216 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1924 



to the center, even though cracks may develop. As a matter of fact 

 these processes, while interesting, are subject to very sensitive con- 

 trol and are impractical where large masses are to be fused. 



To obtain masses quite free from bubbles, it has been found best 

 to raise the temperature rapidly to 1,400° or 1,500° C. at which point 

 the pieces begin to coalesce. At about 1,750° C. the quartz is 

 thoroughly fused though it is still vei-y viscous. In fact, the vis- 

 cosity is high even though the temperature be well over 2,000° C. 

 Vaporization of fused quartz is rapid at 1,600° and at 1,750° C. the 

 loss due to evaporation is very great. Further increase in tempera- 

 ture results in no great gain in fluidity. 



HOMOGENEITY OF FUSED QUARTZ 



The difficulties of obtaining perfectly homogeneous fused quartz 

 free from striae, strain, bubbles, and double refraction must be ap- 

 parent to anyone who has worked on this problem, and discouraging 

 perhaps to those who have tried to buy such material. It is a little 

 too early to state in what quantities such a product can be produced, 

 but we have manufactured quartz of this quality which contained 

 only two or three bubbles visible to the eye. This quality, however, 

 has not as yet been placed on a commercial basis. 



PROPERTIES AND APPLICATIONS OF CLEAR FUSED QUARTZ 



The fact that for a great many purposes clear fused quartz can be 

 used up to 1,000° C. without injury; that its coefficient of thermal 

 expansion is so small as to make it almost negligible; and that it 

 will transmit light rays even into the extreme ultra-violet with very 

 little absorption, gives to it a great utility value — not only to the 

 scientist but the manufacturer as well. 



The specific gravity of clear fused quartz is 2.21. Its coefficient of 

 thermal expansion is 58 by 10'*, which is about 1/17 that of plati- 

 num and 1/34 that of copper ; so small that a rod of quartz 1 meter 

 in length will expand only about six-tenths of a millimeter for a 

 1,000° C. rise in temperature. The small probability of fracture 

 under sudden changes in temperature because of this property makes 

 it especially desirable for many uses. Furthermore, where it is used 

 as a mirror in reflecting telescopes this very small expansion or con- 

 traction with change in temperature causes almost no distortion of 

 the image and, consequently, much greater accuracy is made possible. 

 This property also makes the grinding of a lens or mirror less tedious 

 and costly, as it is not necessary to await the cooling of the shape in 

 order to get the desired curve. It is possible to heat a tube of clear 

 fused quartz, say five-eighths inch in diameter, to the melting point 

 and plunge it into ice cold water without fracturing. 



