178 Subsurface Geologic Methods 



Mounting the Cover Glass 



The concluding operation in preparing a thin section is to mount the 

 cover glass. The simplest method of mounting the cover glass is to use 

 cooked balsam dissolved in xylene. To cement the cover glass, place a 

 thin, even layer of prepared balsam on the surface of the section. Put the 

 cover glass in place and press down sufficiently with the eraser end of a 

 pencil to remove all air bubbles and excess balsam. After removing the 

 squeezed-out balsam, the slide is complete. Several days may be required 

 for the xylene to evaporate and for the balsam to set; but with proper pre- 

 cautions, the section can be used immediately. Until the balsam is dry, the 

 slide should be stored in a flat position. 



Cover glasses are obtainable in various sizes and thicknesses. For 

 most purposes the No. 1 thickness is preferable. 



Thin Sections of Fragments or Heavy Concentrates 



The investigator may find that fragmental materials and heavy-min- 

 eral concentrates contain minerals which defy identification by physical 

 or optical means in their present state. If the material is such that a thin 

 section may aid identification, it is not difficult to make one from even 

 rather finely divided substances. The process and materials used are much 

 the same as those used in making an ordinary thin section, except that no 

 sawing is required and the mounting procedure is repeated for the second 

 time. 



To make a thin section of a heavy-mineral concentrate, place a petro- 

 graphic, glass, mounting slide on the hot plate. Put raw Canada balsam 

 on it and cook. Immediately before the balsam is sufficiently cooked, sprin- 

 kle an excess amount of the heavy concentrate into the hot balsam and 

 permit the grains to settle. An excess is required as some material will be 

 lost in a later transfer. For this step, the cooking of the balsam is not 

 critical as this material is merely a holding mount. Remove the slide from 

 the hot plate and allow to cool. 



The next step is grinding a flat, permanent-mounting surface on the 

 mineral grains with a 150- to 200-mesh abrasive. The amount of grinding 

 necessary depends upon the grain size; if the grains are of uniform size, 

 one should grind about halfway through them. The technician must learn 

 the required amount to be ground by experimenting. An inspection under 

 a low-power microscope may prove useful in determining whether or not 

 the surface on the grains is suitable. If not, a transfer to 320- or even 

 600-mesh abrasive may be essential. (The entire grinding procedure may 

 be carried out on glass plates with the desired size abrasive.) 



Assuming that the ground surface is now flat and polished for mount- 

 ing, the slide is cleaned of all foreign material. Now moisten a cloth with 

 xylene and slowly dissolve the excessive cement from between the grains; 

 leave only sufficient balsam to hold the grains in place. 



The next step is remounting the grains on a new petrographic slide 



