Paul Herman on thu 28 jan 10
Hi Bruce,
Oh man, there is hardly a worse problem than dunting, especially after
an arduous wood firing marathon.
It could be caused by excessive cristobolite, or even a regular alpha/
beta quartz inversion, but I think it sounds like cristobolite.
Could you show us the formula for your clay body? What percentage of
Custer are putting in there? From your description I'm highly
suspicious of the local clay. I suppose you've not had it analyzed?
The local clay might contain a large percentage of fine (or otherwise)
silica. Ball clays are usually quite high in silica. One of my
favorite explanations of silica and all of that is in Hamer and
Hamer's dictionary for Potters.
I've found, and others here on Clayart have noted, that if you have at
least 10% feldspar in the mix the dunting "usually" goes away. I've
also found that clay bodies that are high in iron are more susceptible
to dunting, if fired in cone 10+ reduction. I wish I knew why the iron
did that. Is there anyone out there who can explain the iron effect to
me? How dark is the body when fired in reduction? The slow cooling of
bigger wood fired kilns would make the cristobolite problem worse, and
slow cooling is one of the things that makes wood fired work look so
nice. The extra time allows crystals to grow, and makes the stuff
pretty.
A seat-of -the-pants kind of response might be to replace some of the
local clay with a cheap local/regional kaolin, and make sure you
include at least 10% feldspar. I've always envied you east coast
potters for your extensive local clay deposits. We don't have much of
anything but igneous rock and volcanic ash out here for the most part.
When I first started wood firing my old clay bodies from the gas days
went through a fit of dunting, and I think replacing iron bearing
clays with kaolin was largely responsible for the cure.
I sympathize with your troubles, because I've struggled with that
problem in the past (knock on stoking wood.)
best wishes,
Paul Herman
Great Basin Pottery
Doyle, California US
www.greatbasinpottery.com/
On Jan 28, 2010, at 4:34 PM, Bruce Bowers wrote:
> Hi Folks.
> I just unloaded my wood kiln and had a huge amount of dunting.
> The kiln was sealed as usual and the entire cooling procedure was as
> it
> always is.
> I use a lot of local clay, but I add Tenn. Ball #5, Foundry Hill
> Creme, and
> Custer Feldspar in order to mix up the particle sizes, to make use
> of the
> long melting range of feldspar, and to make the body more plastic.
> I suspect the high silica content is responsible for the problem.
> I usually fire to about cone 12 (front) and cone 10 in the back. In a
> typical firing, I would be using a lot more commercial clay and so
> no dunting
> problems.
> This time, the back of the kiln only went to cone 9 and I had
> absolutely no
> dunting in the back tiers.
> This is an open firebox (no baffle wall) tube kiln and the highest
> degree
> of dunting happened to the hottest pots that were near the firebox.
> Is it possible that cristaboltite inversion that happens above about
> cone 9
> is the culprit?
> Therefore, the cooler pots did not go through this phase of 3% volume
> shifting........is this why no dunting?.......same couple local
> bodies as at
> front,
> Lastly, the mine where I dig my clay is not far from the site of the
> old
> Jordan Clay mine. Jordan clay was notorious for dunting.
> Any ideas on the problem and the cure?
> Bruce Bowers
> _www.bowerswoodfireandpics.com_ (http://www.bowerswoodfireandpics.com)
Lee Love on thu 28 jan 10
Hi silica clay bodies are prone to dunting if slow cooled. MicK
Cassion crash cooled his kiln to 1000*C at the end of his woodfiring.
My friend in Mashiko, Euan Craig, who studied with students of
Casson, crash cooled his hanjiki (half Aussie porcelain and half
shigaraki stoneware body) because it was high silica and also because
it brings out red color in flashed clay.
I found other mentioning of it here (search "crash cooling" at this s=
=3D
ite):
http://myweb.tiscali.co.uk/rikthepotter/nwp/claywork/2001spring/petra.htm
A quote from the page:
"Jeremy had spent time working at Winchcombe Pottery under Ray Finch
where he became obsessed with usefulness. He became keen on soda
reduction firing as well as glaze technology and making glazes from
found materials.
He said that one of the important firing techniques they learnt was
crash cooling the gas kiln from 1300 oC to 1000 oC in 1 - 1=3DBD hours.
This technique brightened colour and promoted good crystal formation
in the clay which made the body stronger and helped prevent dunting."
