iandol on wed 18 apr 01
Dear Barney Adams,
I think on this point, if the modulus of rupture (MOR)is equal =
throughout your clay, in a stress fight the thicker section will always =
survive the tensional stress which builds in the base. The problem will =
be worse when the clay wall dries faster than the base. It is a good =
idea to invert pots or ensure there is a free flow of air across the =
base by standing on slatted shelves.
Once clay becomes rigid, shrinkage in the bottom will increase the =
tensional stress until that force exceeds the MOR. Any segregation, =
porosity or stress raisers in the fabric of the clay will initiate a =
crack. Even a sharp piece of grog dragged across the bottom of a pot as =
it is severed from the wheel will a cut a grove which will act as a =
stress raiser. Often the first cracks are microscopic and not easily =
seen. But as clay warms up during firing such a crack will propagate =
along any path of weakness.
I like the idea of impressing the base upwards just after a pot has been =
taken from the wheel. Shrinkage of the wall exaggerates the curvature =
but when the wall becomes rigid that inner softer clay can shrink =
backwards towards being flat because the curve gives freedom of =
movement. This seems to relieve the tensional stress and it does prevent =
"S" cracks.
Thank you for your input to this thread.
Best regards,
Ivor Lewis. Redhill, South Australia.
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