Why is my clay bloating?

[LindsayM]

Hello, I wonder if you can help me.  I'm having an issue with quite a number of my pots bloating when they are fired and I'm not sure if it's the clay, the glaze, or the firing schedule.

The bloating first happened a couple of months ago.  I have a newish kiln.  It's a Skutt KMT818.  It's more efficient than my last kiln which was old and took ages to reach cone 6. 

I've noticed when I use cones in the kiln that the pre-programmed cone 6 temperature (cone fire mode) actually gets to cone 7.  So I wondered if the clay was just overfired.  

I mainly use Scarva Earthstone ES10 Extra Smooth.  And it can be fired up to 1280C, so it shouldn't be overfired at cone 7 should it?

The other thought that I had is that I reclaim my clay.  Is it possible that the clay is bloating because it has been reclaimed and doesn't contain quite the right balance of fine and bigger particles?

My pottery supplier suggested doing a slow cool.  So I tried that yesterday.  I manually set the kiln for 1222C.  Did a 5-minute hold at target temp, and then  I ramped down at 65C an hour to 800C.  All the extra heatwork from the slow cool meant the cone 7 cones had fallen when I opened the kiln.  There was less bloating in this kiln load, but still a few pieces had bloated. 

Does anyone have any helpful advice?  Very many thanks   

[Bill Kielb]

5 hours ago, LindsayM said:

Does anyone have any helpful advice?  Very many thanks   

This looks like one of those miracle clays but is really a cone 9/10 clay to get full verification. So it ought not bloat at cone 6/7. Can you post a picture or pictures of the defects you are seeing?

[Hulk]

Hi Lindsay, welcome to the Forum!

Slowing your bisque - allowing more time to burn out impurities may help.
Choosing clay(s) that mature at your target temperature, I'll recommend that!

Are you firing your wares twice - bisque fire, then hotter glaze fire? 
...or are you single firing?

Either way, faster ramp up to target temp means less time to burn out impurities; your prior kiln allowed much more time.
Slowing your bisque fire, particularly around 1500-1800°F might help.
Adequate oxygen may also be important*.
If the clay hasn't changed, and you're firing to the same target temperature (verified by cones), the firing schedule is a likely culprit.
I'm seeing much less defects** since adding significant delays (on the ramp up and the ramp down) to my bisque schedule.

Bloating (digitalfire.com)
* Note the last sentence here: Kiln venting system (digitalfire.com)

**bloating in dark clays, particularly where the walls are smidge thicker;
"large particle defect" - random craters;
pinholes
 

[neilestrick]

As for cone 7 going down, the hold at the peak is going to add a little bit of heatwork, as is doing a slow cool from the peak. If you don't need the hold at the top for glaze reasons then don't do it. If you want to do a slow cool for crystal development in your glazes, drop to 1050C at full speed, then do the slow cool. It would be a good idea to do a firing with just a few shelves in the kiln and cones at two levels, Medium speed to cone 6, and see what the cones show. From there you can do a thermocouple offset or a cone offset to dial it in so it's firing accurately. After that you can deal with the glaze issue.

[Min]

9 hours ago, LindsayM said:

Scarva Earthstone ES10 Extra Smooth.

Given this is a smooth white claybody that fires up to 1280C it could very well be the reclaimed clay just wasn't wedged properly. With pyroplastic claybodies if you don't get all the air bubbles out the clay what can happen during glaze firings is the air pockets expand and cause a bloat (sometimes called a bleb). Are you getting the problem more often with the recycled clay? 

[LindsayM]

On 5/18/2024 at 12:07 PM, Bill Kielb said:

This looks like one of those miracle clays but is really a cone 9/10 clay to get full verification. So it ought not bloat at cone 6/7. Can you post a picture or pictures of the defects you are seeing?

Thanks for your reply, here is a picture.  

Edited May 28 by LindsayM

[LindsayM]

On 5/18/2024 at 3:21 PM, Hulk said:

Hi Lindsay, welcome to the Forum!

Slowing your bisque - allowing more time to burn out impurities may help.
Choosing clay(s) that mature at your target temperature, I'll recommend that!

Are you firing your wares twice - bisque fire, then hotter glaze fire? 
...or are you single firing?

Either way, faster ramp up to target temp means less time to burn out impurities; your prior kiln allowed much more time.
Slowing your bisque fire, particularly around 1500-1800°F might help.
Adequate oxygen may also be important*.
If the clay hasn't changed, and you're firing to the same target temperature (verified by cones), the firing schedule is a likely culprit.
I'm seeing much less defects** since adding significant delays (on the ramp up and the ramp down) to my bisque schedule.

Bloating (digitalfire.com)
* Note the last sentence here: Kiln venting system (digitalfire.com)

**bloating in dark clays, particularly where the walls are smidge thicker;
"large particle defect" - random craters;
pinholes
 

I have been using the scutt cone 06 schedule.  But before I had the scutt I would do 100C to 600C and then 150C to 1000C.  Would that be a good thing to try?  I was thinking I could try bisque firing to 04.  Do you think that would help?

