How often do you replace your quarry tiles?  I bought two last week and treated them like some recommended by heating them up 350F for an hour and then 500F for half an hour.  One broke on the first day I actually used them and the other one broke today while I was arranging them for a second use.  It took no force at all, it just snapped.  Is this normal?  Is there anything I can do to make them last longer?  TIA

June