Burness Speakman on wed 16 jul 03
Finally got a wheel and I don't know sqwuat about throwing. Made some
lovely batter bowls and every one of them exploded. They had to be
too big on the bottoms, but just what causes them to go boom in
bisque firings?
Brian Haviland on wed 16 jul 03
At 06:38 AM 7/16/03 -0400, you wrote:
>Finally got a wheel and I don't know sqwuat about throwing. Made some
>lovely batter bowls and every one of them exploded. They had to be
>too big on the bottoms, but just what causes them to go boom in
>bisque firings?
>
>__>snip
>
>Did you let them completely dry out before bisq..You might also let them
>candle on low keeping temp below 210 degrees for a few hours so ALL the
>water is out of the piece before you ramp up on the heaters..Good luck
>with next try..
>
>Brian
>____________________________________________________________________________
>Send postings to clayart@lsv.ceramics.org
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Lois Ruben Aronow on wed 16 jul 03
On Wed, 16 Jul 2003 06:38:59 -0400, you wrote:
>Finally got a wheel and I don't know sqwuat about throwing. Made some
>lovely batter bowls and every one of them exploded. They had to be
>too big on the bottoms, but just what causes them to go boom in
>bisque firings?
>
Everyone at once: not dry enough!! When you put something in the
bisque, make sure it is completely dry. A good way to check this is
to see if it is not cold to the touch. Even if it feels dry, it is
wet if it is cool to the touch.
My other guess is that it is too think on the bottom. Or most likely,
a combination of the two.
************
www.loisaronow.com
=46ine Craft Porcelain and Pottery
***************************************
Lois Ruben Aronow
Modern Porcelain and Tableware
http://www.loisaronow.com=20
MarjB on wed 16 jul 03
Moisture - your pots were not completely dry ? There is chemically combined
water that has to be released at a slow rate - firing too quickly through
that temperature range ? Cooling too quickly ? MarjB
----- Original Message -----
From: "Burness Speakman"
To:
Sent: Wednesday, July 16, 2003 6:38 AM
Subject: Sooo? What makes my big bottom bisque go Booooom.
> Finally got a wheel and I don't know sqwuat about throwing. Made some
> lovely batter bowls and every one of them exploded. They had to be
> too big on the bottoms, but just what causes them to go boom in
> bisque firings?
>
>
____________________________________________________________________________
__
> 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.
>
>
John Rodgers on wed 16 jul 03
In a word.....WATER!!
Your clay was not dry enough when you tried to bisque it. Thick bottoms
are more problematic that thin bottoms, but it can be done. I purposely
make thiick bottoms at times but it takes longer to dry and risk of
explosion or cracking is greater.
What I do is using a needle tool I check to see how much clay is in the
bottom. Just stick the tool down into the clay, put your finger against
it at the clay line and pull it out. It should measure about 1/2 inch.
If you plan to trim a foot ring into the bottom or otherwise carve a
foot on the piece, then leave a little more. The carving during the
greenware drying stage will reduce the thickness later.
After the carving and finishing, I let things dry about two weeks. I
live in Alabama where the humidity is really high and nothing ever
really completely drys out. When I lived in New Mexico, things would dry
completely in three days.
As long as a piece of work feels cool, even if it appears dry, IT IS NOT
DRY!!!! Can't emphasize that enough. After some time, when I judge a
piece ready to bisque fire, I load the kiln, prop the lid open about two
inches, and turn on the bottom ring of elements in my electric kiln. (
I have individual switches for each pair of elements in each tier in the
kiln - I really like this feature). With your particular kiln, you will
want to set it on the lowest setting possible. Let the kiln stay on
overnight. This is called candling - the word is really more applicable
to fuel fired kilns than electrics but the result is the same......
slowly drives out the moisture without exploding everything.
After candling over night, I turn up the heat a bit at a time. I leave
the lid cracked open until temperature reaches 500 degrees then I close
the lid. I like a very slow bisque firing. Moves all the moisture out,
and gives the organics in the clay a chance to burn out without rapid
outgassing which also can destroy a pot.
If you have a kiln with a computer, you can easily set up your kiln to
handle all this automatically except for the lid closing.
Hope this helps,
Regards,
John Rodgers
Birmingham, AL
Burness Speakman wrote:
>Finally got a wheel and I don't know sqwuat about throwing. Made some
>lovely batter bowls and every one of them exploded. They had to be
>too big on the bottoms, but just what causes them to go boom in
>bisque firings?
>
>______________________________________________________________________________
>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.
>
>
>
Tamara Williams on wed 16 jul 03
Also, could be that the bottom of the inside of the pot wasn't =
compressed and there was an air pocket..=20
----- Original Message -----=20
From: MarjB=20
To: CLAYART@LSV.CERAMICS.ORG=20
Sent: Wednesday, July 16, 2003 10:24 AM
Subject: Re: Sooo? What makes my big bottom bisque go Booooom.
