Al Strauss on wed 2 jun 04
Help! Recently I have had a problem where the bases of large bowls and
vases explode during the biscuing cycle. Either the entire base
disintigrates or a horizontal slice of the base breaks off leaving the rest
of the base intact. I am firing cone 6 clay bodies to cone 04. I compress
the base as I am throwing. I stilt the base of each piece off the shelf to
try to even the temperature between inside and outside. My electric kiln
firing cycle is 5 hours to 1100f and 3 hours to 1940f. The kiln is then
turned off and cools. The lid is not opened until completely cool. Any
suggestions appreciated.
Thanx
AL Strauss
Hendrix, Taylor J. on wed 2 jun 04
I rushed a bisque of some raku pieces once. I had taken the precaution
of putting the pieces in the oven for a few hours at about 250 deg F.
They came out of the oven nice and toasty dry...or so I thought. Two of
my pieces shed their bottoms in pretty much the same manner as you
describe. Upon completing the postmortem, I determined that the
greenware was still damp enough to cause steam to fracture the over
thick bases. These two no-nos ( thick bottoms and damp greenware) were
the cumulative cause of my bisque going boom boom. I have no idea of
some of your particulars, but there, you have mine. Your 8 hour bisque
is approx the same as mine and doesn't raise any red flags for me
concerning steam cracking, dunting, etc.
Taylor
Waco, Texas, USA
http://www3.baylor.edu/~Taylor_Hendrix/tjpots.htm
-----Original Message-----
From: Clayart [mailto:CLAYART@LSV.CERAMICS.ORG] On Behalf Of Al Strauss
Sent: Wednesday, June 02, 2004 11:05 AM
To: CLAYART@LSV.CERAMICS.ORG
Subject: Exploding Bottoms
Help! Recently I have had a problem where the bases of large bowls and
vases explode during the biscuing cycle. Either the entire base
disintigrates or a horizontal slice of the base breaks off leaving the
rest
of the base intact. ...
dneese on wed 2 jun 04
AL,
I don't believe that there was anything wrong with your throwing. In most
cases the pieces was not completely dry. It may look dry as the rest of the
piece but usually there is some small amount of moisture in those bases and
feet that causes bottoms to shatter or explode as the temperature rises.
When the ware gets dry enough to support itself without warping place them
up on something where air can circulate under the base for several days. I
have an old metal wire bread tray that I rest large bowls or jars on.
Sometimes I will place the tray over the open top of my electric kiln on
very low for a day or two to make sure all the moisture is drawn off. Large
platters and plates are placed on a square of drywall board. Dry wall seems
to "wick" the moisture out allowing even drying. Can't be in a hurry with
those larger forms. It will happen every time I get in a rush. I throw
bigger things first in a throwing cycle allowing them to completely dry
before loading in the kilns as I am this week.
Dale Tex
"across the alley from the Alamo"
San Antonio, Texas USA
Maurice Weitman on wed 2 jun 04
Hi, Al,
A quick response based on what you've presented.
I think your bisque firing schedule is much too=20
brisk, especially up to 220=BA or so. Most bisque=20
schedules I've seen call for either a very slow=20
(70=BA/hr.) rise to 200=BA-220=BA, or a bit faster=20
(100=BA/hr.) with a 30 minute hold there. My=20
(cone 04) bisque schedule takes twelve hours,=20
compared to your eight.
Large pots may not be able to get rid of enough=20
moisture without a slower rise than yours.
Regards,
Maurice
At 12:05 -0400 on 6/2/04, Al Strauss wrote:
>Help! Recently I have had a problem where the bases of large bowls and
>vases explode during the biscuing cycle. Either the entire base
>disintigrates or a horizontal slice of the base breaks off leaving the rest
>of the base intact. I am firing cone 6 clay bodies to cone 04. I compress
>the base as I am throwing. I stilt the base of each piece off the shelf to
>try to even the temperature between inside and outside.
>My electric kiln firing cycle is 5 hours to 1100f and 3 hours to 1940f.
Catherine Yassin on wed 2 jun 04
In a message dated 6/2/2004 4:24:23 PM Central Daylight Time,
strauss3@OPTONLINE.NET writes:
> Help! Recently I have had a problem where the bases of large bowls and
> vases explode during the biscuing cycle. Either the entire base
> disintigrates or a horizontal slice of the base breaks off leaving the rest
> of the base intact. I am firing cone 6 clay bodies to cone 04. I compress
> the base as I am throwing. I stilt the base of each piece off the shelf to
> try to even the temperature between inside and outside.
