LJTCW@AOL.COM on sun 23 sep 01
I have gotten several requests for the name of my problem clay. I use=20
Rustone from Columbus Clay. I bisque at -05 and glaze at -7. I have used=20
this same clay body for years and have not had the problem before. I have=20
new elements, fire the same way, use witness cones, in a clay pad, not free=20
standing ones and checked the kiln sitter calibration. The thrower at=20
Columbus said that he was having the same problem with a Laguna clay but did=
=20
not mention which one.=20
He also said he did some reading and that it could be a "lime popout". but=20=
I=20
really think it is just BLOAT.
I am leaving for Tokyo to visit my son on Tuesday and will sign off, but=20
please keep the answers coming so I can read the archives when I get back.
Thank you all for your suggestions and comments.
Louise
Message # 1 Sent: Saturday, September 22, 2001 1:04 PM
To: CLAYART@LSV.CERAMICS.ORG
Subject: claybody problems, bloat
I am having a problem with my clay body, it is bloating in the glaze firing.
The clay as it comes from the bag seems a bit different from the previous=20
batches as it seems to " tear" instead of just break apart when I am weighin=
g=20
it out.=A0 There are visible pieces of white material ( fire clay?)when=20
trimming the bottoms.
I bisque at bar 05.=A0 I have an electric kiln with 3 switches and use all a=
t=20
low for 2 hours, medium for 2 and then turn it up.=A0 I have a kilnvent whic=
h=20
draws air from the top to the exhaust at the bottom.=A0 I do not crack the k=
iln=20
open at the beginning. My clay body is a brown color( I do not want to say=20
which supplier or kind for obvious resons).=A0 I know it uses Hawthorn fire=20
clay and Gold art. =A0 The rest is a mystery to me.
I have a lot more pieces of bisque ware to glaze. =A0 Does anyone have a=20
suggestion to avoid the bloat on these pots?
Louise
Louise Jenks
Turtle Clayworks
Cincinnati OH USA
Michael Wendt on sun 23 sep 01
Louise:
Bloating in clay bodies made by commercial manufacturers sometimes is the
result of "change over clay". If they rely on pugging out the lowfire batch
that ran in the pug mill rather than disassembly and thorough cleaning, then
some of the transition clay will contain low fire strands or layers that can
contribute to this problem. Maybe these are the white material you are
seeing?
Larger manufacturers can afford to have more than one pug mill and can thus
keep their low, medium and high fire clays segregated. Check with Columbus
clay and see if they run all their clay through the same pug mill and if so,
what their cleanout procedure is.
In any case, laminar wire wedging can reduce or eliminate many tearing
problems associated with two different clay bodies being mixed. To do this,
you need a canvas topped wedging table and a wire frame to hold an "E"
string from a guitar at a 45 degree angle. Cut your piece in half with the
wire and slam on piece on top of the other to create a new piece with two
layers. Each time you laminate the pieces by cutting and slamming to the
table, you double the number of layers so that by the time you reach 25
doublings, you have 33,554,432 layers. This assures that the clay is very
uniform. Do this just before you throw with the piece and see if it helps
with the tearing. It should also help with the bloating by evening out the
composition so there aren't pockets of different materials.
Regards,
Michael Wendt wendtpot@lewiston.com
Louise wrote:
Message # 1 Sent: Saturday, September 22, 2001 1:04 PM
To: CLAYART@LSV.CERAMICS.ORG
Subject: claybody problems, bloat
I am having a problem with my clay body, it is bloating in the glaze firing.
The clay as it comes from the bag seems a bit different from the previous
batches as it seems to " tear" instead of just break apart when I am
weighing
it out. There are visible pieces of white material ( fire clay?)when
trimming the bottoms.
Ron Roy on mon 24 sep 01
I have been dealing with cone 10 bodies now for 40 years - as a studio
potter, as a clay supplier and now as a consultant to clay suppliers.
Bloating - particularly with iron bearing stoneware clays - is mostly due
to reduced iron during the bisque firing cycle.
What happens is - the red iron (Fe2O3) which is not much of a flux is
turned into FeO which is a powerful flux.
Hamer gives an excellent description of the process.
All this is not to say high fire bodies cannot be contaminated - any
supplier who does not clean out their mill before switching to high fire
bodies should have learned that is not acceptable after the first time they
did it.
The sage advice is still - buy your clay ahead of when you will have to use
it and run a few tests to make sure it's right. Remember - if they do
anything at all - it's only clay replacement.
RR
>I agree that bloating is usually caused by getting lowfire talc based clays
>mixed into stoneware bodies. 2% of talc in a cone 10 body can cause it to
>bloat because of the eutectic between talc and most feldspars. Bloating is
>rarely caused by firing schedule alone.
Ron Roy
RR# 4
15084 Little Lake Rd..
Brighton,
Ontario, Canada
KOK 1H0
Residence 613-475-9544
Studio 613-475-3715
Fax 613-475-3513
Brant Palley NMCLAY.com on mon 24 sep 01
I agree that bloating is usually caused by getting lowfire talc based clays
mixed into stoneware bodies. 2% of talc in a cone 10 body can cause it to
bloat because of the eutectic between talc and most feldspars. Bloating is
rarely caused by firing schedule alone.
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