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Pottery Cracking In Glaze Firing


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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

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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

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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.

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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!

post-63237-0-22331500-1397762845_thumb.gif

post-63237-0-22331500-1397762845_thumb.gif

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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.

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 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.

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  • 5 years later...

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!

IMG_1576.jpg

Edited by Sopita on the Rocks!
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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!

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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! 

 

Screen-Shot-2019-12-23-at-13.28.07.jpg

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.   

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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!

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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.

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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.  
 

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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.

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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.

Edited by Bill Kielb
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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!

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39 minutes ago, Bill Kielb said:

Ordinary cooling speeds would be the operative word here.:D

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.

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