jamie shaw Posted October 30, 2016 Report Share Posted October 30, 2016 Any help with this would be really appreciated. I'm having problems with my stoneware ceramics warping. I cast them in stoneware slip and after drying I bisque them to Cone 06 - 1000 deg. C. I then dip glazed them i na Cone 7 stoneware glaze, left them to dry and fired them as follows: Preheat over 2 hours (60 deg c an hour) Fired to Cone 7 - 1240 deg. C Soak for 15 minutes They were all fired on kiln stilts. I've attached images of two pieces: Image 1 is a profile shot of a piece and is about 3mm in thickness Image 2 is a shot of the bottom of the piece in Image 1 Image 3 is a profile shot of a piece and is about 6mm in thickness Image 4 is a shot of the bottom of the piece in Image 3 You can see some warping in image 2 and some serious warping in image 4 (they are supposed to be rectangular). Does anyone have any advice of how to avoid this please? Is this something to do with the thickness of the pieces? Thanks again for any help. all the best Jamie Link to comment Share on other sites More sharing options...
Min Posted October 30, 2016 Report Share Posted October 30, 2016 Hi, and welcome. this: "They were all fired on kiln stilts." is my guess. Why fire on stilts when you are firing to ^7 and the base of the pots isn't glazed? any chance they were dried unevenly when they came out of the molds? (maybe apply the glaze a bit thiner on the inside to avoid that crawling going on there) Link to comment Share on other sites More sharing options...
glazenerd Posted October 30, 2016 Report Share Posted October 30, 2016 What size kiln? 2265F with 15 min hold in a smaller kiln could/would jump another 1/2 cone. Agree with Min on the stilts issue. However, see some things here that suggest this clay was over-fired as well. Thick glaze on thin walls can also cause warping as the glaze begins to contract upon cooling. Glaze application strikes me as being uneven: heavy in some places, light in others. Perhaps the perfect storm of all three: over-fired clay, stilts, and glaze application. Nerd Link to comment Share on other sites More sharing options...
bciskepottery Posted October 31, 2016 Report Share Posted October 31, 2016 Ditch the stilts. Instead, use either a layer of grog on your kiln shelf or make some clay slats or coils that the houses can sit on while being fired. Your houses expand and contract during firing; giving them an easy surface to move on will help reduce the chance of warping. Coils or slats made of the same clay body will expand/contract at the same rate. Coils and slats will also allow for more even cooling. Link to comment Share on other sites More sharing options...
neilestrick Posted October 31, 2016 Report Share Posted October 31, 2016 Putting a bottom slab on it will solve the problem. Or just set it on a sacrificial slab that can shrink with the piece. Any time you have an open bottom like that it will distort if it's sitting right on the shelf. It's only over fired if it's a cone 6 clay. Link to comment Share on other sites More sharing options...
glazenerd Posted October 31, 2016 Report Share Posted October 31, 2016 Neil: the warping can be attributed to stilts perhaps, and glaze application as well. But cupping in addition to warping is usually a good indication the clay has been cooked to high. Nerd Link to comment Share on other sites More sharing options...
GiselleNo5 Posted October 31, 2016 Report Share Posted October 31, 2016 Does your stoneware slip have grog? I have done some slip casting with smooth slip, no grog and had similar issues with warping over my stilts. Instead I have moved to slump or hump molds or wheel throwing with a groggy clay for those kinds of forms and have had no further issues. Link to comment Share on other sites More sharing options...
neilestrick Posted October 31, 2016 Report Share Posted October 31, 2016 Neil: the warping can be attributed to stilts perhaps, and glaze application as well. But cupping in addition to warping is usually a good indication the clay has been cooked to high. Nerd I just meant that we don't know what cone the clay is rated for so over firing may not be an issue. And open bottom forms are notorious for movement even if everything else is good. Link to comment Share on other sites More sharing options...
glazenerd Posted October 31, 2016 Report Share Posted October 31, 2016 gotcha, just reread the post.... where did U put my glasses? Link to comment Share on other sites More sharing options...
