Case Posted February 18, 2016 Report Share Posted February 18, 2016 First off, thanks for reading. I work with Cone 5 porcelain and fire in a Skutt Km-1027 with a KilnMaster. I use the Cone Fire Mode and I bisque fire to Cone 04 on medium speed with no hold time. For glaze ware, I fire at Cone 5 at medium speed with no hold time. I let it cool to 100 degrees F before I open the lid to remove the pieces. I'll start the kiln at 6:30 am one morning and won't open it to empty it out until 8am the following morning. This process and materials have worked great for me so far. I use dry mix glazes and always work with the same type of clay, B Mix from Freeform here in San Diego. This last load produced a problem I haven't experienced so far. I fired 8 bowls and 12 mugs and 6 of the mugs developed cracks all the way through them after I pulled them out of the kiln from being glaze fired. The cracks were not there during the greenware or bisque stages. One has even cracked into multiple pieces. I'm dumbfounded on whats causing it. If anyone has any idea what I can do to help remedy the situation going forward, please let me know. Thanks! Link to comment Share on other sites More sharing options...
High Bridge Pottery Posted February 18, 2016 Report Share Posted February 18, 2016 Is it a new glaze combo this one just on the inside? Could be the glaze is stressing the clay. Link to comment Share on other sites More sharing options...
Guest JBaymore Posted February 18, 2016 Report Share Posted February 18, 2016 Looks like dunting cracks. Can be caused by a high thermal expansion (COE) glaze on one side of the form and a low expansion one on the other. Sometimes if the glazes are just a certain thickness the pieces clay body is strong enough to hold together. So you don't notice the issue. But get one or the other glaze thicker than "normal" past application .. and "boom". In either case.... not a good situation to have anyway. Will eventually fail. Or as Ray is saying above...... a low expansion glaze only on the inside..... can do the same thing. Sometimes bisque issues can cause this situation if the ware does not have enough oxygen present during the bisque firing (kiln without active local vent, tighter stacking than "normal", faster firing than "normal", lower cone firing than "normal", etc .) The problem shows up in the glaze fire... not the bisque fire. best, .......................john Link to comment Share on other sites More sharing options...
Case Posted February 18, 2016 Author Report Share Posted February 18, 2016 Thanks for responding! High Bridge - Not a new glaze combo and they were not coated particularly thick on the inside versus the outside. I've done maybe a 100+ pieces in this combination over the last year and this was the first time I've seen it. There had been warping before with bowls and cups but the issue was so rare and usually just one piece per load that I attributed it to something else entirely. Thanks, I'll keep this in mind when choosing glazes going forward! John Baymore - It could definitely be that there was not enough oxygen present during the bisque firing. I'll make sure its properly vented on my next load and see what happens. Link to comment Share on other sites More sharing options...
Guest JBaymore Posted February 18, 2016 Report Share Posted February 18, 2016 The bisque issue affects the strength of the underlying clay body. Weakens it. So it will allow a problem that is borderline already to show up. Unless you have a massive cristobalite issue with that body.... then the glaze's impacts on the body are still the likely culprit. What are the glaze's recipes? best, ..................john Link to comment Share on other sites More sharing options...
neilestrick Posted February 18, 2016 Report Share Posted February 18, 2016 Could also be another B Mix issue starting. It's happened in the past. I'd start with the glaze, though. Link to comment Share on other sites More sharing options...
High Bridge Pottery Posted February 18, 2016 Report Share Posted February 18, 2016 It does have a strange uneven speckle so could be a reduction issue. Unless that's the normal look for B mix. Link to comment Share on other sites More sharing options...
Roberta12 Posted February 18, 2016 Report Share Posted February 18, 2016 Would there have been anyone else who would have opened the kiln before you? While it was still very hot? Roberta Link to comment Share on other sites More sharing options...
Guest JBaymore Posted February 18, 2016 Report Share Posted February 18, 2016 Could also be another B Mix issue starting. It's happened in the past. I'd start with the glaze, though. Good point....... one would hope the manfrs were on top of it before it got out. More potash spar issues, maybe? best, .................john Link to comment Share on other sites More sharing options...
Guest JBaymore Posted February 18, 2016 Report Share Posted February 18, 2016 It does have a strange uneven speckle so could be a reduction issue. Unless that's the normal look for B mix. Looks like what might be called a stony matte glaze outside and a semi gloss on the inside. Right there maybe root of the issue. But as we all know diagnosing from photos is tough. best, .....................john Link to comment Share on other sites More sharing options...
glazenerd Posted February 19, 2016 Report Share Posted February 19, 2016 Neil: I am sorta leaning to the clay: at least wondering at this point. I recall B-mix having a custer feldspar problem in the past. I cannot see the cracks close enough to tell but will ask: if the crack lines have a sharp edge then they cracked on the way down (cooling.). If they do not have a sharp edge, then they cracked on the way up. Most glazes are applied at 0.15 to 0.25 grams per SI> not sure that is enough to cause those kinds of cracks. The other issue I question: " I bisque at medium speed with no hold"..... Quartz inversion occurs at 578C (1073F) give or take a few degrees. I assume medium is 180F an hour: that seems to be the industry standard. Not impossible to get fissures in the clay if you blow through this phase too quickly: which would show up later during the glaze cycle. I always program an inversion ramp from 1000-1100F @ 100F an hour. Yes, perhaps overly cautious, but I have blown enough pieces during inversion to slow down. Porcelain and Stoneware in my experience are more susceptible to fissures during inversion than other clays. Nerd Link to comment Share on other sites More sharing options...
neilestrick Posted February 19, 2016 Report Share Posted February 19, 2016 Upon closer inspection, I think they look more like a clay issue than a glaze issue. They just don't look like I would expect them to look when being pulled apart by glaze. This is just a gut feeling thing, though, so no real evidence to support that. Was this a new box of clay? Anyone else using that clay in your studio? Any possibility the kiln fired a bit hotter or colder than usual? Unfortunately this may just be one of those freak things that can't be solved until it happens again. Link to comment Share on other sites More sharing options...
Min Posted February 19, 2016 Report Share Posted February 19, 2016 Since it sounds like you are using commercial dry glazes it would be harder to rule in dunting without knowing glaze recipes. Did you fire any pieces with the same glaze on both the inside and outside? If you have some undamaged pieces glazed the same way made from an older box of clay and some of the latest batch I would put them in the freezer overnight then pour boiling water into them and see what happens. (in the sink) (btw B Mix isn't porcelain) Link to comment Share on other sites More sharing options...
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