--
Lee, a Mashiko potter in Minneapolis
http://mashikopots.blogspot.com/
"Ta tIr na n-=3DF3g ar chul an tI=3D97tIr dlainn trina ch=3DE9ile"=3D97tha=
t is, "T=3D
he
land of eternal youth is behind the house, a beautiful land fluent
within itself." -- John O'Donohue
Michael Wendt on thu 28 jan 10
Bruce,
I would send a sample to Ron Roy for dilatometry if he would
be willing.
If it is cristobalite formation, increasing feldspar to turn
the cristobalite into glass might solve the problem.
Also, you might try the Helmer wood fire clay I make to see
if you like it.
A box costs $9.50 for 38 lbs and the shipping costs $10.20
anywhere in the 48 states, Alaska or Hawaii.
Regards,
Michael Wendt
Bruce wrote:
Hi Folks.
I just unloaded my wood kiln and had a huge amount of
dunting.
The kiln was sealed as usual and the entire cooling
procedure was as it
always is.
I use a lot of local clay, but I add Tenn. Ball #5, Foundry
Hill Creme, and
Custer Feldspar in order to mix up the particle sizes, to
make use of the
long melting range of feldspar, and to make the body more
plastic.
I suspect the high silica content is responsible for the
problem.
I usually fire to about cone 12 (front) and cone 10 in the
back. In a
typical firing, I would be using a lot more commercial clay
and so no dunting
problems.
This time, the back of the kiln only went to cone 9 and I
had absolutely no
dunting in the back tiers.
This is an open firebox (no baffle wall) tube kiln and the
highest degree
of dunting happened to the hottest pots that were near the
firebox.
Is it possible that cristaboltite inversion that happens
above about cone 9
is the culprit?
Therefore, the cooler pots did not go through this phase of
3% volume
shifting........is this why no dunting?.......same couple
local bodies as at
front,
Lastly, the mine where I dig my clay is not far from the
site of the old
Jordan Clay mine. Jordan clay was notorious for dunting.
Any ideas on the problem and the cure?
Bruce Bowers
Vince Pitelka on thu 28 jan 10
Bruce Bowers wrote:
"I just unloaded my wood kiln and had a huge amount of dunting. The kiln wa=
s
sealed as usual and the entire cooling procedure was as it always is. I use
a lot of local clay, but I add Tenn. Ball #5, Foundry Hill Creme, and Custe=
r
Feldspar in order to mix up the particle sizes, to make use of the long
melting range of feldspar, and to make the body more plastic. I suspect the
high silica content is responsible for the problem. I usually fire to about
cone 12 (front) and cone 10 in the back. In a typical firing, I would be
using a lot more commercial clay and so no dunting problems. This time, th=
e
back of the kiln only went to cone 9 and I had absolutely no dunting in the
back tiers. This is an open firebox (no baffle wall) tube kiln and the
highest degree of dunting happened to the hottest pots that were near the
firebox. Is it possible that cristaboltite inversion that happens above
about cone 9 is the culprit?"
Bruce -
Cristobalite inversion happens at 430 degrees Fahrenheit, and as far as I
now, there is no cristobalite inversion at high-fire temperatures. Even if
there was, the clay is quite pyroplastic at cone 9, and a 3% change in
volume would have little or no effect. Also, if the clay was actually
cracking at cone 9, which is pretty unlikely, it would be the kind of rippe=
d
open warped cracks with melted glaze/ash along the edges, which has nothing
to do with dunting.
Dunting is clean, hard-edged tight cracks that happen in cooling below red
heat. If it is only the highest-fired ware that is dunting, then it sounds
like the problem is over-vitrification. How much feldspar did you add?
What kinds of local claybodies are you using? Are there any high-iron
bodies? You might want to do an absorption test on a piece of one of the
pots fired in the back of the kiln that only went to cone 9. I realize tha=
t
you'll probably have to break a pot, but it will be worth it. If the clay
fired to cone 9 has extremely low absorption, then it is a sure bet that th=
e
clay fired to cone 11 or cone 12 is over-vitrified.
If you are adding enough feldspar, then high-silica is not the problem,
because the feldspar will combine with the silica to form glass, which is
the basis of all ceramic firing processes. Cristobalite problems happen
when there is excess free silica which is converted to cristobalite above
2150F.
Hope this is helpful information -
- Vince
Bruce Bowers on thu 28 jan 10
Hi Folks.
I just unloaded my wood kiln and had a huge amount of dunting.
The kiln was sealed as usual and the entire cooling procedure was as it
always is.
