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marti h

Air Bubbles In Clay

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meisie    1

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? 

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Pyewackette    1

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!)

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Benzine    610

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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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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Pres    896

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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neilestrick    1,381

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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Pres    896

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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Pyewackette    1

 

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 ...

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meisie    1

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. :-) 

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Pugaboo    438

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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Pres    896

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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neilestrick    1,381

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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Pugaboo    438

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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Mart    23

 

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?

 

 

Huh.. one month and dehumidifier running? Talk about overkill and massive waste of energy :)

Set your kiln to 82C (180F) and let it sit there for an hour or 2 and it will be fine.

I use ambient to 82C in 30 min and then few hours for drying (only if needed)

 

If you have a top loader, you can stick your hand in there and feel is it "steamy" or not.

 

I had this 45x38 cm (before firing) coil built vase in the kiln for 4 h ... just in case, because I finished it only few days earlier and it was dry outside but not in the inside.

 

post-19541-0-99306100-1381995117_thumb.jpg

 

So I used:

* ambient to 82C - 0.5 h

* 82C-82C - 4 h

* 82C-600C - 5 h

* 600C-1200C - 4 h

* 1200C-1200C - 0.25h

* cool off

ready

 

I did not to bisque fire, because there was no need and this clay is fine at 1200C

 

Someone told me that preheating for longer than 20 minutes was a waste of time and energy, ...

If it still steams, it needs more time. Period. :)

And the energy wasted is way less than "wasted" on building the piece, blowing it up and then building another one and so on...

post-19541-0-99306100-1381995117_thumb.jpg

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Pugaboo    438

I should note I ALWAYS have a dehumidifier running in the studio. :) Georgia seems to be quite humid or at least my studio and garage are. I dry pieces in a cabinet to slowly let them dry and I like to err on the side of caution. I use little loafers and several of the potters at the group studio have horrible issues with cracking while drying. I dry slowly and have had no issues with cracks during drying. I am hoping this means that drying LL is better done slowly rather than quickly. OR I've just been lucky so far! ;) Non sculpture pieces I can dry in a week sometimes less if small ornaments sometimes a tad more if a large coil vase. With sculptures I try to be very careful with as they are a lot of work to get to that point and it seems silly to risk all that work by rushing the drying process.

 

I tend to preheat for 2 hours and then slow bisque. I've done a 4 hour preheat when I had some extruded pieces and I could not check the insides of to tell if dry. I've thought about skipping the 2 hour preheat but so far have not felt like I knew what I was doing well enough to KNOW everything was 100% dry. Maybe one day when I am braver and know my clay a bit better I will. Oh I should note I've fired less than 10 bisque loads so am still new to firing.

 

Well that's it I'm off to my 2nd craft fair with pottery in the morning! The weather looks like it's going to be nice so keep your fingers that the sales will be good.

 

Terry

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Isculpt    96

Terry, y'know how potters are always telling you to test, test, test?  Well, as a sculptor, I have to say that the idea of making a sculpture for the purpose of testing whether a preheat is necessary sounds just a little bit painful.  But I inadvertently ran just such a test.  My work never blows up or blows out a section when I do my usual 8+ hours of preheat, but when my kiln broke, I took my work to a community pottery place for firing.  I assumed that since all work fired there is handbuilt, and most of it is made by newbies, they would preheat the kiln.  When I went back for my work, I found that 4 out of 6 sculptures (about 2 weeks' worth of work) had blown. THEN I found out that they don't preheat.  I know some people insist that preheating is unecessary, but I have learned my lesson and I don't think I'll run THAT test again!

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Pugaboo    438

Isculpt - Thank You! I'm sorry to hear your pieces didn't make it through their kiln but I'm glad you said something here. With sculptures I think I will continue to err on the side of caution in my own kiln AND I will remember your woes and not fire sculpture pieces anywhere that I cannot verify a paranoid kiln director is in charge. How awful for you to lose all that work!

 

Terry

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