Strange blow outs during bisque fire

[Hulk]

Interesting!

Griffith, looks like there's a milky translucent white shiny coating on the clay within the defect area.
Oops, that is glazed; the chip blew out in the bisque fire, per your first post.

Is the pot thickest where the defect occurred, and if so, how thick?
The long preheat should eliminate water to steam as the cause, however, depends on how damp it was going in, how hot and long the drying phase.

[Gpw]

Preheated for 12 hours and slow fire digital.   Air dried for several weeks,   I had another  bowl that had the same kind of defect.   The thickness is consistent from bottom through the walls about the thickness of a pencil.   No one who has posted seems to have had this experience.   Just the expected defects from wet bottoms, too fast firing,   The second bowl had almost a perfect circle blow out  but half way between the bottom and rim.    I guess it is a ‘who knows’ just a fluke.    I was hoping for an answer that was more specific to this kind of event so it won’t be a future risk.     All in all thanks to everyone for all the information. 

[Min]

I know you said the pots were dried for several weeks and they were candled but when looking at the images this looks like a blowout from an air pocket that contained moisture within. This isn't the same as an air bubble.

My hunch is that when you were centering and opening up the mass of clay there was some clay pushed down that overlapped the existing clay and formed a pocket. If the clay contains a fair amount of ball clay or other fine particle material and was also heavily ribbed over this would exacerbate the problem of allowing the moisture to escape. Think about a fully enclosed form, yes we can fire them without them exploding or having blowouts but it takes far longer for them to reach a bone dry state right the way through the clay wall than the same form with an opening in it. No clue what the weather and humidity is like in Northwest Arkansas, perhaps it's a contributing factor, don't know.

 

[Hulk]

With the image you might yet get some more information/ideas, please do check back Griffith.

Added: If not trapped moisture flashing to steam ->boom, then some bits/particles off-gassing whose escape route is progressively closed off as the clay sinters?

Curious, looks like a large flake, and given the uniform  thickness of a pencil, also curious how a large piece has come off and no damage to the interior? ...almost like there was a layer, like a pastry, or crust, that flakes as it bakes.

Edited January 1 by Hulk
minor edit

[Bill Kielb]

Only time I have seen unique circular and spiral failures were from the trimming and burnishing at the green stage, which in those cases very excessive. Was hoping your picture would reveal the actual crack / blowout.  Sorry, that’s all I have at the moment. Anything special to add about the throwing trimming process?
 

“Pictures definitely key here as I have seen circular failures and delaminating especially in bowls where the interiors were overworked in the green stage. Pictures and a little about any special throwing, trimming, ribbing, burnishing etc….  could help narrow this down.”

[Bill Kielb]

2 hours ago, Griffithpotteryworks said:

Preheated for 12 hours and slow fire digital.   Air dried for several weeks,   I had another  bowl that had the same kind of defect.   The thickness is consistent from b

Any reason you preheat this for so long?  Is this your normal practice?

[Gpw]

These were with some other very large forms and I was trying to be over cautious.     

[Gpw]

Thanks.    I will take more care to avoid pockets when adding more clay.   Happy throwing to you all during this new year. 

[Babs]

Is it a damp area? Loiks like the outside profile and inside profile neans a pretty thick area which if any dampness lurked inside the pot wall at that point could cause a blow out if it was close to an element, flame? 

[Kelly in AK]

Thank you for the photos. I believe you made the pots too thick. The outside profile vs. the inside shape shows it, to me, clearly. If the bowl is in fact 20” wide, the foot must be well over an inch thick in some areas. 

There are many variables (relative humidity, clay body, temperature of candle, etc.), an eight hour candle ought to have taken care of most of them, but honestly it’s just too thick and blew up. Water did that. 

[Min]

@Kelly in AK +1 about inside and outside profile and thickness.

@Griffithpotteryworks, stick a couple thumbtacks into the pot from the inside at the midway point plus near the bottom of the side wall then trim down until you reach them. Remove the tacks then smooth over the holes they made. It's a good way to get a feel for how much clay can be taken off. Tap the pots as you trim them so you can hear how the clay sounds as it gets thinner then after a few pots you won't need the tacks.

Edited January 1 by Min

[Hulk]

Caliper tool that indicates thickness at the other end (double ended calipers*), it's a something I wanted/needed for some time now, for I have poor judgment of thickness past the reach of thumb and first two fingers.

That middle one looks handy for wares the left one doesn't fit.
I like the thumbtack idea Min!
 

*There are many options "out there" - try "double ended calipers" also turners/turning/bowl/bowl turners caliper(s)

[neilestrick]

Looks like typical thickness blowouts to me. I've seen them in that location before. I'm surprised that the preheat didn't prevent it, but that could be related to the location in the kiln, stacking, and how quickly the firing ramped after the preheat.

[Pres]

Looking at your pictures I agree with others that the form was thrown too thick, and needed more aggressive thinning to make up for the difference.  I Love throwing large bowls, and other forms for the kitchen, and these require a major amount of wedging of the clay, Mastering the clay on the wheel(coning up and down), and aggressive pressure from the base up into the cylinder walls. Throwing a bowl starts with a rounded interior when opening up, and most of the pressure during pulling is from the outside with more of a flat fingers on the interior keeping the curve of the base into the cylinder then when sufficient pulls have thinned the base the walls are shaped from the inside. My favorite tool for this is a large wooden spoon with the handle cut off and the edges rounded out where the handle had been attached. Whether a dish(flat interior bottom) or bowl (rounded interior bottom) the trimming is important to make certain there are no areas in the base that are thicker than normal fo any thrown piece.

All this IMHO, meant to be helpful, not critical.

best,

Pres

[PeterH]

Re: memory in clay, just remembered this example.

Control of desiccation crack pattern using memory effect of clay paste
https://www.gruppofrattura.it/ocs/index.php/esis/ECF18/paper/viewFile/6064/1949

[Mark C.]

The pot looks really thick where the blowout occurred. That to me means it was to damp for your firing cycle (to fast ). Moe  dry time and  less thick pot (more trimming)