Sheryl VanVleck on fri 9 nov 01
I am sure others are more expereinced than I am; however, I teach adult and
children pottery. At the Recreation center in town we only use 4010, high
fire, clay. It is also one we use at the college. We use ^06, little pots
of Duncan glazes on it at the Recreation Center and the high fire ^10 glazes
at the college. We have had some pitting at the Recreation Center but I
believe this is due more to the firing methods that are used there (the
ceramics teacher does a lot of the firing as does the boss of the art/dance
program). Also they are in the habit of glazing the total pot, bottom
included and just stilting everything. I am trying to stop this practice,
with little success.
So, that is just my experience. Limited though it may be.
Sheryl VanVleck; Wyoming
the snow has gone and the deer are in our corral out back.
H.M. Buchanan on fri 9 nov 01
Sheryl, Why are you discouraging stilting? Firing a high fire clay at a
lower temperature leaves it very porous and glazing the bottom will prevent
the ware from soaking up water. Some glaze faults, crazing and shivering,
that show up long after firing are caused by the body absorbing water.
Judi Buchanan, Flutter-by Pottery, Heidelberg, Mississippi
----- Original Message -----
From: "Sheryl VanVleck"
Also they are in the habit of glazing the total pot, bottom
> included and just stilting everything. I am trying to stop this practice,
> with little success.
Craig Martell on fri 9 nov 01
Judi sez:
> Some glaze faults, crazing and shivering, that show up long after firing
> are caused by the body absorbing water.
Hi:
Bodies that are not totally vitrified can exhibit "delayed crazing" due to
expansion from absorbed water. Glazing the bottom of a piece may not
remedy this. It won't hurt though.
Shivering is a different situation though and it is not caused by the clay
being under vitrified and absorbing water. In the case of shivering, the
glaze is actually too big for the pot and is under such intense compression
that it shatters and takes some of the claybody off as well. The causes of
shivering are usually glazes with a very low calculated expansion or from
high fired ware that is deficient in feldspar, and or, too high in free
silica which causes the piece to develop too much
cristobalite. Cristobalite puts ware through a second silica inversion at
about 435 degrees F and this will put a lot of glazes under too much
compression to stay on the pot.
later, Craig Martell in Oregon
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