iandol on wed 19 jun 02
Dear Bruce Girrell,
Straight from the cuff, I prefer to use a good white firing ball clay =
to induce or restore plasticity to tired reclaimed clay. I would do the =
same if a clay were adjusted with non plastic material. I dislike the =
idea of Bentonite because in excess it induces thixotropy and hence a =
sensitivity to external and internal stress. As you are using Porcelain =
there may already be sufficient Bentonite present.
Rules of thumb would be difficult because you have to make predictions =
about the specific surface area and relative density of the powdered =
material being added, then compensate for that by calculating the =
interstitial volume of the voids which are to be filled and relate this =
back to a weight of clay.
But I think there is a fundamental issue which has to be examined which =
relates to the definition of "Thermal Shock Resistance". Until this is =
clearly defined then there will always be a degree of uncertainty about =
the behaviour of any ceramic material when it is subject to extremes of =
heating or cooling. Regardless of the Chemistry we may try to employ, =
Physics will always defeat us. We can match Coeffs. of =
Expansion/Contraction and remove silica and its phase changes but we =
cannot increase thermal conductivity to a point at which all of the =
material will simultaneously achieve uniform temperature as the heat =
source is applied or eliminated. I think this is a design problem which =
cannot be solved with traditional ceramic materials.
Have a look in the first chapter of Glen Nelson's book and in the =
appendix for ideas of clay and glaze. Kingery et al give a good =
treatment of thermal properties.
Best regards,
Ivor Lewis. Redhill, South Australia
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