Anne Pfeiffer on tue 8 jan 08
I have used Helios porcelain for several years in my electronic cress kiln. I used to fire it to cone 10 successfully. I switched to mid-range about 2 years ago. Suddenly my bowls are warping to ovoids in the glaze firing, some dramatically. I suspect the kiln is firing unevenly, since it seems odd to me that the same clay would warp at mid-range when it didn't at high fire. I recently replaced the elements. I bisque to 04, glaze firing is 5 with a 10 minute hold on a slow ramp. I'm not doing any glaze firings until I fix it because I'm losing too many pieces, not just a few once in a while, but more than 1/2 a load. Any ideas?
Thanks,
Anne
Arnold Howard on tue 8 jan 08
From: "Anne Pfeiffer"
To:
I switched to mid-range about 2 years ago. Suddenly my
bowls are warping to ovoids in the glaze firing, some
dramatically. I suspect the kiln is firing unevenly
---------------
Anne, I suggest that you place witness cones on each shelf
and fire test pieces. The kiln may be firing to a different
cone than the cone you programmed.
Sincerely,
Arnold Howard
Paragon Industries, L.P., Mesquite, Texas USA
ahoward@paragonweb.com / www.paragonweb.com
Snail Scott on wed 9 jan 08
> Date: Tue, 8 Jan 2008 03:52:57 -0800
> From: Anne Pfeiffer
> I have used Helios porcelain for several years in my electronic cress
> kiln. I used to fire it to cone 10 successfully. I switched to
> mid-range about 2 years ago. Suddenly my bowls are warping to ovoids
> in the glaze firing, some dramatically. I suspect the kiln is firing
> unevenly...it seems odd to me that the same clay would warp at
> mid-range when it didn't at high fire...
When you switched to mid-range firing, you didn't just
underfire the same ^10 clay, are you? I assume that you
actually switched to a mid-range clay which shares the
same name.
And that's the issue: when a clay body is marketed in two
versions for different temperatures, they aren't the same
clay body at all. It's not like buying the same sweater in
two different colors. This is especially true of clay bodies
that were developed for ^10 and were reformulated for
^5 or so by simply adding fluxes. You get less alumina
this way, and a proportionately greater tendency to warp.
There may also be a very narrow window of optimum
heat-work, and overshooting by even half a cone can be
disastrous for some of these bodies. It may also be the
case that the midrange formulation is more sensitive to
uneven drying, causing warping as soon as the work
becomes pyroplastic, but due to stresses that already
existed in the form.
I seriously doubt that your kiln is to blame. If you aren't
using witness cones on multiple shelves, though, now
would be a good time to start. Even if you are getting
excellent results, wouldn't it be nice to really know what
conditions are giving you those results? You don't need
to be able to see the cones through the peepholes for
this purpose; the data will be waiting for you when you
unload.
Try lowering your firing temperature a smidge to start.
Do not be lured into a false sense of absolute truth just
because the kiln readout matches whatever cone rating
is stamped on the clay box! First, your kiln may not be
perfectly calibrated. Second, the clay may say "^6" or
whatever, but it may really be happier at ^5-3/4 with a
shorter soak, or ^5 with a much longer soak, or some
other fine-tuning of the actual circumstances.
Also, make sure you are drying your work very, very,
very evenly and slowly. No leaving it out in the open
air. Put it in a damp box or some such, or under well-
wrapped plastic. Set the bowls on their rims so that
the bottoms won't stay damp longer than the tops. Put
a dry sheet of newspaper of paper towel or a dry rag
into the plastic and change it daily to let it absorb and
remove dampness without causing unequal drying.
You can't baby this stuff too much!
Try one of these fixes, then the other, so that you know
which issue, if either (or both), is the real problem. Don't
do the both at once until you've tried them separately.
Then you can decide what changes will work best for
you.
I've never used the Helios in either of its formulations,
but mid-range porcelains are notoriously fussy for the
reasons mentioned above. You will be able to make it
work eventually, but don't expect it to behave like the
^10 clay of the same name; it's not the same clay.
-Snail
Anne Pfeiffer on wed 9 jan 08
Thanks Scott.
You might be right that it is not the kiln, because the glazes are coming out the way they always do. No change at all. This Helios is labeled as ranging 6-10. (I fire electronic 5 with a 10-min. soak). I have used it mid-range for 2.5 years with no warping. This warping is new and sudden. (Which made me think it was the kiln.) I did email Highwater, but they haven't answered. Do you think there might be a chance that one batch of Helios was not mixed properly at Highwater? I'm not jumping to conclusions, just curious if that is a possibility. (In my email I just asked their advice about the warping, I didn't suggest it was their fault.)
