Potters & Sculptors - Making Rock from Mud
Hi all - turning to the experts for advice. Lately I've been making large dinner plates using Standard 112 clay. I bisque at 06 and the plates come out great. Then I glaze and fire to cone 6 in my Skutt kiln on medium speed. I don't use any ramp holds or special cool down program. The plates are always warped.
I make a lot of pottery, but this doesn't seem to happen on bowls or cups, etc... Any thoughts? To hot, to fast maybe? Jack
Whenever I see warping my first guess is either 'clay has memory' or it was not dried slowly enough. If your plates dry too quickly they may not be drying evening and this can cause warping that shows up immediately or during the firing. I dry slowly keeping them covered longer than I think is necessary just to be safe. I believe firing too quickly can also cause warping so perhaps try a ramp program or your slow setting.
Thanks Gina, my plates do dry quickly. I generally throw a plate, trim the following night and bisque fire seven days later. Is that too fast?
I don't know if that is why they are warping but they are probably drying quicker than mine do. Do you cover your plates as they dry? I cover them loosely with plastic. Also what they are sitting on as they dry will affect the drying. You want them to dry evenly which is why I dry slowly. I found this article at John Hesselberth's website that talks about other reasons for warping.... http://www.frogpondpottery.com/articles/preventing-cracking--warpin...
Thanks, I'll read the article. I throw on plaster bats and dry right on them. Generally, I don't cover anything, though I'm reading more and more that I should and my pots should take a few weeks to dry - not a few days... Thanks for the advice.
I cover almost everything at least for a day. If it has an addition like handles, etc, if it is flat like plates or if is uneven in thickness, I always cover. Some things can be dry enough for bisque in a week but I usually like to take more time with the drying. Good luck with your plates, I hope your next batch will be warp-free!
I use 112 exclusively - fire the same basically. I make dinnerware and sell a lot of plates. In the studio where I teach , my students also ude 112. Overtrimming and throwing too thin cause most of the warping with this clay body from my experience. Uneven drying - they warp. Proper flipping from front to back for balanced dring hels keep them from wearping.
Post some photos that show how thick they are - rim, juncture of rim to base and so on. BTW - you won't see any warping in the bisque - it's just in the glaze firing that it will develop.
I make a lot of dinner plates and salad plates. I have a small carved decoration around the edges. I saw the same problem, they bisque just fine, then warp in the ^6 fire. I purposefully broke a couple of them and discovered that the sides were noticeably thinner than the bottom and some of them were just too thin. Since I addressed those issues, watch the over-trimming, I haven't had problems. Also, have you run cone packs throughout the levels in your kiln lately? I had a friend that noticed plate warping and he found that he kiln was firing too hot towards the top. He hasn't had issues since he replaced some of the old coils.
Thanks for the response. I've never run any cone pack tests, but I have to go to the supply house this week and I'll pick some up. Most of the plates that warped were plates that I made from "rolled out" clay formed over a mold. So, I suspect the thickness was generally even. I'll try some more, let them dry (covered) for several weeks and glaze fire on slow - rather than medium - and see what happens. Everything else I make fires beautifully. jack
Using clay with grog surely reduces warping problems, which is essential as I do sculptural work rather than plates.
Common wisdom, as passed on to me by people with ceramic school training, seems to be that flat items like plates should be fired at cone 6 flat on the shelf. But I've become disenchanted with this advice.
I've found round disk-like or bowl-like objects more frequently fire flat at Cone 6 when:
A.) the plate or bowl like object is supported only in the center. This puts tension from gravity around the rim and often closes cracks which might have developed during the bisque firing. If the bottom is glazed, use a ceramic triangular double-side point.
B.) the plate like object is supported only on the rim, with stackable plate setters, which creates a similar gravity tension in the opposite direction to offset stresses built up in the clay.
Plate setters (stackable)
It's no surprise this is the sort of refractory ware used by porcelain factories.
Clay platelets do accumulates differential stresses, "remembering" where and in what direction they have been stressed while being made or while being dried - porcelain is the worst. In the Cone 6 firing, the clay becomes plastic enough for the internal stresses to relax, revealing the warping. Allowing gravity tension to become greater than these internal stresses helps a lot.
I'm sure you're beyond this, but t's my observation that warping at our studio is often associated with "clay smoothers". They're those people who simply can't resist wetting their nearly completed work so they can more perfectly smooth out the surface of the clay - which of course expands the surface they wet creating additional stress within the piece. I sometimes suggest burnishing with the back of a spoon, they seem to be impressed by that slick smooth feeling on their fingers after they wet their ware.
I did one short experiment with a clay body recipe for Cone 8 Bone China. In order to become translucent and vitreous, the entire object becomes so soft it has to be supported by a refractory object formed in the shape you're trying to achieve. In other words it's similar to slumping glass. Bone china ware like this is then fired again with a glaze having a much lower cone. In a factory setting the ware is static-charged and the glaze is applied as a dry dust which is attracted to the charged plates.