Jon Pacini on mon 6 aug 01
Greetings all----Mariko wrote-------
When a clay supplier sells a claybody, which =
has a cone range from 6 to 11, and the ave. absorption is less than l% =
at both low and high end of the cone range as described in their =
catalog, what are the deciding factors in choosing the cone? Is it =
possible to have a same absorption at such a wide temp. range?
I fired a test tile, which is made from this claybody(porcelain), to =
Cone 9 in a gas reduction kiln, and the tile bent almost to 2 o'clock =
position, while at Cone 6 the tile remained straight up. Would someone =
please explain this phenomenon and pros and cons of using this kind of =
claybody?--------------
Back in the darkages a chart on the ceramic lab wall said, "greater
than 10% absorption earthenware, 10%-1% stoneware, less than 1% porcelain".
It's becoming more common to see ^06 clays earthenwares fired as high
as ^1 or 2. At ^06 their porosity may be as high as 19%, at ^2 less than 1%.
So I guess you could say this body would have a firing range of ^06-2. I
would however expect increased shrinkage and increased distortion as the
temperature increases.
The same phenomenon would hold true for stonewares. A stoneware with a
porosity of 8% at ^5 could be fired to ^10 and retain a porosity of greater
than 1%. This body would have a firing range of ^5-10.
If the porosity drops under 1% I would expect brittleness to become
prevalent and a tendency to bloat. Brittleness because of the total
vitrification of the clays taking place, Bloating because fireclay based
stonewares usually contain high concentrations of organics that tend to get
trapped and expand as the body softens and becomes glasslike.
To have a clay body that at ^6 has 1% absorption and fires to ^11 and
still has 1% absorption and none of the shrinkage and distortion
characteristics associated with this rise in temperature would be very rare
indeed.
From what you have posted, the body does seem to get more distortion as
you fire it hotter. I would venture to guess that it's porosity is somewhat
variable also.
If I were to use such a body I would first off, test for shrinkage,
distortion and absorption over the range I wanted to work at. Then test for
glaze fit. Crazing that occurs on a body with less than 1% absorption is
much more expectable than one with 6%, as the latter will allow water to
pass through it. If you're producing vases or tableware, finding glazes that
fit on a porous body is imperative, on a body with less than 1% absorption
less so.
If you are doing flaring cantilevered forms you may want to steer clear
of bodies that experience excessive distortion.
So the advice from this quarter is to test, test, test.
Jon Pacini
Clay Manager
Laguna Clay Co
--------where the threat of thundershowers matches the humidity to the
temperature--100
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