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BELL SYSTEM TECHNICAL JOURNAL 



tion merely by heating to 350°C. to 450°C. for a short time^ (see Fig. 4.2). 

 The clarity of cleared stones, even though originally very dark, rivals that of 

 normally clear quartz, thus leading to the belief that most commercial quartz 

 is colored to a slight degree.- Further, it is claimed that irradiation of either 

 cleared or normally clear stones with radium rays causes them to become 



Fig. 4.2 — Coloration in quartz ma}' be cleared by heating to temperatures below 500°C. 

 A, B and C are adjacent portions of one dark, smoky stone, only C having been cleared. 

 The five, small blocks were likewise cut from a single stone, the second and fourth having 

 been partially cleared at 350°C and fully cleared at 500°C. 



smoky. It is then questionable whether there is a material difference in 

 clear and smoky quartz, or only a difference in condition of the material. By 

 the limited studies that have been made light smokiness has not been found 

 to have any definite effect on finished plates. It might be added that such 

 quartz plates as are normally heated to 400°C. to 500°C. in the process of 

 manufacture, would of course be cleared of any smokiness originally present. 



' Stones so cleared 8 years ago are still clear. 



2 Arkansas quartz (not used commercially) is noted for its unusual clarity. 



