Most wood stoves in our area have some refractory bricks or stove bricks in them somewhere, and these bricks do provide an important role in the wood stove performance. The generally are made from clay and they all will eventually crack. That is actually nothing to be worried about, in fact is is normal, heating and cooling cycles – expansion and contraction. If you went through that you would crack as well! Yes the bricks are in contact with the intense heat from the fire day in and day out, cold to hot to cold again, expansion, contraction over and over again. Over time they loose their strength and crack and eventually crumble. Sometimes even a log thrown in carelessly may even contribute to early failure of the bricks. However the bricks, along with the rope gaskets, baffle places, stove glass all are consumable parts and will fail. Some show their failure in obvious fashion and others not so much until you move them. We generally carry bricks and will replace them when onsite as part of the service of the chimney sweep. Some may charge for this, generally we simply do as part of the service if we are performing a sweep at the same time. So keep in mind it is not something to seriously worry about – simply call your sweep and get them replaced with your next sweep.