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Air Bubbles In Clay


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#21 marti h

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Posted 21 April 2011 - 01:46 PM

Yes, I have been wedging to get air bubbles out before hand-building. I have ALWAYS been told that air bubbles would "blow out" in the kiln. Are you saying that the bubbles disappear as the clay dries?

#22 marti h

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Posted 21 April 2011 - 01:48 PM



I am unable to wedge clay due to arthritis in hands & wrists. Using a slab roller to remove bubbles from recycled clay takes forever. Any suggestions?



I also have extreme arthritis, including plastic joints and steel pins and have thrown pots for the past 16 years with it. My first advice to you is to learn to use SOFT clay. By using clay that is wetter, you take an amazing amount of stress off of your joints. In addition, the clay is more homogeneous when wetter without hard spots. The premixed, bagged commercial clay is way too dry for me and it must be wedged before using because it has hardened bands of platelets caused by the extrusion process. They are slight, but very noticeable, especially on larger pieces.

The best clay for me is recycled slop. I mix it in 5 gal buckets with a mixing bit in my drill. Let it stand for days so all of the lumps become saturated and it is at the same consistency. This is easiest if you let all of your scrap dry out completely, then add water to it. If you don't have large plaster slabs made up, pour the slop out in a thin layer on a swept concrete driveway on a warm day. Depending on temp and humidity, it dries to a nice consistency in a few hours to a day. Check it often and then scrape up with a 5 inch paint scraper and place in bags or a bin. You will find this clay works perfectly for arthritic joints! It is far better and more even in moisture than any store bought clay.

Also, I hope you are not wedging to get "air bubbles" out of clay. These are only a problem in the throwing process where they cause a bump every time the wheel makes a revolution. I pop them with a needle tool. Air pockets will not cause any problems in the kiln, provided the work is absolutely bone dry. Don't ever throw out slop! It is pure gold and only takes a little of your time to recycle. Nobody should be so busy that they can not come to know intimately the substance we work with daily. I also dig my own clay and have gotten to know it intimately, as all professional potters should. Hope this helps! There is a way for you to create. Overcome obstacles by creative thinking and arthritis will never master you.




#23 Chris Campbell

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Posted 21 April 2011 - 03:57 PM

"Yes, I have been wedging to get air bubbles out before hand-building. I have ALWAYS been told that air bubbles would "blow out" in the kiln. Are you saying that the bubbles disappear as the clay dries? "

If you got six potters in a room you would get six opinions on this.
I believe the blow out problem comes from the clay not being dry rather than "air bubbles".
Touch the vessel to your check and if it feels at all cool, it is not dry.
Thick objects are hard to dry and often are deceptively dry looking.
Bubbles don't disappear but most clay is porous enough to let trapped air leak out during bisque.

.... Now for the other five opinions .... : > ) .... offered with a smile

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#24 JBaymore

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Posted 30 September 2013 - 10:59 PM

You can fire totally enclosed forms.  It is water turning to steam in the small pockets that is the issue, not trapped air.  If pieces blow in this manner.... it still had physical water of formation in it.

 

best,

 

...................john


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#25 Pugaboo

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Posted 01 October 2013 - 06:22 PM

My experience with bubbles in clay is limited but that said... I bought a new extruder and used the square hollow plate to make some little pencil boxes. The first batch I pulled had an issue where the clay rejoined as it extruded out. I smoothed the areas over and then set them aside to dry. I got paranoid a few days later thinking they would blow up in my kiln if I missed a single bubble. I was going to throw the whole lot out but decided to talk it over with my teacher and he suggested letting them dry for a longer time then firing them together in a group on a shelf just in case one of them did blow hopefully the pieces around it would stop any fragments from hitting my elements. I actually sat them aside for a couple weeks and then bisque fired them and had no issues what so ever with any of the dozen or so pieces blowing up. I fixed the extruder issue by sanding the inside corners of the plate.

I was terrified of air bubbles when I started working with clay but now like with the extruded boxes if I have any doubt about a piece being air bubble free I simple set the piece aside for an extended period. That doesn't mean I am not careful a wrongly placed air bubble can leave an unattractive mark on a flat surface. But I no longer worry about those mysterious hidden air bubbles waiting to burst out and destroy my work and kiln.

