Liz Harris on wed 23 mar 05
Hello all,
I emailed Vince Pitelka off-list with this problem but haven't heard back--I apologize if
I went through the wrong channels. He had posted some helpful suggestions back
in 1998 regarding this issue, but I need some more information.
I have been bisque firing 19-21 inch diameter cone 6 white stoneware bowls to
cone 06 in my toploader electric and have lost nearly all of them to cracking.
Often, there is a nearly circumferential crack around the footring, and sometimes
there will be smaller fissures on the outside of the bowl which cross
the throwing lines but don't extend all the way through the clay. Sometimes the
cracking is not apparent until after the glaze firing, and there may be glaze
inside the crack. During firing, all the bowls sit on single shelves. Similar bowls
made of cone 10 stoneware and fired by a local co-op have not had this problem.
I suspect the root cause may be bisque dunting due to uneven temperature changes
across the large pots (shelf cones throughout the kiln are perfectly
mature). I plan to fire more slowly and to cone 04. I would also like to take
advantage of Vince's advice to nestle the foot in coils of soft clay, but I am a little
nervous about putting moist clay in the kiln. For how long and at what temperature
should I preheat the kiln in order to dry out the coils? Can I allow the coils
to dry on the kiln shelf at ambient temperature and then load the pots on top of
them? What about making little bisque-fired rollers? Do I need to preheat if
the firing schedule is long and slow enough? How slow should my firing schedule
be for pots of this size? If I choose to elevate the pots during the glaze firing, as well,
how do I keep the coils from sticking to the foot?
I'm new to firing my own kiln, so I appreciate any advice.
Thanks!
Liz Harris in the chilly CT springtime, where I am taking two steps forward for every
one step back!
Craig Martell on wed 23 mar 05
Hi:
Raise the bisque temp to cone 04 and the cracks will probably stop.
regards, Craig Martell Hopewell, Oregon
Cindy in SD on wed 23 mar 05
Dear Liz,
You can put the clay coils into the kiln in their plastic state. They
will dry while you are heating up the kiln. Since you are firing such
large pieces, I recommend you candle the kiln at 5-10 degrees below your
boiling temperature (don't forget--you must adjust for your altitude)
overnight or longer. After that, fire the kiln up at a slow rate. I
would start at 100 F per hour (but you might want to be even more
conservative) up to a little over quartz inversion temp. I don't
remember what temp that is and I confess I'm too lazy to go look it up
at the moment. (I have been on "vacation" building my studio for nearly
3 years, and my memory is a little fuzzy.) If you don't have the info,
just do a quick search of the archives--you should find it fairly
easily, along with lots of other useful information. After this point,
you can speed the kiln up a little, maybe to 150 F/hr. You will need to
experiment to find what works best for you and your clay.
Best of luck,
Cindy in SD
John Kudlacek on thu 24 mar 05
Dear Liz,
Assuming "circumferential" cracks are those which follow the horizontal
contour of the bowl in a concentric fashion I offer this:
I have experienced such cracking and do not trace it to the firing but to
the throwing. If I do not compress the clay adequately and leave a lot of
waste to trim from the bottom of the bowl I get this type of cracking.
Some of the time it appears before the bisque. It is my experience that
when the firing was at the root of the problem, vertical rim cracks
occured. I eliminated that by firing the plate or bowl, upside down on a
post or soap with a cushion of ceramic fiber between the piece and the
post. Apparently this provides for much more even heating of the piece
than when sitting on a shelf. Hope this helps.
John in Topeka where the buds suggest spring is trying to happen.
Liz Harris on thu 24 mar 05
Hi John,
I like your suggestion about firing the bowls suspended and upside down. I've made
a couple changes and am firing a load right now. I'll try your idea if I continue to
have trouble.
Many thanks!
Liz
Liz Harris on thu 24 mar 05
Hello Cindy,
Thank you for the firing information. I have candled a load of ware last night and am
firing today--I will let you know how it turns out!
Hope you're having fun building your own studio; I'll be embarking on that project
over the next year. Good luck!
Many thanks,
Liz
Louis Katz on fri 25 mar 05
I have not seen the cracks, could be cooling, but if the are circular
and near trimmed feet than I would guess that you dried the pots to
trimming stage quickly and that the inside was soft when you trimmed
off the stiffer skin. This only happens in clays that driy on the skin
significantly quicker than the inside ( the water does not move through
these bodies easily). It also happens more often in winter when the
relative humidity and drying temperature both drop. Water seems to move
through the clay better at higher temperatures.