Maybe I am drying the bowls too fast, but they are not warped after the bisque fire at all. Why would the warping from drying too fast happen after that? But, I'll try some bowls, letting them dry very slowly.
There is also a possibility that I am trimming slightly thinner than I used to. Would that cause it? I also changed my stilts to another style, just flat high-fire discs that I made with kiln wash on them, thinking it was having trouble shrinking against the stilts... Nope, still warped dramatically, like you were purposely trying to make ovals.
I'll also try lowering my temperature, but my temmoku glaze might not get as many spots...
I am going to try witness cones in different places in my kiln, and I also am going to make a call to Cress.
But, you might be right, it might be the clay! I have loved it for several years. I love throwing it. I love how the glazes work on it. But, these bowls are ruined, not just slightly asymmetrical, but squashed into an obvious oval.
Thanks,
Anne
----- Original Message ----
From: Snail Scott
To: CLAYART@LSV.CERAMICS.ORG
Sent: Wednesday, January 9, 2008 12:28:48 PM
Subject: Re: Helios clay warping in electronic kiln
> Date: Tue, 8 Jan 2008 03:52:57 -0800
> From: Anne Pfeiffer
> I have used Helios porcelain for several years in my electronic cress
> kiln. I used to fire it to cone 10 successfully. I switched to
> mid-range about 2 years ago. Suddenly my bowls are warping to ovoids
> in the glaze firing, some dramatically. I suspect the kiln is firing
> unevenly...it seems odd to me that the same clay would warp at
> mid-range when it didn't at high fire...
When you switched to mid-range firing, you didn't just
underfire the same ^10 clay, are you? I assume that you
actually switched to a mid-range clay which shares the
same name.
And that's the issue: when a clay body is marketed in two
versions for different temperatures, they aren't the same
clay body at all. It's not like buying the same sweater in
two different colors. This is especially true of clay bodies
that were developed for ^10 and were reformulated for
^5 or so by simply adding fluxes. You get less alumina
this way, and a proportionately greater tendency to warp.
There may also be a very narrow window of optimum
heat-work, and overshooting by even half a cone can be
disastrous for some of these bodies. It may also be the
case that the midrange formulation is more sensitive to
uneven drying, causing warping as soon as the work
becomes pyroplastic, but due to stresses that already
existed in the form.
I seriously doubt that your kiln is to blame. If you aren't
using witness cones on multiple shelves, though, now
would be a good time to start. Even if you are getting
excellent results, wouldn't it be nice to really know what
conditions are giving you those results? You don't need
to be able to see the cones through the peepholes for
this purpose; the data will be waiting for you when you
unload.
Try lowering your firing temperature a smidge to start.
Do not be lured into a false sense of absolute truth just
because the kiln readout matches whatever cone rating
is stamped on the clay box! First, your kiln may not be
perfectly calibrated. Second, the clay may say "^6" or
whatever, but it may really be happier at ^5-3/4 with a
shorter soak, or ^5 with a much longer soak, or some
other fine-tuning of the actual circumstances.
Also, make sure you are drying your work very, very,
very evenly and slowly. No leaving it out in the open
air. Put it in a damp box or some such, or under well-
wrapped plastic. Set the bowls on their rims so that
the bottoms won't stay damp longer than the tops. Put
a dry sheet of newspaper of paper towel or a dry rag
into the plastic and change it daily to let it absorb and
remove dampness without causing unequal drying.
You can't baby this stuff too much!
Try one of these fixes, then the other, so that you know
which issue, if either (or both), is the real problem. Don't
do the both at once until you've tried them separately.
Then you can decide what changes will work best for
you.
I've never used the Helios in either of its formulations,
but mid-range porcelains are notoriously fussy for the
reasons mentioned above. You will be able to make it
work eventually, but don't expect it to behave like the
^10 clay of the same name; it's not the same clay.
-Snail
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Lynn Goodman Porcelain Pottery on wed 9 jan 08
On Jan 9, 2008, at 3:27 PM, Anne Pfeiffer wrote:
> Thanks Scott.
> You might be right that it is not the kiln, because the glazes are
> coming out the way they always do. No change at all. This Helios
> is labeled as ranging 6-10. (I fire electronic 5 with a 10-min.
> soak). I have used it mid-range for 2.5 years with no warping.
> This warping is new and sudden. (Which made me think it was the
> kiln.) I did email Highwater, but they haven't answered. Do you
> think there might be a chance that one batch of Helios was not
> mixed properly at Highwater? I'm not jumping to conclusions, just
> curious if that is a possibility. (In my email I just asked their
> advice about the warping, I didn't suggest it was their fault.)