I also have wrist issues and have found wedging really soft clay to be the charm for me. I fill 2 gallon baggies full of clay scraps and water and throw it on the ground repeatedly until well mixed and then dump it out and wedge it.

I also want to try the cut and slap wedging method and have watched the video several times. Next time I wedge I am going to try it even though I don't have the strung wire frame. To just try it out it will be slower but I think using a regular wire to slice the clay will work just fine.

Terry
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#26 meisie

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Posted 01 October 2013 - 07:05 PM

Years ago when I did pottery in high school (1970's) the teacher taught us to wedge clay by essentially slamming it on a wedging table over and over again. We didn't do any of the kneading type of wedging. The slamming type of wedging does not seem to be taught anymore? But I would assume it could still work? 



#27 Sojourner

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Posted 02 October 2013 - 12:47 PM

It works, same as always.  I don't know why it seems to have "fallen out of fashion".  It's also useful if you're using recycled clay or clay you dug yourself as cutting it on the wire will help to remove any fiber, sticks, small stones, or other-things-that-ought-not-to-be-in-the-clay (such as small pieces of broken off wire from careless potters who previously worked and threw the clay into the recycle bucket, LOL!)



#28 Benzine

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Posted 02 October 2013 - 12:56 PM

I would say the "slam" type of wedging fell out of favor with instructors, because none of them wanted to have a class of twenty students slamming clay onto tables.....especially with high school students......
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#29 Diane Puckett

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Posted 02 October 2013 - 06:12 PM

In this area, there is a potter under every rock. I really wish some entrepreneurial type would put a pugmill on a trailer and go from one studio to the next. They could have a great pugmill, earn extra money, and offer a tremendous service.
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#30 Pres

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Posted 02 October 2013 - 09:45 PM

I would say the "slam" type of wedging fell out of favor with instructors, because none of them wanted to have a class of twenty students slamming clay onto tables.....especially with high school students......

Yeah, those HS jocks loved to throw the clay hard enough that little missiles of clay flew out at the perfect height to get everyone at waistline. Add the noise of this to all the girls screaming ewww and you have pandemonium. Of course there would always be the one person that would sit while wedging and get some right in the face.  Yeah, I banned this technique in the Ceramics 1's.


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#31 neilestrick

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Posted 03 October 2013 - 10:21 AM

In regards to the air bubbles blowing up clay, that is the number one piece of misinformation in ceramics. Air bubbles do not cause pots to blow up. Consider this: every time you score to join pieces together, that joint is full of small air bubbles, and they don't blow up. As John said, you can fire closed forms without them blowing up. I've had lots of people tell me that their closed pieces blow up if they don't put a relief hole in them. The problem is not that the air is expanding so much that it blows up the pot, but rather that closed forms take forever to dry, especially without a relief hole. Even though they feel dry on the outside, they are still wet on the inside, and that moisture turns to steam and blows up the pot.

 

As water turns to steam, it expands approximately 1700 times its original size. That builds enough pressure in the walls to blow up a pot. Air, however, only expands about 5 times its original size going from 70F degrees to 2230F degrees (cone 6), as computed by Charles' Law, which does not increase the pressure enough to blow up a pot.

 

So why do thick pots blow up? Even bone dry pots have some water in them. It's impossible to have a pot with 0% moisture when it's sitting in a studio with 70% relative humidity. So that little bit of remaining moisture must evaporate out of the pot before it turns to steam. With thin pots the heat of the kiln penetrates all the way through the pot easily, drying it out by the time the kiln is hot enough to create steam. Thick pots need more time for the heat to penetrate and dry them out. So the firing speeds that we normally use are too fast, and the remaining water in the pot turns to steam and BOOM! Thick pots need a slower ramp up in temperature during the first 200 degrees, to make sure they dry out. This is why people preheat overnight.


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#32 Pres

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Posted 03 October 2013 - 11:25 AM

Course as we all know, it doesn't make throwing the pot any easier to have an air pocket in the clay while making a pull, just a little more dicey. -_-


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#33 Sojourner

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Posted 03 October 2013 - 02:04 PM

 

I would say the "slam" type of wedging fell out of favor with instructors, because none of them wanted to have a class of twenty students slamming clay onto tables.....especially with high school students......

