Bim Miles on tue 6 may 97
I need some help! Over the weekend I did a glaze firing (cone 6) in which
almost all pieces had bubbles in the clay, large and small. Somewhere I
think I read about bloating, but I cannot for the life of me remember where.
I looked through the books I own, but most are throwing and decorating
books, and the few that even address firing didn't mention bloating or this
problem I had.
The pieces, mostly red clays, were bisque fired to cone 04. I have been
firing this kiln for about two years, and have had this problem only once
before, and then I used recycled clay, so that is what I blamed. This time
the clay is not recycled. I don't remember the exact clay body, but it was
a cone 4-6 clay. There were a few pieces of porcelain, which were all fine.
There were several cereal bowls of a (normally) beautiful black clay, but
they are all landfill now.
OK, is this bloating? If so, what causes it? I bisque rather quickly (no
problems in the past), overnight on low, all peeps open and lid ajar; an
hour or two on medium, then close the lid and all but one peep and fire on
high until cone falls. Should I slow down my firing? I don't have a fancy
electronic kiln sitter or pyrometer so my schedule needs to be more
"mechanical" (i.e., x hours on low, y hours on medium, etc.)
Any help the experienced electric kiln firers on the list can provide, I
would be eternally indebted.
Bim Miles (bim@orcalink.com), Olympia, Washington, USA
Where Spring is almost here... at least the flowers are blooming!
Tony Hansen on wed 7 may 97
Bim Miles wrote:
>
> I need some help! Over the weekend I did a glaze firing (cone 6) in which
> almost all pieces had bubbles in the clay, large and small.
> The pieces, mostly red clays, were bisque fired to cone 04. I have been
> firing this kiln for about two years, and have had this problem only once
> before, and then I used recycled clay, so that is what I blamed.
While porcelains exhibit warping and melting when overfired, red clays
bloat.
The surface seals and the gases produced internally from decomposition
associated with melting cannot escape. This is worse if the glaze melts
early.
Red clays are often high in lignite and the iron
itself can reduce and melt from the inside out.
It is best to fire red bodies well under the point at which they begin
to
bloat. Some go through drastic color changes over a narrow range of
temperatures, best to avoid these temperatures also.
--
=================================================================
Tony Hansen, IMC thansen@mlc.awinc.com
INSIGHT5/Magic of Fire II demos at www.ceramicsoftware.com
Paul and C2 on wed 7 may 97
Bubbles in clay may mean that your clay body has a lot of organic material
in it and you'd fired the clay too fast between 600-1000 degrees celcius.
The organic materials trapped inside the clay body was not burned off slow
enough, expanded and voila, bubbles! I had same problem with my clay....
Caroline in Hong Kong
>----------------------------Original message----------------------------
>I need some help! Over the weekend I did a glaze firing (cone 6) in which
>almost all pieces had bubbles in the clay, large and small. Somewhere I
>think I read about bloating, but I cannot for the life of me remember where.
>I looked through the books I own, but most are throwing and decorating
>books, and the few that even address firing didn't mention bloating or this
>problem I had.
>
>The pieces, mostly red clays, were bisque fired to cone 04. I have been
>firing this kiln for about two years, and have had this problem only once
>before, and then I used recycled clay, so that is what I blamed. This time
>the clay is not recycled. I don't remember the exact clay body, but it was
>a cone 4-6 clay. There were a few pieces of porcelain, which were all fine.
>There were several cereal bowls of a (normally) beautiful black clay, but
>they are all landfill now.
>
>OK, is this bloating? If so, what causes it? I bisque rather quickly (no
>problems in the past), overnight on low, all peeps open and lid ajar; an
>hour or two on medium, then close the lid and all but one peep and fire on
>high until cone falls. Should I slow down my firing? I don't have a fancy
>electronic kiln sitter or pyrometer so my schedule needs to be more
>"mechanical" (i.e., x hours on low, y hours on medium, etc.)
>
>Any help the experienced electric kiln firers on the list can provide, I
>would be eternally indebted.
>
>
>Bim Miles (bim@orcalink.com), Olympia, Washington, USA
>
>Where Spring is almost here... at least the flowers are blooming!
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