ETCH TECHNIQUE 39 



While agitation of the acid bath during etching does speed up the re- 

 moval of quartz from the surface, it does not appear to speed up the de- 

 velopment of the etch-figures here considered. However, moderate agita- 

 tion does improve the uniformit>- of etch from one crystal to another, 

 and even over the surface of single large surfaces (especially when such sur- 

 faces are close together). Uniformity of etch is important in examining for 

 twinning. The surfaces to be etched should never be placed in contact with 

 each other, or with other surfaces, so that the acid cannot flow between them 

 (the separation should be at least ^ of an inch). 



The effect of temperature on the etching process appears to be small for 

 the range of room temperatures normally encountered in practice. 



A word of caution chould be added regarding the handUng of hydrofluoric 

 acid and other fluorine etching materials. The dangers are of two kinds. 

 First, fluorine poisoning may result from contact with any fluorine com- 

 pounds, the effects of which may be cumulative. Special care should be 

 taken to prevent inhalation of vapors from all etching baths containing 

 fluorine. Some persons are especially sensitive to fluorine poisoning. 

 Secondly, hydrofluoric acid baths, or any baths containing free HF, may pro- 

 duce acid burns. Commonly such burns are attended by fluorine poisoning. 

 For these reasons etching with all fluorine compounds is preferably carried 

 out in ventilated hoods (with strong air suction through the door), with con- 

 tinually running water for washing, and with rubber gloves, tongs, racks, 

 etc. for handhng the quartz. 



Etching compounds other than hydrofluoric acid have been widely used 

 in etching glass, as is evidenced b}^ the variety of formulae presented in the 

 "Chemical Formulary."-^ Solutions of ammonium bifluoride (NH4HF2), 

 with additions of various amounts of free hydrogen fluoride, sodium bi- 

 fluoride, sugar, and other materials have long been used on glass. One of 

 the possible advantages of such formulae for etching quartz is the elimination 

 of the dangers of acid burns and strong fumes that may be obtained with 

 hydrofluoric acid (care must still be maintained to prevent fluorine poison- 

 ing). A number of these formulae have been made up and tested on quartz. 

 The preliminary conclusions are as follows. 



The etch-figures that may be developed by the bifluoride compounds on Z 

 and X-cut sections of quartz are not the same as those developed by hydro- 

 fluoric acid. The results approach each other, however, for excessively long 

 etching in both cases. To obtain usable etch-figures on X-cut sections with 

 the bifluoride requires considerably longer etching time than with hydro- 

 fluoric acid, or an elevation of the bath temperature to about 45°C. The 

 addition of hydrofluoric acid to the bifluoride formulae speeds up the de- 

 velopment, but partly negates the safety advantage of the bifluoride bath. 



^1 Published by the Chemical Publishing Co., Brooklyn, N. Y. 



