Could that brick have been stacked on edge with another brick stacked
against it in the middle? Vitrification can partially extend into covered
areas. I'd like to see the brick.

Anyway. I'm sure 'sacrificial' bricks were sometimes used for  lining fire
channels to protect the other bricks but I do know for certain that it was
common practise to place  underfired bricks into the next burning. A
careless apprentice may have stacked a vitrified brick in by accident for
refiring.

Cheers,   Robert

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