Hi Anne,
You should talk to Highwater-- not because I think it's their fault,
but because there could be a problem with one of the materials that
they use to make their clay. Everything in a claybody is mined. That
means that one of the materials could be a little bit different if
mined in one part of the pit, as opposed to being mined from another
part of the pit; that could be enough to flux out your clay more than
usual, causing the warpage. Highwater would not necessarily know that
there's a problem unless they are told by their customers. If they
are not answering e-mails, you should pick up your phone and call
them-- their tech will want to know what's going on!
Good luck,
Lynn
Lynn Goodman
Fine Porcelain Pottery
Cell 347-526-9805
www.lynngoodmanporcelain.com
Veena Raghavan on wed 9 jan 08
Hi Anne,
I use Helios in cone 9-10 reduction. My bowls are are all finely carved and
with thin rims. I have found that these bowls, when made out of Helios, warp a
lot. I have noted on Clayart, sometime ago, that others had the same problem,
and it is also the case in the Center where I do my reduction firing. When I
make the same bowls out of Coleman's cone 10, this does not happen. I too like
Helios, but keep it for other forms.
Good luck with getting a good solution.
Veena
In a message dated 1/9/2008 3:49:59 PM Eastern Standard Time,
annepfeiffer@VERIZON.NET writes:
>
>
> But, you might be right, it might be the clay! I have loved it for several
> years. I love throwing it. I love how the glazes work on it. But, these bowls
> are ruined, not just slightly asymmetrical, but squashed into an obvious
> oval.
>
VeenaRaghavan@cs.com
Tom at Hutchtel.net on wed 9 jan 08
Subject: Re: Helios clay warping in electronic kiln
> Thanks Scott.
>>kiln.) I did email Highwater, but they haven't answered. Do you think
>>there might be a chance that one batch of Helios was not mixed properly at
>>Highwater? I'm not jumping to conclusions, just curious if that is a
>>possibility. (In my email I just asked their advice about the warping, I
>>didn't suggest it was their fault.)
>
Try talking to Jennifer at Highwater. She's been very responsive and
knowledgeable for us. Once, when we were having problems with thermal
shock, they changed to recipe (for Hestia) for us. Later they told us that
they had made the same change in standard Hestia....and I'll bet no one was
ever told there was a change. To my knowledge it was the addition of more
spar to get a bit more melt. But who knows.
Tom Wirt
Hutchinson, MN
twirt@hutchtel.net
www.claycoyote.com
John Boyd on wed 9 jan 08
I use Helios in my little train kiln. I don't have a bisque kiln so all of
it is currently bisqued with wood. I suspect Helios has a bit of sodium
flux, maybe even nepheline syenite and it can be very fussy with the warping
and cracking you mention. To combat this, I use almost no water when
throwing it and I never let any water accumulate on the pot. If there is a
dribble, I wipe it immediately. I use a metal rib in almost all of the
shaping to wick moisture and fine particles from the surface, which would
shrink. It likes to dry slow; after any step in the making process I seal
it up in the damp closet. I also allow the pot to become near bone dry in
the damp closet before I take it out, so it is as gradual as possible. The
wide and low bowls made with Helios that come from the little train kiln are
worth the fuss. I would document your making and drying process and see if
there is a problem there.
Sincerely,
JB
Jeanie Silver on thu 10 jan 08
Ann
You mentioned something that was kind of a red flag for me...you said that
you stilted these bowls with homemade solid disc stilt(if I remember
rightly) That can cause major problems in distortion that would not show
up with balanced, three-prong stilts..think of the difference between a
pizza crust spinning in midair, lightly twirled by fingers amd a pizza crust
plopped down over a raised fist....I have had very good results making
stilts in the shape of a child's paper-cutout crown...the prongs of the
crown can encounter the bowl at the strongest point of the curve..with
careful construction, the bowl is evenly cradled,,,good luck
Jeanie in Pa.
Anne Pfeiffer on thu 10 jan 08
Thank you Veena.
I wonder why my bowls were OK for awhile? I guess I'll have to try other clays for bowls.
----- Original Message ----
From: Veena Raghavan
To: CLAYART@LSV.CERAMICS.ORG
Sent: Wednesday, January 9, 2008 6:08:10 PM
Subject: Re: Helios clay warping in electronic kiln
Hi Anne,
I use Helios in cone 9-10 reduction. My bowls are are all finely carved
and
with thin rims. I have found that these bowls, when made out of Helios,
warp a
lot. I have noted on Clayart, sometime ago, that others had the same
problem,
and it is also the case in the Center where I do my reduction firing.
