Tony Ferguson on tue 5 jul 05
John,
It sounds like a crystolbolite problem and or firing too fast, possibly bisque but it doesn't sound like it. Feel free to send me a few pics too and of the cross section of the clay wall. I've had some problems and I can tell you the solutions I've found. Some glazes can also tear pots apart. I have some tricks to fix this as well. I notice you have 13% spar in your body. How long was your firing? Give me a call we can talk about it.
Tony Ferguson
John Anthony wrote:
Hi all-
I have had a problem in the last two firings of my woodkiln that
I have never
encountered before. Certain pots, some handbuilt, some thrown, have
succumbed
to dunting. They are always on the top shelf in the front (towards the
firebox). Other pots in other areas of the kiln
with the same claybody and glazes have not exhibited this problem. I
have read up on dunting- checked my Hamer,
and there has been some recent discussion here of the problem. Hard
to get a real consensus
on a likely cause. Last firing I thought perhaps I had gone up too
fast; this time was very careful to go through 1000F slowly.
I took the door down when the tempo was about 260 F on the top, gave
it an hour then unloaded. The vases from the top shelf
were making loud cracking sounds- not like glaze pings, like loud
cracks. I put one tall torso-shaped vase
aside to watch, and it literally tore itself apart while I was
looking. Spiral crack started from the narrow bottom and went about
half
way up and around. Then the wider top just came apart- spiral cracks
with lateral cracks between. I should note that on this particular
pot
(as with one that came apart similarly last firing) there was only
glaze on the outside top one third of the pot -
the bottom of the pot was a Tile 6 slip. However a rectangular
handbuilt bottle that was completely glazed also pulled apart.
These pots were slow bisqued (about 11 hours) to 06, and unloaded from
the bisque when completely cold.
The liner glaze on the pots is "The"
which is:
Custer 23
OM4 23
Silica 23
Whiting 23
Talc 8
add bentonite 2%
The outer glaze is Oribe:
Custer 29
Whiting 21
Talc 8
EPK 12
Bone Ash 1
Silica 24
Cop Carb 5
The clay body is
Fireclay 36
Ball Clay 20
Kaolin 16
Silica 3
G200 13
30/48 grog 6
48 Kyanite 6
So I guess my question would be, for any who wish to help me out with
some advice here, what is the most likely culprit - bisque defect,
glaze fit,
firing problem, all of the above? Thanks in advance for any input.
cheers
John Anthony
http://www.redhillpottery.com
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Tony Ferguson
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Paul Herman on tue 5 jul 05
Hello John Anthony,
I'm inclined to suspect the clay body. You don't say what fireclay you
use, but is it high in iron? Iron in the clay body does strange things
in reduction. I think it interferes with feldspar's ability to reduce
cristobolite. I don't know why.
In trying to perfect a red woodfired clay, my "fake Bizen" there has
been some spiral dunting, much to my chagrin. For what it's worth, my
white firing clays never dunt. Another for-what-its-worth, the inside
glaze may be under too much compression under the circumstances, try a
real crackley one for the liner.
If anyone can shed more light on the Iron thing, please do!
Best,
Paul Herman
Great Basin Pottery
Doyle, California US
http://www.greatbasinpottery.com/
----------
>From: John Anthony
>To: CLAYART@LSV.CERAMICS.ORG
>Subject: dunting question
>Date: Tue, Jul 5, 2005, 8:24 AM
>
> Hi all-
> I have had a problem in the last two firings of my woodkiln that
> I have never
> encountered before.
John Anthony on tue 5 jul 05
Hi all-
I have had a problem in the last two firings of my woodkiln that
I have never
encountered before. Certain pots, some handbuilt, some thrown, have
succumbed
to dunting. They are always on the top shelf in the front (towards the
firebox). Other pots in other areas of the kiln
with the same claybody and glazes have not exhibited this problem. I
have read up on dunting- checked my Hamer,
and there has been some recent discussion here of the problem. Hard
to get a real consensus
on a likely cause. Last firing I thought perhaps I had gone up too
fast; this time was very careful to go through 1000F slowly.
