38 BELL SYSTEM TECHXICAL JOIRXAL 



microscope slides for a standard length of time. The lead-glass slides be- 

 come coated with a white lead-fluoride deposit to a depth dependent mainly 

 upon the HF content of the acid. The optical density of this deposit is 

 measured with a specially adapted photometer. The photometer readings 

 are correlated with required etching-times to give the desired etch-figures; a 

 different etching-time being required for different kinds of sections, slabs, 

 etc. Use of this means of controlling the etching time has greatly improved 

 the regularity with which good etch-figures are produced in the shop. 



Commercial hydrofluoric acid from a number of different suppliers has 

 been analyzed for purity, and tested for the development of etch-figures. 

 It appears that when such acids are brought to the same concentration (by 

 addition of water if necessar^^) there is no difference in their effectiveness, 

 nor are they inferior to pure reagent acid. Commonly the acid is supplied 

 as 48% solutions in lead or hard rubber drums, or as 60% in steel drums 

 (usually the concentration is a few per cent higher than labeled). The dif- 

 ference in packaging is of no importance in the results obtained, provided 

 the concentration is properly reduced. 



There are two important factors regarding the starting concentration of 

 hydrofluoric acid baths. In the first place, acids stronger than 50%, though 

 reacting vigorously with the quartz (and removing material rapidly), do not 

 give good etch-figures. Secondly, strong acids not contained in sealed 

 containers lose strength very rapidly by gassing of the HF gas. Hence un- 

 used fresh acid should be kept well stoppered. Before use the acid should be 

 diluted to a concentration of 45% to 50%. This may be accomplished by 

 adding about | volume of water to one volume of 60% acid, or | volume of 

 water to one volume of 55% acid. 



Concentrated hydrofluoric acid loses HF by gassing more rapidly than it 

 loses water by evaporation. This preferential loss of HF continues until 

 the HF concentration is reduced to 35% or less,^^ and is not completely over- 

 come by covering the bath without sealing. In fact, in practice, it appears 

 that about as much HF is lost by gassing as is used in etchmg the quartz. 

 Thus the bath should be kept as tightly covered as is practicable. 



Whereas, in the past only lead and hard rubber have been used for fabri- 

 cation of acid baths and racks, it appears that for concentrations not greater 

 than 50% HF, copper, nickel, and brass may be used as well (steel is inferior 

 at low concentrations). Lead-tin solders may not be used, but silver solder 

 is satisfactory'. Thus shop acid equipment ma\' be easily fabricated out of 

 common fabricating materials.-^ 



" At room temperatures there appears to be a constant-concentration mixture at some 

 concentration below the 35% concentration of the constant boiling mixture, the exact 

 value depending upon the temperature of the solution and the ambient humidit\^ 



2" PolystjTene is a good material for use in fabrication of vessels for handling HF and 

 its reaction products in the laboratorj-. 



