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Hello All!

I've been trying to make cake plates for a while in my new studio, they just keep breaking in the kiln and I can't figure out why.  I've attached a photo from my sneak peak earlier,  I dried it super slow and there were no signs of stress after bisque firing. My pottery dictionary doesn't seem to match this type of crack, so I thought I'd ask you wise people.

Thanks in advance!

 

Sarah

 

cakeplate.jpg

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Looks like a dunt to me. If the broken edges have sharp glaze cutoff then the cracking happened on the cool down, if the glaze rounds over the broken edge then it happened on the heating up. It looks like an uneven cross section but it's hard to tell what is the plate part and what is part of the pedestal. What's your firing (and downfiring) schedule? 

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Hi Sarah:

i would be interested to know your bisq firing schedule?  Given the size and weight of these pieces: quartz inversion comes to mind. Stress fissures can occur in the bisq firing in the quartz inversion range (563c/ 1064f +/-) that may not appear until the added contraction of glaze makes the visible.  If you have any bisq plates left; plunk the edges with your fingernail or softly with a metal tool. ( softly). If they ping they have fissures, if they pong ( lower tone) they are solid.

another issue concerns me: why am I seeing a ridge line in the center of body?  Is that the photo angle, light reflection? Or am I seeing a distinct ridge in the clay body? I am also seeing discoloration?

T

 

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It does look like a dunted piece.How thick is it ???It looks like 3/4 of an inch? and whats the bottom foot look like??Photo of bottom please

need to see and know more to determine cause .

Is this a cone 6 fire as well?.

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7 hours ago, LeeU said:

Are these cake stands? Looks rather thick--is there a pedestal under there? 

Yes, there's a pedestal underneath, its seems to have peeled off a bit of the pedestal with it so it looks thicker, it's 1/3 inch thick by the pedestal

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7 hours ago, Min said:

Looks like a dunt to me. If the broken edges have sharp glaze cutoff then the cracking happened on the cool down, if the glaze rounds over the broken edge then it happened on the heating up. It looks like an uneven cross section but it's hard to tell what is the plate part and what is part of the pedestal. What's your firing (and downfiring) schedule? 

I think it happened in cool down in that case, it also came off the shelf quite easily. It has peeled off a bit of the pedestal with it so it looks thicker. Firing was to 1260 with 30 min soak, I don't have a downfiring schedule should I?? I just leave it be until it's cool enough which usually takes 24hrs

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6 hours ago, glazenerd said:

Hi Sarah:

i would be interested to know your bisq firing schedule?  Given the size and weight of these pieces: quartz inversion comes to mind. Stress fissures can occur in the bisq firing in the quartz inversion range (563c/ 1064f +/-) that may not appear until the added contraction of glaze makes the visible.  If you have any bisq plates left; plunk the edges with your fingernail or softly with a metal tool. ( softly). If they ping they have fissures, if they pong ( lower tone) they are solid.

another issue concerns me: why am I seeing a ridge line in the center of body?  Is that the photo angle, light reflection? Or am I seeing a distinct ridge in the clay body? I am also seeing discoloration?

T

 

Bisque firing is 6 hrs to 600°c then fp to 900 with 20 min soak (this was preprogrammed on my kiln) that's a great ping pong tip thank you, I dont have any bisque ones atm but will try it on the next batch. As for the other concerns I can't  see any discolouration or a ridge, maybe it's the photo? I used flecked clay maybe its that? Or my amateur eye

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3 hours ago, Mark C. said:

It does look like a dunted piece.How thick is it ???It looks like 3/4 of an inch? and whats the bottom foot look like??Photo of bottom please

need to see and know more to determine cause .

Is this a cone 6 fire as well?.

Its 1/3 inch thick where it meets the pedestal it looks thicker where its peeled from the pedestal I think. Fired to cone 8

20181206_103713.jpg

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Sarah:

" 6 hrs to 600C". I will assume the rate climb is 100C an hour?  If so, that translates to 212F an hour climb by USA measure. Quartz inversion occurs at 563C: at which the clay body goes through its first real change. Molecular water is leaving the clay at the same time silica is expanding. In essence, all the materials that compose a clay body is shrinking, except the silica which is expanding at the same time. Go through this critical temperature too fast, it the stress creates fissures. Go through it way too fast, and it will split the piece entirely.

most pieces do just fine: mugs, bowls, etc are small enough to ramp faster with no effect. Larger, heavier, pieces with large foot rings require a slower speed at inversion. Given the size, weight, and how it rests on the foot ring puts inversion on the top of my list. Typically 50C an hour just prior to 563C, to just past this temp does the job. In addition, given your foot ring style: putting sand, waster slab, or other means to allow the foot ring to move without surface drag on the kiln shelf would help-a lot.

Sometimes stress cracks that occur from quartz inversion are visible after bisq firing, sometimes not. If not, the contraction of glaze makes them very apparent. There is a "quartz inversion" thread somewhere around here. Do a search.

T

 

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The way the crack is angled looks like a thickness issue to me. Of course, that can be related to firing speed. Thicker pots need slower firings. Plus, with a piece like this, the hanging edge is going to heat a lot faster than the center where it's connected to the pedestal, so you've got to fire slower to keep the thermal expansion more even.

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Another thing that might be helpful is to surround the rim of the piece evenly with other, smaller items like small cups or test tiles, or even kiln posts.This will help create a heat sink around the rim, which can even the temperature out and help the piece cool more slowly. 

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10 hours ago, glazenerd said:

Sarah:

" 6 hrs to 600C". I will assume the rate climb is 100C an hour?  If so, that translates to 212F an hour climb by USA measure. Quartz inversion occurs at 563C: at which the clay body goes through its first real change. Molecular water is leaving the clay at the same time silica is expanding. In essence, all the materials that compose a clay body is shrinking, except the silica which is expanding at the same time. Go through this critical temperature too fast, it the stress creates fissures. Go through it way too fast, and it will split the piece entirely.

most pieces do just fine: mugs, bowls, etc are small enough to ramp faster with no effect. Larger, heavier, pieces with large foot rings require a slower speed at inversion. Given the size, weight, and how it rests on the foot ring puts inversion on the top of my list. Typically 50C an hour just prior to 563C, to just past this temp does the job. In addition, given your foot ring style: putting sand, waster slab, or other means to allow the foot ring to move without surface drag on the kiln shelf would help-a lot.

Sometimes stress cracks that occur from quartz inversion are visible after bisq firing, sometimes not. If not, the contraction of glaze makes them very apparent. There is a "quartz inversion" thread somewhere around here. Do a search.

T

 

Amazing thank you so much! Will try all of the above 

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5 hours ago, Callie Beller Diesel said:

Another thing that might be helpful is to surround the rim of the piece evenly with other, smaller items like small cups or test tiles, or even kiln posts.This will help create a heat sink around the rim, which can even the temperature out and help the piece cool more slowly. 

Ooh interesting thanks will def try

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