Timothy Sullivan on tue 19 jun 01
Hello all -
I'm just getting back to pottery full time after a 20+ year hiatus working in
Corporate America. I'm having a problem with Laguna B-Mix cone 10 stoneware
clay warping on large platters (20" + diameter) and bowls with handles, and
am hoping that some of you might be able to help. I love B-Mix for
everything else, but am about ready to give up on it for the platters and
bowls, and am looking for a light firing cone 10 commercial clay that has
more structural integrity. I'm not getting the warping during either drying
or in a bisque, but only in a glaze firing where it looks like the clay is
softening at high temps. I would appreciate knowing about any light firing
cone 10 clays that people have used with success in these kinds of
Thanks in advance -
Tim Sullivan in beautiful Atlanta, GA
L. P. Skeen on tue 19 jun 01
Tim, have you tried Loafer's Glory from Highwater?
Very similar to Bmix and easier to get as well; Highwater has good customer
service. :) Highwater is also not far from you in Asheville, NC and worth
I would appreciate knowing about any light firing cone 10 clays that people
have used with success in these kinds of applications.
Cat Jarosz on wed 20 jun 01
Hi Tim.... you didnt say if you were bridging those platters in a high
fire... hmmm how do I say this... Bridge means span two shelves ??? OR the
shape your shelves are in.... I started having that same problem with
platters and its because my old clay kiln shelves (10 yrs plus ) are
finally warping out even with the flipping back and forth... they are only
12x24 and 3 deep so I have to span the front 2 shelves with the
platters..... my gosh I hope this is coming out right ....just cant seem to
find the right words here... I tried shimming with fiber blanket pieces and
that helped some but got a great TIP from my teacher who I still go to for
everything as we remain good friends.. he said that when that started
happening to him that he bought some small round shelfs and placed the
platters on them and then spaned the shelves !!! worked like a charm...
hope you can understand what I 'm trying to say with my limits , check
your kiln shelves.. CAT
Robert Santerre on thu 21 jun 01
Tim, the biggest secret to non-warping with large platters is to fire them on FLAT
shelves. I fire Highwater's MoonWhite to cone 10. Struggled for a long time with
warped plates and bowls. Bought some new shelves (for other reasons) and Voila! my
plates, bowls and platters now have straight rims. Nothing I did differently, just
firing on new FLAT shelves.
Good luck, Bob
Timothy Sullivan wrote:
> Hello all -
> I'm just getting back to pottery full time after a 20+ year hiatus working in
> Corporate America. I'm having a problem with Laguna B-Mix cone 10 stoneware
> clay warping on large platters (20" + diameter) and bowls with handles, and
> am hoping that some of you might be able to help. I love B-Mix for
> everything else, but am about ready to give up on it for the platters and
> bowls, and am looking for a light firing cone 10 commercial clay that has
> more structural integrity. I'm not getting the warping during either drying
> or in a bisque, but only in a glaze firing where it looks like the clay is
> softening at high temps.
Any clay fired to it's maturing temperature will vitrify, soften and move -
translate slump a bit. If the walls of the form are reasonably even and the
supporting shelf is FLAT the slumping will be even and no warping will occur.
> I would appreciate knowing about any light firing
> cone 10 clays that people have used with success in these kinds of
> Thanks in advance -
> Tim Sullivan in beautiful Atlanta, GA
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> settings from http://www.ceramics.org/clayart/
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DONALD G. GOLDSOBEL on thu 21 jun 01
To eliminate the spanning two shelves problem, I use a scattering of large
particle grog under the platter and they do not warp or crack. It may be
due to the circulation of heat above and below all of the platter. If you
set it on the shelves directly, there is a space where the shelves meet and
where the temp is not the same as the part in contact with the shelf which
could lead to warpping or cracking.
>Send postings to firstname.lastname@example.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
LOGAN OPLINGER on fri 22 jun 01
Hi Cat, Tim,
Yes, spanning shelves, whether they are warped or not, will lead to warping if the shelves are not perfectly even with each other.
Tim, I don't recall now if you said your platters have a foot rim, or short stubby legs or not. An uneven foot rim, or a foot rim or legs not properly placed to support the weight of the plater can cause warping.
Warping can also be caused by an application of kiln wash on the shelves with high and low spots. As a platter shrinks during the firing, the high spots can catch on the foot rim or legs.
Some remedies which work, depending on condition of shelves, size of shelves, style of work, size of work, type of clay, etc.:
1. As Cat mentioned, perfectly flat kiln shelves. Also, an even, flat application of kiln wash.
2. If foot rims or legs are used, placement to support the weight of the platter.
V V < rim or leg at transition point.
3. A sacrificial FLAT slab of clay coated with kiln wash placed under the platter. The slab of clay will shrink along with the platter. Kiln wash prevents the two from fusing together.
4. A thin, flat layer of medium to coarse grog placed on the kiln shelf before the platter is placed on the shelf will help with two problems. The grog can help even out the surface of a warped or uneven kiln shelf. The particles of grog act like ball bearings, and allow the platter to shrink without catching and draging on the surface of the shelf. Be careful loading so the particles of grog are not knocked down onto other glazed pieces.
I hope this helps.
