Seeing is believing

[yappystudent]

So, I got tired of hearing from a certain group of people I know in real life about how silly I am to think that a bubble left inside of clay won't explode during firing. Literally eye-rolling when I tried to tell them YOU GUYS said on my ceramic arts daily forums that that is a myth. I tried to explain this is trapped  moisture and not trapped air. I took a further risk as both they, and I, have taken the inevitable college pottery classes where about 10% of the work blows up, and the teacher blames it on the students not having learned how to wedge properly, when really,  -I'm guessing anyway,  it's work rushed into the kiln so students can have it fired before next week's class. 

So I made a little marble with a huge air pocket inside it, let it dry out about 3 weeks just to be safe, pretty damp here on the coast, and put it in with a load of my work. So I wouldn't forget which one it was I marked it with a little bomb-shaped impression . After bisque firing I took the doubters outside, had them examine the marble for holes, cracks, etc (they confirmed there were none) then opened it up with a hammer, showing them that it was in fact, completely hollow. Sometimes I really would rather be right than be happy! -actually I'm pretty happy about it, go figure. 

Every ceramics teacher should do this instead of giving misinformation about moisture, I literally did not know that this was wrong until I joined the forums. 

[Joseph Fireborn]

I just had this same discussion on reddit. 

I had to explain that hot air doesn't explode a pot, but the moisture inside. Someone said that hot air does expand(ie internal combustion). I had to link information that explained that hot air will expand a flexible surface, such as a hot air balloon. However, it will not expand a rigid surface such as a clay pot. The air will move around trying to stretch the pot but it cannot. It is definitely the moisture in clay that explodes, nothing else. I think people got it in the end but who knows, the next 1000 new potters will think the same thing.

[firebob]

There are to many people will keep teaching the air pocket is causing the problems for this myth to ever go away.

The only issue I know about an air pocket is they can make it harder to to dry out the work in a short amount of time. 

[Pres]

Yeah, I was one of those in the day. Told that by a professor that should have known better since he got his MFA at the great Alfred.  Sometimes I wonder if that wasn't proliferated to get us to learn how to wedge clay. At any rate it took me about 2 years to realize no air bubble problems, but definite drying problems. I have seen more flaky shards of clay in the kiln over the first 10 years to tell me to really make absolutely certain that the pot is bone dry. I also learned if I had any concerns to water smoke an hour or so longer.

 

best,

Pres  

[High Bridge Pottery]

Nice work with the test to prove your point  Always worth testing even if the information comes from knowledgeable sources (within reason)

[neilestrick]

Air going from room temperature to 2200F only expands about 1.5 times. Water turning to steam expands 1,700 times. The issues with an enclosed air pocket, like say a hollow sculpture with no air hole in it, is that it takes forever to dry. Like a couple of weeks or more. The outside can be bone dry and the inside can still be leather hard. Put that in the kiln and BOOM! Must be because there was no air hole, right?  The benefit of an air hole or two is that it will dry a little bit faster, and prevent deformation during firing. I've had porcelain pieces without air holes deform slightly because of the increased air pressure inside and the softness of the clay at peak temp. Good job blowing some minds @yappystudent!

[Sheryl Leigh]

Good for you, testing and teaching!

[hitchmss]

Water reaches its boiling point at 212 F. Teach your students the stages of firing and what occurs. Like Neil said, any moisture still present expands rapidly.  Go through water smoking stage slowly and evap all that moisture off and you're good to go. I've single fired work that was glazed and fired in the same day (glazed, loaded immediately and began firing). Go slowly through that first stage though. You can show your kids evidence of moisture that is still present in even bone dry work by holding a mason jar loosely over a spy hole at the early stages of firing (100-200 deg), or using a mirror held over the spy hole. Moisture will condensate on the glass surface. Good way to check if you've dried your work out long enough before ramping up too!

[Babs]

doglover, remember to vacuum you kiln thoroughly afterwards, esp if electric, check the elements and grooves, no fancy work near your experiment.

yeh wet stuff ok if you know it's there....and know what to do

[Viking Potter]

Dog lover, I think you may be confused about the exploding clay myth.  Your other posts would suggest that you are testing to see if wet clay that is reused and has air bubbles trapped in it will explode, separate or otherwise come apart while it sits or while you are using it to make a project. It will not.  But, as you have discovered, if you keep it in a sealed container, it will start to smell pretty funky.  It is still usable, but there are things you can to do minimize the smell.  You would not want to use clay with a bunch of air trapped in the body of the clay, so you would work out the air bubbles by wedging and kneading the clay before  you build with it.   The reason you want to get rid of the air bubbles is because they can appear in the surface of your project and, as you dry the piece they can appear as holes when the thin skin over the bubble dries faster than the rest of the work.  
I build birdhouses around a blown up balloon.  In other words, I intentionally trap a large air bubble inside my project.  As the clay dries, it squeezes down on the balloon, but no explosion.  I have had the trapped air deform a thin section of my project and even sometimes separate a weak area of my work, but that is due to the change of pressure on a particular weak spot as the volume inside the drying piece changes.  These kinds of cracks happen to every kind of clay work where the difference in shrinkage causes stress in the clay. 

The other concern with air bubbles exploding clay is the mistaken belief that air, when heated in the firing process in the kiln will  build such pressure that it will explode or otherwise  break the piece.  As discussed in this thread above, that is scientifically unlikely.   It is certainly possible that air bubbles would adversely effect the consistent drying of a piece, but it is likely the presence of moisture in or around an air pocket that would cause any damage.   Again, if you build a sealed sphere, dried it  till there was no moisture in the work and then fired it, you would have a hollow ceramic sphere with no damage, and in fact that is what Yappystudent did in the experiment as stated above.