I use a lot of local clay, but I add Tenn. Ball #5, Foundry Hill Creme, and
Custer Feldspar in order to mix up the particle sizes, to make use of the
long melting range of feldspar, and to make the body more plastic.
I suspect the high silica content is responsible for the problem.
I usually fire to about cone 12 (front) and cone 10 in the back. In a
typical firing, I would be using a lot more commercial clay and so no dunti=
ng
problems.
This time, the back of the kiln only went to cone 9 and I had absolutely no
dunting in the back tiers.
This is an open firebox (no baffle wall) tube kiln and the highest degree
of dunting happened to the hottest pots that were near the firebox.
Is it possible that cristaboltite inversion that happens above about cone 9
is the culprit?
Therefore, the cooler pots did not go through this phase of 3% volume
shifting........is this why no dunting?.......same couple local bodies as a=
t
front,
Lastly, the mine where I dig my clay is not far from the site of the old
Jordan Clay mine. Jordan clay was notorious for dunting.
Any ideas on the problem and the cure?
Bruce Bowers
_www.bowerswoodfireandpics.com_ (http://www.bowerswoodfireandpics.com)
Edouard Bastarache on fri 29 jan 10
" If you are adding enough feldspar, then high-silica is not the problem,
because the feldspar will combine with the silica to form glass, which is
the basis of all ceramic firing processes. Cristobalite problems happen
when there is excess free silica which is converted to cristobalite above
2150F.
Vince,"
in reduction, ferric oxide will do the same to convert
free silica to fayalite and prevent the formation of cristobalite.
Gis,
Edouard Bastarache
Spertesperantisto (Lernu-Paris)
Sorel-Tracy
Quebec
http://edouardbastarache.blogspot.com/
http://www.flickr.com/photos/30058682@N00/
http://cerampeintures.blogspot.com/
http://albertpaintings.blogspot.com/
http://perso.wanadoo.fr/smart2000/index.htm
>
> Hope this is helpful information -
> - Vince
>
David Woof on fri 29 jan 10
Hi Bruce=3D2C
Stop all that inversion=3D2C cristobalite technical blather you have been r=
ea=3D
ding for a moment=3D2C reread your post and start with your additions of lo=
ca=3D
l clay. I think you will find answers there when you take time to reflect a=
=3D
nd reason on the information you provided. Many times the questions answer=
=3D
the question.
=3D20
I like how you jumped in and are learning on the job.=3D20
=3D20
Local clays contain all sorts of mineral soluables=3D2C fluxing agents and =
su=3D
ndry minerals that can contribute to dunting when overfired.
=3D20
I really have to go now=3D2C so maybe someone else will speak to this furt=
he=3D
r and eloquently or I'm sure at least coherently technical.
=3D20
Best=3D2C
=3D20
David Woof
________________________________________________________________________
16. Dunting Problem
Posted by: "Bruce Bowers" Sailoor47@AOL.COM=3D20
Date: Thu Jan 28=3D2C 2010 5:08 pm ((PST))
=3D20
Hi Folks.
I just unloaded my wood kiln and had a huge amount of dunting.
The kiln was sealed as usual and the entire cooling procedure was as it
always is.
I use a lot of local clay=3D2C but I add Tenn. Ball #5=3D2C Foundry Hill Cr=
eme=3D
=3D2C and
Custer Feldspar in order to mix up the particle sizes=3D2C to make use of t=
he
long melting range of feldspar=3D2C and to make the body more plastic.
I suspect the high silica content is responsible for the problem.
I usually fire to about cone 12 (front) and cone 10 in the back. In a
typical firing=3D2C I would be using a lot more commercial clay and so no d=
un=3D
ting
problems.
This time=3D2C the back of the kiln only went to cone 9 and I had absolutel=
y =3D
no
dunting in the back tiers.
This is an open firebox (no baffle wall) tube kiln and the highest degree
of dunting happened to the hottest pots that were near the firebox.
Is it possible that cristaboltite inversion that happens above about cone 9
is the culprit?
Therefore=3D2C the cooler pots did not go through this phase of 3% volume
shifting........is this why no dunting?.......same couple local bodies as a=
=3D
t
front=3D2C
Lastly=3D2C the mine where I dig my clay is not far from the site of the ol=
d
Jordan Clay mine. Jordan clay was notorious for dunting.
Any ideas on the problem and the cure?