Yes bisque firing and then glaze firing to cone 6 (not single firing).

Would you mind sharing your bisque firing schedule?  

For the glaze fire, I did see this schedule on digitalfire https://digitalfire.com/schedule/c6dhsc  Do you think that might help?

Thank you so much for your help.  

[LindsayM]

On 5/18/2024 at 3:56 PM, Min said:

Given this is a smooth white claybody that fires up to 1280C it could very well be the reclaimed clay just wasn't wedged properly. With pyroplastic claybodies if you don't get all the air bubbles out the clay what can happen during glaze firings is the air pockets expand and cause a bloat (sometimes called a bleb). Are you getting the problem more often with the recycled clay? 

That is very interesting.  I do seem to be getting bloating bubbly things on the reclaimed clay.  I'll try wedging it for longer.  Wish I had a pugmill!!

[LindsayM]

Thanks everyone for your very helpful info!

[Min]

8 minutes ago, LindsayM said:

That is very interesting.  I do seem to be getting bloating bubbly things on the reclaimed clay.  I'll try wedging it for longer.  

IMHO that really does looks like a bloat/bleb from an air bubble in the clay wall. When the glaze melts it seals the clay over so the gas can't escape and you get those bloats/blebs. 

After wedging your usual way try slicing a few chunks of clay up into thin slices and look for air pockets. Keep doing this until you can wedge without getting them. Try cut and slam wedging also if you don't already do this.

I'ld also suggest doing some absorption tests on your claybody given it is being fired below it's maturity, how to that here if you need it, about 2/3 the way down.

[LindsayM]

1 hour ago, Min said:

IMHO that really does looks like a bloat/bleb from an air bubble in the clay wall. When the glaze melts it seals the clay over so the gas can't escape and you get those bloats/blebs. 

After wedging your usual way try slicing a few chunks of clay up into thin slices and look for air pockets. Keep doing this until you can wedge without getting them. Try cut and slam wedging also if you don't already do this.

I'ld also suggest doing some absorption tests on your claybody given it is being fired below it's maturity, how to that here if you need it, about 2/3 the way down.

Thanks Min, that's very helpful.  I spiral wedge big amounts and then rams head wedge when they are smaller quantities.  So, mostly the bubbles have gone.  But occasionally I find a little air bubble has got through the wedging.  I'll do a bit cut and slam too and see if that helps.  Thanks again -)

[Hulk]

"Would you mind sharing your bisque firing schedule? "

I'd programmed a bisque schedule to match up with my notes from fully manual kiln.
The main thing being long hold at 1500°F ...if you're using the same clay (same batch), and firing rate (new kiln) is the only change...

# rate target hold
1  100 200 :15
2  300  1500  :60
3  160  1945  :00
in °F
Where I'll skip the first segment when the kiln was ran up to 200°F the night before.
I'd forgotten that I'd combined the two half hour holds to one longer hold...

When I've had bloating (dark red clay, black clay), they appeared where the wares were a smidge thicker.
Your blemishes, looks like they are in the thinner/thinnest part.
The "adequate oxygen" would be (could be) an important part of "burning out impurities" idea.

Particles of other clay and/or tiny air pockets, both seem reasonable theories, particularly as you are firing well below the clay vendor's published limit.

 

 

 

Edited May 20 by Hulk
units

[LindsayM]

14 hours ago, Hulk said:

# rate target hold
1  100 200 :15
2  300  1500  :60
3  160  1945  :00
in °F

 

 

 

Thank you so much.  When you say you sometimes skip the first segment, do you mean that if you preheat the day before, you just go straight into segment 2?  Thank you

[Hulk]

Yes, saves some time - can start later in the morning and still finish firing before four o'clock in the afternoon, when the electric rates go up.

I'm watching the portable pyrometer for the evening before ramp up; it's easy to run well past 200°F!

[Kelly in AK]

Scarva, from what I see, doesn’t publish absorption numbers for this clay body. It’s a worthwhile test to do yourself. I believe you’re firing this clay at its upper limit. As @Min said, gas that’s trapped will form a bloat. It may well be from the clay sealing itself as much as the glaze sealing gasses in. It’s noteworthy your photos show single large bloats/blebs, rather than several evenly distributed ones. 

[PeterH]

1 hour ago, Kelly in AK said:

Scarva, from what I see, doesn’t publish absorption numbers for this clay body. It’s a worthwhile test to do yourself.

https://digitalfire.com/glossary/clay+body+porosity
Clay Body Porosity
In ceramic testing this term generally refers to the pore space within a fired clay body. It is measured by weighing a specimen, boiling it in water, weighing it again, and calculating the increase in weight (thus it is also known by the term absorption). As ceramic clay bodies vitrify in a kiln they densify and shrink (thus reducing pore space). The % porosity of a body is thus an indicator of its degree of vitrification.

https://digitalfire.com/test/shab
SHAB - Shrinkage/Absorption Test