chantalqtal Posted November 23, 2021 Report Share Posted November 23, 2021 So, hi again...I´m not really a newbie to porcelain slip casting, but over the last months I´ve had terrible outcomes in my firings. I´m using the same porcelain body as always to which I add about 5% molochite. I only fire around 1250-60ºC....I´ve tried all kinds of stuff (everything I can think of or have heard of) to diminish stress on the greenware during drying and the issue even seems to have become more accentuated since I have my new oven ( higher number of crooked items after firing), which is a 80l top loader with extra heating on its floor: my standard cylindrical cups and mugs have oval mouthes after been fired, and sometimes even worse. I have run out of ideas to solve the problem , and for other questions that I still have, I just can't find answers online. can anybody suggest a good book that talks about the importance of the shape of the design for porcelain ware, e.g footrims,their sizes, hights - angles and curves of the walls etc... lets see what you come up with - thanks Link to comment Share on other sites More sharing options...
Bill Kielb Posted November 23, 2021 Report Share Posted November 23, 2021 A quick suggestion post some example pictures of the defect, as well as your firing schedule bisque and glaze. Maybe describe your process a bit more. As much information as you can so folks can provide informed feedback. Do you fire with cones? Maybe your drying process, bisque firing schedule, glaze firing schedule, applied decorations …… Link to comment Share on other sites More sharing options...
Min Posted November 23, 2021 Report Share Posted November 23, 2021 If the only variable is the kiln you are firing in I would verify the firing temperature with witness cones. Could be you are just firing higher than in your previous kiln. Link to comment Share on other sites More sharing options...
chantalqtal Posted November 23, 2021 Author Report Share Posted November 23, 2021 24 minutes ago, Min said: If the only variable is the kiln you are firing in I would verify the firing temperature with witness cones. Could be you are just firing higher than in your previous kiln. actually I did that on the first firing..and it was fine, but yes, I´m going to repeat Link to comment Share on other sites More sharing options...
chantalqtal Posted November 23, 2021 Author Report Share Posted November 23, 2021 4 hours ago, Bill Kielb said: A quick suggestion post some example pictures of the defect, as well as your firing schedule bisque and glaze. Maybe describe your process a bit more. As much information as you can so folks can provide informed feedback. Do you fire with cones? Maybe your drying process, bisque firing schedule, glaze firing schedule, applied decorations …… I've taken some pics, but the allowed size is too small for uploading even one...im onto it Link to comment Share on other sites More sharing options...
Mark C. Posted November 24, 2021 Report Share Posted November 24, 2021 6 hours ago, chantalqtal said: I've taken some pics, but the allowed size is too small for uploading even one...im onto it downsize the photos in another program to post-On my mac i choose them as small. With a PC break the window in 1/2 and use the smaller 1/2-just kidding Link to comment Share on other sites More sharing options...
chantalqtal Posted November 24, 2021 Author Report Share Posted November 24, 2021 I fire to about cone 7, glaze only the inside of my cups and polish the outside after bisque and after high firing. I have a whole shelf full of textured cups poured in layers, that are just fine, its just lately and seemingly since the purchase of my new oven that half of the cups are warped, even plain ones without hand applied textures. Link to comment Share on other sites More sharing options...
JohnnyK Posted November 24, 2021 Report Share Posted November 24, 2021 Are you, by any chance using a Giffen Grip to trim those cups? If not, how do you trim them. The shape of the rim in the second photo might indicate rim pressure from whatever you are using to hold the pieces when trimming... Link to comment Share on other sites More sharing options...
Mark C. Posted November 24, 2021 Report Share Posted November 24, 2021 My guess is it looks like a matt or No glaze at all is on the outside and the inside is a glossy glaze-this can set up tension that may be warping the cups Tell us about the inside outside glazes? Link to comment Share on other sites More sharing options...
PeterH Posted November 24, 2021 Report Share Posted November 24, 2021 On 11/23/2021 at 12:12 PM, chantalqtal said: my standard cylindrical cups and mugs have oval mouthes after been fired Is that just after the glaze firing, or is it also visible after the bisque? Link to comment Share on other sites More sharing options...