Bruce Bowers
_www.bowerswoodfireandpics.com_ (http://www.bowerswoodfireandpics.com)
=3D20
=3D20
=3D20
_________________________________________________________________
Your E-mail and More On-the-Go. Get Windows Live Hotmail Free.
http://clk.atdmt.com/GBL/go/196390709/direct/01/=3D
Ron Roy on fri 29 jan 10
Hi Bruce,
Thats what it sounds like to me - if you want to send a sample for me
to measure in my dilatometer - that will confirm it. Let me know and
I'll send the instructions - it's easy.
You probably need to read the article on the subject by Peter Sohngen
in Studio Potter Volume 28 #1.
In essence his research (I did the dilatometery) shows you need at
least 10% spar and go easy on micro fine silica (what we usually get
in the bag.)
Graded silica - like 200M with few fines is not a problem.
The more iron involved the more complicated the problem becomes.
Ron Roy
Quoting Bruce Bowers :
> Hi Folks.
> I just unloaded my wood kiln and had a huge amount of dunting.
> The kiln was sealed as usual and the entire cooling procedure was as it
> always is.
> I use a lot of local clay, but I add Tenn. Ball #5, Foundry Hill Creme, a=
nd
> Custer Feldspar in order to mix up the particle sizes, to make use of th=
e
> long melting range of feldspar, and to make the body more plastic.
> I suspect the high silica content is responsible for the problem.
> I usually fire to about cone 12 (front) and cone 10 in the back. In a
> typical firing, I would be using a lot more commercial clay and so no dun=
ting
> problems.
> This time, the back of the kiln only went to cone 9 and I had absolutely =
no
> dunting in the back tiers.
> This is an open firebox (no baffle wall) tube kiln and the highest degree
> of dunting happened to the hottest pots that were near the firebox.
> Is it possible that cristaboltite inversion that happens above about cone=
9
> is the culprit?
> Therefore, the cooler pots did not go through this phase of 3% volume
> shifting........is this why no dunting?.......same couple local bodies as=
at
> front,
> Lastly, the mine where I dig my clay is not far from the site of the old
> Jordan Clay mine. Jordan clay was notorious for dunting.
> Any ideas on the problem and the cure?
> Bruce Bowers
> _www.bowerswoodfireandpics.com_ (http://www.bowerswoodfireandpics.com)
>
Ron Roy on sat 30 jan 10
Hi Ed,
Where did you get this information - my experiments show the opposite.
I have read also that iron oxide helps the production of cristobalite.
It certainly lowers the melting point of clay bodies and therefore
limits the amount of spar you can use.
RR
Quoting Edouard Bastarache :
> in reduction, ferric oxide will do the same to convert
> free silica to fayalite and prevent the formation of cristobalite.
>
>
> Gis,
>
> Edouard Bastarache
Vince Pitelka on sat 30 jan 10
Edouard Bastarache wrote:
" . . in reduction, ferric oxide will do the same to convert
> free silica to fayalite and prevent the formation of cristobalite."
And Ron Roy wrote:
Where did you get this information - my experiments show the opposite.
I have read also that iron oxide helps the production of cristobalite.
It certainly lowers the melting point of clay bodies and therefore
limits the amount of spar you can use.
Ron -
Do you know the mechanics by which iron oxide promotes cristobalite? Your
knowledge of the chemistry of clays and glazes is a mountain compared to my
speed-bump of understanding, but it would seem that any flux that combines
with silica and creates a glassy phase will help prevent the formation of
cristobalite. What am I missing? (Obviously, a lot, but it always helps me
to pin it down).
- Vince
Vince Pitelka
Appalachian Center for Craft
Tennessee Tech University
vpitelka@dtccom.net; wpitelka@tntech.edu
http://iweb.tntech.edu/wpitelka
Ron Roy on mon 1 feb 10
Hi Ed,
I assume we are talking about cone 9 and up in reduction - my
measurements are made on cone 10 reduction fired clays.
My information does not support this - glad to run some samples
through my dilatometer - just let me know if you want to do it - I'll
send instructions on how to prepare the samples.
See page 68 in Micheal Cardew's Pioneer Pottery - "Iron oxide, both in
the ferric and the ferrous forms, also acts in a body as a
'mineralizer' promoting the crystallisation of cristobalite.
Do you have any dilatometer charts of those clay bodies? Do you have
any of the glaze recipes you used so we can check the expansions?
Crazing glazes don't break clay with cristobalite in them for instance.
RR
Quoting Edouard Bastarache :
> Directly from Smart.Conseil in France.
> The best ceramics technologist I know of.
