Strange blow outs during bisque fire

[Gpw]

Recently I have had two strange blow outs during a bisque firing.  These pieces were large wheel thrown bowls ( approx. 20 wide and 9 tall) fired at cone 04 digital with a preheat of 8 hours.   The "defect" was a circular blow out of about 2 inches in diameter and pretty much a perfect circle.   The blowout did not go completely into the piece.  There were no other signs of cracks or defects.    Since this defect has occured twice, I need to find out what I am doing incorrectly so that I can prevent reoccurance.  Thanks.

Edited December 31, 2023 by Griffithpotteryworks
Adding photo

[Min]

Hi and welcome to the forum!

Could you post a picture of the blowouts? Also, where in the kiln were the large bowls were in relation to spyholes?

[Babs]

Are you wedging on plaster?, big grog in your clay?

[Bill Kielb]

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.

[Jeff Longtin]

A common culprit for these kinds of problems is too much water and/or not enough compression. 

Do you use lots of water during the throwing process? If a pool of water sits in the bottom of the form, as you throw, this can lead to cracking/blowout problems.

Compressing the clay, with a rubber kidney tool, is a good way to strengthen the bottom of the form.

As Bill suggests overworking a form can also stress the clay. Using a kidney tool shaped like your desired shape can reduce the amount of time your fingers are actually working the clay.

[Harold Roberts]

Just saw this post and I would add that excess water could well be the problem. I glaze all but my very large pots green, that means I don't bisque them. I use a clay body that has no grog in it and no fireclay. Porcelains and slipcast pots work well in once fired pottery because the are very tight and don't absorb water quickly. I have to be very careful when I apply my glazes because I can get the blow outs you describe.

[Hulk]

Jeff, Bill,

Curious what "overwork" means, exactly?
I'm not recalling hearing/reading what is overwork in general, nor overwork specific to throwing forms.
Is it where the clay has been pushed to the point of shearing?

I'd observed, early on, that clay doesn't seem to recover well from being pushed to the point of shearing - best to remove, rewedge (thoroughly), then start over, which I'd classed with the more common problem, a) bubble, blob, dry or wet spot - any inconsistency - can cause the clay to flow unevenly, and somewhat less common b) where the clay moves on the bat, as problems that regularly occur on my wheel and on the others around me (Wheel I and Wheel II classes), but not discussed much.
...my thought was, why not experience these on purpose and thereby learn to recognize?
Push until the clay either shears or disengages from the bat; repeat until both are clearly achieved.
Put a few BBs (small, round, smooth - no sense getting hurt) in the clay, throw; try again with a soft bit of clay folded in; try again with a stiff bit folded in.
...ok, now don't do that/them.

Edited December 27, 2023 by Hulk
define

[Bill Kielb]

5 hours ago, Hulk said:

Curious what "overwork" means, exactly?

my reference was for greenware.
In my experience I have seen interesting/ unique  spiral cracks occur where excess working / burnishing has taken place. My recollection is it was someone who wanted the perfect bowl interior and so would trim and heavily burnish the inside of the greenware. It looked great but the difference in density plus the propensity to cause -for lack of better words - local de-lamination made for some interesting circular or spiral failures upon firing. Saw this same failure at the bottom of several cups - same technique, almost too much  trimming and aggressive burnishing to make it look super smooth. When fired and failed the failures appeared to be spiral or follow the circular path of the work that had been done pictures here might help a lot though.

As far as density, compression …… I find clay most logical if I treat it like other material properties we accept. Compression is a real thing, real compression though. Clay memory to me is not a mysterious thing. If Crease a piece of paper, one side is stretched and the other compressed. If I stretch one side of something while throwing I have to work really hard to get everything back to the same uniform density and thickness otherwise it will return in the fired ware. Not really memory to me.

Edited December 27, 2023 by Bill Kielb

[Jeff Longtin]

Hulk/Tom,

When people throw pots they generally lubricate the clay with water. (So the clay slides between their fingers.) The water will often collect at the bottom of the form.

If the thrower repeatedly tries to "improve" the form they may leave a lot of water at the base of the piece. If they don't keep this accumulated water to a minimum the clay may become over saturated and separate once the piece dries/is fired.

An easy way to rule this out is to throw the form quickly and keep the water to a minimum. If this type of cracking persists it may be a clay formula problem.

Rather than try to throw the perfect shape, and over work the clay, throw a pot as best you can, in a shorter amount of time, and then trim it to your desired shape. 

 

[Hulk]

Thanks Bill, thanks Jeff.
I interpreted overwork as forcing the clay enough past its limit of elasticity, which I'd called shear or partial shear.
For Bill, excessive trimming/burnishing.
For Jeff, extended throwing time, where many passes, wet the whole time, leads to over-wet issues.

