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On 10/30/2019 at 9:01 PM, liambesaw said:

rujbepusyz.jpg

From Tony Hansen's digitalfire site.

https://digitalfire.com/4sight/glossary/glossary_dunting.html

Usually from cooling too fast through quartz or cristabolite inversion, or mismatched glaze coe.  

Nice crack, seems like we need to test actually.
I have an interesting question, when I gas fire, early on, the top and bottom of my kiln can be as much as 300 degrees different. Cristobalite issues happen around 428 degree F and quartz inversion happens at about 1063 degrees F  or in the Hansen model maybe a 50 degree range. Cristobalite, 3%, change, quartz inversion 1-2% change. It seems impossible that my pots would ever survive being  significantly different temperatures from Top   to bottom, inside to out and firing let’s say 300  - 400 degrees per hour.  The gradient in the kiln alone should be way too extreme for pots to survive this, right? I mean different parts of my pots growing / not growing 1-3% while other parts not should be a killer at those firing speeds shouldn’t they?

I should expect really high failure rates, yet I cannot remember a single one actually. What gives?

Do this nearly every firing for more than 100 firings, yet no damage. What gives?

 

Edited by Bill Kielb
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I've seen tall pots, like 24" or more, that were fired 2-3 cones different top to bottom, without any problems. It's a gradient, so the pot can handle it. It's not like there's a line in the middle of the pots where one half was cone 11 and the other half was cone 8. Plus with your average 4-6" tall pot, the difference would be much smaller. I'm not sure how this applies to this thread, though.

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17 hours ago, Babs said:

Ok we need some great photos of definite dunting to compare:-))

And a very close up shot of this flaw.

Maybe the sharpness that is in question would show the thinned glaze is melted but it is the tearing further of the rough clay body on the second firing which attracts our eye

Add to that it is common practice to glaze fire much more rapidly does aggravate the size of the preexisting crack , invisible to our eye at bisquing.

Seems reasonable to. Me and yes in general glaze firings are much faster and materials shrink significantly as they truly melt. Not sure about the picture though so a decent close up would help 100%. I think the sharpness and irregularity of same is probably most indicative. The flaw I believe everyone agrees was there during the  firing. Now just Speculation of how bad it may have been. Micro crack that didn’t fully fuse verses a crack that was more of a crease through most of the firing then failed on cooling. I think this is leading to an indication of failed on cooling if sharp irregular edges  rather than sharp irregular edges indicating it was an in fire fluid occurrence.

never know though, test, test, test I guess.

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1 minute ago, neilestrick said:

I've seen tall pots, like 24" or more, that were fired 2-3 cones different top to bottom, without any problems. It's a gradient, so the pot can handle it. It's not like there's a line in the middle of the pots where one half was cone 11 and the other half was cone 8. Plus with your average 4-6" tall pot, the difference would be much smaller. I'm not sure how this applies to this thread, though.

Just a question pertinent to the quartz inversion mentioned

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11 minutes ago, Bill Kielb said:

Just a question pertinent to the quartz inversion mentioned

Probably because quartz and cristabolite inversion are only an issue in certain clay bodies.  High iron and "translucent" porcelains with excess unfluxed silica.  I linked the article that mentions that.

Pretty rare as far as I'm concerned, I don't mess with trying to find the edge, so I play it safe.  

You can see in the article how he induced dunting on some tiles by piling tiles over part of a tile which allowed the exposed part to cool naturally and the rest of the tile was held hot by the tiles piled on top.  

Edited by liambesaw
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3 hours ago, Bill Kielb said:

Nice crack, seems like we need to test actually.
I have an interesting question, when I gas fire, early on, the top and bottom of my kiln can be as much as 300 degrees different. Cristobalite issues happen around 428 degree F and quartz inversion happens at about 1063 degrees F  or in the Hansen model maybe a 50 degree range. Cristobalite, 3%, change, quartz inversion 1-2% change. It seems impossible that my pots would ever survive being  significantly different temperatures from Top   to bottom, inside to out and firing let’s say 300  - 400 degrees per hour.  The gradient in the kiln alone should be way too extreme for pots to survive this, right? I mean different parts of my pots growing / not growing 1-3% while other parts not should be a killer at those firing speeds shouldn’t they?

I should expect really high failure rates, yet I cannot remember a single one actually. What gives?

Do this nearly every firing for more than 100 firings, yet no damage. What gives?

6EB140E3-36A9-4AB4-AF79-3D30B4A808DC.jpeg

@Bill Kielb, time to start a new thread on forms and amounts of silica, flux and the formation and effect of cristobalite. Let's not make @tomhumf's question more complicated than it is. Thank you!

 

Edited by Min
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I see cracks like this in my students' work on a regular basis. The bowl got flexed when it was too dry, and a tiny crack forms. You can hear it before you see it if you tap the pot. They ask if they can go ahead and glaze it anyway and I tell them it'll just open up but they want to try it and it comes out just like this. Sometimes this happens when we don't see a crack before glazing, but it's more than likely that the crack, or at least the stress, was already there, and has nothing to do with quartz inversion or cristobalite.

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Wow this got a lot more feedback than I expected! Seems the consensus is the firing is unlikely the cause, more probably the manufacture. This batch was some mugs with custom name stamps added on a round of clay. Something new for me.  

I did some re-wetting of the mugs with a spray bottle. This has never been a problem before but maybe I stressed them and caused cracks when they had got too dry.  And stamping the letters may have been too rough. 

I'll just try and work with them wetter and carefully going forward and see if I have any more problems. 

Thanks

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