red bricks, black on the inside?

[ColJack]

I have been chasing a switch box into a wall today, the brick was red/orange of a normal brick once the plaster was off, but about 7-8 mm into the brick, it was black .. quite hard too... what type is it?
strange thing is I seem to have gotten "splinters" off the brick?
can't see anything but feels like i've a splinter in the inside joint of one of my fingers..

 

[JohnD]

a badly-bumed brick.

In older houses, when brick quality was inconsistent, the best bricks were used for outside facing work, and poorer bricks were used inside. The worst ones were used in gardens for rockeries or rustic walls.

 

[ColJack]

no i mean the brick would be red / orange on the outside of it, but have a black "filling" if you like..

 

[hotrod]

a badly-bumed brick

sounds painful

 

[JohnD]

burned

 

[Shytalkz]

Kiln bottoms.

 

[^woody^]

Pockets of clay bake/burn unevenly. The splinters are where the burnt clay has fractured into what is effectively glass shards