From my pov, each a legit concern!
In my experience, some clays tolerate more wet time than others. I do like to rib away the slip on the outside of forms, and sponge away most of the slip/wet from the insides as well. "Wetting" the clay from the outside is arrested soon as the second pass with the metal rib is complete, which is part of my process now - when wetting ceases, drying begins! A form that may not be prime for flaring out - too wet - can be ready not long after said ribbing.

Clay responds to pressure, pressure and movement, imo.
Where the clay is damp enough to move, trimming and ribbing/burnishing does move the clay, agreed.
The clay moves, round and around. Burnishing definitely involves movement; smearing might be a good word, but it doesn't sound particularly good.
Where the smear begins to separate later on, ah!
I don't see where clay volume is reduced by applying pressure, nor where moisture can be squeezed out by applying pressure to clay on the wheel (in a clay filter press, well, that's different, eh?), so the term "compression" I still don't like, but am mostly over it, heh. 

[Bill Kielb]

21 hours ago, Hulk said:

I don't see where clay volume is reduced by applying pressure, nor where moisture can be squeezed out by applying pressure to clay on the wheel (in a clay filter press, well, that's different, eh?), so the term "compression" I still don't like, but am mostly over it, heh. 

It definitely will reduces in volume and dewater as well as force fines into open areas, just happens to be tiny. Maybe “not stretched” is a better thought but that does not address the mistake we see when folks push down so hard the clay begins to squirt out an unconstrained edge. I think I attempted to confirm this with a home made penetrometer btw - not a pretty sight though. 

Edited December 29, 2023 by Bill Kielb

[Jeff Longtin]

Compression - it's not that your compressing/squeezing the clay as much as you're ALIGNING the clay particles. (That's the benefit of a rubber kidney tool.)

In the unused state clay particles are randomly aligned. (Or so I'm told.) Wedging helps to align the particles a bit. Throwing aligns the particles some more, and using a rib tool aligns the particles even further. The act of aligning helps to reduce the chances of cracking. 

Circular cracks indicate that the clay is shrinking in circular directions. If you rib a form, before you pull it from the wheel, you align the particles in a horizontal direction and counter the circular direction the particles want to follow. (If that makes sense?)

Too much water, in the throwing process, contributes to more shrinking. Less water = less shrinking.

(This explanation may have a few hits and misses. I know what works from a usage point of view...the technical explanation has been learned over the years.)

The other aspect, about bowls, that contribute to cracking, is irregular clay wall thickness. As much as possible its best to trim a bowl such that the outside profile matches the inside profile. (That way the clay shrinks evenly.)

 

[Hulk]

Unstretched, de-stretched, aligned, reprogrammed, I like all of them!

I like this also:

"The compression is not to dewater or make the clay more dense. It is to alter the residual stress state of the clay. To reprogram its memory, so to speak."

- Gavin Stairs (on potters.org)

Richard Burkett's shrinkage test, same thread, demonstrates the (dramatic) difference "compression" makes - overall shrinkage 15% for stretched slab, 11% for compressed slab, 12% for "normal" slab - big difference.

If pushing on the clay - against the wheel head as it revolves, but not so much that the thickness is substantially changed* - alters the density by squeezing out water, I should be able to easily squeeze water out of clay?
How? I haven't any luck with that, but I haven't tried applying progressive test to discover the force required to squeeze water from clays quickly, as in the amount of time one spends compressing the base of a wheel thrown ware...
 

*where a ridge of clay moved one way is then reversed.

[Bill Kielb]

8 hours ago, Hulk said:

The compression is not to dewater or make the clay more dense. It is to alter the residual stress state of the clay. To reprogram its memory, so to speak."

I guess to each their own. I think lots of explanations sound complex, most materials don’t really care though. I find treating clay just like we do in construction, the basic properties of materials, has greatest clarity and predictability of outcome.

Residual stress sounds good but does not have any meaning to me, it does not add any clarity.  When I compress am I adding stress? Residual? What is residual?

Memory is too abstract for me as well, one side is stretched, the other compressed. Fold a piece of paper, see why there is a crease there and why it’s difficult to completely remove the crease without altering the local properties of the material to match the surrounding material.  Clay has plastic limits like most other material, exceed them and the results are predictable. -  to me, memory is a cool thought but again does not really add clarity to what is happening. 

Two semesters of hydraulics and hydrology help but definitely not the end all for clay. Still has worked well for me constructing roads, sidewalks earthen dams, sub grades, buildings …. Sneaking up on half century now.

[PeterH]

11 hours ago, Bill Kielb said:

I guess to each their own. I think lots of explanations sound complex, most materials don’t really care though. I find treating clay just like we do in construction, the basic properties of materials, has greatest clarity and predictability of outcome.

