and made of plastic materials (materials with amorphous patterns) or glass 

 containing elements of only low atomic weight. The plastic materials are pre- 

 ferred to glass, as meaurements on Pyrex tubes with wall thickness j ust sufficient 

 to permit careful handling show 40 to 50 percent absorption of the CuKoz 

 radiation. Longer wave lengths are absorbed to an even greater extent. Glass 

 appears to be suitable for MoK °c radiation ; however, as will be shown later, Mo 

 radiation is not desirable for use in the identification of components of mixtures. 

 Another mounting method recommended for long-wave-length studies on 

 materials of low atomic weight consists in mixing the powder of the unknown 

 with about 10 percent (by volume) of gum of tragacanth or collodion and 

 extruding it as a rod approximately 0.5 mm in diameter. 



Sample, Microscope 

 _j_ ^5^slides 



1-inch 



Figure 9-5. Jig for rolling samples to predetermined thickness threads. 



An excellent method of mineral specimen preparation used by some of 

 the most prominent workers in the field, although it is usually not described in 

 standard texts nor recommended in the American Society for Testing Materials 

 procedures (E43-42T, 1942), consists in mixing the powder of the unknown 

 with a minimum of DuPont household Duco cement (or other plastic cements 

 such as ethylcellulose in toluene) and then rolling the plastic mass between two 

 microscope slides to form a thin rod of the desired thickness. Thickness can be 

 carefully controlled by inserting the microscope slides in a jig which holds them 

 a fixed, predetermined distance apart (fig. 9-5). The cement acts as binder and 

 diluent, and if kept to a minimum generally will not affect the background of the 

 diffraction pattern. 



157 



