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Pieces Forming A Crack A Day After Glaze Firing


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#1 DMCosta

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Posted 06 December 2013 - 04:37 PM

Hi, I have just had three pieces come out of my most recent glaze firing and all three looked beautiful coming out of the kiln, then about 24 hours later formed a large ugly hairline crack from the rim of the pot all the way down to the base. I am a relatively experienced potter, I have never seen or had this issue. I am not opening the kiln until it's cool. Anyone know what's going on?

 

One bowl had a commercially prepared glaze, the other two were my own mixed glazes. 

 

Thank for your help,

~Dianna



#2 neilestrick

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Posted 06 December 2013 - 05:06 PM

Glazed inside and out? All bowl forms?


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#3 JBaymore

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Posted 06 December 2013 - 05:54 PM

It is called dunting.

 

Many possible causes.

 

Mose likely (if glazed inside and out) is that the COE (Coefficient of -reversible- Thermal Expansion) of the two glazes is drastically different.,... with one putting the underlying form in significant compression and the other putting it in significant tension. Result....... eventyually the strees is relieved when the form cracks.

 

Be glad that YOU are the one noticing this....... and not your customer.

 

It can also happen if this situation is set up in the firing... and then you pour hot liquid l;ike coffee into a cup.... and then it usually goes "bang"....... and hoopefully NOT burning anyone.

 

If you firing is to stoneware temps and you lengthne the firing above about 2000F..... the developmane of the cristobolaite form of siclica in underfluzed bodies can also cause this. Often seen in pieces that people put into extended firings in anagama kilns firing for 3-7 days.... when that clay body is OK in short 18-20 hour gas firings.

 

best,

 

................john


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Professor of Ceramics; New Hampshire Insitute of Art

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#4 jolieo

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Posted 07 December 2013 - 07:08 PM

Hi I understand the idea of Coe because I was and am enamored with glass. Is there no way to know the Coe of a glaze? With glass , if the Coe is within a few points of each other, things might work out. Might. How about the coe of clay , it is basically the shrinkage, right? So if I did strips of my clay , measured them precisely , then fired them , I would have some idea of what to expect from the clay for the temp I fired to. Is there no test for the glaze ? Thanks jolie



#5 JBaymore

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Posted 07 December 2013 - 07:22 PM

Just like in glass work...... you want the COEs to match up pretty closely.

 

Firing shrinkage and COE are not related in clay bodies.

 

To figure glaze COEs you can use glaze chemistry software to reasonably accurately predict them.

 

No one has yet figured out how to model the complex activities of clay bodies. COEs on clay bodies are measured... using something called a dilatometer.  Dilatometers are the province of industrial ceramics labs.  You can do glaze with a dialatometer too .....a little more difficult.

 

There is a "quick and dirty" way to establish a clay body's COE. You mix up a set of testing glazes that have steadily increasing COEs. Then glaze some test tiles with the glazes... and the one in the center of the "shivering on one end and crazing on the other" curve is close to the actual COE.

 

best,

 

..............john


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#6 jolieo

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Posted 08 December 2013 - 09:12 AM

Thanks to both of you! I think I am getting myself into a can of worms! But I do love murky and complicated. Ok - being new I am pretty safe going w store glazes that " fit" my clay right? I usually jump into the deep end but that is usually into a pool or at most a lake, and of course this is an ocean. So I am taking it real slow. Thanks jolie

#7 JBaymore

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Posted 08 December 2013 - 02:32 PM

Test Tiles are an under-appreciated ceramic tool.

 

When you put a lot of time into something, why not also make some test tiles? They'll be bisqued and ready for glaze long before your sculptural piece is dry. You can try out the glazes you want to use on your test tiles and discover problems before you apply the wrong glaze to your art.

 

Norm, you are describing one of my all time "pet peeves" that I see SO often with students. They suddenly decide that it would be a "good idea" to use some new slip, glaze, wash (or whatever) on a lot of pieces for their final critique/show of the semester/year/ college career. Never tested them.... but hey........ let's go for it.

 

Then when it (invariably) screws up... it is not their fault... it was just a "bad firing", or was their kiln partner's fault for firing wrong at one point in the schedule, or something like that.

 

best,

 

......................john


John Baymore
Immediate Past President; Potters Council
Professor of Ceramics; New Hampshire Insitute of Art

http://www.JohnBaymore.com




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