When I
make the same bowls out of Coleman's cone 10, this does not happen. I
too like
Helios, but keep it for other forms.
Good luck with getting a good solution.
Veena
In a message dated 1/9/2008 3:49:59 PM Eastern Standard Time,
annepfeiffer@VERIZON.NET writes:
>
>
> But, you might be right, it might be the clay! I have loved it for
several
> years. I love throwing it. I love how the glazes work on it. But,
these bowls
> are ruined, not just slightly asymmetrical, but squashed into an
obvious
> oval.
>
VeenaRaghavan@cs.com
______________________________________________________________________________
Clayart members may send postings to: clayart@lsv.ceramics.org
You may look at the archives for the list, post messages, or change
your
subscription settings here: http://www.acers.org/cic/clayart/
Moderator of the list is Mel Jacobson who may be reached at
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Snail Scott on thu 10 jan 08
On Jan 9, 2008, at 11:00 PM, Automatic digest processor wrote:
> Date: Wed, 9 Jan 2008 12:27:37 -0800
> From: Anne Pfeiffer
> Thanks Scott.
> You might be right that it is not the kiln, because the glazes are
> coming out the way they always do. No change at all. This Helios is
> labeled as ranging 6-10. (I fire electronic 5 with a 10-min. soak). I
> have used it mid-range for 2.5 years with no warping...
Sorry, Anne, I didn't realize that the Helios was one of
those so-called 'wide-firing-range' clays, rather than a
clay with two different temperature formulations.
So, apparently it wasn't a clay switch that caused the
problems, nor (obviously) overfiring. That leaves two
possible contenders: uneven drying, and a change (or
error) in clay-body composition). If you haven't changed
your studio methods, it seems unlikely that catastrophic
warping like you described would be due to drying,
though it never hurts to take it slow. So, changes to the
clay itself begin to seem like the likeliest contender.
Continue to try contacting the manufacturer, and record
the batch number from the packaging - they'll want to
know what it is. Bad batches are rare from most makers,
but they can happen.
-Snail
Ron Roy on thu 10 jan 08
Hi Anne,
It would be unusual for a cone 10 clay to warp at cone 6 - I would contact
High Water first.
Make sure you fire with cones to see what temperature you are really getting to.
If you do have to work with a "soft firing clay" make sure the shelves are
flat and smooth - any clay that softens enough will take the shape of the
shelf or any bumps on it.
Differences in trimmed thickness will have an effect as well. Even having
one side closer to the elements can do that.
You will find that mixing that clay with a more refractory clay will help
cure the problem.
RR
>I have used Helios porcelain for several years in my electronic cress
>kiln. I used to fire it to cone 10 successfully. I switched to mid-range
>about 2 years ago. Suddenly my bowls are warping to ovoids in the glaze
>firing, some dramatically. I suspect the kiln is firing unevenly, since it
>seems odd to me that the same clay would warp at mid-range when it didn't
>at high fire. I recently replaced the elements. I bisque to 04, glaze
>firing is 5 with a 10 minute hold on a slow ramp. I'm not doing any glaze
>firings until I fix it because I'm losing too many pieces, not just a few
>once in a while, but more than 1/2 a load. Any ideas?
>
>Thanks,
>Anne
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Ron Roy on thu 10 jan 08
Sounds like it's a bad batch to me - mistakes happen - materials change -
call High water and ask if they have tested that batch for absorbency.
By the way any clay that is describes as having a rage of cone 6 to 10 is
misleading. You cannot have a clay that will be properly vitrified (not
leak) at cone 10 and not have it leak at cone 6 - and if it's properly
vitrified at cone 6 it will be over fired at cone 10.
RR
>Thanks Scott.
>You might be right that it is not the kiln, because the glazes are coming
>out the way they always do. No change at all. This Helios is labeled as
>ranging 6-10. (I fire electronic 5 with a 10-min. soak). I have used it
>mid-range for 2.5 years with no warping. This warping is new and sudden.
>(Which made me think it was the kiln.) I did email Highwater, but they
>haven't answered. Do you think there might be a chance that one batch of
>Helios was not mixed properly at Highwater? I'm not jumping to
>conclusions, just curious if that is a possibility. (In my email I just
>asked their advice about the warping, I didn't suggest it was their
>fault.)
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Anne Pfeiffer on thu 10 jan 08
OK. So, Jennifer did email me back yesterday, but her email ended up in the Little Willy folder, found it today.
