Porcelain pieces cracking in kiln during glaze firing

[AnnK]

Hi!  This is my first time posting here and I'm feeling a bit frantic .. hoping someone can help me solve this problem.  I have a tiny Skutt 8x4 LT ceramic kiln.  I'm having an issue with my Laguna ^5 Porcelain cracking during the glaze firing.  Here is a video of where the cracks are occurring.  *EDIT* Here are several pieces, from 2 different firings that have cracked.   I did commit a mortal ceramic sin & peeked into the little kiln when it was at about 330°F (which I'm embarrassed to admit is a bad habit of mine, but maybe this time I pushed it too far).  I've never had these cracks in my stoneware pieces and I know porcelain and ball clay are more prone to crack from thermal shock.  

 

 

[AnnK]

I have some other photos of more pieces that cracked during this most recent firing.  I will upload them shortly.  I'm pretty sure it's due to thermal shock.  

[Callie Beller Diesel]

Because they keep happening in the exact same place, I’m inclined to think its a construction flaw creating an initial weakness. Are these thrown or slipcast or? 
 Because the cracks look rough, that does mean the cracks are happening in the cooling part of the cycle, which means it’s cooling off too fast around quartz inversion. That’s 1063F. Small kilns do heat and cool quickly, and sometimes you need to program some ramps in to slow that down. If it’s a manual kiln, you can turn it back on for 20 minutes or so at the critical spots. 

[Mark C.]

I think its how thay are made -as Callie said how are they made?.I think she coverd it in above pots.

also stop peeking and let them cool slow.I do not thinks they are cracking from your peeking-they are cracking when hotter as they cool fast.

[AnnK]

1 hour ago, Callie Beller Diesel said:

Because they keep happening in the exact same place, I’m inclined to think its a construction flaw creating an initial weakness. Are these thrown or slipcast or? 
 Because the cracks look rough, that does mean the cracks are happening in the cooling part of the cycle, which means it’s cooling off too fast around quartz inversion. That’s 1063F. Small kilns do heat and cool quickly, and sometimes you need to program some ramps in to slow that down. If it’s a manual kiln, you can turn it back on for 20 minutes or so at the critical spots. 

They are thrown.  I've never had this issue with these pieces when I was using stoneware... only when I switched to porcelain did these cracks begin to appear.   The kiln is not manual, so I would need to program it as you described, which I've only done once before.  

[AnnK]

1 hour ago, Mark C. said:

I think its how thay are made -as Callie said how are they made?.I think she coverd it in above pots.

also stop peeking and let them cool slow.I do not thinks they are cracking from your peeking-they are cracking when hotter as they cool fast.

They are thrown.  Just frustrating... I feel so defeated.  

[Min]

Break some open and look at the cross section. Are they thrown as one piece and trimmed or are you joining pieces?

Frustrating....understandable but it comes with the territory. It’s solveable.

[AnnK]

1 minute ago, Min said:

Break some open and look at the cross section. Are they thrown as one piece and trimmed or are you joining pieces?

Frustrating....understandable but it comes with the territory. It’s solveable.

They are thrown as one piece and trimmed.  I'll try to crack one open and see what it looks like.  

[Min]

It could just be that they are trimmed too thinly where they are cracking, micro crack from uneven drying that opens up during firing.

[AnnK]

1 minute ago, Min said:

It could just be that they are trimmed too thinly where they are cracking, micro crack from uneven drying that opens up during firing.

They are solid... I know they are thick but I make sure to let them completely dry out before firing and also use a preheat.  

[AnnK]

 

 

Also, on the bottom, I use a little drill tool to carve out just a bit of clay to help with drying since they are solid.  

 

[neilestrick]

@AnnBollin Porcelain can't handle being that thick. And I'm surprised that the stoneware pieces are surviving at all. clay really shouldn't be thicker than maybe 3/8" in any form, unless you're doing large scale sculpture. Is there any specific reason you're making them solid? If they were hollow you could use 1/4 as much clay, dry them faster, save a lot of energy firing them because of the lower mass, and not have to worry about them cracking or blowing up. I think you've just gotten lucky so far.

[Callie Beller Diesel]

Throw them like you do your stash boxes in that third video, but on a smaller scale. You can still do them off the hump. And maybe use the paintbrush handle to make the grooves instead of that super sharp wooden knife. Or consider rounding the tip on the wooden knife. I think the sharp edge of that tool is doing something weird compression wise. 

[AnnK]

8 minutes ago, neilestrick said:

@AnnBollin Porcelain can't handle being that thick. And I'm surprised that the stoneware pieces are surviving at all. clay really shouldn't be thicker than maybe 3/8" in any form, unless you're doing large scale sculpture. Is there any specific reason you're making them solid? If they were hollow you could use 1/4 as much clay, dry them faster, save a lot of energy firing them because of the lower mass, and not have to worry about them cracking or blowing up. I think you've just gotten lucky so far.

I prefer the weight of them as solid pieces.  I must've been really lucky ... I've made a lot of these and never had any blow up or crack until switching to porcelain.  I guess I need to just stop pushing the limit and play by the rules.  Haha

[AnnK]

7 minutes ago, Callie Beller Diesel said:

Throw them like you do your stash boxes in that third video, but on a smaller scale. You can still do them off the hump. And maybe use the paintbrush handle to make the grooves instead of that super sharp wooden knife. Or consider rounding the tip on the wooden knife. I think the sharp edge of that tool is doing something weird compression wise. 

Thank you!  I'll give that a try.  I really appreciate you taking the time to help me solve this problem!

[Mark C.]

The solid nature is whats the issue. You have been really lucky that you had any survive. Porcelain that thick cracks and your did as well. Hollow them out more-fill with a heavy weight if you need that later. No amount of firing tweaking will fix this -they are just to thick.

use a larger drill bit to ream them out

[Bill Kielb]

9 hours ago, AnnBollin said:

They are thrown as one piece and trimmed.  I'll try to crack one open and see what it looks like.  

Might be a nice time to learn throwing as a closed form. With trapped air you could virtually eliminate all trimming and get your  near final shape when throwing. When dry enough cut a small hole in the bottom, push the bottom in ever so slightly and ya got a straight flat surface every time.

[AnnK]

22 minutes ago, Bill Kielb said:

Might be a nice time to learn throwing as a closed form. With trapped air you could virtually eliminate all trimming and get your  near final shape when throwing. When dry enough cut a small hole in the bottom, push the bottom in ever so slightly and ya got a straight flat surface every time.

Thanks.  I actually do make a different variation of these that is a lidded pot (thrown as a closed form).