a specimen prep- tion will be carried out using the conven- 

 aration are limited by many factors, some of tional combinations. It may be added that 

 which can be anticipated and others which not all specimens are strictly opaque or 

 are not easily predicted. One of the foremost transparent, and they may be viewed profit- 

 problems is learning to distinguish true struc- ably in either or both illuminations. All of 

 ture, especially that which is significant, these various situations will be discussed 

 from the above-mentioned artifact intro- here. 



duced by the sample treatment. Obviously, Specimen Preparation for Reflected Light. 

 an accurate prediction of the type of artifact Metallographic, some mineralogical, and 

 likely to be encountered in a specimen would ceramic specimens fall into this category, 

 be quite helpful. This is possible to a certain Several available publications describe in 

 degree in almost any situation, but never detail the conventional techniques in this 

 entirely so. Recognition of artifact is of prime area (19, 20) Briefly, the sample is mounted 

 importance, both during specimen prepara- in a plastic material which aids in holding it 

 tion and in the interpretation of data. It is during subsequent grinding, polishing, etch- 

 hoped that some assistance will be provided ing, and viewing. The mounting medium 

 b}^ mentioning some of the inherent artifact may be thermoplastic and, therefore, can be 

 hazards incidental to the preparative meth- formed in molds under pressure and tem- 

 ods discussed in this section. Nevertheless, perature, or thermosetting, as for example, 

 major emphasis is devoted to the proper the epoxy resins, which can be polymerized 

 choice of technique from the perspective of at low temperatures and under no pressure 

 sample character. to produce a suitably hard mount of soft 



Logically, specimen preparations may materials. These mounting materials should 



be classed in two categories according to be examined critically to look for materials 



whether the material is transparent or which may be mistaken for constituents 



opaque. The importance of this classification of the sample. For example, some thermo- 



is brought out by the existence of the two setting formulations contain highly reflective 



major types of microscopes, which have been particulate structures. The effects of the 



designed for the express purpose of examin- mounting temperatures and pressures upon 



ing opaque or transparent specimens. There the sample also deserve consideration. They 



are other qualities of samples which deter- may induce changes which render the speci- 



mine specimen preparation, some of which men useless. 



are size, hardness, roughness, solubility. Sectioning of materials after mounting can 



elasticity, fluidity, and crystallinity. How- be done with a cutoff wheel. The grinding 



ever, none of these has the marked effect on and polishing can be carried out with abra- 



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