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Clay bloating - so many tests including hotter bisque. Please help!!


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Hello!

I have researched (including looking at older posts here), talked to my clay manufacturer and others with solutions, & tried a bunch of tests and keep having bloat issues in my stoneware. I do not want to give up my clay, but.....

I will lay out the issues/tests as succinctly as possible. I would be grateful for suggestions. 

**My clay is a soft white stoneware with TX talc and a very small amount of molochite. Cone 6. 
**I bisque at 04, since hearing of this solution from another well seasoned potter who uses same clay and the head clay person at my clay company.  Burn those organic materials!! This seemed to work at first and definitely reduced the bloating. But it’s back. 
***I thought it might be a chemical reaction in the glaze combination. I overlap 2 glazes from 2 different companies for my white pieces. So I tried a new white to match companies.... There was no bloat!! But the color combination  comes out different (no surprise).  I also overlap 2 glazes from the same company for other pieces and get some bloat, but not as much. Thats why i tried a new white ....thinking chemicals maybe. 
***Even though I think it is something in the clay, I have not given up on it because this mostly happens on my bowls! Rarely  on a mug.  Maybe it’s a Physics issue in my throwing. Haha. Ugh. 
***I then thought it may be the molochite. I have used the clay without it but not recently-thats my next test.  
***The molochite (i am assuming) has been showing up since last years 2019-2020 batch ONLY while trimming ....tiny hard bits that carve a line in the clay...... Grrrr. Called the company at the end of my last load of clay and they said they would say some to the warehouse(?). I thought it was a fluke, but it’s there again. 
****Now I wonder if the hard bits of molochite are creating the bloat. I know I should let them know or change clay types....but new clay means testing a lot of glaze and loosing customers if I have to switch everything.

If you have read all of this Thank You!! Any suggestions????? I am stubborn and don’t give up usually but I am at my wits end on this subject. 
 

Sincerely,

Brigitte
 

 

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It's less about bisquing higher and more about bisquing slower.  Try to really slow things down from 1200⁰f to 1700⁰f and see how it goes.  Bloating is typically from OVER firing, or firing too fast to cone 6.  The outside of the clay becomes glassy and gas from inside the clay cannot escape any longer.

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Yes, pictures please. Is this  a custom mix? If so, post the recipe. If it truly is bloating, I would say the talc is probably the issue, either from a formula standpoint or it could also be due to poor mixing, since the talc is probably in a very small percentage. Molochite won't cause bloating, as it is very refractory.

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Bisque firing more slowly and providing plenty of oxygen may help - is your kiln equipped with a powered vent? If no, are you bisque firing with a peep removed?

One clay I've tried bloated here and there - much less in thinner pieces - no bloat when bisque held at 1500F (on the way up and down), glaze fire to cone 5 instead of 6.

Good luck!

Bloating (digitalfire.com)

Body Bloating (digitalfire.com)

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Thank you everyone for your input. I definitely think it may be overfiring after reading the digital fire articles. And I may try 05 bisque instead of 04. One step at a time.  
To answer a couple questions....
**I am not sure if we are able to slow the bisque firing between 12/1500-1700f electric kiln.   I will look into that. But we always use slow bisque and slow glaze. Maybe the program does slow it down around 1200  or 1500....have to read. LL kiln. 
**I am not consistent in using orton cones. Kick myself often!! My co-workers do so at least we have some consistency. The cone 6 cone is never over droopy but they have different clay etc. And have never had a bloat issue. 
**My glazes are commercial. 
**Our kiln has a powered vent and I do not take a peep out ever except the rare times we need to help cool down when it gets to 500 f or less. 
**I will test a cone 5 glaze fire next!!!

***my pics are too large! The site won’t allow them  :( Pretty sure after seeing many other examples it is bloating and blistering. If I can fix my photos I will send them  

I will give an update after testing. Thank you!!!!

Brigitte

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Thanks for the update Brigitte!

The powered vent should be sufficient for supplying oxygen; when the kiln is getting hot - over 1200F or so - the exhaust should be warm (don't breathe it!).

A cone* on each shelf might help in the deduction/analysis, e.g. the cooler/warmer shelf has more/less bloats...

Try using the snip (Snipping Tool) on your pictures - choose the salient portion, save as .jpg to get under the file size limit.

*at least a target cone, if not a full pack. My target (cone 5) tells me enough, so long as I'm close...

Edited by Hulk
file size limit
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At home I have photoshop cc and I am in the middle of changing over to a new computer so all I have is this phone for my photos today. Only one photo will crop enough to show.  
I will send a late entry if I get my photoshop downloaded.
 

I was thinking the same (Hulk) about putting a cone on each shelf (we usually do a small range of 5, 6 & 7 ...) and taking notes showing what shelf each piece is on. Mainly because it doesn’t happen all the time or not to all the pieces in one firing. A bit tedious to do, but now I think it would have saved me time. :) 
 

Fingers crossed!

 

3A087B50-F41C-4990-8CA7-847593773DC2.jpeg

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Is your clay body a custom mix? If so, can you post the recipe? If not, what is the brand/#?

Could be bloats, or could be air bubbles. With porcelain, because it gets so soft in the firing, large air bubbles near the surface will look like bloats. I don't know if your clay gets that soft, though.

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The clay is a soft stoneware very close  to a porcelain texture. I also use porcelain and have no issues. They are both from Sheffield Pottery. The stoneware is    #W1A with molochite. 
I also get what looks like blistering. There is no evidence  after the bisque firing that these exist. Super smooth. Are you talking about good ‘ol bubbles you sometimes  find while throwing? I usually pierce those or pop them or put it back to wedge. I don’t have them often. 

