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Rebekah Krieger

Broken :(

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Bob Coyle    113

I think the "water smoke" stage( 1100-1200 F)  puts more of a stress on piece than the loss of surface water that happens around 200-250 F. You need to go slow  through both transitions, but I have lost more pots by pushing the ramp past the water smoke stage than going too fast on the 200 F boil out.

 

The thermocouple you showed will work to give you a general idea of what the temperature is inside your kiln. It is not exact though. It depends on lots of factors, but it may give you a ballpark idea of where you are in the firing cycle, and that is good enough.

 

I try to slow the ramp down so that the temperature hovers around 200F or at least an hour. Then when it gets to around 1100-1200 F I try to slow things down for another hour or so. I do the mirror test. I close the kiln and leave the the top peep hole open and hold a mirror in front of it and try to maintain the temperature till it no longer show strong water condensation. This means sticking with the kiln while it is around these two temperatures, but since I have been doing this, I haven't lost a pot because of a blow.

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Chris Campbell    1,088

It just does not look like a kiln firing/ dampness problem ... It looks like a clay problem ... nothing exploded it just fought with itself. To me it looks like two different clays not totally wedged together but thrown in one piece and shrinking at different rates. Something's gotta give.

Exploding steam looks like something exploded and the evidence is usually all over your kiln. This is a stress crack.

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OffCenter    82

 

It's better to err on the side of caution, so for that I'd continue slow firing after 250 up to about 1000. In a programmable kiln just set it on slow bisque. That is slow enough for thick things like sculpture and large tiles as long as it really is completely dry. You probably already know this, but a little silica on the shelf under the tile makes it easier for the tile to move on the shelf as it shrinks without cracking.

 

Jim

 

any size silca?  and no i didnt know... first big tiles.... i plan on letting them dry for a few weeks at minimum.... thanks again.....

 

im with jim its..... noon somewhere and....  no one is looking  not to mention throwing and trimming skill improves

 

 

A larger mesh if you have it, but the same size silica you use in glazes will work just fine. Alumnia hydrate works, too. Think of it as tiny refractory ball bearings that eliminates some of the friction of a large tile sliding on the shelf as it shrinks. If I remember correctly you're in Florida. A dehumidifier might be a worthwile purchase. It has been raining almost every day for about a month here in Georgia so I keep one going in my studio all the time.

 

Jim

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Min    783

It just does not look like a kiln firing/ dampness problem ... It looks like a clay problem ... nothing exploded it just fought with itself. To me it looks like two different clays not totally wedged together but thrown in one piece and shrinking at different rates. Something's gotta give.

Exploding steam looks like something exploded and the evidence is usually all over your kiln. This is a stress crack.

 I agree with Chris, it doesn't look like an explosion happened to the bowl unless you had some pots actually blow shards off.

 

My go to info on all types of cracks is The Potters Dictionary of Materials and Techniques by Frank Hammer. On page 82 he has a picture of cracks like in your bowl, brickwork pattern that is in the lower section of the bowl, hairline,  and is a dunt caused by cooling the bisque too quickly. Stacked pots with a lot of mass on the shelf cooling much slower than the top section of the pot. Any chance you cooled quickly and or had stacked something heavy in the bowl? Were the pots on the top and bottom shelves more damaged than the ones in the middle of the kiln?

 

It wouldn't hurt to candle overnight and fire slowly until red heat until you have solved this but it really looks like a cooling dunt.

 

Min

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nairda    7

Low tech way to determine if moisture is still present before putting the peeps in and turning elements above Low:  hold a shiny metal object or even a small mirror near the top open peephole.  If moisture is present it will fog the metal or mirror.  Just hold the object in a way that you don't burn your fingers.

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I don't know how I could possibly cool slower... the cone shut the kiln off at 7pm and i didn't open it until 10 the next morning, it was not warm when I opened it. It completely cooled itself with the lid shut. The kiln is in my garage with the door shut so there was not risk of draft. The peep holes stayed closed.  I have no way of turning the kiln back on "low" to let it cool slower without actually opening the kiln and inserting a new cone.. (which is out of the question).  

 

I didn't mix 2 clays - I am 90% sure I used fresh out of the bag and wedged clay.  I have never put a piece of clay on the wheel that has not been wedged.  

 

 

Rather than beginning with the lid propped open with a piece of kiln furniture, do you suggest I leave the lid wide open for the first hour to allow it to heat slower?

 

There could have been another piece on the inside but I would have been smaller. I don't recall exactly.  

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Pres    896

The reason I asked about something setting inside, if the shrinkage rates were different-espacially with a piece inside, there could be pressure from the inside object on the outside object.

 

After thinking about it, if you cooled slow, and this is the only piece that cracked that way, I'm lost. Seems like more would have cracked due to cooling dunts.  Final thought-call it a fluke, and don't worry about it unless it happens frequently.  As to firing with lid off, or propted, either works, It is the length of time that matters. Others have said put something shiny by the peep like a mirror. I think that is a good idea for someone who is unsure. Myself, I just put my dry hand next to the open area of the kiln whether propted lid, open peep, or lid off. I can feel the moisture on my hand, and know not to turn up the kiln until that moisture is no longer present. Even then I do not close everything up, leaving peeps open for a while checking them for moisture as I slowly turn up the kiln. When no moisture is felt for a while, I put in the plugs.

