Nice videos, FreeThinker. It really makes it clear how dedicated you are to doing it right. And the battery story, humorous but what a lesson to us all.
I mentioned sometime back the possible use of a sapphire rod. Actually there are fairly cheap sapphire windows:
They claim good transmissivity from 200 to 5,500 nm (0.2 to 5.5 um). Your bright yellow is producing a lot of energy in the mid visible, that is 500 nm, and of course a lot more in the longer wavelenths including "mid-IR". By the way, I strongly recommend you wear ordinarly oxy-acetylene welding googles when looking at your reactor.... lenses of eyes are fairly routine to replace now, but not retinas!
One could close off your opening allowing much closer positioning for the pyrometer / IR photometer using such a window (assuming the IR transmission was good through the photodiode's spectral range--or even a true pyrometer... Anyway, a sapphire window could be attached at the outside surface of your firebrick, occluding the "hole" or maybe even at another position in the "hole" to create a double walled conduction and convection block. An Al2O3 tube with two sapphire window ends and an evacuated interior would likely be better than a solid rod-- far less conductive and far broader spectral transmissivity, and perhaps cheaper than a sapphire rod. Such an "IR / Vis / UV light pipe", cemented right into the "hole" itself should allow very close image coupling to your IR scope. The much thinner pair of end panes would attenuate less of the thermal signal than a solid rod, regardless of the spectrum. The idea being to get the optical / pyrometric device to "see" only the inside of the reactor rather than a bunch of additional cold brick with a little hot spot.
I have considered sapphire glass in my double end open variant of the tube with the purpose of doing crude spectroscopy, but the special size requirements made them costly, believe minimum order was 10, not to mention the challenge of making a spectrometry, for my special interest in UV. Final kill, apart from cost and lead time was that sapphire would not cut it as it attenuates quickly at 200nm and the breakpoint is really quality related and the producer did not want to give me full spec on the pieces. Anybody has a high resolution spectrometer in 100-400(or about)nm lying about in the basement? No?
BTW the special range for the Voltcraft Pyro is 8-14um.