Marcia Selsor Posted October 31, 2016 Report Share Posted October 31, 2016 I agree the open bottom is the issue and possibly the casting was not thick enough for the form. Maybe it needs some engineered structural help in designing the cast form... like more of a ridge around the base. Link to comment Share on other sites More sharing options...
jamie shaw Posted October 31, 2016 Author Report Share Posted October 31, 2016 Thank you all for your amazing advice. I really appreciate the time you took to respond. Here are some thoughts based on your advice: I think the drying was slow and even. They were in a warm room although 3 yards from a radiator so that may have dried one side quicker than the other. It's something I'll avoid next time. I'll go thinner with the glaze on the inside. The kiln is small - not sure the size exactly but it is small. A Skutt 714. As for the different thickness of glaze - I was experimenting with 1, 2 and 3 dips in the glaze bucket on different parts of the piece so they were actually thicker in places. I fired on stilts as even though I wiped the glaze off the bottom I was still concerned the glaze would drip onto the kiln shelves but the shelf wash would have prevented sticking to the shelf I could try to find a groggy slip - I guess they need to be poured into a plaster mould really considering the shape. I'll figure out a way of getting a ridge around the base - maybe by adding coils after they come out of the mould. I've ordered a full set of orton cones so that I can test how high the kiln is actually firing. I put new elements in recently so wonder if that's an issue. So...I will cast (with groggy slip if I can find it), evenly dry the pieces, put just 2 coats of glaze on the outside (1 inside) then ditch the stilts and rest the houses on both sacrificial slabs and the shelf too. Amazing help - thanks so much all of you. Jamie Link to comment Share on other sites More sharing options...
Babs Posted October 31, 2016 Report Share Posted October 31, 2016 If heavy pots place fine grog orsilica sand under your pots when firing Link to comment Share on other sites More sharing options...
Min Posted October 31, 2016 Report Share Posted October 31, 2016 "So...I will cast (with groggy slip if I can find it), evenly dry the pieces, put just 2 coats of glaze on the outside (1 inside) then ditch the stilts and rest the houses on both sacrificial slabs and the shelf too." couple suggestions, - if you want to try a slip with grog just try adding a little to the slip you already use, or you could use some fine silica sand, might not be necessary though if you fire on a waster piece. - I would run some glaze tests with 1, 2 and 3 dips of your glaze(s) on test tiles the same thickness as your finished pieces will be. Count off the seconds you dip for and measure the specific gravity of the glaze so you can replicate the results. - Make a waster or sacrificial piece of the same clay you use for the houses, it can be really thin, dry it between boards so it stays flat. Bisque it then brush with either kiln wash or just some alumina hydrate mixed with wax resist and brush it on the waster piece and place your glazed house on that. You could add some silica sand or grog to the wax too if you find it necessary. Picture below of lid of butter dish fired rim side on waster piece, usually don't warp at all fired this way. Same sort of shape idea as your houses. Link to comment Share on other sites More sharing options...
neilestrick Posted November 1, 2016 Report Share Posted November 1, 2016 Don't change your slip until you try the other fixes first. Link to comment Share on other sites More sharing options...
jamie shaw Posted November 5, 2016 Author Report Share Posted November 5, 2016 "So...I will cast (with groggy slip if I can find it), evenly dry the pieces, put just 2 coats of glaze on the outside (1 inside) then ditch the stilts and rest the houses on both sacrificial slabs and the shelf too." couple suggestions, - if you want to try a slip with grog just try adding a little to the slip you already use, or you could use some fine silica sand, might not be necessary though if you fire on a waster piece. - I would run some glaze tests with 1, 2 and 3 dips of your glaze(s) on test tiles the same thickness as your finished pieces will be. Count off the seconds you dip for and measure the specific gravity of the glaze so you can replicate the results. - Make a waster or sacrificial piece of the same clay you use for the houses, it can be really thin, dry it between boards so it stays flat. Bisque it then brush with either kiln wash or just some alumina hydrate mixed with wax resist and brush it on the waster piece and place your glazed house on that. You could add some silica sand or grog to the wax too if you find it necessary. Picture below of lid of butter dish fired rim side on waster piece, usually don't warp at all fired this way. Same sort of shape idea as your houses. That's very helpful and thanks for taking the time to upload the pictures. I will definitely be trying the waster piece and looking at specific gravity / timing of glazes. Thanks for your advice. Link to comment Share on other sites More sharing options...
jamie shaw Posted November 5, 2016 Author Report Share Posted November 5, 2016 Don't change your slip until you try the other fixes first. Will do. Thank you. Link to comment Share on other sites More sharing options...
jamie shaw Posted November 5, 2016 Author Report Share Posted November 5, 2016 If heavy pots place fine grog orsilica sand under your pots when firing Thanks. I'll try it with some test pieces. All the best Link to comment Share on other sites More sharing options...
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