One last point because I forgot to mention it in the video! The reason why people think poking a hole in hollow forms saves them from exploding is because that the humid air can escape though the hole! So I do think it's worth putting a hole in hollow forms if possible (like I put a hole in the donut saucers I made a while back).

In middleschool, I eventually managed to make one nice bowl on the wheel. Someone else's project exploded in the kiln and trashed my bowl. Decades later and Im still upset about this.

My pottery instructor was adamant about explosions being a result of moisture and not air. It just makes sense. Water expands so much more than air when heated (about 1600 x by volume, i think), which is why steam was used to power much of the industrial revolution.

This may have already been said somewhere in the comments- But, trapped moisture becomes trapped steam which will definitely cause an explosion

As a former pottery teacher, yes, this is a lie I’ve told many times and I knew it was a lie. I was made to tell this lie by the studio owner. They had a weekly firing schedule and if your piece wasn’t dry, 💥! I also had to tell the lie that you couldn’t fire clay that was thicker than your pinky finger for the same reason. Again, because the studio would not allow the time for the work to dry. So glad I work for myself now.

This gave me such an irrationally odd feeling that the air inside the ball is trapped in there forever!! This is air that has been floating around our world for hundreds or thousands of years and now it's trapped in the ceramic ball for a LONG time! An unexpected moment for me, I suppose because everything else in pottery that I've encountered has an air hole!

I will say that as a public school teacher, some of our biggest enemies are time constraints and environment. Ideally we would like to get everything to the correct hardness/dryness before firing but in some schools there's just not enough space to store work to dry for that long (students also want their finished work super quickly) or the production is rushed because classes are too short and so you get pieces with bits and parts at different dryness and thicknesses. And school teachers, from my experience, also don't often get to control the temperatures in their rooms so you never really know when something is fully dried. I also remember a custodian came in overnight and shut the kiln off which absolutely ruined the students work and upset the temp in the kiln/drying room.

Yes! AMAZING - I can't tell you how many times I have to tell people this - and from now on I will send them this video to demonstrate the fact.

Great video, and I think you're entirely right. One easy(-ish) way of demonstrating this: take an empty pot, cover its opening with a taped down plastic bag or similar. Heat the pot and see how much the bag inflates. Then try the same with a few splashes of water in the pot, allowing the water to boil away. You should see the bag inflate much, much more with the latter, as the thermal expansion of air is _nothing_ next to the volume change of water passing from liquid to gas.

Heated air does expand, but moisture (water) turning to steam is a massive increase in volume. Basic physics says the change of state is far more significant in putting pressure on the clay form than simple gaseous expansion. Thanks for the experiment!

I think the reason that your 'wet' pots survived was that they had enough surface area to mass for all the steam to safely escape. The steam needs to be confined for long enough to build up pressure and then that pressure needs to exceed the structural integrity of the clay. I have a feeling if you repeated the ball experiment but fired it at the same moisture level as those pots it would almost certainly explode regardless of how gentle the cycle you use is.

Back in high school i made an absolutely amazing relief tile. I was very careful to ensure there were no air bubbles. Because I procrastinated, I kinda rushed it through drying. Thankfully it never fully exploded, but now I know why it cracked along the two pieces I put together despite my desperate efforts to prevent damage. The acrylic paint hid the flaws and I got a good grade.

Hi again Mia, yes, yes, YES.....I have long held the belief that it's the moisture, NOT the air that causes, not just explosions but big cracks too. Especially in bottoms, which are often thicker than the piece in progress & dry slower. Great video

One thing I've experienced with wild clays where no temper is added is that, even after thorough oven drying (500F), wild clays normally include a bunch of organic material - roots, bug junk etc. Upon kiln firing, the surface can sinter to the point it's airtight, trapping organic gasses that expand and cause the ceramic to bloat and/or explode. Shocked the heck out of me when it happened but it was a good lesson. Added temper would have allowed the clay to breath, preventing the problem.

Wow. I really want to show this video to pottery instructors. I suspect some might be interested in testing further while i think several i know will become all stubborn and double down on the trapped air idea. Thanks for makingb this controversial video

What might be an interesting experiment would be to make several near identical pieces sequentially so they would have dried a different number of days when you fire them at the same time. Mark each one to differentiate them. (2 days, 3 days, 4 days) See which explodes and which do not. Of course one exploding could break others. Not sure how to control for that unless you had multiple kilns with identical temperatures.

