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Temperature Most Likely To Explode?


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I am bisque firing and I had some small sculptural pieces that I allowed to dry for a long time.  I thought I heard 3 snap sounds, my husband said he did not hear them.  Now I wonder if I had some pieces explode.  I went down to my kiln and it was already at 1740 degrees.  At one point I thought I remember reading that any bisque explosions would happen during the beginning of the bisque process as water tries to escape. 

 

Is there a temperature range that I should know about for the most common explosions? I read a book about the firing process etc from the library but I cannot for the life of me remember.  I will find out the truth either late tonight or tomorrow morning.

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My students and coworker have heard sounds from the kiln, they swore were pops from explosions. I opened the kiln to find everything just fine.

 

There are two main points, when wares can explode. Around 212 F, when the physical water turns to steam and finds a way out of the clay, and Round 900 F, when the chemically bonded water leaves the clay. after that point, things shouldn't explode, but they still could crack.

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Guest JBaymore

I should have my students get on here and answer this posting...... last Monday's 3 hours of class time was totally on the firing of clay and glazes and developing firing schedules.

 

The typical audible "pops" (and carnage) happen when the COLDEST PART of the kiln chamber moves through the state change of water from the liquid to gasseous phase at 212F (100C). This location is not necessaruily what the PYROMETER or the CONTROLLER readout is saying..... because often the temperature at the location of the thermocouple(s) is not representative of the whole chamber's load at that point in time.... due to something called "thermal lag" (which is normally present in all kilns ... and can vary with stacking and firing rate changes).

 

The so-called "water smoking period" when chemically combined water in the clay crystals is driven off (and clay becomes ceramic) is not a specific tempeature.... it happens over a range depending on the specific clay minerals present...... and is from about 896 F (480C) to about 1292F (700C).... just below the temperature that sintering in clays begins.

 

When the chemically combined water is being driven off TYPICALLY unless the cycle is VERY fast thru there, or unless the walls are very thick (many inches) it is usually not going to develop an sudible "pop".

 

And my description of the classic 212 F bisque firing water blow up is a sound sort of like a giant piece of popcorn popping inside a big cloth bag. Mmmmmmmuuuummmmmpppphhhhhh!

 

best,

 

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

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I once had a students pot explode so fiercely that I though someone had set off a small explosive in the building. Scared the crap out of me. In my experience, when pots blow up the kiln is usually around 400-550F degrees. It takes time for the heat to penetrate and actually turn the water to steam, so the kiln generally gets well beyond 212F before the pots get there.

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For a bisque firing I usually get the kiln to about 180 degrees F and let it sit for an hour or so. The 212 degrees is the temp of water boiling, putting it in lay terms. Then another ramp up to 200 degrees F and sit for an hour then proceed with the rest of the firing. I do the same with a glaze firing. Can never be too careful.

 

Hey Neil, just noticed you are affiliated with L and L. I love mine!

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For me, stuff usually blows up below 500*F if the item was too wet/thick and we climbed too fast.  Otherwise, I have had items blow up in the quartz inversion range when the chemical water is being driven off.  These are usually very thick pieces. These pops are the ones that I've been around to hear, and they definitely sound violent.  I guess that's why modern kilns all have metal jackets on the outside of them :)

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