246 



Subsurface Geologic Methods 



holder, and the two charts are synchronized, the furnace is started. The 

 heating rate has been standardized at 12° a minute, as this gives sensi- 

 tive control, produces adequately sharp peaks, and is close to the heat- 

 ing rate used by a number of other workers in this field. The record 

 is made from 100° to 1,050° C. At the beginning and end of the run 

 the button switch activating the solenoid pens is pushed, thus fixing the 

 temperature on the multiple-differential thermocouple record. When 

 1,050° C. is reached, the furnace is raised from the specimen holder, and 



300 400 SOO 6 00 



0«gr«tt C«ntigrad« 



) 200 SOO 400 500 600 700 



Dtgrttt Ctntigroda 

 Figure 103. Theoretical thermal curves. 



the samples are removed by compressed air while they are still hot. This 

 procedure prevents caking, which occurs in certain specimens upon 

 cooling. 



The temperature thermocouples are calibrated and recalibrated occa- 

 sionally with the alpha-beta quartz change. It has been found after trying 

 many thermocouples that the quartz-inversion peak occurs on a differential 

 curve within 5° of 579° C. (This is higher than the equilibrium value.) 



