Victoria E. Hamilton on mon 31 jan 05
Hello -
The only thing I know about Laguna B (6 or 10) is that you have to throw
even or it cracks. Trimming even later does not help.
Victoria Hamilton
Millennia Antica Pottery
Seattle, WA
-----Original Message-----
From: Clayart [mailto:CLAYART@LSV.CERAMICS.ORG] On Behalf Of Sam or Mary
Yancy
Sent: Monday, January 31, 2005 18:44
To: CLAYART@LSV.CERAMICS.ORG
Subject: Re: Cracking during bisque firing
Last time i used b-mix - cone five, ALL pieces cracked on firing to cone 6.
Bisk fire was ok. Never figured it out, used several differnet commercial
glazes, --- but cost me an bunch in xmas sales/gifts. will not use b-mix
now. Sam in Daly City
MudPuppy wrote:I seem to be having alot of problems
with my bisque firings lately...
Here's what's happening, but I have posted photos on my website journal to
give you a clearer idea. You can see them at
http://studio.box49.com/e107_plugins/userjournals_menu/userjournals.php?user
.MudPuppy
Ware goes into the kiln looking great, but when I open the kiln the next
day, the ware is full of cracks, both encircling the entire bowl and/or
cracking the piece right in half.
Most often, the cracks appear only on the inside of the bowl. Some pieces
are cracked clear through, but many are not.
While some pieces are trimmed and have a pretty traditional foot, others are
not trimmed at all and sit on 3 points of clay pushed in with my thumb.
I am wondering if the problem is related to the cold weather and my kilns
size...its a small converted electric updraft kiln-aprox 9 cubic feet
inside, fueled by 2 propane burners in the kilns floor.
I bisque to 850 celcius and then just shut the kiln off. I usually leave the
ware in the kiln with just the pilots lit till the temp reads about 500
farenheight, then slowly raise the temp over about 6 hours. Since kiln space
is precious, I nest some pieces. Sometimes one inside the other, but
sometimes I place large pieces over smaller ones and fire them on thier rims
rather than on thier feet with small pieces inside. I am also seeing some
cracking in handbuilt stuff...I am wondering, since this is a new problem in
the last month or so, if it could be related to our cold temps. We have gone
weeks without breaking freezing. So could my kiln and ware be cooling too
quickly and causing the cracking?
I am wondering what a good cooling down schedule might be for my bisque, or
if anyone can see a more obvious problem with either my greenware or my
firing.
Thanks!
Cathi Newlin
MudPuppy@box49.com
http://studio.box49.com
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____________________________________________________________________________
__
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MudPuppy on mon 31 jan 05
I seem to be having alot of problems with my bisque firings lately...
Here's what's happening, but I have posted photos on my website journal to
give you a clearer idea. You can see them at
http://studio.box49.com/e107_plugins/userjournals_menu/userjournals.php?user.MudPuppy
Ware goes into the kiln looking great, but when I open the kiln the next
day, the ware is full of cracks, both encircling the entire bowl and/or
cracking the piece right in half.
Most often, the cracks appear only on the inside of the bowl. Some pieces
are cracked clear through, but many are not.
While some pieces are trimmed and have a pretty traditional foot, others are
not trimmed at all and sit on 3 points of clay pushed in with my thumb.
I am wondering if the problem is related to the cold weather and my kilns
size...its a small converted electric updraft kiln-aprox 9 cubic feet
inside, fueled by 2 propane burners in the kilns floor.
I bisque to 850 celcius and then just shut the kiln off. I usually leave the
ware in the kiln with just the pilots lit till the temp reads about 500
farenheight, then slowly raise the temp over about 6 hours. Since kiln space
is precious, I nest some pieces. Sometimes one inside the other, but
sometimes I place large pieces over smaller ones and fire them on thier rims
rather than on thier feet with small pieces inside. I am also seeing some
cracking in handbuilt stuff...I am wondering, since this is a new problem in
the last month or so, if it could be related to our cold temps. We have gone
weeks without breaking freezing. So could my kiln and ware be cooling too
quickly and causing the cracking?
I am wondering what a good cooling down schedule might be for my bisque, or
if anyone can see a more obvious problem with either my greenware or my
firing.
Thanks!
Cathi Newlin
MudPuppy@box49.com
http://studio.box49.com
Earl Brunner on mon 31 jan 05
They look like thermal shock to me. It's possible that the kiln is dropping
in temperature too fast.
Earl Brunner
Las Vegas, NV
-----Original Message-----
From: Clayart [mailto:CLAYART@LSV.CERAMICS.ORG] On Behalf Of MudPuppy
Sent: Monday, January 31, 2005 11:46 AM
To: CLAYART@LSV.CERAMICS.ORG
Subject: Cracking during bisque firing
I bisque to 850 celcius and then just shut the kiln off. I usually leave the
ware in the kiln with just the pilots lit till the temp reads about 500
farenheight, then slowly raise the temp over about 6 hours. Since kiln space
is precious, I nest some pieces. Sometimes one inside the other, but
sometimes I place large pieces over smaller ones and fire them on thier rims
rather than on thier feet with small pieces inside. I am also seeing some
cracking in handbuilt stuff...I am wondering, since this is a new problem in
the last month or so, if it could be related to our cold temps. We have gone
weeks without breaking freezing. So could my kiln and ware be cooling too
quickly and causing the cracking?
I am wondering what a good cooling down schedule might be for my bisque, or
if anyone can see a more obvious problem with either my greenware or my
firing.
