Spontaneous combustion and rocket mass heaters

Hi All;     Observed an interesting thing recently. Our RMH is in our greenhouse/studio and during the cold part of winter it burns from apx 7 am- apx 10 pm , every day. (Still uses less than 5 cord all winter)  After 3 core rebuilds/changes I settled with an all heavy firebrick core. I tried casting fireclay/perlite, I tried insulated firebrick, none gave me the feed tube longevity that the heavy use required. Being in a plastic greenhouse I needed mass more than super high temps.  My cast core would regularly reach 1100 F at the barrel top, but cooled too quickly.  The core with insulated firebrick reached the same barrel top temps (but not quite as quickly)  My current core is all full thickness heavy firebrick.  Barrel top temps routinely run 800-900. With constant monitoring and small wood I have gotten it up to 975 F tops.   The greatest thing for us is , that core hold heat !  Lots of heat ! -15 below zero outside , 45 F inside and the soft clay brick surrounding my core will have a surface temp of 150 F or more !  The mass temps are less than that ( maybe 110 near the transition area and as low as 90 at the far end)   For the last few weeks our temps have been well below zero at night and 2 or 3 above zero during the day....  COLD ....  We try to stay home as much as possible when the weather is like this. The other day we both had to leave for what we thought was a quick trip to town and back (50 mile rnd trip)  instead of taking just 1.5 hrs like it should have it was over 4 hrs later when we returned . Upon arriving home I trotted over to the studio to check on the rmh,  toasty warm inside (68 F) no fire ... not even any coals left in the ash ... darn ! Guess i'll have to build a new one ... I placed 2 decent sized sticks in the feed tube and was reaching to stuff kindling in when I was called into the house... 45 minutes later I went back to the greenhouse expecting the temp to have started falling ... instead what do I find but BURNING WOOD ! How cool is this !No coals, No paper , no kindling , not even small wood, just two pieces of normal size rmh wood that had been leaning up against my peter channel cheerfully burning !  Over the next week I experimented and was able to reproduce my results !  A little back to school studying and I have learned a few things about wood. At 212 F or 100 C the water in wood boils and evaporates. At 248-300 F -120-150 C wood chars. At 300-580 F or 280-480 C ( 451 F is an accepted number) wood has ignition . The heavy firebrick in my feed tube holds heat so well that my room temp firewood will rise in temp thru evaporation, charing and finally ignition !!! OR SPONTANEOUS COMBUSTION !  Yes my wood is dry when I cut it in the spring , it sits split all summer stacked in a shed , so it is as dry as it can be , although it is at below zero temp when I bring it inside . My  wood inside averages 70 F,  some that leans against my mini bell is over 100 F (burn that side of pile first )  Who knew that you could just drop wood into an empty firebox ... walk away ... and come back to a burning fire !  YES ! Rocket mass heaters rock !

Happened to me before. Even with a cleaned out j. Tho, not 4 hours. More like two.