Residual stress sounds good but does not have any meaning to me, it does not add any clarity.  When I compress am I adding stress? Residual? What is residual?

Memory is too abstract for me as well, one side is stretched, the other compressed. Fold a piece of paper, see why there is a crease there and why it’s difficult to completely remove the crease without altering the local properties of the material to match the surrounding material.  Clay has plastic limits like most other material, exceed them and the results are predictable. -  to me, memory is a cool thought but again does not really add clarity to what is happening. 

Two semesters of hydraulics and hydrology help but definitely not the end all for clay. Still has worked well for me constructing roads, sidewalks earthen dams, sub grades, buildings …. Sneaking up on half century now.

Just my take on this.

Certainly as a term "memory" has its problems, not least the implication of some sort of consciousness.

However "local platelet alignment" is a real property, that can cause the previous history of the clay to influence its future behaviour. An effect that sometimes has practical significance.

Techno File: Clay Memory
https://ceramicartsnetwork.org/ceramics-monthly/ceramics-monthly-article/Techno-File-Clay-Memory#

Crack H in Hamer & Hamer
http://ceramicsfieldguide.org/pdf/materials-handouts/ClayCracks.pdf

PS

"Squeezing out water" may sometimes relate to changing platelet alignment so there is less inter-platelet  space to hold residual water.

A similar effect occurs during hard-panning, but in this case caused by excessive deflocculation rather than physical manipulation. 

 

Edited December 30, 2023 by PeterH

[Bill Kielb]

On 12/30/2023 at 6:45 AM, PeterH said:

However "local platelet alignment" is a real property, that can cause the previous history of the clay to influence its future behaviour. An effect that sometimes has practical significance.

I certainly would agree local platelet alignment could affect something but it’s a bit too mysterious and implies unrepairable for me. How to align them without redistributing smaller particles as infill also something of a mystery.

I always wondered why the Hammer and Hammer drawing did not have platelets aligned around the 90 degree corners in the direction it was bent?  Anyway, simple in my mind is better than what could or might be possible or could influence to happen onward to infinity.  If I bend something it will crease, the material has changed locally into some realignment  to some extent (The outside of the bend has stretched and the inside has compressed and or buckled).  I am not sure the material remembers who or why it was bent, just the effect of the extent dependent on the properties of materials. That its current structure is a reflection of its prior handling and its properties I guess could be termed memory. Memory implies there is no path to normalizing things again it may just reappear because it remembers. IMO I don’t prefer the term in clay it does not clarify how one should manage the material to avoid the “memory” so for me it is ok, but in my experience can add mystery to handling and shaping clay. I think more useful is how to avoid, what to avoid and what work might really be required to fix the material distortion. 

On the other hand, Memory for me is effectively descriptive and helpful in heavily cross linked plastics such as pex piping which will tend to return to original shape even after severe distortion. Go figure, maybe just me.

Edited December 31, 2023 by Bill Kielb

[neilestrick]

Lots of great discussion here, but we still don't know what the blowouts look like, where they were on the piece, etc. OP has abandoned us. Personally, I've never considered compression to be the cause for blowouts. They are typically caused by foreign bodies in the clay or moisture. I don't see why compression, platelets alignment, etc, would cause a section to violently separate from the piece.

[Gpw]

Are your blow outs on the sides of your work?    These are not in the bottom like those that happen when water is left sitting in the bottom.  Thanks 

 

working on loading pics.   Blowout was on the side only, not a spiral or blow out on the bottom.  Been throwing for 35 years and never seen this before.   Thanks 

[Min]

39 minutes ago, Griffithpotteryworks said:

working on loading pics

If you are having problems posting pictures try emailing them to yourself which will automatically reduce their size then try posting those ones.

[Gpw]

Are your blow outs on the sides of your work?    These are not in the bottom like those that happen when water is left sitting in the bottom.  Thanks 

[Gpw]

Are your blow outs on the sides of your work?    These are not in the bottom like those that happen when water is left sitting in the bottom.  Thanks 

[Gpw]

This is on the side.    Specific details are in the very first post.   Thanks.  

[Min]

On 12/4/2023 at 8:57 AM, Griffithpotteryworks said:

two strange blow outs during a bisque firing

Thanks for posting the photo but I'm a bit confused as to what I'm looking at. It looks like there is glaze on the circular area?

Could you take  another photo that shows the entire pot? Was there any relationship between where in the kiln the bowls were in regards to the spyhole(s)? Spyhole(s0 left open during firing and cooling? New clay, recycled clay?

[Gpw]

No relation to spy hole.  Yes it is glazed.   

[Gpw]

No relation to spy hole.  Yes it is glazed.   It was glazed after the blow out.