ETCH TECHNIQUE 



19 



pits are quite distinctive, when well developed. Use may be made of a 

 microscope or a high powered projector to view the figures. The pit out- 

 lines may be aligned with lines ruled on the eye-piece or on the screen, and a 

 tLxed marking device may be used to mark the quartz surface with orienta- 

 tion lines. Twinning may be detected by the appearance of different etch- 

 pits as the specimen is moved about. For example, on an electrically 

 twinned X-cut surface both X-cut views of Fig. 5.5 could be found. How- 

 ever, the location and marking of twinning boundaries involves a tedious 

 exploration of the surface, since only a minute portion is viewed at any one 

 time. This exploration may be eliminated if the surface is first viewed by 

 reflection methods where the whole surface and extent of twinning is at once 

 seen (as in Fig. 5.1) and marked. 



i". 



-35 CUT 



+ 35, AT-CUT 



Fig. 5.6 — Etch-pits on the etched surface of a +35° AT plate, and on an analogous but 

 wrong sensed —35° plate. This difference in etch-pits may be used in the manufacturing 

 process to determine the right and wrong sensed regions of twinned AT slabs. 



A special case where the microscope or projector method might be em- 

 ployed is in the examination of thin AT, BT, CT or DT slabs for twinning 

 and sense of cut. Here the slabs are known to be cut with a reference edge 

 parallel to an electric axis, and with the major faces inclined at 35° to 55° 

 (depending upon the variety of slab) from the optic axis, the sense of the 

 inclination being positive for the AT and CT slabs, and negative for the BT 

 and DT. The effect of electrical twinning on such etched surfaces is shown 

 in Fig. 5.6. The etch-pits of the good +35° AT-portion of the slab are easily 

 distinguished from the analogous —35° (bad) portions. This difference is 

 similarly distinguishable in the other cuts. 



Actually, orientation and twinning are seldom analyzed by the method 

 described above, i.e. by examining their appearance in the microscope, or 

 by projection on the screen. The method appears to be far less practical 

 than other methods which depend upon the gross effect, of hundreds of simi- 



