Chilly Posted March 19, 2014 Report Share Posted March 19, 2014 So, I emptied the kiln this afternoon - a ^6 firing. One mug cracked, some mugs OK, some stuck to their coasters. Spent a while with the angle grinder and a diamond disk and cleaned the mugs. While working on the last one, I glanced at the first one I'd fixed and there, from top to bottom, was a crack. That made 2 cracked mugs from one firing. I'd also made a spatula rest Mark IV. Mark I is in daily use but is ^04 and I wanted a stoneware one. Mark II had a glaze failure and it has bubbles all over it. Mark III jumped out of my hands on the way to the kiln and shattered. Mark IV emerged from the kiln in glorious perfection. I smoothed it's feet, washed it and carefully carried it into the kitchen. The other half picked it up and studied it - he is after all the one who will use it - and it slipped out of his hand and crashed into the sink. I know what I'll be making at pottery class tomorrow...... more mugs and Mark V spatula rest. Any suggestions as to why the mugs cracked? One was rolled and formed into a tube, the other was slip-cast. Both are ^6 clay, both had ^6 white gloss commercial glaze on the inside and outer rim, both had (different) commercial ^6 glaze on the outside. Both cracks appear to be on cooling, sharp edged. I've had three mugs crack all with the same glaze, one hand-made, the other two slip-cast from different moulds. I'm beginning to suspect the glaze. Will have to make some more samples. Link to comment Share on other sites More sharing options...
High Bridge Pottery Posted March 19, 2014 Report Share Posted March 19, 2014 R.I.P Mark VI in all his glorious perfection. Long live Mark V A picture of said cracked mugs would be good. Have you used the white on the inside with no problems without the other glaze? Link to comment Share on other sites More sharing options...
Diane Puckett Posted March 20, 2014 Report Share Posted March 20, 2014 Ugh. I hate days like that. Are you letting the kiln cool to room temperature before opening it? Link to comment Share on other sites More sharing options...
BeckyH Posted March 20, 2014 Report Share Posted March 20, 2014 Some glazes just don't like some shapes, and will pull them apart in the kiln. Time to try a different glaze. Link to comment Share on other sites More sharing options...
Evelyne Schoenmann Posted March 20, 2014 Report Share Posted March 20, 2014 ... and maybe the cracked mug was too near a heating coil? (uneven distribution of heat). I'am sorry for the accident to Mark IV. I hope Mark V will survive and live a long time! And yes, Diane is right: always have patience long enough to let the kiln really cool down. Good luck with the next firing! Evelyne Link to comment Share on other sites More sharing options...
Min Posted March 20, 2014 Report Share Posted March 20, 2014 Both are ^6 clay, both had ^6 white gloss commercial glaze on the inside and outer rim, both had (different) commercial ^6 glaze on the outside. Both cracks appear to be on cooling, sharp edged. I've had three mugs crack all with the same glaze, one hand-made, the other two slip-cast from different moulds. I'm beginning to suspect the glaze. Will have to make some more samples. Pictures would help, but it sounds like a dunt from using a glaze with a lower expansion on the inside of the mugs and a higher expansion glaze on the outside. Can you tell if the ones that cracked had a thicker glaze on the inside than the ones that didn't? I would freeze any surviving mugs overnight then pour boiling water into them. If they also crack I would put it down to the expansion of the glazes. (Maybe it would help to rename "Mark" to George ) Link to comment Share on other sites More sharing options...
Chilly Posted March 20, 2014 Author Report Share Posted March 20, 2014 Pictures of the mugs. Also two pictures of a fragment of another "cracked" mug, showing the thickness of the inside and outside glaze. Looks like the inside glaze is thicker, but it is also much more stable. Three of the four outside glazes are really runny, which is obviously not helping. Because I'm working with very small numbers of items and glaze, I'm pouring the inside glaze, and brushing the outside. The red in particular is a real **** to apply, but it's getting better with the addition of Epsom Salts. What's weird is that two of the mugs have almost identical shaped cracks - one was hand-built, the other slip-cast. I used to like playing with clay, it's the only craft material that you can keep re-using (until fired). With fabrics and other materials, once you've cut something out, you can only make something different if it's smaller. Now I'm getting frustrated. Link to comment Share on other sites More sharing options...