Moisture - your pots were not completely dry ? There is chemically =
combined
water that has to be released at a slow rate - firing too quickly =
through
that temperature range ? Cooling too quickly ? MarjB
----- Original Message -----
From: "Burness Speakman"
To:
Sent: Wednesday, July 16, 2003 6:38 AM
Subject: Sooo? What makes my big bottom bisque go Booooom.
> Finally got a wheel and I don't know sqwuat about throwing. Made =
some
> lovely batter bowls and every one of them exploded. They had to be
> too big on the bottoms, but just what causes them to go boom in
> bisque firings?
>
>
=
_________________________________________________________________________=
___
__
> 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.
>
>
=
_________________________________________________________________________=
_____
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.
John Jensen on wed 16 jul 03
I notice a lot of responses suggesting that your pots may not be dry
enough. This may be true, it is certainly important in most cases that
your pots be bone dry before you begin to raise the temperature of the
kiln above the boiling point of water. If they are damp you can dry
them by keeping the temperature of the kiln below 212 F for a couple of
hours. But even if your pots are bone dry when you fire, they may be
subject to explosion if they are too thick and the firing is too fast.
Certain chemical changes occur during the firing of the clay which
results in the formation of steam throughout the clay...if the pot is
thick and the firing is fast then this steam formation will result in a
forceful explosion (Booooom!) You can fire very thick pots if you fire
slowly. I regularly fire sculpture as thick as four inches, but I raise
the temperature very slowly and may take a day and a half to get to Cone
04 (for example). Such a temperature increase is in the range of 40
degrees per hour at first and then as much as 100 degrees per hour in
the later stages of the firing. I wouldn't go by these numbers as
gospel, but rather as a suggested guide line. If your pots are as thick
as an inch you could fire it faster.
John Jensen, Mudbug Pottery
mudbug@toadhouse.com , http://www.toadhouse.com
Snail Scott on wed 16 jul 03
At 06:38 AM 7/16/03 -0400, you wrote:
>...what causes them to go boom in
>bisque firings?
Moisture; just moisture; nothin' but moisture.
Thickness, per se, doesn't kill pots, but thickness
makes it hard for moisture to escape before it turns
to steam. It's the sudden expansion of water into
steam that causes explosions. If the work needs to be
thick (for whatever reason), it can be fired safely,
but this entails keeping the temperature below the
boiling point of water for long enough that the water
can escape even from the thickest spots. Remember,
even bone-dry clay will never be drier than the
ambient humidity. There's always some water there,
not even including the chemically-bound water. Give
it the candling time it needs, then bring it up nice
and easy.
Some kilns have instructions on them, saying, "candle
for an hour, then fire on 'low' for an hour, then
'medium' for an hour, etc... That's fine for slip-cast
work, which I suspect is still the biggest market for
electric kilns, and the one they had in mind when they
wrote that. It's probably OK, too, for small, thin
pottery. But beyond that, let the work tell you how
fast you can fire, not some generalized scheme. One
firing schedule does NOT fit all!
-Snail
william schran on wed 16 jul 03
The simple answer - water. The bowls were not completely dry. When
you think your pots are dry, hold the rim against your cheek - feel
for temperature/moisture. Now hold the bottom against your cheek,
feel the same? - it's probably dry. Fell colder/wetter? - not dry
yet. Bottoms last area to dry.
Bill
>Finally got a wheel and I don't know sqwuat about throwing. Made some
>lovely batter bowls and every one of them exploded. They had to be
>too big on the bottoms, but just what causes them to go boom in
>bisque firings?
>
>______________________________________________________________________________
>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.
pdp1@EARTHLINK.NET on wed 16 jul 03
Hi Burgess,
Could be the remaining moisture in their unusual thickness
has not had time to cook off at the liesure it prefers...if
it gets hurried, it can take the Clay with it.
I am guessing...maybe take them up to heat more slowly...see
if that does it.
Too...how 'thick' are these bottoms?
Even for a stout 'Batter Bowl' the bottoms need be no more
thick say, than 3/8ths of an inch, to my reconning...
Phil
lasvegas
----- Original Message -----
From: "Burness Speakman"
To:
Sent: Wednesday, July 16, 2003 3:38 AM
Subject: Sooo? What makes my big bottom bisque go Booooom.
> Finally got a wheel and I don't know sqwuat about
throwing. Made some
> lovely batter bowls and every one of them exploded. They
had to be
> too big on the bottoms, but just what causes them to go
boom in
> bisque firings?
>
>
____________________________________________________________
__________________
> 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.
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