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Crack Question


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#1 capt don

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Posted 03 August 2013 - 11:01 AM

Everyone was so helpful with my last question so maybe I can get some help with this problem. This bowl that cracked was a cone 5 clay bisque fired to cone 04. It was dried for a couple weeks and was bone dry. The bottom is probably 1/4 inch thick with a coil rim around the base and the sides taper up from that thickness at the bottom to very thin at the top. Any idea what I did wrong?

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#2 Benzine

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Posted 03 August 2013 - 02:06 PM

Hmmm, it could just be to variations in thickness.  Did you let the kiln cool completely before unloading?  I've had things get stress fractures, from not doing so before.


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#3 Chris Campbell

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Posted 03 August 2013 - 02:12 PM

It could also be a small hairline crack you did not even notice before firing ... you might have slightly bumped it at anytime, looked and not seen anything and kept on. The form looks OK and the thickness seems right.

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#4 oldlady

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Posted 03 August 2013 - 04:25 PM

did you at any time try to pick it up without supporting the bottom first?  lifting it one handed by the rim could have done this.


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#5 capt don

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Posted 05 August 2013 - 08:59 AM

Thanks for your help everyone. About 2 hours after the kiln cut off, I pulled the side inspection plugs in the kiln but didn't open the top till morning about 10 hours after it cut off. Maybe it cracked on the side near the plugs.



#6 OffCenter

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Posted 05 August 2013 - 10:23 AM

Thanks for your help everyone. About 2 hours after the kiln cut off, I pulled the side inspection plugs in the kiln but didn't open the top till morning about 10 hours after it cut off. Maybe it cracked on the side near the plugs.

 

Probably not.


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#7 Benzine

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Posted 05 August 2013 - 10:52 AM

 

Thanks for your help everyone. About 2 hours after the kiln cut off, I pulled the side inspection plugs in the kiln but didn't open the top till morning about 10 hours after it cut off. Maybe it cracked on the side near the plugs.

 

Probably not.

 

Yeah, I would say the same.

 

When I've had cracks like that, it was because I cooled the entire object too quickly, and was probably even foolish enough to take it out of the kiln, and set it on a much cooler surface......I'm like a little kid on Christmas, when it comes to peeking at my work.


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#8 mregecko

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Posted 05 August 2013 - 11:41 AM

In my experience, cracks like that happen from cooling issues too. When I do thin raku platters, built in a similar fashion, they crack radially going outwards after being removed from the kiln.

 

I know some people elevate flat pieces like this in the kiln so that they have even airflow all around, and thus even cooling. 

 

So yeah, I'd guess the problem was in the cooling as well.



#9 Mark C.

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Posted 07 August 2013 - 12:10 AM

This piece looks thinner than a potato chip on the edge. That makes it very easy to crack just as a real life potato chip. Add this to a thickrt bottom and handling in and out of kiln-also was it a trimmed chip as this also make it easy to crack it. Next time treat it very carfully as any mistreatment will crack that edge.

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