Min Posted November 24, 2021 Report Share Posted November 24, 2021 Has your casting slip changed? Do you mix it yourself or is it a commercial slip body? Any new materials? I know you said you checked the new kiln with witness cones but did you check what cone your old kiln went to before you stopped using it? Perhaps the old kiln wasn't firing as hot as the new one. If there is no change to the slip then I would try firing a cone cooler and see if the warping still happens. If its better then check the clay absorption to make sure it isn't too porous. I don't see a problem with the shape of your pots. Wouldn't hurt to try a few cast a bit thicker also. Link to comment Share on other sites More sharing options...
chantalqtal Posted November 24, 2021 Author Report Share Posted November 24, 2021 52 minutes ago, PeterH said: Is that just after the glaze firing, or is it also visible after the bisque? nope, they're perfect looking after bisque 28 minutes ago, Min said: Has your casting slip changed? Do you mix it yourself or is it a commercial slip body? Any new materials? I know you said you checked the new kiln with witness cones but did you check what cone your old kiln went to before you stopped using it? Perhaps the old kiln wasn't firing as hot as the new one. If there is no change to the slip then I would try firing a cone cooler and see if the warping still happens. If its better then check the clay absorption to make sure it isn't too porous. I don't see a problem with the shape of your pots. Wouldn't hurt to try a few cast a bit thicker also. there's only one pic showing warping , the others were to show what kind of work I do. have contacted my provider who says nothing has changed, they were even going to check the specific batch once again... its a comercial slip powder, that I then prepare. I had fired with cones in the old oven a few times, and would swear I´m firing quite a bit lower now, as already said, will use witnesses again in next firing and set the temp 5ºC lower Link to comment Share on other sites More sharing options...
chantalqtal Posted November 24, 2021 Author Report Share Posted November 24, 2021 33 minutes ago, Min said: Has your casting slip changed? Do you mix it yourself or is it a commercial slip body? Any new materials? I know you said you checked the new kiln with witness cones but did you check what cone your old kiln went to before you stopped using it? Perhaps the old kiln wasn't firing as hot as the new one. If there is no change to the slip then I would try firing a cone cooler and see if the warping still happens. If its better then check the clay absorption to make sure it isn't too porous. I don't see a problem with the shape of your pots. Wouldn't hurt to try a few cast a bit thicker also. 1 hour ago, Mark C. said: My guess is it looks like a matt or No glaze at all is on the outside and the inside is a glossy glaze-this can set up tension that may be warping the cups Tell us about the inside outside glazes? yes as I wrote, I don't glaze inside, I've also been to the lab where they prepare the comercial glazes...they said it shouldn't be a problem... Link to comment Share on other sites More sharing options...
chantalqtal Posted November 24, 2021 Author Report Share Posted November 24, 2021 1 hour ago, JohnnyK said: Are you, by any chance using a Giffen Grip to trim those cups? If not, how do you trim them. The shape of the rim in the second photo might indicate rim pressure from whatever you are using to hold the pieces when trimming... 1 hour ago, JohnnyK said: Are you, by any chance using a Giffen Grip to trim those cups? If not, how do you trim them. The shape of the rim in the second photo might indicate rim pressure from whatever you are using to hold the pieces when trimming... haha, don't know what a gif fen grip is, so id say no - I use a thin metal blade - Link to comment Share on other sites More sharing options...
Bill Kielb Posted November 24, 2021 Report Share Posted November 24, 2021 1 hour ago, chantalqtal said: use witnesses again in next firing and set the temp 5ºC lower Just assuming here that 5c lower is in reference to top temp which then you would verify the heatwork effect with a cone. 1 hour ago, chantalqtal said: haha, don't know what a gif fen grip is, so id say no - I use a thin metal blade - Since you likely trim on the rim, it certainly is possible to distort them but that would also likely show up in the bisque. Do the distortions correspond to cups with 1 or two sided applied decorations rather than those than run continuously around? Last question, just tossing the idea out there: is your firing schedule the same, meaning the program as well does it finishes on time and did your own kiln run the same schedule but take significantly more time to complete? Link to comment Share on other sites More sharing options...
Mark C. Posted November 24, 2021 Report Share Posted November 24, 2021 I sent you a Pm with a person who may be able to help you better-contact her Mark Link to comment Share on other sites More sharing options...
chantalqtal Posted November 24, 2021 Author Report Share Posted November 24, 2021 34 minutes ago, Bill Kielb said: Just assuming here that 5c lower is in reference to top temp which then you would verify the heatwork effect with a cone. Since you likely trim on the rim, it certainly is possible to distort them but that would also likely show up in the bisque. Do the distortions correspond to cups with 1 or two sided applied decorations rather than those than run continuously around? Last question, just tossing the idea out there: is your firing schedule the same, meaning the program as well does it finishes on time and did your own kiln run the same schedule but take significantly more time to complete? I really try to trim carefully, Ive thought about it a lot, wondering sometimes if I do it at exactly the right moment. At the moment, I´m not decorating at all, just to avoid wasting all that time on decoration until I´ve solved this issue - so even the plain cups distort.... The firing schedule starts and finishes exactly as programmed. Link to comment Share on other sites More sharing options...
Babs Posted November 24, 2021 Report Share Posted November 24, 2021 Did you see Mark's post above. Link to comment Share on other sites More sharing options...
chantalqtal Posted November 24, 2021 Author Report Share Posted November 24, 2021 1 hour ago, Babs said: Did you see Mark's post above. yes -thanks have already contacted them :-) Link to comment Share on other sites More sharing options...
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