These images illustrate the opening of the top of a newly built heater. It took a day to open and then close.
The heater in question had been fired four times when the phenomena below was observed.
When looking up the fire tube at the refractory capping slabs,
what looked like parallel steam cracks were visible as white lines that had not been smoked over by the first fires. There were also crack lines adjacent to the two larger cracks.
The slabs had to be removed and replaced, or so it seemed.
Four rows of facing brick were removed from three sides of the heater.
The common concrete capping slab weighs at least 80kg and was poured in place.
To be removed it had to be sawn into three manageable pieces.
The first of the offending refractory slabs is removed for close inspection.
The Slab has absolutely no fissures what so ever. The indentations that had not caught the smoke,
and so looked like cracks, were actually just superficial indentations made by the edges of the plastic used to line the moulds.