Great video! I live in humid climate, I use little grog, and sometimes I fire childrens work, sometimes closed spheres. So I candle for quite a while. Sometimes 24 hours. And I ramp up slowly. I hate cleaning and would rather take it slow.

I'm still going to dry for a long time and I am still going to put a hole. At my school studio the tech always checked each piece before it went into the kiln and would reject pieces he didn't trust. Over the 3 years I was at that studio I saw plenty of pieces spontaneously disassemble and take out other peoples work. It was always an event which resulted in an after action meeting in the classroom. We were using a gas kiln, not electric, but we would often use electric for the bisque depending on how many pieces there were.

Hi Mia. Great video. While I agree about the water vs air, I think it might be more complicated. I had 2 small but thick sculptures that were never fired in my first studio. Sat at home drying over 5 years. I drilled into the bottoms and both pieces bisque fired fine. Glazed one and it blew up in the glaze fire. Definitely no trapped water in it. Only 1-1.5" thick. Sometimes there's no easy answer.

Great video! You really got me thinking. Try making a solid clay ball the same size as the one you bisque-fired. My guess is that the trapped water in the solid ball will cause an explosion. Just Kidding. Don't do this, it will definitely explode. The operative concept is not water versus air, but "trapped" water. Air can expand and contract, it does that all the time inside of things. It doesn't cause explosions. Water turning into a gas however is a different story. Much more pressure. Much more expansion. All happening very quickly because of the latent heat of water, i.e. heat/energy builds up in liquid water until right at a specific critical point, the water becomes a gas, all at once. Boom.

I don't know how you ended up in my suggested videos, but so far I'm glad I discovered your channel. It would appear that every once in a while, the algorithm does something right =)

What I've learned through my journey of being a self-taught hobbyist potter is that what makes the pieces explode is the humidity/water that's inside that when heating eventually makes the water boil and because of that, it explodes

I was told this when I was in ceramics class. I always thought the fast shrinkage while firing had something to do with it, along with the trapped moisture.

Thanks this was super helpful!

Coming from someone who ran steam engines: It makes sense that water is the main problem. Using ideal gas law( PV = nRT ), trapped air will apply around 5 times its pressure in a kiln at 1500K compared to room temperature around 300K, assuming the air pocket is impenetrable and stays the same volume. For every liter of water, at atmospheric pressure it turns into 1600 liters of steam at 373K, and only gets worse with ideal gas law as the temperature increases. It only takes a few drops of water to exceed the expansion rate of the air inside the sphere.

Very well explained. Ty

I knew it, thank you for proving this. Now i an reference this video the next time someone tells me my sculpture needs holes poked into it. Thanks for being brave and confident and testing this out!

I remember spreading this misconception at a summer camp I did pottery at. We only had a week with the students so there was no way we'd be able to fully dry it for them to fire it correctly.

Thank you so much for the time you took to maje this video!!!

So nice to see someone brave enough to carry out this experiment! I've suspected this for a long time for this simple reason: Sometimes I find and pop air bubbles while throwing. But that means I probably sometimes DONT find all the air bubbles. And yet I never have explosions. I wonder though, if you took the clay up past the temperature where it vitrifies, could that trap the air and cause an explosion?

Great video. I like your teaching style. I would be interested to see the sphere be glazed and go through a glaze firing.

I don't know if anyone said this yet, but I appreciate the scientific approach you used and the honesty when the pots didn't explode. Some people would not have, and that's fair, no one wants to be wrong, but I think it's important to show how the experiment went, even if it wasn't according to plan.

Thank you! Phenomenal video. Great presentation and instruction, I'm very thankful.

Fun fact...Clay is made up of tiny plates that tangle and stack together. Water acts as a lubricant between them, letting them slip past one another, allowing it to be formed. As the water evaporates, they lock together, forming the shape, but also leaving tiny voids, leaving the clay porous, but also potentially trapping water in pockets. The air bubbles that cause the problems, also create that same humid microclimate, inside them, which also functions to keep the inside of the bubble slightly cooler....And since all the clay around them is dry, it sinters (slightly melts, fusing together) first, and forms what is essentially a cyst around both water and the air, sealing it to varying degrees, depending on the porosity of the clay, and when the heat reaches that cyst and evaporates the water, and heats the air, causing it to expand as well. Sometimes that only results in a weird little bubble in the clay, or a crack in the body of the work...But if it seals, you get an explosion, from gas, both the air, and the water flashing to steam, expanding too fast to escape through what porosity is in the clay. That's why you wedge clay before you use it, to get rid of those small to medium sized bubbles, and to evenly distribute the moisture, so it doesn't so easily collect in those ready made pockets. Ironically, the size of that sphere might have contributed to it's survival, since there is more room for the expansion, and more area of porous clay, to let it out without breaking. Though if it were made of a less porous clay, or were glazed over the entire surface, I would bet you'd run into serious issues. In truth, modern kilns, with their better temperature controls, make actual explosions a pretty rare thing...their programable slow heating programs make sure that the entire body of the clay is dry, and will let the trapped air and water slowly exit through the pores in the clay.