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2 hours ago, Brigitte Flick said:

Are you talking about good ‘ol bubbles you sometimes  find while throwing? I usually pierce those or pop them or put it back to wedge. I don’t have them often. 

Yes, normal air bubbles. They'll show up in porcelain after the glaze firing if they're near the surface. I've never seen it in stoneware, though, even super smooth stoneware, since it doesn't get nearly as glassy as porcelain. Just a theory. The shape of your bubbles is more like what I see with air bubbles, though. The elongated shape of the bubble is indicative of something in the clay that was stretched out during the shaping process. It was there before firing. Bloats that are the result of a formula problem or over-firing don't behave like that because the whole pot is the problem, not a specific spot. If it was truly a clay formula problem, I would expect bubbles that were somewhat evenly dispersed, smaller, and more of them, like the pot has chicken pox. That's also why I was thinking it might be a mixing problem, like blobs of talc that didn't get distributed well. A cone 6 body should only have 1-3% talc in it, so if it wasn't well mixed it could be in little clumps in the clay body, resulting in these bloats. It would be easy for everything else to look well mixed but the talc not be because there's so little of it in there. Do you have this problem with recycled clay, or only fresh? Have you broken them open to see what's inside the bubble?

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On 3/27/2021 at 11:55 AM, Brigitte Flick said:

this mostly happens on my bowls! Rarely  on a mug

Are you wedging the clay for the mugs or using it straight from the bag? I agree with Neil, to me it looks like air bubbles in a pyroplastic clay. These air bubbles don't usually show up in bisque unless they are quite large, they expand during the hotter glaze firing. I've had them with Plainsman 370 clay, sometimes called a porcelain, sometimes a porcelaneous stoneware.

edit: If the walls on the bowls are thinner than the mug walls that can make a difference too. I've found that the thinner the walls are the worse the problem with bubbles can be. 

Edited by Min
added a thought
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Posted (edited)

Mixing problem: I listed in my first post that one of my worries is that I find little super hard pieces of (?molochite? Talc?) while trimming. They pull at the trimming tool and carve lines. It is nothing from my studio and only shows up in greenware. I pull them out and repair when I come across them, but who know how many are imbedded in the clay. Totally annoying and I called the company. They hadn’t heard any complaints other than mine. These little specks are what I was thinking may be part of the issue....blowing up into bloats later in the kiln. 
My white bowls (and one batch of clear) have the chicken pox look. Thinner than mugs/tumblers.
I wedge all my clay whether its a mug or bowl etc. This is not a porcelaneous stoneware....my other clay is. (I am on the hunt for a more true porcelain. ) I thought I was finding more issues with reclaim, but I am not so sure anymore, so I only use it on smaller pieces.

My bowls tend to be a bit thinner than my mugs and when a mug gets thinner near the lip I was getting blisters, but only with certain glazes. My last batch of mugs were perfect! I made them a bit less thin at the top because I read an article on lips and blistering type issues. 
The mugs were in the same kiln as a few large bowls and one had the large bubbles in the pic. (All same glaze)

I found  a couple of very small bloats/bubbles on other pieces but very unassuming luckily. 

Thank you for your continued input!!

I have a few pieces to fire tonight or tomorrow. I am going to try cone 5 and see how that goes. 
:(

 

Edited by Brigitte Flick
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Is there any way of constructing a test-tile with some of these hard pieces inserted into them, and their location marked (eg by inscribing a circle round them, perhaps by the end of a tube)?

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Posted (edited)

Yes! Good idea! I can make some scrap pieces and while trimming I will find them for sure. Use an Amaco glaze pencil to circle spots...or something darker.  Excellent. Maybe I will hold off on my kiln firing.  

I did pull out an overfired  orton cone 6 today (I fired for my co-workers)  It was a 6 & 7. Both bent to “ground. Slightly hotter (2236) than top temp if the computer is correct. 

i will compare these with 5 & 6 cones in my cone 5 firing. 
 

:)

Edited by Brigitte Flick
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2 hours ago, Brigitte Flick said:

I did pull out an overfired  orton cone 6 today (I fired for my co-workers)  It was a 6 & 7. Both bent to “ground. Slightly hotter (2236) than top temp if the computer is correct. 

Not sure what you mean by"bent to ground"? Cone 7 tip touching kiln shelf or slumped flat? 

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3 hours ago, Brigitte Flick said:

Yes! Good idea! I can make some scrap pieces and while trimming I will find them for sure. Use an Amaco glaze pencil to circle spots...or something darker.  Excellent. Maybe I will hold off on my kiln firing.  

I did pull out an overfired  orton cone 6 today (I fired for my co-workers)  It was a 6 & 7. Both bent to “ground. Slightly hotter (2236) than top temp if the computer is correct. 

i will compare these with 5 & 6 cones in my cone 5 firing. 
 

:)

Maybe take a photo before and after firing. Just in case the results are worth reporting to your supplier.

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SUCCESS with cone 5!!
Not a single bloat or blister!!! But I won’t count my chickens until I fire another group with more white pieces.  
The color is a tiny bit different but it”s fine. 
The orton cones bent as they should. Cone 5 was bent over (curled) to the base it was sitting on and the cone 6 was just bent slightly.  
So to answer Min’s question about “bent to the ground” I meant they (both orton cones, 6 & 7 in the lst firing) curled over to the base they sat on. 
 
I also broke open some bloats/bubbles and it looks like there was just air pockets. A slit of air.  Again, no photoshop/other editing available right now to get my photo smaller.  I am still going to do what PeterH suggested and circle the hard bits on a test piece.

Thanks so much everyone!! I will give a quick update if the website allows after my next firing with white items.

Fingers crossed,

Brigitte 

 

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