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flyemma

Multiple Clay Explosions In Bisque

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Guys I'm having a serious issue, almost every bisque I have done recently in my new kiln sitter is having explosions. I have been letting my clay sit out longer and get even more bone dry yet it still keeps happening. I've made sure no air bubbles except for one piece had them however this is becoming an issue. Any help or something someone might recommend would be great! With the kiln being a kiln sitter I don't have much control to do holds I can only control the switches on the way up which I have lengthened a lot but once I hit the second switch everything goes boom, very discouraging

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wow, only had one piece explode in the 7 years I have been doing pottery. My sister in-law had carved a head once that exploded because she didn't hollow it out but that's it and we (two) of us are production so a lot of pottery. We use porcelain and let everything sit for a few days getting bone dry and bisque, no candling but I am on an electronic.

 

My 2 cents though is If you have not had this problem I wouldn't start fussing with your routine that has stood the test of time, that will drive you crazy and seems like it would be unlikely to be the problem if your stuff is bone dry when you put it in. If you change something external and it makes what's really happening not occur then you are going to doom yourself to some odd routine. There was a poster on here I recall that solved a similar problem by some month long completely unnecessary drying routine and she is now convinced that pottery is THAT fussy. It is fussy but not fussy. I vote for the new sitter being screwed up. Can you take it back? 

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Well I finally have a space where I have recently made my own personal studio and I have a smaller kiln sitter kiln for that space and I just can't figure out what has been doing it. Last week was my first explosion is all the years Ive been throwing, now its a non stop streak. I know the air bubbles are totally me if their is some, but I also have been letting my pieces dry out much longer to keep from the moisture to causing it, Im just so lost as to what else it might be. For a smaller kiln I think an hour per each switch is plenty of enough time to warm up and let any extra moisture escape. So frustrating! 

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Your work is still not dry when its going in kiln

You can let it dry more or you just have to go longer on the one switch on as low setting as you have-I also suggest a digital pyrometer which is cheap-that way you know whats happing inside . My kiln is like yours -all manual and I never blow stuff up but the pots are dry going in or I let it sit with one switch on low for 3-4 hours.

You are going to fast-its pretty simple go slower

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pssst, air bubbles don't blow up pots. Damp pots do. Set your kiln sitter for 12 hours,  turn on the bottom element to the lowest setting and leave the peeps out. If you are making regular thickness pots then try this candling overnight or at least for a few hours. (don't forget to turn the dial on the sitter back up again before you bisque fire so it doesn't time out)

 

edit: I just re-read your second post, I missed this bit the first time:  "For a smaller kiln I think an hour per each switch is plenty of enough time to warm up and let any extra moisture escape" doesn't matter what size the kiln is the clay still has to go through the same processes, think you are firing too fast also. 

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I have the model k-810 forgot to type that, its about a foot by a foot on the inside, the model does not have a time on it so I'm relying on myself recording the time length etc every time. I do let the kiln sit on the first switch for an hour and by the time I either close the lid and or get to the second switch something blows up the following few minutes, do you think all the steam still isn't out by then? Im sure its about the 200 degrees it needs by then. I have a temp gun that reaches 640 degrees that I have been using to monitor the warm up.

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Small kilns like that can heat up really fast! Yes you are ramping up too fast.

 

I started out with a small kiln with manual switches, low, medium, and high. For bisque firings, I would fire on low for 3 hours, followed by medium for 3 hours, then high.

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Is there a vent or peep hole open? If the moisture never escapes the kiln it can become a problem.

Holding a piece of glass above the peep hole can let you see the steam leaving. Keep on low until there is none.

 

I typically candle my bisque at 100°C for ~8 hours because I frequently fire wet student work on a timeline. There are some other loading tips for doing this. Wet pieces stay off the bottom shelf. They go near the outside as much as possible. Less stacking and bottoms up when possible.  Losing just one piece costs more than electricity for the entire firing.

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I use a small kiln too and do one switch an hour but don't have problems. Sounds like it's still too wet and you may be having problems now as a smaller kiln will get hotter quicker (depending). Put em inna over first and go to 180 degrees for two hours or so. My stuff don't hit a shelf till it's dry. I'm not big on candling; it's just a personal choice.

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Flyemma, I think by now you know that your kiln fires too fast, but I thought that maybe you will benefit from knowing what happens in the early stages in the kiln. This is according to "Ceramics Faults and Remedies" by Harry Frazer, a book that I still after years in clay consider the most important ceramics book on my shelf: 

20-200 C ( 68-392 F) - the remaining pore water evaporates. So when you think your pot is dry, it still contain about 1% moisture. 