When gases (air) and liquids (water) is heated it expands. However because gases are compressible they become pressurised but water can’t be compressed so it forces out causing an explosion

I get where the myth comes from. It's basic physics: pV=nRT slightly simplifies explanation: n is the amount of gas molecules and atoms, which obviously doesn't change in a closed space. R is a constant which means it doesn't change either. V is the volume, so another constant in this case. T is temperature and p is pressure. Therefore if we raise the temperature, we also raise the pressure. The idea that trapped air can cause explosions is the opposite of far-fetched! So either the higher pressure is not enough to lead to an explosion or the clay isn't airtight and therefore not actually a closed space. I don't know much about pottery, but if water can evaporate from the material, then I wouldn't be surprised if it also lets though air molecules and atoms.

Yes! Great tute!!!!!

I was taught to use the very old kiln that I bought used as a dryer. Prop the lid use just one circuit. Hands are toast these days. Slip is the way for me now. I make my own molds on a good day. Good video. Stay safe.

Great video! Great that you busted this myth! I’ve also learned that it’s the air. I still thought so until now! Thanks a lot! It’s time to start talking about moist and humidity instead of air now in the videos. Grusse auf Finnland!

It's even crazier in metal foundry working. They are normally water free, unlike pottery. But a little water touching hot metal evaporates so fast that it explodes. Your mild program might be drying the pottery while it's in the kiln, which is a more expensive but faster drying process. You can technically even use the kiln as a dehumidifier, but kiln will throw away some heat with the steam. Dehumidifier with a compressor is much more efficient. Kiln has a dehumidifier COP(coefficient of performance) of around 1 at most. Dehumidifier with a compressor can have something like 3. It means, it's like 1/3 cost of drying.

I always was told this. But also that thick things exploded. So those dont go hand in hand. What you are saying explains so much. The effects you could make with air bubbles below a thin layer of clay....

Happy Birthday Mia !!

Hi Mia, always withinthe kiln opening I waited any "wow" scenario and there was nothing 😂 you are the best ❤ thank you for inspirative video and time to spent with this test !!!!! 😊

Great information! Thank you,,,,

Great video! Maybe you could make two identical spheres let one dry and fire one after a week when it "looks dry" to see if its the water that makes it explode

There was a maker at Open Sauce who makes double walled insulated ceramic coffee mugs that don't explode! Used a mix of 3D printing and traditional slip casting to make them.

In highschool I had an overdue art assignment, a life size sculpture of a hand, that needed to be ready to load in the kiln by end of the next school day or I'd get a zero... And I hadn't even started it. The art teacher was incredulous and half mocking when I said I was taking a lump of wet clay home and that I'd bring it in before school fully bone dry. She claimed it was impossible, and *insisted* it would explode when I shared how I planned to dry it in the oven I went home and built it as if half protruding from the table to eliminate the bulk of the meaty parts of the palm, so it was just under an inch at the thickest. After dinner I stuck the not even leather hard piece on some metal screen onto the oven grates, and turned the oven to 175F with the door cracked a bit for humidity to vent. Before bed I set it to 200F, woke up a few hours later and set it to 225F, and a few hours more and set it to 250F. If it was going to explode it likely would have already at that far past the boiling point, so when I actually got up to get ready for school and set it to 450F for a last hour, it was an overcautious step done mostly to be a smart@$$, since I then brought it to school insulated in a towel and still burning hot so that she had to acknowledge it was irrefutably bone dry. (and unexploded)

In Highschool i made and Eyeball and had to convince the Teacher that my project would not explode before they would fire it. It was a hallo sphere (the Eye) with the eyelid wrapped around the whole thing. So it looked up when placed but was still basically round. My dad did ceramics and i worked one of his friends shop as well. So i told my Teacher i would just take it home and fire it. He finally gave in but placed it on the top shelf by itself just in case. Came out great.

Really interesting video. Great to see you being sponsored. Hope Babbel sponsor more videos.