I tell my students to take three days to get troublesome bodies to
trimming stage rather than the just one day that we would all prefer.
More nonplastics in the clay will help I think but I do not have a good
idea of what other steps to take to avoid this problem other than
taking longer to dry at a higher temperature.
Louis
Ron Roy on fri 25 mar 05
Hi Liz,
No doubt the cracking is happening on the way down about 570C and it is due
to the effect of the quartz inversion at that temperature.
Many times it is simply due to how fast your kiln is cooling - slow cooling
from 650C down to 550C will stop it for instance.
Thicker rims will also help. If your bowls are thicker at the bottom that
makes the problem more likely as well.
Getting the ware separated from the kiln shelf is another way.
Those of use who fire porcelain are at greatest risk because there is
usually so much silica in porcelain bodies - se we bisque higher to get
more strength.
RR
>I have been bisque firing 19-21 inch diameter cone 6 white stoneware bowls to
>cone 06 in my toploader electric and have lost nearly all of them to cracking.
>Often, there is a nearly circumferential crack around the footring, and
>sometimes
>there will be smaller fissures on the outside of the bowl which cross
>the throwing lines but don't extend all the way through the clay.
>Sometimes the
>cracking is not apparent until after the glaze firing, and there may be glaze
>inside the crack. During firing, all the bowls sit on single shelves.
>Similar bowls
>made of cone 10 stoneware and fired by a local co-op have not had this problem.
>
>I suspect the root cause may be bisque dunting due to uneven temperature
>changes
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
Fax: 613-475-3513
David Cowdrill on sat 26 mar 05
I have had "circumferential cracking" problems which sound similar to those Liz described. I make
soup bowls about 6 inches in diameter and lose about 1/3 of them to cracks roughly horizontal
about an inch outside the foot ring. It has occurred with two different stoneware bodies and a
porcelain, normally during bisque. It has happened during summer session at Alfred and in my
studio, so I do not think it is related to clay body or firing . Only a throwing or trimming problem
remains but I have no clue as to what to change. Would appreciate some help.
Kathi LeSueur on sun 27 mar 05
David Cowdrill wrote:
>I have had "circumferential cracking" problems which sound similar to those Liz described. I make
>soup bowls about 6 inches in diameter and lose about 1/3 of them to cracks roughly horizontal
>about an inch outside the foot ring. It has occurred with two different stoneware bodies and a
>porcelain, normally during bisque. It has happened during summer session at Alfred and in my
>studio, so I do not think it is related to clay body or firing . Only a throwing or trimming problem
>remains but I have no clue as to what to change. Would appreciate some help.>>>>>>
>
I've had these cracks also. I think they are present, but often hard to
see, while the ware is green. My solution is to dry upside down. As soon
as the rim can support the form I turn it over. For pieces where I'm
concerned about the stress on the rim creating cracks I use a different
method. I take large plastic planters (10 or 12 inch pots from
landscaping), place a piece of carpet foam over the top and then place
the bowl upside down on it. I haven't had cracking since using this method.
Kathi
>
>
Liz Harris on sun 3 apr 05
Hello all,
Well, I recently bisque fired another load of large ware, making some modifications.
I elevated the bowls above the kiln shelf using coils of moist clay, candled the kiln,
and then fired it slowly to cone 04. Two out of three pieces came through intact. I
then glazed them and fired again, this time to cone 6; both pieces cracked. Rats.
Had a little chat with my clay supplier the other day. Seems that I have been using a
clay which is not recommended for large work (purchased prior to my throwing large
pieces). Live and learn!
I now have in my possession two new clays, both with alot of grog, which are highly
recommended for ware of this size. I'm very hopeful...will post results.
Liz
Lee Love on mon 4 apr 05
Liz Harris wrote:
>Well, I recently bisque fired another load of large ware, making some modifications.
>I elevated the bowls above the kiln shelf using coils of moist clay,
>
Hi Liz,
Did you put greenware on coils of moist clay? If so,
the feet may have absorbed moisture from the soft clay and expanded
while the rest of the pot stayed the same size. That could cause cracking.
--
Lee In Mashiko, Japan
| |
|