I took the door down when the tempo was about 260 F on the top, gave
it an hour then unloaded. The vases from the top shelf
were making loud cracking sounds- not like glaze pings, like loud
cracks. I put one tall torso-shaped vase
aside to watch, and it literally tore itself apart while I was
looking. Spiral crack started from the narrow bottom and went about
half
way up and around. Then the wider top just came apart- spiral cracks
with lateral cracks between. I should note that on this particular
pot
(as with one that came apart similarly last firing) there was only
glaze on the outside top one third of the pot -
the bottom of the pot was a Tile 6 slip. However a rectangular
handbuilt bottle that was completely glazed also pulled apart.
These pots were slow bisqued (about 11 hours) to 06, and unloaded from
the bisque when completely cold.
The liner glaze on the pots is "The"
which is:
Custer 23
OM4 23
Silica 23
Whiting 23
Talc 8
add bentonite 2%
The outer glaze is Oribe:
Custer 29
Whiting 21
Talc 8
EPK 12
Bone Ash 1
Silica 24
Cop Carb 5
The clay body is
Fireclay 36
Ball Clay 20
Kaolin 16
Silica 3
G200 13
30/48 grog 6
48 Kyanite 6
So I guess my question would be, for any who wish to help me out with
some advice here, what is the most likely culprit - bisque defect,
glaze fit,
firing problem, all of the above? Thanks in advance for any input.
cheers
John Anthony
http://www.redhillpottery.com
David Beumee on wed 6 jul 05
Hi John,
I don't see either of the glaze recipes with ingredients of particularly low expansion material that might put the body in enough compression to cause dunting, and your claybody appears to have a high enough spar content to prevent formation of crystobalite, which can cause dunting. The only way to actually find out if cristobalite is the cause is to have dilitometry done on the body, or find out through Jim Robinson's Glaze Test Series if the spar content has lowered the thermal expansion of the body sufficiently to keep GTS 9 and 10 on without shivering.
You say you took the door down when the temperature on top was 260 F on top. If the pyrometer is reading the air temperature inside the kiln, the pots and shelves will be hotter, and the pots go through cristobalite inversion at 225 degrees C, about 420 degrees F. You could be taking down the door when the pots are going through cristobalite inversion. If the cracks show a grainy surface, likely they are caused by cristobalite inversion. If the cracks show a glassier, smoother fracture, they could be quartz inversion cracks.
David Beumee
-------------- Original message ----------------------
From: John Anthony
> Hi all-
> I have had a problem in the last two firings of my woodkiln that
> I have never
> encountered before. Certain pots, some handbuilt, some thrown, have
> succumbed
> to dunting. They are always on the top shelf in the front (towards the
> firebox). Other pots in other areas of the kiln
> with the same claybody and glazes have not exhibited this problem. I
> have read up on dunting- checked my Hamer,
> and there has been some recent discussion here of the problem. Hard
> to get a real consensus
> on a likely cause. Last firing I thought perhaps I had gone up too
> fast; this time was very careful to go through 1000F slowly.
> I took the door down when the tempo was about 260 F on the top, gave
> it an hour then unloaded. The vases from the top shelf
> were making loud cracking sounds- not like glaze pings, like loud
> cracks. I put one tall torso-shaped vase
> aside to watch, and it literally tore itself apart while I was
> looking. Spiral crack started from the narrow bottom and went about
> half
> way up and around. Then the wider top just came apart- spiral cracks
> with lateral cracks between. I should note that on this particular
> pot
> (as with one that came apart similarly last firing) there was only
> glaze on the outside top one third of the pot -
> the bottom of the pot was a Tile 6 slip. However a rectangular
> handbuilt bottle that was completely glazed also pulled apart.
>
> These pots were slow bisqued (about 11 hours) to 06, and unloaded from
> the bisque when completely cold.
>
>
> The liner glaze on the pots is "The"
> which is:
> Custer 23
> OM4 23
> Silica 23
> Whiting 23
> Talc 8
>
> add bentonite 2%
>
>
> The outer glaze is Oribe:
> Custer 29
> Whiting 21
> Talc 8
> EPK 12
> Bone Ash 1
> Silica 24
> Cop Carb 5
>
>
> The clay body is
>
> Fireclay 36
> Ball Clay 20
> Kaolin 16
> Silica 3
> G200 13
> 30/48 grog 6
> 48 Kyanite 6
>
>
>
> So I guess my question would be, for any who wish to help me out with
> some advice here, what is the most likely culprit - bisque defect,
> glaze fit,
> firing problem, all of the above? Thanks in advance for any input.
