RAW QUARTZ, ITS DEFECTS AND INSPECTION 359 



wide variation of the types, sizes and concentrations of defects, and the 

 further variation of the types, sizes, and requirements to be met in the 

 finished plates. However, an analysis of the factors involved leads to some 

 important conclusions. The various factors may be correlated as follows. 

 Fine textured inclusions are less objectionable than coarse textured inclu- 

 sions (i.e., smokiness, and blue needles and phantoms, than white, or bubble 

 needles and phantoms). Isolated defects are less objectionable than con- 

 centrated defects of the same texture size (i.e., isolated bubbles, than bubble 

 phantoms). Cracks should never be permitted in finished plates. Twin- 

 ning will be discussed in detail in Chapter V. 



Fig. 4.15. This stone could be economically processed by cutting off the base rear 

 line C-C. A few scattered bubbles appear at D, fractures and bubbles fill the base B, 

 and A is a bubble veil. 



Further, a given defect is more likely to be tolerated in: (1) large piezoids 

 (finished piezoelectric elements) than in small ones; (2) in low-frequency- 

 mode plates (CT and DT types) than in high-frequency-mode plates {A T 

 and BT types); (3) in plates to be operated at low amplitudes of vibration 

 (filter elements, and oscillator plates with low drive) than those driven to 

 maximum amplitudes (oscillator plates with high drive); and (4) plates 

 having low rather than high-quality requirements (on activity, temperature- 

 coefficient, frequency adjustment). 



Thus, blue needles have long been permitted in some types of large, low- 

 frequency-mode filter elements. While breakage has resulted from the use 

 of blue needles in high-frequency oscillators with very high drive, it is not 



