Annie12345 Posted April 16, 2014 Report Share Posted April 16, 2014 I've been firing in my electric kiln for a couple of years now. The last batch of items fired came through the bisque firing without any problem. When I did a 'slow glaze' firing with them two of the pieces cracked. This is the first time I've had this happen. Any ideas why they would crack in the glaze firing but not in the bisque? Both pieces were on the top shelf and near each other and both pieces had the same type of glaze. It's been about 40 years since I've had a ceramics class and am making pieces and running my little kiln from long ago memory. I have no clue what happened. thanks. Annie Sequim, WA Link to comment Share on other sites More sharing options...
Pres Posted April 16, 2014 Report Share Posted April 16, 2014 Bad news is that cracks often can occur for different reasons, but the good news is that most cracks are identifiable as to the reason the crack happened. Please post a few images showing the pots that cracked. We may be able to narrow things down better if you would do that. best, Preston Link to comment Share on other sites More sharing options...
Annie12345 Posted April 17, 2014 Author Report Share Posted April 17, 2014 If I can figure out how to post a photo, I'll do it! Hmmm.... so far no luck. Any clues on how to post photos, they would be appreciated. Annie Link to comment Share on other sites More sharing options...
Chris Campbell Posted April 17, 2014 Report Share Posted April 17, 2014 When you post your reply look at the bar below to the right where it says "More reply options" Hit that and a new dialogue box comes up and at the bottom you will see instructions to attach files. Browse to find them on your computer then hit attach this file. Then just hit "Add reply". That's all there is to it. Link to comment Share on other sites More sharing options...
Annie12345 Posted April 17, 2014 Author Report Share Posted April 17, 2014 AHA! I firgured out the photo posting procedure! Here are my cracked pots. My question is why did they crack in the glaze firing and not the bisque? I'm using an electric kiln and it was firing at O6. The two pieces were near each other, on the top shelf. Nothing else in the kiln cracked. Other items were made with the same clay and same glaze. It's a mystery to me, and I'd like to understand it. Thanks! Link to comment Share on other sites More sharing options...
Min Posted April 17, 2014 Report Share Posted April 17, 2014 Any chance you cracked the lid while the kiln was still hot? My guess is that it looks like a cooling dunt perhaps in combination with having a thick glaze layer on what looks like a very thin clay thickness. Link to comment Share on other sites More sharing options...
mregecko Posted April 17, 2014 Report Share Posted April 17, 2014 Hard to tell for sure, but it looks from the photo like they cracked once the glaze was already hardened... When cracks occur in the middle of the firing, the edges of the crack are usually more rounded as the melted glaze's surface tension rounds out corners. I'm guessing it's a cooling dunt as well. Even slightly faster cooling on a thin form, that may have a little bit of glaze tension, can cause dunts. Link to comment Share on other sites More sharing options...
Annie12345 Posted April 18, 2014 Author Report Share Posted April 18, 2014 "Dunt" is a new word to me. Not sure what it means, but I get the general idea. I did open the kiln a crack (sort of like shaking the Christmas presents before Christmas) but it was already down to 135. Guess I need to wait longer to peek. Thanks for the education!!! Annie Link to comment Share on other sites More sharing options...
ayjay Posted April 18, 2014 Report Share Posted April 18, 2014 I did open the kiln a crack (sort of like shaking the Christmas presents before Christmas) but it was already down to 135. Guess I need to wait longer to peek. Thanks for the education!!! Annie I don't think opening the kiln for a peek at135, (either Fahrenheit or Centigrade) is going to be the sole cause of the problem - I've often been guilty of peeking, normally at about 250°C ( domestic oven temperature) but I've also done it at anywhere between 600°C and 100°C and I'm yet to experience anything like you are showing, so I'd say there must be some other contributing factor. Link to comment Share on other sites More sharing options...
Sopita on the Rocks! Posted December 23, 2019 Report Share Posted December 23, 2019 Hello, this is my first time posting here, so hi to everyone!! I see this post is from a couple of years ago, but I'd like to -if possible- get some help regarding this same issue. I had cracks on two pieces that I was really looking forward to, and this was during the second, firing. I thought they were safe since they survived their bisque firing so beautifully they're stoneware, and fired on an electric kiln to 1240˚C I'm attaching a photo of one of the pieces, a tart dish (the leaf on the background is the other cracked one, but you can't "appreciate" the crack from the photo). Any clues as to what I should check/stop doing? Thanks in advance! Link to comment Share on other sites More sharing options...