Thanks!
Cathi Newlin
MudPuppy@box49.com
http://studio.box49.com
Sam or Mary Yancy on mon 31 jan 05
Last time i used b-mix - cone five, ALL pieces cracked on firing to cone 6. Bisk fire was ok. Never figured it out, used several differnet commercial glazes, --- but cost me an bunch in xmas sales/gifts. will not use b-mix now. Sam in Daly City
MudPuppy wrote:I seem to be having alot of problems with my bisque firings lately...
Here's what's happening, but I have posted photos on my website journal to
give you a clearer idea. You can see them at
http://studio.box49.com/e107_plugins/userjournals_menu/userjournals.php?user.MudPuppy
Ware goes into the kiln looking great, but when I open the kiln the next
day, the ware is full of cracks, both encircling the entire bowl and/or
cracking the piece right in half.
Most often, the cracks appear only on the inside of the bowl. Some pieces
are cracked clear through, but many are not.
While some pieces are trimmed and have a pretty traditional foot, others are
not trimmed at all and sit on 3 points of clay pushed in with my thumb.
I am wondering if the problem is related to the cold weather and my kilns
size...its a small converted electric updraft kiln-aprox 9 cubic feet
inside, fueled by 2 propane burners in the kilns floor.
I bisque to 850 celcius and then just shut the kiln off. I usually leave the
ware in the kiln with just the pilots lit till the temp reads about 500
farenheight, then slowly raise the temp over about 6 hours. Since kiln space
is precious, I nest some pieces. Sometimes one inside the other, but
sometimes I place large pieces over smaller ones and fire them on thier rims
rather than on thier feet with small pieces inside. I am also seeing some
cracking in handbuilt stuff...I am wondering, since this is a new problem in
the last month or so, if it could be related to our cold temps. We have gone
weeks without breaking freezing. So could my kiln and ware be cooling too
quickly and causing the cracking?
I am wondering what a good cooling down schedule might be for my bisque, or
if anyone can see a more obvious problem with either my greenware or my
firing.
Thanks!
Cathi Newlin
MudPuppy@box49.com
http://studio.box49.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.
Jennifer Boyer on tue 1 feb 05
I looked up the dunting section in the Hamer book and also the temp you
are bisking to. You say you bisk to 850 C. Hamer refers to a 800 C(1472
F) bisk as being "very soft" and that it doesn't give the clay the
strength to withstand quartz and cristobalote inversions during
cooling, resulting in dunting cracks. Depending on what cones you use,
850 C translates to cone 012 to 015, which sounds to me like you're not
bisking high enough. Try to bisk at least to the equivalent of of 09, a
temp which works well with my clay. Some swear by 06!
Jennifer
On Jan 31, 2005, at 2:46 PM, MudPuppy wrote:
> I seem to be having alot of problems with my bisque firings lately...
> Here's what's happening, but I have posted photos on my website
> journal to
> give you a clearer idea. You can see them at
>
>
> I am wondering if the problem is related to the cold weather and my
> kilns
> size...its a small converted electric updraft kiln-aprox 9 cubic feet
> inside, fueled by 2 propane burners in the kilns floor.
>
> I bisque to 850 celcius and then just shut the kiln off. I usually
> leave the
> ware in the kiln with just the pilots lit till the temp reads about 500
> farenheight, then slowly raise the temp over about 6 hours. Since kiln
> space
> is precious, I nest some pieces. Sometimes one inside the other, but
>
> _______________________________________________________________________
> _______
> 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.
>
>
************************
Jennifer Boyer
Thistle Hill Pottery
Montpelier, VT
http://thistlehillpottery.com
Ivor and Olive Lewis on tue 1 feb 05
Dear Cathi Newlin
Looking at your pictures this seems to be a case of Dunting during
cooling, which means that your kiln is cooling very fast.
As you describe it, I would ask if you are preventing drafts from
circulating by securely sealing the top vent of you kiln and also
stuffing the fireports with fibre to prevent any form of up draft.
No other suggestion, except that you have a very tender clay and
firing to a higher cone might be advisable. I would suggest no lower
than Cone 08
Best regards,
Ivor Lewis.
Redhill,
S. Australia.
Jon Pacini on fri 4 feb 05
Greetings All ---Hi Cathi
Sorry to chime in so late on this ---I haven t been on the computer much
this week.
I see lots of bisque dunts and some of these on your site do appear to be of
that nature. If you are indeed bisqueing at 850 *C, then you re not getting
the ware hot enough to develop the strength it needs to withstand cooling. I
am echoing the sentiment that a number of others have posted.
White Stoneware/porcelain bodies seem to exhibit this tendency more so than
Buff or Red ones. Most white body formulas I see have added silica, and I
would attribute the problem to silica content. I usually recommend bisque
firing white stonewares and porcelain type clays to ^04. Not that you can t
get away with firing lower, but you ll have a lot less trouble with your
ware if you bisque at that temperature.
Some of the cracks are opened up so much that they look more like stress
cracks set up during drying. Keep in mind that dense bodies without much
sand or grog need to dry evenly. Very evenly. If the bottom stays moist and
the rim changes color then you will get these types of jagged cracks across
the bottom.
What happens is that the rim has dried and set and the bottom is still
shrinking. Though usually they will crack when green, many times they won t
crack till they are fired.
Some food for thought----
Best regards
Jon Pacini
Clay Manager
Laguna Clay Co.
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