Absolutely wonderful video! I’m glad you did this. I’m a Hand building Instructor, and I always tell my students this. Good idea to do with students to show them firsthand that it’s the moisture in a piece, not air ❤ The leatherhard- 😂 cool!!!

I'm just watching the videos about pottery, never did any. But I knew about water being the main reason behind cracks and explosion. I think most people who explain those things on yt do say those. Maybe sometimes they say air as a quick comment, but I have never seen anyone proposing that air and not water is the cause behind explosion. BUT I do believe that sometimes air can cause cracks, maybe even explosions. But only if you heat or cool down piece very quickly, which everyone is trying to avoid anyway.

I only threw for a couple of years over 20 years ago, but the *only* time I had a failure was pretty obviously from trapped air. A largish flake popped off a mug and left what was obviously an air pocket. I mean, it was a little hollow. My fault for not noticing it in the throwing (I was just a bit beyond rank beginner,) so it *can* happen, just not catastrophically like the pix you showed in your video. I've actually still got the mug, I turned it into a pencil mug.

I have only just found your channel and I love it!

Babel needs to sponsor more videos 🎉🎉🎉🎉

In my elementary school art class, one of my classmates' projects exploded in the kiln, smashing several others. The teacher decided, in the interest of "fairness," that nobody would receive their finished project, and presumably destroyed them all. I was so pissed. Still am, truth be told. If you have the choice between doing a good thing for some people or doing nothing for all people, and you choose to do nothing, you seriously need to reevaluate your priorities.

A big part of it is that the phase change from liquid to gas does way more expansion than how much air expands when heated up. Air going from 30°C to 1100°C will only expand by a factor of 4.5x, which isn't all that much pressure and is certainly tolerable by clay. But water going from 99°C to 100°C will expand by a factor of around 1600x, and then the heat from 100° to 1100° is another 3.5x on top of that - that's an expansion ratio of around 5600x, all told, and is an *enormous* amount of pressure to hold.

This was a great explanation of explosions in the kiln. You are correct the culprit is water, not trapped air

Thank you so much for doing this experiment! I've been marvelling at some ceramics at a local gift shop that are completely closed forms and wondering how they didn't explode! My big takeaway is to dry a closed form piece for 2-4 x as long before firing as I would one of my normal pieces. (My environment is fairly dry and I can fire within 4ish days on average).

I did learn at pottery that indeed water caused the explosions, and that the inside spaces would dry slower.

I had a friend purposely plug his clay sculpture to try cause an explosion and we were all surprised when it didn’t explode. Now I know why

Omg yes! I’m so over this myth!!

Thank you 🙏🏼 best information ever 😀☺️😁

Water does expand into steam a lot greater than just air alone, but if your clay thickness is able to withstand the expansion before it cracks slowly or explode rapidly it won't break. So try the experiment with an air cavity on a thinner clay wall instead. And perhaps fire it up rapidly instead of slow and gradual which won't allow hardening and expansion at an equal pace.

in school when i did pottery we stored pottery waiting to be fired in the kiln room for about 2 weeks(minimum) before firing, the kiln was almost always firing something so the room stayed around 120f and was quite dry thanks to dehumidifiers

Hello, I am from Kiev and I want to tell you my sad story about my first clay art. I remember, when I was studying at an online school, we went on a field trip to a pottery club. It was a very nice place and the decorations made it look like a real Ukrainian village house with a garden. Everyone who wanted, got a piece of clay and we started. I made a miniature little jug and a few cups, which looked very well. But since it was not a very professional pottery club and children came here just to play with clay and have fun, there was no baking included with the tour. Also, the day we went there was white clay instead of brown clay. And it was very sad and unfair, because it meant that I could not bake it properly since we, like many other people had just an ordinary oven which was of course, not hot enough for white clay. And even though unlike other children, I made a miniature, but profeccional realistic jug, I was never able to fully complete it. If only the tour guide understood the pain... But at the end I coloured my tiny tea set with some paint, and even though it could not hold any water, the result was still beautiful.

Great video! I reckon you should do another proof where you run a harsher fireing program the explodes the wet clay but the same program doesn't explode the trapped air. Because technically you have only proven that your fireing program prevents explosions

I did some rough calculation, and if you heat up the air inside the sphere from 30 degrees to 900 degrees the pressure inside would be around 3.9 times larger. If you have 1mL of water inside a 1L closed space the pressure would double (in addition to the increase from the air heating) which results in pressure around 8 times larger. Since there isn't a lot of air inside even a small amount of water has a significant effect on the pressure once it turns into steam.