Yeah, those HS jocks loved to throw the clay hard enough that little missiles of clay flew out at the perfect height to get everyone at waistline. Add the noise of this to all the girls screaming ewww and you have pandemonium. Of course there would always be the one person that would sit while wedging and get some right in the face.  Yeah, I banned this technique in the Ceramics 1's.

 

Wow, nothing like that ever happened in my ceramics class in high school, which is where I learned the technique ...



#34 meisie

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Posted 14 October 2013 - 11:39 AM

We also did etching in high school and etched into the zinc plate with acid. That's not done anymore either. Probably with good reason, I always listened to my teacher. :-) 



#35 JBaymore

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Posted 14 October 2013 - 12:21 PM

Great summation there Neil.

best,

.............john
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#36 Pugaboo

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Posted 14 October 2013 - 04:29 PM

So the firing speeds that we normally use are too fast, and the remaining water in the pot turns to steam and BOOM! Thick pots need a slower ramp up in temperature during the first 200 degrees, to make sure they dry out. This is why people preheat overnight.


Neil,
What length of preheating would you recommend for hand built items including coil and slab?

Also I just finished a sculpture of a dog sitting it is about 8x8 inches. I tried to get much of it to the same thickness throughout but there were places where it just had to remain a bit thicker. I plan to let it dry for about a month in my studio with a dehumidifier running to try and lower the overall humidity. When I go to bisque it how long of a preheat would you suggest I try?

Someone told me that preheating for longer than 20 minutes was a waste of time and energy, you don't seem to agree with this thought since you mentioned people preheating overnight. You will have to excuse me as these two opposite thoughts have me a bit confused and I am trying to understand. Your previous answer tells me you know a bit about the process clay goes through in the kiln so I am hoping you won't mind clarifying the air bubble, moisture content and preheating issue.

I really appreciate your input.

Terry
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#37 Biglou13

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Posted 14 October 2013 - 09:16 PM

Clay blowing up Because of pockets......it's propaganda!
Caution big brother is watching.
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#38 Pres

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Posted 14 October 2013 - 10:43 PM

If I could stick my two cents in here, even though Pugaboo directed the question to Neil.  Every bisquefiring requires a little decision making by the potter dependent on: 1)type of clay and ware being fired. 2)atmospheric conditions the week before the firing and during. 3)the perceived dryness of the ware. 4)the pack of the kiln.  I list these factors because in the case of 1) a dense lightly grogged clay will fire differently than a heavily grogged body.  Also handbuilt ware is usually heavier than wheel thrown ware. In the case of 2) August days with high humidity will cause the ware to have a higher content of atmospheric moisture, and there fore not as dry as when you have a nice warm day in the early Summer.  When dealing with pottery you have to judge how dry the ware is as in 3). Here if you have loaded a kiln with ware that is not quite bone dry an adjustment to the water smoking cycle is needed. When packing a kiln, some people will stack pots wall to wall, floor to ceiling in a bisque fire. A dense kiln pack means that if one piece is not dry when firing and does blow up, then the load could collapse to some degree due to an empty space. Also a heavily packed kiln will not allow steam to off as well as one that is lighter packed.

 

All of these things need to be taken in to account, and probably some that I have not mentioned. However, one gets to successful firing by evaluating whatever they are firing and how they load, and then gaining experience for the next load and the one after that.


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#39 neilestrick

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Posted 15 October 2013 - 09:25 AM

Preheating overnight will cost very little in electricity, like less than a dollar. It's a small price to pay (literally) to save your pots from explosions. I don't preheat every batch I fire. In fact I rarely preheat. But when in doubt, I preheat. There's no sense risking an explosion.


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#40 Pugaboo

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Posted 15 October 2013 - 05:10 PM

Thanks Neil and Pres! That's kind of what I have been doing when in doubt I do a longer preheat my longest so far has been 4 hours. I tend to let my stuff dry for a longer time than most of the local potters and even then I am paranoid and try to err on the side of caution. I haven't had anything blow up (YET knock on wood) but when I mentioned to another potter that I did a 4 hour preheat they said WHY? You are just wasting energy grumble grumble! Lol So when I read your response I though hmmm maybe you would know something on this.

Thank you both for your information it helps immensely.

Terry
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