David Hendley on fri 6 aug 99
I don't know of a way to predict slumping of a claybody other than
running tests.
If the clay component of the body is called 'fireclay' or 'kaolin' that
will give you a clue that it might withstand a lot of heat, but, still, only
testing will tell you for sure.
Then, if it includes a flux, such as feldspar, you know it will mature and
slump at a lower temperature than the clay alone.
If a claybody is slumping, then I'd say, by definition, it is close to or
at its maturation temperature. If it is actually melting, then it is
past its maturation temperature, and is actually severly overfired.
If your claybody is slumping, and you don't want it to, you have two
choices: lower the firing temperature, or change the claybody
by using less flux or a more refractory clay. Feldspar is the most
common flux in stoneware bodies because it melts slowly, to allow
a lot of leeway. Some other fluxes act suddenly, which makes them
more difficult to use.
It is normal and expected for some claybodies to slump, most notabily
porcelain. If you know what to expect, you can still work with a
slumping claybody by planning ahead, by designing your work
accordingly or by using supports during firing. The rims of porcelain
plates are routinely supported during firing.
I don't really know, but I don't think the glaze affects slumping of the
claybody one way or the other. I guess if the clay was really thin
and the glaze was really thick on both sides, the 'clay-glaze interaction
zone' could be large enough that there would not be much clay-only
left to support the weight of the piece.
David Hendley
Maydelle, Texas
hendley@tyler.net
http://www.farmpots.com
At 02:15 PM 8/3/99 EDT, you wrote:
>----------------------------Original message----------------------------
>------------------
>How thin is too thin?
>
>I've been through dozens of resources trying to find information on
predicting
>slumping in a clay body. Tichane's book is one of the few that even mention
>slumping, but still not much of a help.
>
>What I'm looking for is...if a piece slumps in a firing at its "normal"
>maturation temperature, can you fire it lower and keep it from slumping (yes,
>I'm trying this...)? If it's actually melting when it slumps, is this
partial
>or total vitrification, even though the clay is not getting to its maturation
>temperature? Does having a glaze on (appropriate for the firing temp)
increase
>the risk of slumping (it seems to in my tests)? Does a mat or gloss glaze
make
>a difference (haven't tried this yet)?
>
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