Cracks in the bricks/vermiculite plates may happen due to bumps from the wood. There is no need to replace the bricks even though they are cracked. They do still insulate the fire chamber.

The purpose of the bricks is to insulate the fire chamber to increase the combustion temperature. However, if they start crumbling, the insulation disappears. This may damage the cast iron and they do in this case have to be replaced.