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Rebekah Krieger

Broken :(

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Ok so the last firing I did I had my first kiln explosion - most everything in the kiln was ruined.  This morning I opened the kiln to everything in tact except for 1 item. This bowl! The crack does not appear on the inside of the bowl, just the outside. Does anyone have an idea of what could be causing this? I heated the kiln VERY slow this time just to avoid the explosion. This was a bisque 04 

post-19612-0-02706200-1373558890_thumb.jpg

post-19612-0-02706200-1373558890_thumb.jpg

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Explosions are caused by only one thing: steam. Either your pots aren't dry enough before firing, or they are too thick for the speed at which you are firing. Even bone dry pots contain some moisture, which must be driven off before it turns to steam in the walls of the pot. Thick pots need more time at pre-steam temperatures to drive off the last little bit of moisture. Do a preheat before the firing to dry them out. If it's a manual kiln, put the bottom element on low for a few hours with the lid cracked. If it's a digital kiln use the preheat function or program it into a custom schedule.

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Next time  dry pots more 1st then leave lid open for more than 1 hr and leave on low for a few hours untill steam is all gone. You need to get the water out. I tend to dry them before loading them so I do not have to go so slow.

Mark

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My kiln just has a "high, medium, low" nob -  I had the kiln propped open on low for an hour before I shut it. Maybe the humidity in the air is preventing things from drying as much as I thought they were. ..

 

Yep, the crazy humidity earlier this week prevented things from drying much at all. Also put your pots in the kiln upside down whenever possible. Explosions mostly happen at the bottom. Also dry them upside down prior to going in the kiln.

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It may be worthwhile for you to buy a pyrometer. That way when you get past the boiling point (say 250 degrees just to be sure) then you can turn up the kiln. Take a couple of hours to get to 250. Yes, there is chemical water that leaves the pot at something like 800 degrees but unless your kiln fires a lot faster than normal and what is in the kiln is a lot thicker than most pots, that can be ignored.

 

Jim

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It may be worthwhile for you to buy a pyrometer. That way when you get past the boiling point (say 250 degrees just to be sure) then you can turn up the kiln. Take a couple of hours to get to 250. Yes, there is chemical water that leaves the pot at something like 800 degrees but unless your kiln fires a lot faster than normal and what is in the kiln is a lot thicker than most pots, that can be ignored.

 

Jim

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It may be worthwhile for you to buy a pyrometer. That way when you get past the boiling point (say 250 degrees just to be sure) then you can turn up the kiln. Take a couple of hours to get to 250. Yes, there is chemical water that leaves the pot at something like 800 degrees but unless your kiln fires a lot faster than normal and what is in the kiln is a lot thicker than most pots, that can be ignored.

 

Jim

So if I plan on firing a large 14x18 x 1 inch thick tile. I need to slow. Bisque fire down? For thick pieces like that how slow /long do you go? (Programmable kiln)

 

Thanks good info!

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I am not even sure how to use it properly. It came with my kiln. 

 

That is indeed a pyrometer and thermocouple.  You just have it inserted into the kiln while firing.  There are a couple different ways you can do this.  The easiest, is to bore out the middle of one of your peep covers, and insert it through there.  The other way, is to make a more permanent whole through the kiln skin and brick, and insert the thermocouple there.  I recommend the former, as it's easier to do, and if you don't like it, you are only out one peep cover.

 

 

all i gotta say is "BLAAAHHH!" I try to be so careful - the unpredictability with pottery is sometimes so frustrating! If it wasn't 11:45 am I would be drinking heavily! lol  

 

You live in Wisconsin, so you can drink any time of day.

 

 

Yes, you can drink at any time of day there, but I'm fairly certain that the law requires it be PBR.

Rebekah Krieger and Pres like this

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rebby, I agree with Chris C., those cracks do not look like a crack from thickness, but something else. so a few questions-did you place anything inside of that bowl during the firing? Did you fire the bowl right side up, or upside down? Is your kiln shelf kiln washed? Did the bowl have anything unusual happen during its construction(warped and reshaped, dented and smoothed etc)?

As everyone here has said, the humidity which raises the atmospheric moisture takes a long time to remove in firing. I bisqued a load the other day, and actually (watersmoked) candled the kiln overnight with the lid off(bottom switch on low) putting the lid down early in the morning with bottom two switches on low-top off. I left all peep plugs out. 2 hrs later, I put the top switch on low, and then 1 hr later put the bottom on 30, middle on 20 top left at low. Slow rise til red heat in the kiln, then two bottom switches to 75 or 3/4 top to 1/2  then at red orange heat all switches to high until 06 was at 90 degree angle.  Hope this will help.

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It may be worthwhile for you to buy a pyrometer. That way when you get past the boiling point (say 250 degrees just to be sure) then you can turn up the kiln. Take a couple of hours to get to 250. Yes, there is chemical water that leaves the pot at something like 800 degrees but unless your kiln fires a lot faster than normal and what is in the kiln is a lot thicker than most pots, that can be ignored.

 

Jim

So if I plan on firing a large 14x18 x 1 inch thick tile. I need to slow. Bisque fire down? For thick pieces like that how slow /long do you go? (Programmable kiln)

 

Thanks good info!

 

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

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rebby, I agree with Chris C., those cracks do not look like a crack from thickness, but something else. so a few questions-did you place anything inside of that bowl during the firing? Did you fire the bowl right side up, or upside down? Is your kiln shelf kiln washed? Did the bowl have anything unusual happen during its construction(warped and reshaped, dented and smoothed etc)?

As everyone here has said, the humidity which raises the atmospheric moisture takes a long time to remove in firing. I bisqued a load the other day, and actually (watersmoked) candled the kiln overnight with the lid off(bottom switch on low) putting the lid down early in the morning with bottom two switches on low-top off. I left all peep plugs out. 2 hrs later, I put the top switch on low, and then 1 hr later put the bottom on 30, middle on 20 top left at low. Slow rise til red heat in the kiln, then two bottom switches to 75 or 3/4 top to 1/2  then at red orange heat all switches to high until 06 was at 90 degree angle.  Hope this will help.

 

I don't think anything was in the bowl.. It's hard to remember but there could have been a mug in it.  It was right side up.  My kiln shelf could probably use another coat of kiln wash. I am more picky about it for glaze firings. I don't recall anything strange with construction..  Honestly when I try to make larger pieces sometimes they don't work out and I trim off the top half of the piece to make a smaller one.  I don't think it flopped.  

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The pyrometer looks fine-its the tip that burns up and in that photo the tip is not visable-I would insert this in middle of kiln -Spy plug is fine if you do not use it. As you have a manual kiln like I do all you can do is set at low leave the lid cracked a bit for a few hours before closing. That crack does look like a stress crack.

Happy firing

Mark

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I am not even sure how to use it properly. It came with my kiln. 

 

That is indeed a pyrometer and thermocouple.  You just have it inserted into the kiln while firing.  There are a couple different ways you can do this.  The easiest, is to bore out the middle of one of your peep covers, and insert it through there.  The other way, is to make a more permanent whole through the kiln skin and brick, and insert the thermocouple there.  I recommend the former, as it's easier to do, and if you don't like it, you are only out one peep cover.

 

 

all i gotta say is "BLAAAHHH!" I try to be so careful - the unpredictability with pottery is sometimes so frustrating! If it wasn't 11:45 am I would be drinking heavily! lol  

 

You live in Wisconsin, so you can drink any time of day.

 

 

Yes, you can drink at any time of day there, but I'm fairly certain that the law requires it be PBR.

 

 

And I think the drinking age is now 12 or 13, and you can get your motorcycle license at age 9.

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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

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