200 C - 500 (392 F- 932 F) the kiln can fire somewhat faster.......since you are limited in measuring equipment, I will keep it at slow until 400 C (752 F)

At 470 C - 700 C ( 878 F -1292 F) Chemically bound water ( that is the water that sits around each particle)   and gasses to begins go and the first changes to the silica in the clay starts to happen. ( you can fire faster now)

Between 650 C-1000 C (1202 F - 1832 F) all the carbon goes, but it is still worth it to keep the rate of increasing heat steady, because you may trap carbon when the fluxes starts to fuse. That may cause bloating in the glaze fire ( or in the case of porcelain black spotting)

If you understand what happens in your kiln, you can manipulate the rate at which it must fire better. 

The way I tested for dryness in my early days was to hold a pot against my cheek. If it feels cool, it needs more drying time. Also: make sure you have even walls and basis all over. If the base is thicker than the walls, you may think it is dry when it is not and even if it will not blow off, it may still cause cracks. 

Hope this helps. 

Antoinette Badenhorst

www.porcelainbyAntoinette.com

www.Teachinart.com

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The thing with air bubbles is:  

 

They do not cause explosions in themselves.

 

BUT.

 

Imagine a 4 inch sphere, made of half-inch thick clay.  Leave it to dry, the moisture on the outside will start to dry and shrink, and the clay gets a kind of crust, making it harder for the moisture to travel from the inside of the sphere to the outside where it can evaporate.  

 

No matter how long you leave the sphere, the inside will still be damp.  

 

That's why people recommend having a small hole.  It's so the moisture can escape, both during the drying process and while firing.

 

Nothing to do with the air inside getting hot and expanding and exploding.  That's a pottery myth.

 

 

So, like MatthewV says, candle it at the lowest setting for 8 hours.

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Actually the small hole is mostly to relieve pressure. As the clay shrinks, the shape will become more spherical as the internal pressure increases. If there is a thin area, it will bulge at that spot. The relief hole is important from the wet to leather-hard stage too.

 

If I was to attempt a perfect sphere, I would try not adding a hole.

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Glazenerd, this may be a good time to explain what "quartz inversion @ 1063 F" means. I just listed it as "changes in silica". According to my references quarts, crystobalite and silica glass is the "silica" that will start to change chemically- very scientific description that I think is too high to come by for the moment - but if my understanding is correct, then it is the Silica binding with oxygen that breaks down and bind with other ions to form an irreversible bond. This process can ALSO not be pushed, because it will weaken the final fired product. 

Maybe  you have a better way to explain, this is my simple interpretation.........

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You can fire fast, but your pieces need to be able to handle it. I often do 5 hour bisque firings, but my pieces are thin and I make sure they are super dry, and I use porcelain which doesn't have much stuff to burn out like some stoneware clays do. If your pieces are blowing up with a typical firing schedule, it could be that they are too thick. So you either need to make them thinner or slow down the firing to accommodate that thickness.

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Antoinette:

 

That is a good working description.  Potters would have a hard time with the concept because we measure heat in regards to temperature. Temperature actually measures the movement of atoms. The faster an atom moves; the hotter it becomes. Matthew V just posted an inversion thread: his analogy works well.

 

Nerd

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At 573C   alpha quartz (trigonal) converts to beta quartz (hexagonal). The inversion temperature is the beginning of the "Phase change" for silica (quartz) also called flint. The changes in crystal structure lead to changes in the specific density: an increasing temperature corresponds to increasing vibrations of the atoms in the crystal lattice, and as these need more and more space, more open crystal structures are favored. At 573C, is when the vibrations of the atom reach their peak: as alpha becomes beta. Once that "earthquake" ends at 573C, the pieces can be heated at a higher rate.

 

Rule of thumb:   the larger the pieces, or more contact the foot ring makes with the shelf: the slower the temp climb during inversion. The temperature after can increase because the transition has taken place.

 

Nerd

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Reluctant to annoy the gods with saying it but I have not had anything explode. Had a few pieces break but with no damage to the surrounding pieces. I have a Skutt 1027 with a sitter, start with the bottom on low, 1 hour later turn on the mid to low,1 hr later the top. Next Hour bottom to med and so on. After all the switches are on med for an hour I turn them all to high untill the sitter shuts off or my cones bend .

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 Im just so lost as to what else it might be. For a smaller kiln I think an hour per each switch is plenty of enough time to warm up and let any extra moisture escape. So frustrating! 

 

Small kilns heat up much faster than large kilns, which means you need to slow down ramping up. You need to give time to let the water escape the clay before the clay starts to vitrify.

 

Let it candle for several hours on low before your start ramping up. Better to spend a couple dollars on electricity than to lose a whole kiln load.

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The "pops" you hear immediately after turning up the temp definitely mean you have turned it up too soon. As a learner, that was the most agonizing time, waiting for the pops, worried that it was too soon, and knowing that the pops are confirmation and that, now, it's too late to fix.

Even if you're really really sure the pots are dry, if you haven't pre-dried them in your oven on low, or just the pilot light in my old stove, overnight, or candled your bisque for 2-3 hours (until no steam shows on a mirror held to peep hole), you're going to have that anxious minute after the first turn-up.

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