> This was proved by my own works in New France.
> One year I tested many cone 9-10 redux clays, out of 250 that all
> contained Ferric Oxide,
> 33% were without any type of spar.
> And no problemo.
> I posted these results on Clayart a few years ago.
>
> Smart told me the explanation.
> If you fire in redux at cone 9, ferric oxide will be transformed
> into black iron oxide, a very powerfull flux that will attack the
> free silica and turn
> it into fayalite, preventing the formation of cristobalite.
>
> The other ceramicist who understood my results,
> was Gary Shafer, then living in the panhandle of Ohio,
> on the pottery chat, smart guy. He said something like :
> "You use Ferric Oxide as a fluxing material instead
> of spars, firing in redux."
> The key point is to fire in reduction.
>
> Gis,
>
> Edouard Bastarache Spertesperantisto (Lernu-Paris)
>
> Sorel-Tracy
> Quebec
>
> http://edouardbastarache.blogspot.com/
> http://www.flickr.com/photos/30058682@N00/
> http://cerampeintures.blogspot.com/
> http://albertpaintings.blogspot.com/
> http://perso.wanadoo.fr/smart2000/index.htm
>
>
>
>
>
> ----- Original Message ----- From:
> To: "Edouard Bastarache"
> Cc:
> Sent: Saturday, January 30, 2010 2:41 PM
> Subject: Re: Dunting Problem
>
>
>> Hi Ed,
>>
>> Where did you get this information - my experiments show the
>> opposite. I have read also that iron oxide helps the production of
>> cristobalite.
>>
>> It certainly lowers the melting point of clay bodies and therefore
>> limits the amount of spar you can use.
>>
>> RR
>>
>> Quoting Edouard Bastarache :
>>
>>
>>> in reduction, ferric oxide will do the same to convert
>>> free silica to fayalite and prevent the formation of cristobalite.
>>>
>>>
>>> Gis,
>>>
>>> Edouard Bastarache
>>
>>
>>
>
Ron Roy on mon 1 feb 10
Hi Vince,
I don't understand it Vince - I do hear that the real killer for
cristobalite is sodium and potassium - and my body testing sure
confirms that.
I have been making high iron clay bodies for 50 years now - certainly
enough to know that iron oxide fluxing is going to result in lots of
cristobalite.
Have you read Peters article in Studio Potter? Peter Sohngen in Studio
Potter Volume 28 #1. Once you see the dilatometer charts in that
article the picture will be clearer.
I have asked Studio Potter to include this article with their free
articles but they have not done so yet.
It is the best article so far on how to avoid cristobalite in high
fire clay bodies and one of the main reasons is the understanding
about what we are getting in our silica bags. Micro fine silica
converts easily to cristobalite - that is probably the most
significant change in the least 50 years - more efficient quartz
grinding.
Perhaps - when I do some of your clay bodies - you could include a
clay that is fluxed with iron oxide - that should tell it all.
RR
Quoting Vince Pitelka :
> Edouard Bastarache wrote:
> " . . in reduction, ferric oxide will do the same to convert
>> free silica to fayalite and prevent the formation of cristobalite."
>
> And Ron Roy wrote:
> Where did you get this information - my experiments show the opposite.
> I have read also that iron oxide helps the production of cristobalite.
> It certainly lowers the melting point of clay bodies and therefore
> limits the amount of spar you can use.
>
> Ron -
> Do you know the mechanics by which iron oxide promotes cristobalite? You=
r
> knowledge of the chemistry of clays and glazes is a mountain compared to =
my
> speed-bump of understanding, but it would seem that any flux that combine=
s
> with silica and creates a glassy phase will help prevent the formation of
> cristobalite. What am I missing? (Obviously, a lot, but it always helps =
me
> to pin it down).
> - Vince
>
> Vince Pitelka
> Appalachian Center for Craft
> Tennessee Tech University
> vpitelka@dtccom.net; wpitelka@tntech.edu
> http://iweb.tntech.edu/wpitelka
>
ivor & olive lewis on tue 2 feb 10
Ron Roy tells us
<e
ferric and the ferrous forms, also acts in a body as a 'mineralizer'
promoting the crystallisation of cristobalite.>>
Which suggests that the Iron oxides act as catalysts causing excess
vitrified silica to crystallise as Cristobalite, a view that would be
supported by information on the FeO-SiO2-Fe2O3 Phase Diagram.
Best regards,
Ivor Lewis,
Redhill,
South Australia
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