Brian Molanphy on sat 2 feb 02
jon, thanks for your informative post. you wrote in part:
' If you take a raw, plastic, 'pure clay' that shrinks at 20% and add
enough
grog to it to bring the shrinkage down to a managable 12%, the clay part of
the mix still wants to shrink at the 20% rate. But because of the non
shrinking obstacles in the way, in our case "grog", it can't. So then it
ruptures giving those star like patterns. If you heat it and cool it often
enough the little stars become bigger stars and finally you get the "Big
Bang".
Ideally, you could bisque fire some grogless body, grind it up and
add it
to the raw mix, then during the bisque firing it would all equalize.
During the glaze firing, the body and grog would all shrink at the
same
rate and fuse together as one homogenous structure.'
my question is: during bisque firing of a clay that included grog of the
same body, wouldn't 'star patterns' still occur, at least until all the
elements of the body had undergone the same amount of heatwork? i assume
that the raw clay shrinks away from the the grog, even if the grog is the
same composition as the raw clay.
brian
Jon Pacini on mon 4 feb 02
Greetings all---
Brian wrote--my question is: during bisque firing of a clay that included
grog of the
same body, wouldn't 'star patterns' still occur, at least until all the
elements of the body had undergone the same amount of heatwork? i assume
that the raw clay shrinks away from the the grog, even if the grog is the
same composition as the raw clay.
It very well could develop the same stresses during the initial drying
and shrinking phases. Once you equalize the shrinkage in the bisque and the
grog and body start to fuse, you get past the problem of having a point of
fracture caused by the grog.
The key here is that since the body and grog are the same composition
they will fuse together and become homogenous from some point during the
bisque.
Most commercially prepared grogs are nearly completely vitreous and
quite refractory as they are manufactured for applications other than
pottery. They do integrate themselves quite well into most bodies, but they
nearly always remain a "foreign" particle in that body. They do not fuse
with the clay body in the same manner that a grog of the same composition of
the body would.
From a technical stand point when making oven or flame ware (which as I
recall is where this started) you are looking for a body with a low COE. And
commercial grogs do have low COEs. This is because they are high in Alumina,
low in alkaline and their silica is fused.
If your grog is quite course you may run up against this star like
cracking situation, but if the grog is fine mesh, the pluses of the low COE
should tip the scales in favor of the grog.
Something to remember is that there are always trade offs with minerals.
Substituting one for another may resolve a specific problem, but present a
problem in an unintended or unexpected direction.
Jon Pacini
Clay Manager
Laguna Clay Co
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