I was actually taught that it was steam and that we ought to make hollow spheres not solid ones

makes sense that water still actually has a chemical fase to climb and air doesn't

Would like to see you fire a similar ball that is leather hard dryness. Yeah I want to see the explosion, lol. Thanks for the great video!

I took a pottery class my senior year of high school and I only had one project explode, thank god. It was a pretty thick sculpture (seal type thing) so it makes sense it didn’t dry all the way.

A long time after I made it, I broke one of the stoneware pots I made at school, I found quite large voids inside which puzzled me for a while. Til the clay is fully vitrified it remains quite porous, so air and water vapour can pass through it, how else would it ever dry out otherwise? If the temperature rise in the kiln is slow enough, any trapped air will simply diffuse out through the porosity of the clay. It would be fun to see what would happen to your hollow sphere if it had been taken up to a fully vitrified firing.

Thank you I totally agree with you, things can be wet for weeks and you not realize it

Of course water, or most other liquids that vaporize within the temperature range of the kiln. Water expands 1700 to 1 and then more due to superheating in a kiln environment. Extreme example: 1ml of water will become a minimum of 1.7L of steam at atmospheric pressure. Constrain that and the math gets complicated and involves tracing water's phase diagram, but generally speaking it would probably get to around 2-10 atmospheres and/or explode. No, that is not particularly safe at the 10 atmosphere amount even with the walls of a kiln. I am an engineer, though the above amounts to crude calculations and engineer's "professional common sense". Perhaps an old microwave could be a crude hydration tester by heating the piece to a temperature proportional to the water in it for around 30s. In the case of 1ml of water inside, the relatively durable steel inner walls of a microwave could be a durable or cheap and sacrificial test to save the kiln. It could even be used as a fast drier for some clay mixes, though it could crack from fast shrinking on fast drying and that is not advice.

As a teacher, I'll admit that I perpetuated the myth for my first several years teaching. But as other potters/ ceramicists explained; air only expands a couple times its size when heated. Water expands thousands of times. I make sure to dispell the myth now.

Hi Mia, thank you very much for all your videos, you are an excellent teacher. I was told that any little bit of grit or dirt in your clay will cause an explosion. Is this true?

This was very interesting. I do wonder how that sphere would fare in the kiln if you glaze it. I think that, depending on the type of glaze, it would have a good chance of exploding under those conditions. Gasses from the glaze might pressurize the interior leading to an explosion. I would love to see that tried!

I love your channel! Could you teach us how clay is painted, like animals, flowers? Is it with special pens? 💗🌞🤩 thank you!!!

Perhaps in a next experiment try different- shape, clay, wall thikness, temp and glazing

it make much more sense that it's water rather than air, you can see it also in cooking for example in bread baking and egg frying, or even when you add water to a hot oil, the oil escalates the water heating & the water rashes to escape resulting in shooting everywhere

Nice explanation. Wouldn't it be possible to use an accurate balance to figure out when the clay was dry enough? Like when a certain piece has lost 2.0 % of of it's original weight? I don't actually know what the relevant weigth loss is for fresh vs. "burnable" clay.

I was never told this myth so now I want to know

From what I understand of Water is that it will try and escape a heated Environment, so having it trapped in clay will generate pressure and explode. Oxygen can explode when pressurized but isn't the main cause of that pressure in this intent.

At the same time, it's worth remembering that when water is superheated, like it would be inside a kiln, it turns to vapour, which is essentially air (but under pressure), so in a way, the problem is STILL the trapped air, but if the pot were dry, it would not be a problem.

I'm sorry to say but trapped air *can* cause explosions in the kiln; however, not the amount you think. It happens when very very small bubbles of air are cooling. As the kiln powers down, the walls on the inside of the bubble cool much quicker than the rest of it, causing micro fractures that can propagate across the piece as it further cools. In this case, you wouldn't see this phenomena because the surface area you created is able to dissipate the heat and cool very evenly throughout the piece. With that being said, you are 100% correct with the moisture content part :)

If you ever feel like following up with this experiment, how about making another sphere of a similar size? Then fire while it is as un-dried as possible? By minimising other variables, then you can control for just the amount of water in your ceramics and then have better proof of your theory

I was always told to be careful with molds made of plaster of Paris because if any of it broke off and got in to the clay. It would end badly when it was fired.

when leaving space at the start for being wrong, "maybe I'm full of it", you missed an opportunity to say maybe I'm just full of hot air.

