ETCH TECHNIQUE 51 



probably least tolerable in filter plates, which have to meet very special re- 

 quirements.-^ Detailed experimental studies of allowable amounts of twin- 

 ning are of little value since to use the results in a manufacturing process 

 would require a careful inspection of each plate and a difficult classification 

 into groups depending upon the variety, amount, and placement of the 

 twinning. Acceptance or rejection of finished plates on the basis of their 

 final electrical operating characteristics appears to be the only practical 

 means of separating usably twinned plates from unusably twinned plates. 

 This method of selection does not determine whether the rejected plates con- 

 tain twinning or other imperfections (or are misoriented or misdimensioned) 

 and is therefore of little use in analyzing the processing methods to deter- 

 mine best practices. This disadvantage may be eliminated by etching the 

 rejected plates and examining them for twinning (and such other imperfec- 

 tions as show up best after etching). 



The effects of crystal imperfections other than twinning were discussed 

 in Chapter IV, Section 4.9. 



-" See footnote 24. 



