iandol on sun 5 aug 01
Dear Mariko Cruse,
You ask absorption is less than l% at both low and high end of the cone range, =
what are the deciding factors in choosing the cone?>
I think that question can only be answered by the manufacturer of the =
clay.
However, you do raise issues to which I alluded to a few postings ago =
about the distinction between a glaze and a clay body and how one must =
melt to become mature whereas the other should never melt to achieve =
maturity. All clay bodies mature through a process called fluid (or =
liquid) phase sintering, as described in Kingery et al, "Introduction to =
Ceramics". The rate at which this process proceeds depends upon several =
factors; composition, particle size, shape and distribution range, =
degree of packing and melting points of constituents being among the =
most important. Broadly, the higher the temperature, the smaller the =
particles and the closer they are packed, the more rapidly will =
densification occur. However,if the fluid phase (derived from low =
melting point fluxes) starts to dominate the system, pyroplastic =
deformation (slumping) will happen.
This brings us back to the concept of Heat/Work. It should be possible, =
using thermodynamic equations, to calculate the amount of energy which =
must be used to achieve maturity, one approaching theoretical density, =
and derive from this value a time and a temperature for firing, or a =
cone value, but it is beyond my ability to do that. For such information =
you have to ask your supplier or Ceramics Professor.
My opinion is that a clay body with such a wide cone range carries =
penalties due to the compromises which must be made. Clays should be =
selected which retain their material integrity and also achieve near TD =
at the maturing temperatures of the glazes which are chosen. I would =
predict Porcelains which have a high flux load would have a short =
maturity range. Cone 6-11 is approximately 150 degrees celsius. This =
seems excessive for the sintering process.
I hope this information is of value.
Best regards,
Ivor Lewis. Redhill, South Australia
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