Denice Posted December 23, 2019 Report Share Posted December 23, 2019 Did you fire this on the shelf without any stilts beneath it so it could move during the firing? It is to bad about the crack, nice work I am sure you spent a lot of time on it. Denice Link to comment Share on other sites More sharing options...
Sopita on the Rocks! Posted December 23, 2019 Report Share Posted December 23, 2019 Thank you! hmmmm, yes, no stilts! I Just started doing the "dish" thing, so I'm very new to this. And I'm basically trying to self teach myself ceramics on the weekends. Anyways, maybe I need to get some of those? I fired it directly on a shelf. I do have tripods for dishes, but tried that before with a rather thin dish and it curved... So I thought it was best to lay the dishes directly on the shelf! I will try to figure out the Spanish word for "stilts" (I'm from Argentina) and get me some :p It's awfully sad when things survive all the previous steps, only to go horribly wrong in the last one!!!! Hello, this is my first time posting here, so hi to everyone!! I see this post is from a couple of years ago, but I'd like to -if possible- get some help regarding this same issue. I had cracks on two pieces that I was really looking forward to, and this was during the second, firing. I thought they were safe since they survived their bisque firing so beautifully they're stoneware, and fired on an electric kiln to 1240˚C I'm attaching a photo of one of the pieces, a tart dish (the leaf on the background is the other cracked one, but you can't "appreciate" the crack from the photo). Any clues as to what I should check/stop doing? Thanks in advance! Link to comment Share on other sites More sharing options...
liambesaw Posted December 23, 2019 Report Share Posted December 23, 2019 Can we get a close up picture of the crack, where the glaze and clay interface? And was it split like that when you found it, or have you separated the halves for clarity Link to comment Share on other sites More sharing options...
Sopita on the Rocks! Posted December 23, 2019 Report Share Posted December 23, 2019 It was split like that, it's actually not all the way through, it's like it split open... I'm attaching another photo to see if it shows better. Thanks! Link to comment Share on other sites More sharing options...
Bill Kielb Posted December 23, 2019 Report Share Posted December 23, 2019 8 minutes ago, Sopita on the Rocks! said: It was split like that, it's actually not all the way through, it's like it split open... I'm attaching another photo to see if it shows better. Thanks! Just a comment, anytime you fire anything flat and largish try and put something under the pot so it wont catch or drag on the shelf while firing. Popular methods include grog smooth and evenly under the pot or alumina neatly under the pot. The idea is to allow the pot to expand and contract during firing and cooldown without it catching on the shelf. Remember the shelve expands and contracts during firing as well. Wares that are wide as well as wares that may be heavy have a greater chance of catching on the shelf so someway that allows it to move in all directions independently of the shelf helps alleviate this. Link to comment Share on other sites More sharing options...
Sopita on the Rocks! Posted December 23, 2019 Report Share Posted December 23, 2019 6 minutes ago, Bill Kielb said: Just a comment, anytime you fire anything flat and largish try and put something under the pot so it wont catch or drag on the shelf while firing. Popular methods include grog smooth and evenly under the pot or alumina neatly under the pot. The idea is to allow the pot to expand and contract during firing and cooldown without it catching on the shelf. Remember the shelve expands and contracts during firing as well. Wares that are wide as well as wares that may be heavy have a greater chance of catching on the shelf so someway that allows it to move in all directions independently of the shelf helps alleviate this. thanks, will definitely try that! Link to comment Share on other sites More sharing options...
oldlady Posted December 23, 2019 Report Share Posted December 23, 2019 sopita, grog will help a lot. i just wanted to say how much i admire your scraffito. the design is excellent and your execution extremely well done. hope this kind of work will be repeated. alice Link to comment Share on other sites More sharing options...
Sopita on the Rocks! Posted December 23, 2019 Report Share Posted December 23, 2019 Just now, oldlady said: sopita, grog will help a lot. i just wanted to say how much i admire your scraffito. the design is excellent and your execution extremely well done. hope this kind of work will be repeated. alice thank you so much! YES, will definitely persist until I get it right!!! It was so much fun to make, and so good for my practise on being patient, ha! I actually have a new one drying to bisque fire next week or the other, so will be more careful in the future. Link to comment Share on other sites More sharing options...
