Shannon Posted February 9, 2011 Report Share Posted February 9, 2011 Hey, just had a mini-disaster in the kiln, was wondering if anyone had had the same problem. Have a bottom-heavy sculpture that slumped in the kiln after a Cone 6 glaze firing. I think the problem might be that I sprayed the glaze unevenly or didn't let it dry long enough before putting it in the kiln (after 2 hours). Glaze felt dry to the touch, but the pieces slumped unevenly throughout the kiln- pretty sure it wasn't the kiln. Question #1: How long do you usually let a piece dry before glaze firing? My info: Laguna Dover Slip, bisqued, spray-glazed, then glaze fired at Cone 6. Piece is 7" tall, 7" in diameter, with a cone shape heavier at the bottom with a 1/4" foot. Using an Electric Skutt Kiln, have access to a gas-firing kiln Question #2: What would a good high bisque/low glaze fire combination be? My info: Things I've read recommend a bisque as high as cone 6, with a glaze fire at 04. Thanks in advance for your help! Tearing my hair out over here! Link to comment Share on other sites More sharing options...
Chris Campbell Posted February 9, 2011 Report Share Posted February 9, 2011 I have not heard of slumping being caused by glaze application. It is more a result of the clay body needing more support or being fired higher than it is rated. The support can come from the shape of the form, it's thickness or from adding firing support. Were you slip casting these? If you had a part of the form that was thinner than the rest, it could collapse. Since it happened all through your kiln, there might be a common cause in your technique. Can you show us what the shape was before being fired so we can see how it slumped? Link to comment Share on other sites More sharing options...
wyoartist2 Posted February 9, 2011 Report Share Posted February 9, 2011 Hey, just had a mini-disaster in the kiln, was wondering if anyone had had the same problem. Have a bottom-heavy sculpture that slumped in the kiln after a Cone 6 glaze firing. I think the problem might be that I sprayed the glaze unevenly or didn't let it dry long enough before putting it in the kiln (after 2 hours). Glaze felt dry to the touch, but the pieces slumped unevenly throughout the kiln- pretty sure it wasn't the kiln. Question #1: How long do you usually let a piece dry before glaze firing? My info: Laguna Dover Slip, bisqued, spray-glazed, then glaze fired at Cone 6. Piece is 7" tall, 7" in diameter, with a cone shape heavier at the bottom with a 1/4" foot. Using an Electric Skutt Kiln, have access to a gas-firing kiln Question #2: What would a good high bisque/low glaze fire combination be? My info: Things I've read recommend a bisque as high as cone 6, with a glaze fire at 04. Thanks in advance for your help! Tearing my hair out over here! Link to comment Share on other sites More sharing options...
wyoartist2 Posted February 9, 2011 Report Share Posted February 9, 2011 Hello! I don't know if this helps or not, but the only time that I had something slump in the kiln was when my auto-shut-off malfunctioned and the pieces were fired too long. Good luck! Link to comment Share on other sites More sharing options...
Marcia Selsor Posted February 9, 2011 Report Share Posted February 9, 2011 I agree with Chris. I think it is the form's extreme underbelly and extreme flat shoulder that may have been weak. I doubt the glaze application had anything to do with it. Marcia Link to comment Share on other sites More sharing options...
Pres Posted February 9, 2011 Report Share Posted February 9, 2011 Hello! I don't know if this helps or not, but the only time that I had something slump in the kiln was when my auto-shut-off malfunctioned and the pieces were fired too long. Good luck! On this line here(auto-shut off malfunction) make certain the rings on your sectional kiln are lined up vertically, if not the sitter will not swing down to shut the kiln off, but the guard timer will. However, your pots don't seem to have any dunting or other overfire characteristics, so I would assume as many of the others here that it is in the design of a slip cast piece. Link to comment Share on other sites More sharing options...
kmcl Posted February 9, 2011 Report Share Posted February 9, 2011 It looks like your bisque firing is too hot. It should be somewhere in the ball park of cone 06 to cone 04. Not cone 6. The scale goes like 024 (three numbers) 06 (two numbers) cone 6 (one number). The 024 is not as hot as the cone 06 and the cone 06 is not as hot as cone 6. The glaze sticks best to clay fired from 04 to 06. The glaze firing is usually hotter, but is glaze dependent. If the glaze matures at 024 or some low temperature, you would glaze the bisque and fire to the proper temperature for the glaze. It looks as you did a high temperature bisque and then refired the glaze at a cooler temp. That probably didn't cause the slumping, but if the clay wasn't suitable for the higher temperature, it may have started to melt, like a cone does. Link to comment Share on other sites More sharing options...
Shannon Posted February 13, 2011 Author Report Share Posted February 13, 2011 I have not heard of slumping being caused by glaze application. It is more a result of the clay body needing more support or being fired higher than it is rated. The support can come from the shape of the form, it's thickness or from adding firing support. Were you slip casting these? If you had a part of the form that was thinner than the rest, it could collapse. Since it happened all through your kiln, there might be a common cause in your technique. Can you show us what the shape was before being fired so we can see how it slumped? Hi Chris, Thanks so much for your reply. I don't know if it helps, but I attached a photo of the lamp bases before they were bisqued. They were all uniform-looking, no drooping or slumping before and after the bisque firing. I bisqued at Cone 06, then glaze fired at Cone 6. I think the small foot at the bottom might have something to do with it. Also, I've been thinking of increasing the time over which the glaze firing cools. Any suggestions? Thanks for your time and help! Link to comment Share on other sites More sharing options...
Shannon Posted February 13, 2011 Author Report Share Posted February 13, 2011 Thank you everyone for your time and help! Will be doing some test firings soon, hopefully will figure out a solution. Cheers, Shannon Link to comment Share on other sites More sharing options...
Chris Campbell Posted February 13, 2011 Report Share Posted February 13, 2011 I don't think your slumping has anything to do with the glaze or the cooling schedule. It's the form which does not seem to be able to support itself. Once you take a form out so far from it's center and put so much of its mass above it ... you have to consider the fact that it has to hold itself up. Are they slipcast? If so, you may be casting them too thick so the base cannot support the weight in the heat of the firing ... so it sags. Have you ever fired this shape successfully with a different clay body? Are they all sagging the same way? Link to comment Share on other sites More sharing options...
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