FROST and POTTERY

This is what a pot has to withstand at below freezing temperatures. Can you make out the crystals of frost in the soil?  Pots must be made of a clay that fires densely enough to absorb no water, or else this is the result.  This is a commercial pot from China.  The shape is fine - the sloping walls directing  the expanding ice upwards rather than outward.  But the porosity of the clay is the weakness here.  When choosing a clay I choose the lowest porosity stoneware I can.  I fire it to the point where it becomes as hard as stone.  None of my pots have suffered any damage from freezing.