Hi Anne,
Hmmn, I haven't had anybody bring up this issue recently and especially
not with cone 5. Have you added a kiln vent or changed the way you
finish the rims? Are any other forms warping? The new elements could
have an affect, try to load the bowls in the middle of the shelves away
from the kiln wall. Could you email me a picture?
Thanks,
Jennifer Hoolihan
Thanks for everyone who has given me advice. I will take it all in and digest it.
Anne
----- Original Message ----
From: Snail Scott
To: CLAYART@LSV.CERAMICS.ORG
Sent: Thursday, January 10, 2008 10:09:07 AM
Subject: Re: Helios clay warping in electronic kiln
On Jan 9, 2008, at 11:00 PM, Automatic digest processor wrote:
> Date: Wed, 9 Jan 2008 12:27:37 -0800
> From: Anne Pfeiffer
> Thanks Scott.
> You might be right that it is not the kiln, because the glazes are
> coming out the way they always do. No change at all. This Helios is
> labeled as ranging 6-10. (I fire electronic 5 with a 10-min. soak).
I
> have used it mid-range for 2.5 years with no warping...
Sorry, Anne, I didn't realize that the Helios was one of
those so-called 'wide-firing-range' clays, rather than a
clay with two different temperature formulations.
So, apparently it wasn't a clay switch that caused the
problems, nor (obviously) overfiring. That leaves two
possible contenders: uneven drying, and a change (or
error) in clay-body composition). If you haven't changed
your studio methods, it seems unlikely that catastrophic
warping like you described would be due to drying,
though it never hurts to take it slow. So, changes to the
clay itself begin to seem like the likeliest contender.
Continue to try contacting the manufacturer, and record
the batch number from the packaging - they'll want to
know what it is. Bad batches are rare from most makers,
but they can happen.
-Snail
______________________________________________________________________________
Clayart members may send postings to: clayart@lsv.ceramics.org
You may look at the archives for the list, post messages, or change
your
subscription settings here: http://www.acers.org/cic/clayart/
Moderator of the list is Mel Jacobson who may be reached at
melpots2@visi.com
Anne Pfeiffer on fri 11 jan 08
Thanks Jeanie. I thought the stilts I was using, with the little points, might be catching on the foot of the bowls, keeping them from shrinking evenly, which is why I tried the flat, thinking they could shrink without any resistance against the foot. But, it made no difference. So, then I ordered the long triangles (they are called triangles, but look like triangles from the side, but from the top a long, thin rectangle, i.e. no individual points) the bowls also warped on them. Phooey. I'm going back to the point stilts and to try putting my bowls even farther from the elements. After that I'm going to have to try other clays.
----- Original Message ----
From: Jeanie Silver
To: CLAYART@LSV.CERAMICS.ORG
Sent: Thursday, January 10, 2008 10:59:35 PM
Subject: Re: Helios clay warping in electronic kiln
Ann
You mentioned something that was kind of a red flag for me...you said
that
you stilted these bowls with homemade solid disc stilt(if I remember
rightly) That can cause major problems in distortion that would not
show
up with balanced, three-prong stilts..think of the difference between
a
pizza crust spinning in midair, lightly twirled by fingers amd a pizza
crust
plopped down over a raised fist....I have had very good results making
stilts in the shape of a child's paper-cutout crown...the prongs of the
crown can encounter the bowl at the strongest point of the curve..with
careful construction, the bowl is evenly cradled,,,good luck
Jeanie in Pa.
______________________________________________________________________________
Clayart members may send postings to: clayart@lsv.ceramics.org
You may look at the archives for the list, post messages, or change
your
subscription settings here: http://www.acers.org/cic/clayart/
Moderator of the list is Mel Jacobson who may be reached at
melpots2@visi.com
John Britt on fri 11 jan 08
Anne,
I had several thoughts on your problem.
First, since you are firing lower than the peak temperature of Helios it
should not warp. But since it is, it could be that you have improved your
throwing skills to such an extent that the bowls are too thin? I have had
this problem at cone 10 and had to make the pieces thicker. Bowls are
particularly a problem if you pick them up from the bat (I would let them
get stiff and then cut them off). Another possibility is having the rims
too close to the elements (but I assume you know that).
Another thought is that if you are using recycled clay it can cause
problems. I saw a lecture at NCECA on clay bodies and the guy (from
Alfred?) talked about how feldspar needs to be distributed evenly through
out the clay body which happens when you blunge it, but if you use
recycled clay the feldspar can clump together thus causing uneven
groupings of feldspar (flux) which results in warping and bloating.
Don=92t know if this helps but I just thought of you and was trying to help,=
John Britt
www.johnbrittpottery.com
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