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atanzey    6

Any chance it might have gotten picked up one handed, with a thumb on the inside at that point? (Seems low for that.) My first thought was that it might have had a greenware fracture that didn't show until firing.

 

Alice

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Melinda    3

I agree that you should fire slowly, until the water is all out of the pots....keep slow and keep the lid and some peeps open through about 725 F and use that pyrometer with a firing temp chart handy, it really lets you know where you are in the firing if you are firing manually.

 

It only costs pennies for you to extend your firing time a bit, so that you avoid those unhappy "explosions". Better luck with your next firing amd enjoy a cocktail!    Melinda

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OffCenter    82

I agree that you should fire slowly, until the water is all out of the pots....keep slow and keep the lid and some peeps open through about 725 F and use that pyrometer with a firing temp chart handy, it really lets you know where you are in the firing if you are firing manually.

 

It only costs pennies for you to extend your firing time a bit, so that you avoid those unhappy "explosions". Better luck with your next firing amd enjoy a cocktail!    Melinda

 

Keeping the lid open and firing slow to 725 is just a waste of time and energy. If the pots are dry and not extra thick (like sculpture or large tiles) getting past the boiling point is all you have to worry about. Unless you have a kiln that fires a lot faster than a common electric kiln you don't have to worry about the chemical water. For big, thick work speed up after boiling point but not full speed until the chem water is out at 500 or so degrees. This is true even if pots aren't dry and you are candeling. Once you've candeled well past boiling (say 250) turn up the heat.

 

Jim

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Chris Campbell    1,088

It's a crack ... not an explosion. When pots explode, there is a big exploded hole in them. Unless this pot has a hidden crater inside, it looks like a crack.

 

Electric kilns are notorious for cooling too quickly. It has to do with the brick thinness compared to other kilns. Leave it all by itself and it cools unevenly and quickly. Which is why we were so thrilled when we got computers that we could program a controlled cooling. I believe there are also ways to do this manually without having to put in another cone ... Someone here must know how ... I think you somehow bypass the kiln sitter and turn the kiln on and off manually??

 

And ... On a further note ... are exploding pots that common?? As a workshop presenter I can testify that I have pushed the limits without that many issues. Teachers in schools must do this too. Are pots exploding on a regular basis for others?

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OffCenter    82

It's a crack ... not an explosion. When pots explode, there is a big exploded hole in them. Unless this pot has a hidden crater inside, it looks like a crack.

 

Electric kilns are notorious for cooling too quickly. It has to do with the brick thinness compared to other kilns. Leave it all by itself and it cools unevenly and quickly. Which is why we were so thrilled when we got computers that we could program a controlled cooling. I believe there are also ways to do this manually without having to put in another cone ... Someone here must know how ... I think you somehow bypass the kiln sitter and turn the kiln on and off manually??

 

And ... On a further note ... are exploding pots that common?? As a workshop presenter I can testify that I have pushed the limits without that many issues. Teachers in schools must do this too. Are pots exploding on a regular basis for others?

 

Yes, it is a crack and not an explosion, but I was addressing the post that recommended a far too cautious firing to avoid explosions. I think exploding pots are fairly common because some beginners may be overly cautious after the boiling point but not cautious enough before the boiling point and because a pot that looks dry is not completly dry all the way through. Your mention of teachers pushing the limits reminds me of Pete Pinnell saying that he believes the reason teachers still teach falsely that air bubbles blow up pots is because that is easier than admitting that some student work the treacher was firing wasn't completely dry.

 

Jim

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Pres    896

Yeah Jim. . . at fault there. However, it was always easier than explaining air bubble would be damper or that there were other reasons for wedging the clay-which I always explained about mixing to even consistency, drying the clay part way, removing air pockets, and alighning the particles. However, few of those things seemed like enough of a reason for the kids. Very few pots blew in my loads maybe 1% if even that, but it hurt when they did.

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Chris Campbell    1,088

With a HUGE SMILE I will offer that a lot of pottery RULES and myths come from expediency rather than hard scientific fact. Which doesn't mean some are not also true, but how does one know ..... Hmmm ... Should start a new topic re: what is the most outrageous, false pottery rule you ever heard.

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Claypple    29

 ... Should start a new topic re: what is the most outrageous, false pottery rule you ever heard.

Good idea, please do!

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oldlady    1,323

chris, that one is easy.  in 1972 i took my first class. it was in a place 45 miles away.  i got a sitter, drove all that way into the distant countryside and found the place.  the teacher was a substitute who said that you could not combine thrown work with slabs.  i never went back.

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Benzine    610

I propagate a few of the myths myself, mainly to make my students more cautious.  I figure that, if I tell them to do something a certain way, they will cut corners here and there.  So if I didn't "scare" them a little bit, in regards to certain things, I'd have a lot more issues. 

 

I had a coworker, who didn't think you needed to put an air hole, in large hollow sculptures.  So I made a fairly thin one, and didn't put any vent in it at all.  I let it dry a few weeks, and fired it.  There were many pieces after the firing.

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