Anne on mon 8 nov 04
I'm a hs teacher who has been teaching clay for over 10 years. Of couse, we do a
classic coil bowl project. This year I have a new problem. I am getting horizontal
cracks in the commercial glaze around the coil pots. I think it is the new clay we are
using (Laguna white earthenware 100) shrinking not enough for the glaze. We have
used Campbells and Highwater clay for years with Amaco, Spectrum and Campbells
commercial glazes. We are using the same glazes, but a different clay body.Of
couse, the distributer for the clay says absolutely it is not the clay. They have never
had this problem before with other teachers using the same clay with non-Laguna
glazes.
I bisque fire to 04 with a slow ramp in an automatic skutt kiln. The bisqued pieces are
crack free. I glaze fire to 06 with a medium ramp and get these horizontal cracks
every time with each different company's glaze. The Spectrum pot was so bad that it
actually cracked the pot as well, and the pot is in two pieces, when it was fine after
the bisque firing. There are no cracks other than the horizontal ones. This problem
does not happen on small pieces, like pinch pots, only on coil bowls that are at least
7" tall and every single one has had the same problem.
Is it possible that there is a bad batch of clay? Or is the kiln misfiring? Or is the clay
distributer not being completely honest and knows that I have to use Laguna glazes
only with Laguna clay? (Laguna says they don't test their clay with other brands of
glazes, they assume we are using Laguna glazes. But the distributer never said, we
had to invest in all new glazes--they wouldn't have gotten the bid if they had said
that). I have witness cones in right now to test the kiln. Has anyone else had these
uniformly horizontal glaze cracks before? Any advice?
Rob on tue 9 nov 04
Anne -
This is not the first time I've heard of glaze fit problems with Laguna
clays. In fact, I've got customers who have had glaze fit problems using
Laguna Moroccan Sands glazes on Laguna clays. Laguna's response has been,
well, less than helpful.
My recommendation is to see if you can go back to using a clay you know
works - I really like the Campbell's low-fire clays, they seem to play well
with almot all earthenware glazes.
Rob Van Rens
Frederick Clay Art Center
Subject: commercial earthenware glaze cracks
> I'm a hs teacher who has been teaching clay for over 10 years. Of couse,
we do a
> classic coil bowl project. This year I have a new problem. I am getting
horizontal
> cracks in the commercial glaze around the coil pots. I think it is the new
clay we are
> using (Laguna white earthenware 100) shrinking not enough for the glaze.
We have
> used Campbells and Highwater clay for years with Amaco, Spectrum and
Campbells
> commercial glazes. We are using the same glazes, but a different clay
body.Of
> couse, the distributer for the clay says absolutely it is not the clay.
They have never
> had this problem before with other teachers using the same clay with
non-Laguna
> glazes.
>
> I bisque fire to 04 with a slow ramp in an automatic skutt kiln. The
bisqued pieces are
> crack free. I glaze fire to 06 with a medium ramp and get these horizontal
cracks
> every time with each different company's glaze. The Spectrum pot was so
bad that it
> actually cracked the pot as well, and the pot is in two pieces, when it
was fine after
> the bisque firing. There are no cracks other than the horizontal ones.
This problem
> does not happen on small pieces, like pinch pots, only on coil bowls that
are at least
> 7" tall and every single one has had the same problem.
Ron Roy on thu 11 nov 04
Hi Anne,
You are right - it is a fit problem - and the clay is contracting too much
compared to the glaze - it is called shivering. It is important that you
understand - even small pieces can be a problem. Sometimes small sharp
flakes of glaze will pop off pots when there is this kind of clay/glaze fit
mismatch.
If it is happening with all the glazes you use I would also suspect the
clay body - if you would be willing to make a test bar of the clay and send
it to me I would like to test it in my dilatometer - that will show if
there is anything unusual about the clay.
Just let me know and I will send you instructions on how to make the
samples. It will not cure you immediate problem but will help avoid it in
the future.
I suggest you contact Laguna directly - your local distributer seems to
have written you off.
RR
>I'm a hs teacher who has been teaching clay for over 10 years. Of couse,
>we do a
>classic coil bowl project. This year I have a new problem. I am getting
>horizontal
>cracks in the commercial glaze around the coil pots. I think it is the new
>clay we are
>using (Laguna white earthenware 100) shrinking not enough for the glaze.
>We have
>used Campbells and Highwater clay for years with Amaco, Spectrum and Campbells
>commercial glazes. We are using the same glazes, but a different clay body.Of
>couse, the distributer for the clay says absolutely it is not the clay.
>They have never
>had this problem before with other teachers using the same clay with non-Laguna
>glazes.
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
Fax: 613-475-3513
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