>
> cheers
>
> John Anthony
>
> http://www.redhillpottery.com
>
> ______________________________________________________________________________
> Send postings to clayart@lsv.ceramics.org
>
> You may look at the archives for the list or change your subscription
> settings from http://www.ceramics.org/clayart/
>
> Moderator of the list is Mel Jacobson who may be reached at melpots@pclink.com.
Ron Roy on wed 6 jul 05
Hi John,
If it were not for the 13% spar I would say cristobalite - but it cannot be
the cause with that much spar - as long as you are sure it all got added to
the batch.
The two glazes do not have a low expansion so I cannot say that is the problem.
When I compare your clay body to other cone 10 bodies (I used Hawthorn,
OM#4 and EPK by the way) I find it short of alumina and silica for a cone
10 body - perhaps the body is so brittle that it cannot handle uneven
cooling.
Mix up a small amount of the following body and see if it does well in that
area.
Fire clay - 33.0
Ball clay - 13.0
EPK - 29.0
G200 - 11.0
Grog - 6.0
Kyanite - 8.0
Total 100.0
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
Fax: 613-475-3513
Ron Roy on wed 6 jul 05
Hi Paul,
I have heard that iron was a catalyst for cristobalite production - I don't
know about that part - what I do know is: Iron is a flux in reduction - so
you have to make the body refractory to compensate for that fluxing - and
still get in at least 10% spar to make sure the cristobalite gets melted as
it forms.
The other important factor is micro fine quartz - get it out - use fine
sand if you have to but ungraded fine silica is a real problem.
See Peter Sohngens article in Studio Potter Vol 28 #1 - I did the
dilatometery so I can vouch for the accuracy.
Peter thinks a little cristobalite is OK - I don't - not in oven ware
anyway - better to know how to fit the glazes properly.
RR
>I'm inclined to suspect the clay body. You don't say what fireclay you
>use, but is it high in iron? Iron in the clay body does strange things
>in reduction. I think it interferes with feldspar's ability to reduce
>cristobolite. I don't know why.
>
>In trying to perfect a red woodfired clay, my "fake Bizen" there has
>been some spiral dunting, much to my chagrin. For what it's worth, my
>white firing clays never dunt. Another for-what-its-worth, the inside
>glaze may be under too much compression under the circumstances, try a
>real crackley one for the liner.
>
>If anyone can shed more light on the Iron thing, please do!
>
>Best,
>
>Paul Herman
>
>Great Basin Pottery
>Doyle, California US
>http://www.greatbasinpottery.com/
>
>----------
>>From: John Anthony
>>To: CLAYART@LSV.CERAMICS.ORG
>>Subject: dunting question
>>Date: Tue, Jul 5, 2005, 8:24 AM
>>
>
>> Hi all-
>> I have had a problem in the last two firings of my woodkiln that
>> I have never
>> encountered before.
>
>______________________________________________________________________________
>Send postings to clayart@lsv.ceramics.org
>
>You may look at the archives for the list or change your subscription
>settings from http://www.ceramics.org/clayart/
>
>Moderator of the list is Mel Jacobson who may be reached at melpots@pclink.com.
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
Fax: 613-475-3513
Paul Herman on thu 7 jul 05
HI Ron,
Thanks for your thoughts.
For the "Fake Bizen" clay, my next move will be to reduce the silica and
increase the spar a bit. The clay is refractory enough that I think I
can get away with more spar. It holds it's shape at cone 13 in the front
of the wood kiln. The red clay content is Carbondale Red, which I
understand is a refractory red clay.
best wishes,
Paul Herman
Great Basin Pottery
Doyle, California US
http://www.greatbasinpottery.com/
----------
>From: Ron Roy
> Hi Paul,
>
> I have heard that iron was a catalyst for cristobalite production - I don't
> know about that part - what I do know is: Iron is a flux in reduction - so
> you have to make the body refractory to compensate for that fluxing - and
> still get in at least 10% spar to make sure the cristobalite gets melted as
> it forms.
>
> The other important factor is micro fine quartz - get it out - use fine
> sand if you have to but ungraded fine silica is a real problem.
>
> See Peter Sohngens article in Studio Potter Vol 28 #1 - I did the
> dilatometery so I can vouch for the accuracy.
>
> Peter thinks a little cristobalite is OK - I don't - not in oven ware
> anyway - better to know how to fit the glazes properly.
>
> RR
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