214 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1924 



highest quality of raw material, of which there is none better than 

 water clear crystals. It is much more difficult to make fused quartz 

 of this high quality from sand even if its purity exceed 99 per cent. 

 The rock crystal used in this work is water clear and contains prob- 

 ably less than two-tenths of 1 per cent impurities. The surfaces are 

 often encrusted with iron oxide and other foreign material and the 

 crystal itself can be seen to contain clusters of small bubbles. The 

 crystal is therefore washed in acids, and then broken up and the 

 unsuitable pieces discarded. 



There are two distinct steps in the preparation of tubes, rods, 

 ribbons, and cane ; the most important of which is the initial fusion. 

 The clean quartz crystals, which are of various sizes, are packed as 

 densely as possible in a graphite or carbon crucible so that during 

 the cracking of the crystals, which is bound to occur as the tem- 

 perature is raised, the parts can not separate and allow any small 

 amount of gas which may be present to enter the many crevices 

 and thus form bubbles. Those tightly packed crucibles are placed 

 in a modified vacuum furnace and the temperature raised as quickly 

 as possible to the melting point. During this fusion the pressure 

 in the furnace is kept as low as possible. The time required for 

 fusion will vary with the conditions and in all cases no more than 

 45 minutes is necessaiy. The energy rate of fusion is from 3 to 8 

 kilowatt-hours per pound of quartz, and the loss of quartz due to 

 volatilization is negligible compared with other charges. The re- 

 sult of this first fusion is a clear, transparent slug containing com- 

 paratively few bubbles ranging in size from a pin point to 2 or 3 

 millimeters in diameter. Whether these bubbles have been formed 

 by a gas or by silica vapor, it must be remembered that they have 

 been formed at a temperature of about 1,800° C; and consequently 

 their pressure at room temperature is very small. This slug is now 

 placed in another graphic crucible which is suspended in a vertical 

 carbon tube furnace. A graphite piston which just fits the crucible 

 is placed on top of the fused quartz slugs and a weight is placed on 

 top of a plunger attached to the piston. The slugs are again brought 

 to fusion, the bubbles are practically collapsed and by the action 

 of the weight the quartz is extruded in the various forms, such as 

 rods, tubes, ribbon, etc. This material is practically free from bub- 

 bles, but because of limiting dimensions it may become necessary 

 to rework some of this, which is accomplished by the usual bench 

 methods with an oxygen-illuminating-gas flame. 



When it is desired to obtain large blocks as free from bubbles as 

 the tubing, cane, and ribbon, another operation is necessary. As 

 before, the quartz is fused in a vacuum furnace which, however, is 

 also designed to withstand very high pressures. As soon as the 



