Michael Wendt on fri 11 jan 08
Anne,
Testing each batch prior to use
is always a good idea.
The practice of stilting any really
vitreous clay body can lead to
warping.
Since you have used this clay
labeled cone 6-10
successfully with stilts in the past
and since it worked before,
I am inclined to agree with
Ron Roy that the batch is
off formulation. A true cone
10 porcelain is fairly underfired
at cone 6 which explains why
you could do this in the past.
Ideally, you should always test
samples of the clay you get from
the beginning, middle and end
of a new batch long before you
use it in production.
Test for shrinkage, warping, porosity
and glaze fit and response. Do this
with a control sample from your
current batch beside the new samples
and witness cones on the same shelf
next to the samples. For how
if you have any scraps from
previous batches, you might
want to compare the character
of the two clays to see if they
are the same fired side by side.
Regards,
Michael Wendt
Wendt Pottery
2729 Clearwater Ave.
Lewiston, Id 83501
U.S.A.
208-746-3724
wendtpot@lewiston.com
http://www.wendtpottery.com
http://UniquePorcelainDesigns.com
There is also a possibility that I am trimming slightly
thinner than I used to. Would that cause it? I also
changed my stilts to another style, just flat high-fire
discs that I made with kiln wash on them, thinking it
was having trouble shrinking against the stilts...
Nope, still warped dramatically, like you were
purposely trying to make ovals.
I'll also try lowering my temperature, but my temmoku
glaze might not get as many spots...
I am going to try witness cones in different places in
my kiln, and I also am going to make a call to Cress.
But, you might be right, it might be the clay! I have
loved it for several years. I love throwing it. I love
how the glazes work on it. But, these bowls are ruined,
not just slightly asymmetrical, but squashed into an
obvious oval.
Thanks,
Anne
Tom at Hutchtel.net on fri 11 jan 08
Subject: Clay testing... was :Helios clay warping in electronic kiln
> Anne,
> Testing each batch prior to use
> is always a good idea.
> The practice of stilting any really
> vitreous clay body can lead to
> warping.
One of the additional ways I test each batch , besides shrinkage and
absorption, is to extrude a 1/4' or 3/16" rod, and place it in a square 'u'
shaped frame with the legs about 6" apart. There's a small notch in the top
side of the legs facing up and that top side is trimmed to a sharp edge. It
can be reused many times. Lay a bisqued rod in the notch and fire as usual.
It will slump, but it should be consistent.
Tom Wirt
Hutchinson, MN
twirt@hutchtel.net
www.claycoyote.com
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