CLEAR FUSED QUARTZ BERRY 215 



material is fused, the vacuum valve is closed and the pressure in 

 the tank is brought up to some value, depending on the object in 

 view, in less than a minute. This pressure collapses the bubbles 

 and makes it possible to obtain very large slugs freer from bubbles 

 than many kinds of the best optical glass. 



Previous attempts to reduce the bubbles by continued heating 

 above the melting point resulted, after a certain stage, only in ex- 

 cessive loss of silica by volatilization. AVe have fused quartz at 

 initial pressures of 600 pounds per square inch, atmospheric pres- 

 sure, and less than one-half millimeter of pressure. In the first case 

 the mass was practically opaque ; at atmospheric pressure it was 

 considerably improved although much inferior to the present quality 

 of quartz; and under vacuum conditions a large mass can be pro- 

 duced, which from the standpoint of number of bubbles is very 

 satisfactory. 



Not the least of the difficulties encountered in this development has 

 been that in connection with the furnace equipment. The vacuum 

 furnace in particular had to be greatly changed and enlarged with 

 the result that we now have probably the largest vacuum furnace 

 in daily use capable of operating at low pressures. Then, in addi- 

 tion to this, the furnace had to be so constructed as to withstand 

 repeatedly on the cover a total pressure of over 1,000,000 pounds 

 (about 600 tons), and of course as the size of the furnace increases 

 these difficulties are multiplied. Special attention must be paid to 

 the design of the resistor unit, to the thermal insulation, to even 

 heat distribution, to the cooling of the terminals, and to the many 

 other factors which present themselves in the use of these two extremes 

 in pressure. 



Wlien the quartz crystal is heated between 500° and 600° C, it 

 undergoes a remarkable physical change, cracking into small pieces 

 sometimes with explosive violence. This is due to the difference 

 of coefficient of expansion along the two axes subjecting the crystal 

 to great strain, and to the decrepitation caused by the presence of 

 water and liquid carbon dioxide held in vast numbers of minute 

 cavities throughout the crystal. The only advantage therefore in 

 using large crystals for fusing lies in the greater ease of keeping 

 the charge free from foreign material before the different particles 

 begin to coalesce. 



Hereaus has heated crystal quartz in very small pieces, about the 

 size of a nut, very slowly so that no cracking occurs and, conse- 

 quently, no bubbles are included in the vitreous pieces. Hersch- 

 kowitsch, on the other hand, has arrived at about the same result 

 by accelerating the heating process so that a film of vitreous mate- 

 rial is formed on the outside and prevents air from penetrating 



