This morning, after running the system for a total of 7 to 10 hours in three main sessions, I took things apart enough to see what there is to see inside. We've been running it pretty hot, with the barrel top temp generally over 700˚ and maxing out the thermometer frequently when wood was added. We're almost at the end of our heating season here, so there's no reason to hold back. Plus it just seems to want to run hot. Anyway, there are some interesting things going on in there.
Here's the end cap/inlet piece pulled off. The grate shows significant corrosion, but it's still pretty much intact. However, the piece of blanket partially covering it has shrunk a bit, fused to adjacent material and become quite crisp. While it lacks resilience, it still has some strength. I was able to carefully invert the unit without dislodging the grate. Here's a closer, albeit slightly out of focus look:
According to the manufacturer, this material contains glassy silica that devitrifies, i.e., crystalizes, when exposed to temps in excess of 1000˚C=1832˚F. This is an oversimplification, as this sort of phase change is a function of both time and temp. Even with the devitrification, it isn't supposed to shrink more than 4% at temps up to 2300˚F — in carefully defined test conditions. the practical working range is supposed to run up to 2100˚, but even that depends on the chemistry of the high temp environment, not to mention the mechanical challenges right under the fuel inlet on top of the grate. In any case, this piece ain't what it used to be.