When air goes from room temperature to the highest temperatures in the kiln, it will expand if it can, but it's not a gigantic change in volume. When water transitions from liquid to gas, it takes about 1400x the volume it previously needed.

Trapped moisture becomes trapped steam which powers the explosion. Great video!

Not exactly the same materials or kiln setup, but in tile manufacturing, the tiles go through a dryer setup before getting fired. It is definitely moisture that causes explosions in the kilns.

If you want an explosion, introduce some water filled cavities into your pottery. Humidity alone is not necessarily enough, it needs to be liquid water that does not have enough space to expand to as it evaporates. Even tiny drops of water stuck between layers of clay are in my opinion quite sufficient.

I wonder if measuring the weight of the moist clay and the same lump thoroughly dried might give a solid understanding. Than one can calculate how much water is stored in it. And with that info one can measure the initial weight of any piece of pottery fresh of the wheel, and calculate how much water needs to evaporate for it to be dry enough to be fired safely.

I have a theory that would be cool to test. I think you are onto something with the inside environment but I think it is less to do with the state of the clay and more to do with the inside humidity, as you mentioned. My hypothesis is, perfect world, you have a completely dry shell but there was somehow a humid environment inside that in some way added no moisture back into the inside wall of the clay, I think it would still break because the moisture is expanding in the heat creating a lot of pressure on the inside of the ball. Drying the clay as long as you did allows the inside moisture to reach equilibrium with the outside environment creating a low moisture inside environment reducing the amount of expansion inside the ball. A perfect world test would be hard to do but you could make a ball with a small hole in it just big enough to put a balloon inside, and then dry it completely the same way you did before, then put a balloon inside and fill the balloon with a half teaspoon of water and fill it the rest of the way with air. The balloon would retain the moisture preventing the clay from gaining additional moisture until time of firing. Then close up the hole and allow it to dry as well so the balloon is inside the completely enclosed dry ball. The balloon will melt causing it to pop in the kiln, this would be intentional, then allowing the warm humid air to come in contact with the ball only after the firing has begun. Then I believe the expanding gasses would hopefully in my experiment still cause the clay to break.

Of course the actual reason is water turning to vapor. I think using a wrong cycle (too hot too fast) will also make things worse. I hear a lot of stories about explosions from school kilns and the like, because they often try to bake overnight instead of a full 24h cycle. Water evaporation increases the volume x1000 (or more) in "normal" pressure environment. This is a number we (firefighters) use when making calculations for sprinkler systems or fires inside solid structures, to avoid overpressure and other issues when too much steam is generated. (superheated steam under pressure is more dangerous than actual flames for a bunch of reasons) When in pottery water is trapped in a small pocket inside clay that has solidified, this also means the pressure in that pocket will go x1000, which no clay will ever hold. If during the evaporation there is still a path for the vapor to escape, for example through porous or unbaked parts (or the poked hole that you mentioned in your pinned comment), and it can escape fast enough, the object will not explode. *_If you want to ensure explosion,_* make a pocket by folding over a layer of clay, compress it without kneading. This will most likely create a pocket. If you do it with extra water on the surface that you fold shut, or you rub more water in to camouflage the seam, you'll have the perfect recipe for kabooom... The essence of what I'm trying to say is, that it'll take a lot of coincidence to explode pottery that is homogenous, regardless of insufficient drying, because the main factors are actual _small_ inclusions or pockets of water (with or without a little bit of air), and too fast evaporation due to faster heating or too high temperature. The larger the cavity, the smaller the chance of explosion. So I feel that the large sphere didn't prove anything, despite your theory being absolutely correct =)) (I also think that the idea that it was air that made the explosion may stem from what they found after an item exploded: the cavity that held the water will have been enlarged before fully firing/baking, so they will find parts of cavities in places of the item where no cavity was intended, making it seem like it was an air bubble that exploded, rather than a waterdroplet-turned-steam bubble.)

Even though i dont do anything with clay. i always thought the whole air causes explosions was bs to begin with. I always heard from everywhere that if you going to use a kiln or a fire for old scholl firing to ensure its bone dry. so i always thought explosions were steam explosions.

If you are unsure of drying progress, log the weight. As the piece dries it will become lighter. When the weight curve bottoms out, you know you're about as dry as you're going to get.

Fascinating discourse. :)

The warning on the kiln gave the location away :D

Is it possible to glaze the ball? Would the glazed surface trap air which would expand in the heat?