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Pottery Cracking In Glaze Firing


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#1 Annie12345

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Posted 16 April 2014 - 12:36 PM

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



#2 Pres

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Posted 16 April 2014 - 12:41 PM

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.

 

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Preston


Simply retired teacher, not dead, living the dream. on and on and. . . . on. . . .                                                                                 http://picworkspottery.blogspot.com/


#3 Annie12345

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Posted 17 April 2014 - 02:21 PM

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



#4 Chris Campbell

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Posted 17 April 2014 - 02:25 PM

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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#5 Annie12345

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Posted 17 April 2014 - 02:31 PM

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!

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#6 Min

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Posted 17 April 2014 - 03:17 PM

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.



#7 mregecko

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Posted 17 April 2014 - 03:29 PM

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.



#8 Annie12345

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Posted 17 April 2014 - 10:04 PM

"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



#9 ayjay

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Posted 18 April 2014 - 04:14 AM

 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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