Magnolia Mud Research Posted December 23, 2019 Report Share Posted December 23, 2019 I have learned (the hard way) to support flat bottomed items such as plates and heavy jars on balls of wadding with a diameter that is no smaller than the thickness of the plate or bottom of the jar. Ball to ball separation around the circle is about twice the diameter of the ball. The balls separates the heating and cooling gradients in the ware from the mass of the kiln shelf and allows each to cool at their own schedule. Link to comment Share on other sites More sharing options...
Min Posted December 23, 2019 Report Share Posted December 23, 2019 @Sopita on the Rocks!, if you run your fingertip across the crack is the glaze sharp on the edges of the crack with no smoothing over of the glaze? Also, how fast do you cool the kiln and how hot was it when you removed the peep hole plugs and cracked open the lid? Link to comment Share on other sites More sharing options...
neilestrick Posted December 24, 2019 Report Share Posted December 24, 2019 That looks like a cooling crack to me. When you have a large flat area in contact with the shelf, the vertical walls are going to cool faster than the flat area. Try slowing down your cooling cycle. Putting something on the shelf to help it move would also be good, as it could be a combination of cooling too quickly and failure to move. If you're working with porcelain, I would use silica sand rather than grog, as it has more rounded edges as opposed to the sharp edges of grog, and will be less likely to leave marks on the clay. I would also avoid using wadding, coils, etc to lift the pot off the shelf as porcelain is prone to warping using those methods. Link to comment Share on other sites More sharing options...
Bill Kielb Posted December 24, 2019 Report Share Posted December 24, 2019 23 hours ago, Sopita on the Rocks! said: thank you so much! YES, will definitely persist until I get it right!!! It was so much fun to make, and so good for my practise on being patient, ha! I actually have a new one drying to bisque fire next week or the other, so will be more careful in the future. Just to mention - placing something that allows freedom of movement (Grog, Alumina, Sand) under these larger pots can be important in the bisque cycle as well. Especially when folks start double and triple stacking inside them because of their size. As other items are stacked they add weight which is taken by the bottom of your pot and increases the drag on the shelf. If you are not loading your own kiln this can be an issue. The difference in shrinkage of the ware and that of the shelf is very significant and is easily experienced by those who have gone to bonded nitride shelves. When a pot has an oops or glaze drip down to a conventional shelf it most often fractures the foot of the pot In some way because the glaze stuck enough to the shelf or even kiln wash to allow enough independent movement of the pot so grinding to save the pot is often difficult as a large chunk of it fractures off. When using bonded nitride shelves, glaze does not stick to the shelf and quit often to the potters delight, no crack, just some grinding because the glaze does not stick to the shelf and therefore the ware can move freely while cooling. Thermal gradients in a cooling kiln exist, but at low temperatures and ordinary cooling speeds usually hard to significantly influence and generally a non issue in my experience. Link to comment Share on other sites More sharing options...
Min Posted December 24, 2019 Report Share Posted December 24, 2019 1 hour ago, Bill Kielb said: Thermal gradients in a cooling kiln exist, but at low temperatures and ordinary cooling speeds usually hard to significantly influence and generally a non issue in my experience. Try propping the lid open or removing plugs at around 400F with a wide flat based pot on the top shelf. Dunt de dunt dunt! Link to comment Share on other sites More sharing options...
Bill Kielb Posted December 24, 2019 Report Share Posted December 24, 2019 3 minutes ago, Min said: Try propping the lid open or removing plugs at around 400F with a wide flat based pot on the top shelf. Dunt de dunt dunt! Ordinary cooling speeds would be the operative word here. Link to comment Share on other sites More sharing options...
neilestrick Posted December 24, 2019 Report Share Posted December 24, 2019 39 minutes ago, Bill Kielb said: Ordinary cooling speeds would be the operative word here. Depends on the size of the kiln and how tight it's packed. My baby kiln can cool from cone 6 to unload in 5-6 hours. My 4 cubic foot can do it in 8 hours if it's packed light. Both of those situations can be too fast for a wide flat piece. Link to comment Share on other sites More sharing options...
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