Matt MacIntire on sat 26 may 01
What happens in reduced clay bodies, with small amounts of iron, that causes
severe thermal shock when pouring in boiling water?
I use a clay with a small amount of iron. It is almost white after bisque,
just a hint of that salmon color. In oxidation this clay is an off-white
cream color. In a wood fired kiln, there is enough iron that the clay body
mostly goes brownish where exposed. In areas of lighter reduction it is
more grayish than brown. This clay doesn't have a lot of iron in it. The
clay is quite vitrified at cone 10, but the doesn't seem over fired. The
clay isn't slumping or blistering at all, but it gets plenty hard and there
is very little absorption after firing.
If I fire this cone 10 clay in oxidation or LIGHT reduction, I don't notice
problems with thermal shock. If the body gets moderate to strong reduction,
the fired pieces can not take much thermal shock. If I pour boiling water
in a piece from a wood kiln and it cracks apart. I have seen this problem
with many (similar) commercial "buff" stoneware bodies. I have also noticed
this doesn't seem to happen with porcelain mugs, nor mugs from the same clay
that hasn't been reduced. Something about the reduction of the iron in the
clay body seems to make it brittle. Does anyone know what causes this?
FWIW... We do some light body reduction, but not the billowing clouds of
black smoke I have seen some folks do. Then for the rest of the firing, the
kiln reduces and clears, reduces and clears as we stoke.
Is there any solution to this problem besides less reduction -- or not using
boiling water?
What is in an ovenware body that makes it more resistant to thermal shock?
It also makes me wonder.... How do people who fire reduction with a high
iron body make any drinking vessels that can hold up to boiling water?
thanks
Matt
Craig Martell on sun 27 may 01
Hello Matt:
I don't think that small percentages of iron are the culprit here. The
thing to look at is the difference in firing and cooling curves between the
kilns you are firing work in. What you want to question is the length of
time the ware is at cone 6 and above. Both in the heating and cooling
cycle. This range is where cristobalite will develop and that form of
silica is what makes ware vulnerable to "thermal shock". The longer ware
is subjected to hi temp, the more cristobalite will develop, as it's an
ongoing process. If the clay you are using is a bit unbalanced and does
not contain enough feldspar to assimilate the cristobalite into the body
glass, you will have trouble especially when the ware goes through a longer
heat and cool at cone 6 and above.
Probably the oxidation fires and lighter reduction fires are quicker up and
down the temp scale so you are converting less silica to cristobalite. In
answer to your comment about porcelain mugs taking the "thermal shock"
better, it's because porcelain claybodies contain so much feldspar that
they are nearing a glassy state and fused silica has a lower coefficient of
expansion that crystalline silica.
regards, Craig Martell in Oregon
Snail Scott on sun 27 may 01
At 07:38 PM 5/26/01 -0400, you wrote:
>What happens in reduced clay bodies, with small amounts of iron, that causes
>severe thermal shock when pouring in boiling water?
Iron becomes a flux in these conditions...
sort of like firing it to a higher temperature,
in effect. To get the same vitrification in a
reduction fire, go to a lower temperature than
you would need for an oxidation fire.
Some minerals, like kyanite and pyrophyllite,
can confer a degree of thermal-shock resistance.
Thermal-shock resistance is not a simple
proposition, though. Many variables are at work.
Note, though, that the thermal conductivity of
boiling water being placed in a cup is very
high, compared with putting the same cup in
210 F air. You could probably put your 'problem'
cup into a preheated oven with no cracking,
because the heat transfer will be much slower.
Pouring boiling water into anything is seriously
stressful! I'd bet that you could put that cup
with cold water into the microwave and heat it
to boiling with no trouble, though, since the
heat rise would be more gradual.
-Snail
Ron Roy on mon 28 may 01
The problem is probably cristobalite production and/or low expansion glazes
- a bad combination.
See Peter Sohngens article in "Studio Potter" magazine - about 2 or 3 years
ago now - I haven't found my copies of Studio Potter yet - after my move -
anyone know which issue that was?
An excellent artical - onthe effect of fine silica on such bodies. I did
the dilatometery for him.
RR
>What happens in reduced clay bodies, with small amounts of iron, that causes
>severe thermal shock when pouring in boiling water?
>
>I use a clay with a small amount of iron. It is almost white after bisque,
>just a hint of that salmon color. In oxidation this clay is an off-white
>cream color. In a wood fired kiln, there is enough iron that the clay body
>mostly goes brownish where exposed. In areas of lighter reduction it is
>more grayish than brown. This clay doesn't have a lot of iron in it. The
>clay is quite vitrified at cone 10, but the doesn't seem over fired. The
>clay isn't slumping or blistering at all, but it gets plenty hard and there
>is very little absorption after firing.
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
Craig Martell on tue 29 may 01
Hello John:
There is a stong possibility that black coring results from carbonates not
being burned out in the bisque firing or from a hasty temp rise in a single
fire situation too. Sometimes it's really hard to sort all this out.
regards, Craig Martell in Oregon
Clay Coordinator on tue 29 may 01
Craig,
I agree with your post. But additionally I have noticed on occassion that
if you really over reduce (if that is possible) early on in the firing (cone
014) you can sometimes black core the piece. I think that this causes the
clay to be weakened and I have even seen severe shivering as a result. The
"skin" (glaze/body interface) of the piece just pulled away.
Later on,
John Britt
Matt MacIntire on tue 29 may 01
Ron,
Thanks for your suggestion about the article. I'll look for it. While
searching the clayart archives I noticed a similar thread about this
cracking from a few years ago in which you also mentioned critobalite.
This is a commercial body, and I have lately realized that I probably ought
not to use something off the shelf. I thought that along with convenience,
I was also getting some sensible formulation, but it appears that this body
is not formulated for my specific needs. Time to make my own.
I'll learn a little bit more about cristobalite and begin to work my way
through some body formulation trials. Thanks for your help.
Matt
iandol on thu 31 may 01
Dear Craig Martell,
In your post on the above topic you say that black coring results from carbonates not being burned out>
I thought black core was a result of incomplete burning of organic =
residues.=20
I am a bit lost here. Perhaps you could explain the chemistry of this =
one. How would I cause this to happen and what proportion of carbonate, =
presumably calcium carbonate, is necessary for it to occur?
Best regards,
Ivor.=20
Ron Roy on thu 31 may 01
Hi Matt,
Yes - if you read Peters article you will be on the right track.
Most commercial clay suppliers would benefit from having their clays dilled
- trouble is a dilatometer costs $16,000 Amr.
I have one and if you need some samples run it costs $75 Amr - $100 Can per
sample - let me know if you need instructions on how to make samples.
RR
>Ron,
>Thanks for your suggestion about the article. I'll look for it. While
>searching the clayart archives I noticed a similar thread about this
>cracking from a few years ago in which you also mentioned critobalite.
>This is a commercial body, and I have lately realized that I probably ought
>not to use something off the shelf. I thought that along with convenience,
>I was also getting some sensible formulation, but it appears that this body
>is not formulated for my specific needs. Time to make my own.
>
>I'll learn a little bit more about cristobalite and begin to work my way
>through some body formulation trials. Thanks for your help.
>
>Matt
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
Craig Martell on sat 2 jun 01
Ivor asked:
>In your post on the above topic you say >black coring results from carbonates not being burned out>
What I really meant was "carbonaceous material", which is another way of
expressing "organics" in the clay. Sorry for the confusion and my poor
choice of terms.
regards, Craig Martell in Oregon
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