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That is an interesting choice of floor insulation Are the air spaces in the "engineered brick" the only insulation/heat break between the oven floor and the slab?
These "engineered bricks" are sold under the brand name "Poroton" (which means something like "clay with pores") in Germany, usually they are used to build houses and have excellent insulating properties, comparable to Hebel/Ytong.
(The coefficient of heat conductivity is about 0,07 W/mK.)
So, yes, these bricks are the only insulation between hearth and slab and I think they are much better than vermiculite.