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Timothy Sullivan on fri 22 jun 01
On bowls with handles, the clay warps, I presume, due to the weight of the
handle relative to the thickness of the bowl's walls. I assume that if I
made the handles small enough and the walls thick enough, that I wouldn't get
the warping. I've made this sort of thing in the past with a fireclay/darker
clay body, but in moving to a smooth, white stoneware this problem develops.
The platters have a round foot, and the warping/sagging occurs out beyond the
foot. Again, I would assume that if I kept moving the foot out toward the
edge of the platter, I would eventually reach a point where the
warping/sagging went away. I'm down to the point where I'm looking for a
clay that has more structural strength, and will allow for some uneven
distribution of weight (i.e. handles on bowls), and will allow for some
reasonable span beyond the foot for platters.
I appreciate the responses regarding the need for flat shelves, but in this
case, I don't think that's my problem.
Are any of you successfully making large platters with a light firing
commercial cone 10 clay?
Thanks for the help
Martin Howard on fri 22 jun 01
That is so true. But beginners, (for that read me some months ago!) so often
put the kiln wash on like paste.
No, several thin, very thin coats is far more effective and will last,
without causing high spots.
Webb's Cottage Pottery
Woolpits Road, Great Saling
BRAINTREE, Essex CM7 5DZ
Lee Love on sat 23 jun 01
At the workshop, large platters are centered on an extra kiln shelf (regular
square shelves) that is placed upon the first shelves so that they don't have to
span a crack.
Lee In Mashiko
bivaletz ginny on sat 23 jun 01
tim, we use laguna white for our platters and such
when we want a light clay body with some tooth to it.
so far it has been about the best we've found to
control warping at ^ 10. good luck
ginny from orcas island, washington.
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Ron Roy on sun 24 jun 01
About time someone said this - an obvious answer. The shelf can be wadded
so it is supported well and won't warp over time.
There are other ways to minimize warping.
Recompression of rims is important - stretched clay likes to unsretched
during drying and firing.
Leaving rims thicker - helps a lot.
Make sure the food rim is trimmed so it is the same level everywhere.
Round the bottom of the foot rim - makes it harder for it to get hung up on
>At the workshop, large platters are centered on an extra kiln shelf (regular
>square shelves) that is placed upon the first shelves so that they don't
>span a crack.
>Lee In Mashiko
15084 Little Lake Rd..
David Woof on sun 24 jun 01
Once we start hanging stuff, ie: platter rims, in space, the laws of physics
and rules of architecture need attentive consideration.
clay does not cantilever at pyroplastic temperatures.
If one thinks of the extending and up raised angle from foot to rim as a
section of an inverted arch and design your form with-in the parameters of
the arch, the weight of the rim is transferred back to the foot. There is
still plenty of room here for design and expression.
if at the top of the rim the angle is again changed, even slightly to a
horizontal, the principle of corrugation stabilizes and evenly distributes
using the epk/alumina hydrate 50/50, wedge in a little fine saw dust, smart
pills (little balls, or wads) that wood and salt firers use on bottoms and
foot rims is a foolproof way of supporting/leveling any form. no muss no
fuss, no kiln wash or grog falling into other pots.
use common sense in spaceing, attach with white glue, and set in place while
wads are still soft.
several wads in the center will give support to mid bottom and since the
wads don't stick, the piece can move on the wads and shrink as it will.
other than fireing on a coat of ITC, I don't wash my shelves, gas or
woodfire. lean and clean, easy to alternate flip the shelves as needed.
Earth and Fire Studios
525 Fiesta St.
Always a little over the edge; reverently takeing an irreverent look at
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weaver on mon 25 jun 01
>>About time someone said this - an obvious answer. The shelf can be
so it is supported well and won't warp over time.<<
I've read contradictory information on loading a kiln. Some sources say
a shelf should be supported only by three legs so that it doesn't rock
and other sources say use four so it doesn't warp. Does using four
really prevent warping of the shelf?
Dave Finkelnburg on mon 25 jun 01
In general, 3 post stacking is better, in my opinion. The longest
unsupported span is shorter that way. Besides, it's more stable without
having to wad the fourth post.
Heat and gravity, as you know, warp shelves. Use 3 posts or 4, mullite
shelves sag over time if you heat them hot enough. I am using 12-inch x
24-inch x 1-inch, mullite shelves at cone 10 reduction right now, so I see
plenty of sagging, IF I put enough load on the shelf.
I stack 3 posts when I can, 4 when I have to because of the dimensions
of the ware. Even with 3 posts I get sagging under the unsupported corners,
or in the middle of the long span, or at all 3 places, with heavy ware.
Keep flipping the shelves every firing and you minimize the warping. It
is very important to keep the posts directly above the posts below. The
cumulative weight of the stack causes most of the sagging/warping in my
While nothing is perfect, 3 posts are better.
Dave Finkelnburg, firing in Idaho
Martin Howard on tue 26 jun 01
Dave says, rightly,
But you cannot do that if you use batwash, because there will be a shower of
flakes each time onto the ware beneath.
So, flip or batwash, not both.
Unless others have a batwash that you can apply to both sides and does not
Webb's Cottage Pottery
Woolpits Road, Great Saling
BRAINTREE, Essex CM7 5DZ