It is my understanding that the primary cause of rebuilds is spalling due to the extreme difference in temperature between the hot face and the air supply side.
That spalling is caused by slag penetrating the brick surface and causing crystalline phase shifts that expand behind the front face of the brick,
which THEN allows for thermal shock to cause delamination and spallation of the slag-penetrated brick face.This is probably the most widely misunderstood and misattributed mechanism of corrosion and wear within the masonry heater community.
Think of it this way: if it were truely just heat differential and thermal shock, then those bricks would be spalling their faces off within the first 3-5 firings. ...but it takes time. And in that intervening time, the low-melting sodium and potassium ash penetrates the brick's microstructure and crystallographically alters it from the inside-out.
Slag isn't just visible clinker on the exterior brick face. The more harmful type is the less-visibly-apparent kind which melts at lower temp (potassium- and sodium-based glass formers) and has far less fluid viscosity than clinker does, so it wicks into the brick interior (
upon every heating cycle) and begins recrystallizing (
upon every cooling cycle) and the altered crystal phases inside the brick expand the brick from within, causing the slagged surfaces to delaminate and spall off.
If you were to examine the spalled surfaces under a microscope, you'd see a growing glassy phase matrix pushing the crystalline phases apart from each other, with potassium aluminate, sodium aluminate, and all number of abberant crystal species within and immediately behind the spalled layer.
If you were to take the entire brick out and cross-section it for view under clear, ample lighting, you'd see a visible coloration difference between the slagged outer layer and the intact, un-slagged interior brick layer behind it, even without a microscope.