Hi AL, I bisque fire cone 6 clay bodies to cone 04 and compress the bottoms
as well. But I do not have the problem you have. How large are these bowls and
how thick are your bottoms compared to the sides? And why do you stilt the
bases? Do you have the same problem with exploding bottoms if you do not stilt
the bottoms? You may want to rethink how you put your clay on your bat (if you
use one). Try and look at the bottom of the clay that gets put on the bat. You
may have a wedging wrinkle that is trapping air. Do you start with round balls
or cone shapes that have a wide flat bottom? I love a good mystery! I hope
you get "to the bottom" of this one... :)
-Cat Yassin
San Antonio
Mary White on wed 2 jun 04
Before you crank the heat up to medium, hold a small mirror up to the
top peephole or the partly open lid. If there's steam, leave it on
low until there isn't. You may not have a mirror handy, but you
probably have a CD. They work just as well.
--
Mary White
Madeira Park, British Columbia
william schran on wed 2 jun 04
Al wrote:>Help! Recently I have had a problem where the bases of
large bowls and
vases explode during the biscuing cycle.<
The pots are still to wet to fire. Try turning the pots upside down
when you think they are dry and let them sit a couple more days
before you load and fire.
Bill
Ivor and Olive Lewis on thu 3 jun 04
Dear Al Strauss,
Having experienced this effect I can assure you that your pieces, when
put in the kiln are not dried as well as they might be.
Extra thickness of clay where the footring inserts into the wall is
always the last part to give up "Free Water". If any of this remains
in the clay then you will get this catastrophe. One gram (1 cc) of
water will generate about one and a quarter litres of steam,
instantaneously as the temperature trips through the boiling point,
more that one thousand times the space of the liquid. This results in
a massive pressure increase, within milliseconds, within the porosity
of the clay which was occupied by the liquid. Result, a big Bang.
either allow more drying time or slow down the rate of heating in the
range 50-150=BA C (125-250=BA F)
Best regards,
Ivor Lewis. Redhill, South Australia
Potters Council Member
Lili Krakowski on thu 3 jun 04
Why is this happening now? No idea. Several ideas. None definitive. =
But your bottoms are too damp.
Place your pots on a stack of newspapers--oh, 8 sheets will do. Put in =
a warmish place. Leave. You will see the next day that there is a =
ring--because there still is water.
Fire your pots s l o w l y. I start my bisque in a 1227 Skutt with all =
plugs out, lid slightly open, only one element on low--and leave for 4 =
five hours.....
Lili Krakowski
Marc Hudson on thu 3 jun 04
Hi Al,
It appears you are firing your bowls before they are completely dry. =20
Expanding water moisture trying to escape the clay body expands too quick=
ly=20
and the clay body cracks. I have very good succdess drying bowls, large =
and=20
small, by turning them over onto their rim top on a tray or shelf. The=20
trapped air inside encourages even drying, ie, the foot and the bowl body=
dry=20
at about the same rate. Make sure the foot of the bowl does not feel coo=
l to=20
your inside wrist (or cheek)before loading into the kiln. Also, try to k=
eep=20
the foot not more than twice the thickness of the walls of the bowl, this=
=20
will help minimize differences in shrinkage between thick and thin.
Hope this helps!
Marc Hudson
Playing with Fire
Espa=F1ola, NM, USA
www.artfulnm.org/hudson.html
---------- Original Message -----------
From: Al Strauss
To: CLAYART@LSV.CERAMICS.ORG
Sent: Wed, 2 Jun 2004 12:05:20 -0400
Subject: Exploding Bottoms
> Help! Recently I have had a problem where the bases of large bowls=20
> and vases explode during the biscuing cycle. Either the entire base=20
> disintigrates or a horizontal slice of the base breaks off leaving=20
> the rest of the base intact. I am firing cone 6 clay bodies to cone=20
> 04. I compress the base as I am throwing. I stilt the base of each=20
> piece off the shelf to try to even the temperature between inside=20
> and outside. My electric kiln firing cycle is 5 hours to 1100f and 3=20
> hours to 1940f. The kiln is then turned off and cools. The lid is=20
> not opened until completely cool. Any suggestions appreciated.
>=20
> Thanx
> AL Strauss
>=20
>=20
_________________________________________________________________________=
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------- End of Original Message -------
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