Pres Posted March 20, 2014 Report Share Posted March 20, 2014 These look like cooling dunts to me. The glazes don't look too thick, and the pieces are of even thickness. So I would say cooling dunts. Link to comment Share on other sites More sharing options...
dave the potter Posted March 20, 2014 Report Share Posted March 20, 2014 I agree, the cracks came in the cooling. Sometimes pots will crack if the have stuck to the shelf(glaze runs, glaze on the shelf, etc). The cooling shrinkage cannot happen correctly since the bottom of the pot will not move. Link to comment Share on other sites More sharing options...
jrgpots Posted March 20, 2014 Report Share Posted March 20, 2014 (Maybe it would help to rename "Mark" to George ) Perhaps the name "Mathusielah I" would engender it with longer life. Perhaps giving it an Irish name would infuse a bit of good luck to the piece. "Dunting" would not be good name nor would "Slip", "Craze", or "Oops" be good proper name for the spatula rest. Jed Link to comment Share on other sites More sharing options...
Chilly Posted March 20, 2014 Author Report Share Posted March 20, 2014 I agree, the cracks came in the cooling. Sometimes pots will crack if the have stuck to the shelf(glaze runs, glaze on the shelf, etc). The cooling shrinkage cannot happen correctly since the bottom of the pot will not move. I confess to opening the kiln at 140C, but on the basis that I thought stoneware is supposed to be "oven-proof", and my oven is usually around 180-220C, I thought 140C was cool enough. But the bit about them sticking makes much more sense. Although one of them wasn't stuck, all the others were as the glazes are sooooo runny. Link to comment Share on other sites More sharing options...
Diane Puckett Posted March 21, 2014 Report Share Posted March 21, 2014 Opening the kiln while it is still hot suddenly exposes the pots to uneven heat, ie one side of the pot is suddenly cooler than the other. Just that can cause dunting. The pots sticking to the shelves combines with the sudden temp change probably did it. Harry Fraser's book, Ceramic Faults and their remedies has a lengthy section on dunting. Link to comment Share on other sites More sharing options...
Chilly Posted March 21, 2014 Author Report Share Posted March 21, 2014 Harry Fraser's book, Ceramic Faults and their remedies has a lengthy section on dunting. I collected this book from the library this morning. Had a quick glance. Going to take some time to absorb, but "sticking" to their coasters has definately got to be one of the causes. Link to comment Share on other sites More sharing options...
Stephen Posted March 21, 2014 Report Share Posted March 21, 2014 Don't let this throw you off though, whatever the problem is you will find it and move on. When something happens in the final step of the process it is just more frustrating because all the work has been done and it came so close to completion. Link to comment Share on other sites More sharing options...
Min Posted March 21, 2014 Report Share Posted March 21, 2014 Pictures of the mugs. Also two pictures of a fragment of another "cracked" mug, showing the thickness of the inside and outside glaze. Looks like the inside glaze is thicker, but it is also much more stable. Three of the four outside glazes are really runny, which is obviously not helping. Because I'm working with very small numbers of items and glaze, I'm pouring the inside glaze, and brushing the outside. The red in particular is a real **** to apply, but it's getting better with the addition of Epsom Salts. What's weird is that two of the mugs have almost identical shaped cracks - one was hand-built, the other slip-cast. I used to like playing with clay, it's the only craft material that you can keep re-using (until fired). With fabrics and other materials, once you've cut something out, you can only make something different if it's smaller. Now I'm getting frustrated. Thanks for posting pictures, it does make it easier to try and figure out what happened. Your red glaze is gorgeous! Agree with it looking like a cooling dunt. It's safer to wait until the pots are cool enough that you can pick them up with your bare hands when unloading. The kiln shelves retain a lot of heat, the top of pots might feel just a bit hot but the bases will be much hotter. The pyrometer is measuring the air temp in the kiln, ware and shelves will be hotter. Ovenware is as much about the shape of the pot as it is about the clay and glaze fit.It can be made with any firing range clay. Frustration and clay seem to go hand in hand. It's hard to do but if you can avoid getting attached to a piece until it has survived the final fire then it's a bit easier. I can't think of anyone I know that doesn't have issues with one process or another. It seems you can just get one problem solved and another will crop up due to materials change or a slightly different technique etc. I'm guessing that a failure rate of about 10% is not uncommon. This is a very humbling art to work in. Link to comment Share on other sites More sharing options...
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