Jones Beene wrote: "If the failure was only pressure-related, it would happen near the middle of the cavity, which is the region of least structural strength against internal pressure - but since the failure (apparently) happened at almost exactly the place where the temperature gradient would be maximized – that explanation seems to fit the circumstances."
I find it far more likely to be determined by a defect in the Al2O3 tube. The ceramic is very brittle. I have had those thermocouple tubes break for no apparent reason when inserting them in a furnace. They also require handling with reasonable care.
