posted 4 years ago
I think your problem is not that you used a clay and wood shavings mix, but that the clay wasn't completely coating all the pieces of shaving, so that fire could spread when it got more oxygen. I expect the shavings were charring all last year, making fine charcoal that caught fire easily. Normally, this would be fine, as the charcoal would eventually turn to ash leaving lots of little insulating voids in the clay matrix. For that matter, a thin clay slip and straw/hay mix might work even better because the voids would be more numerous and smaller so that air would not move as easily from hot side to cold side.
The base is the really problematic part as you know; insulation does not stop heat flow, it only slows it down. Slowing heat transfer during a short hot fire can be sufficient, but when the fire/heat usually lasts for many hours, the heat eventually builds up on the far side. If there was a thin plywood layer with open air under it, that might even let heat dissipate fast enough to avoid charring. The best way, of course, would be to have nothing combustible at all below the floor. Next best would be to have spaced-out bricks under the solid brick floor so that air could cool the bottoms of the floor before heat gets to the wooden support structure.