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BELL SYSTEM TECHNICAL JOURNAL 



removed without danger of the crystal cracking due to cold shock. It is 

 essential to control the temperature of the furnace so that the temperature 

 of the crystal plates does not reach 1063°F, otherwise the crystals may 

 become electrically twinned and consequently useless. In order to avoid 

 shattering of quartz plates due to thermal shocks while heating up and 

 cooling, fairly long cycles have heretofore been specified. However, more 

 recent experience has shown that much shorter cycles can be employed 

 especially where small crystals are involved. Moreover, there are indica- 

 tions that the faster heating, particularly during the last two or three hun- 

 dred degrees temperature rise, results in better spots. During the firing 



Fig. 13.4 — Continuous-belt furnace for firing silver spots. 



operation the crystal plates may be placed on nichrome wire mesh trays or 

 Pyrex dishes provided ample provision is made for air to circulate around 

 the spots. This precaution is essential with the types of pastes employed 

 as the baking reaction must take place in the presence of oxygen so that the 

 lead borate will not be reduced to lead, leaving only a partially bonded 

 mixture of silver and lead on the plate. This type of spot when encountered 

 usually has a dull appearance after burnishing as compared with the bright 

 surface obtained with a good silver spot, and is quite difficult to wet with 

 solder. For firing silver spots, ventilated continuous-belt-type furnaces 

 with open ports are being used with very satisfactory results. Figure 13.4 

 shows a furnace of this type developed by C. J. Christensen for this purpose. 

 After the firing operation, the spots should be examined to ensure that 

 they are satisfactory. Besides a visual inspection, it is desirable that a 



