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Causes for Explosions in Kiln During Firing Process: Article or Project


Introduction by Majik Merlin

A frequently discussed problem during the firing of ceramics, both cast and clay-built techniques, is why items explode in the kiln. There are several reasons this happens but the MAJOR reason (often overlooked) was so well addressed by Andrew Francis, that when approached he allowed his comments to be shared here.

Briefly, why I feel this to be an important topic is because there are a lot of self-taught potters and ceramists out there and so much misinformation or partial information spreading that the root cause is most often overlooked.

I believe Andrew's explanation may help clarify. Andrew is a potter and many of his comments are directed to potters; however, the premise of his words are relevant to slip-cast ceramists as well.

More Details

I (Andrew) thought I'd jump in here and fill a gap in the understanding of the many ways that explosions occur in bisque firings. I read (a) most recent post here (on Facebook) about an explosion with a 'why did this happen?' in which about 25% of responses were 'air bubble' or 'air pocket'. It is incredibly frustrating for many of us who know better to see people repeating what Tim See called 'the oldest myth in the book'. Air bubbles do not cause explosions, moisture does. If you eliminate all air bubbles, you don't eliminate explosions. Here are the typical ways explosions occur in bisque firings:

1 - pot is too wet, placed in a bisque firing, temperature is brought to the boiling point too fast, water in the clay expands exponentially as it turns to steam, explosion occurs.

2 - pot feels dry, ditto #1

3 - Pot feels dry, is heavy, studio tech figures s/he'll extend the warm-up time to account for the extra thickness. S/he does not check for moisture before the schedule proceeds past the boiling point of water, explosion occurs.

4 - pot is thick, clay is not very well prepared, firing schedule lengthened as in #3, pot explodes, people peer into the kiln and note the air pockets revealed by the explosion and blame them for the explosion. The air pockets ***may have contributed to the explosion*** by focusing more moisture in the air pockets and creating explosions where they might not have occurred without the air bubbles - it was still moisture that caused the explosion.

HERE is how I believe the myth about this started - in a well thrown pot with good wall thickness, no air bubbles, etc. a pot can be fired rather quickly because it dries quickly in the firing - the moisture has little distance or resistance to travel out of the pot. In a similarly thrown piece, but with air bubbles, moisture CAN migrate to the air bubbles in the same way it can migrate out of the pot, and can take longer to leave the air bubble causing a pot that would otherwise not have exploded, to explode - this because of the concentration of moisture remaining slightly longer in the air pockets.

YES - the air bubble MIGHT have contributed to the explosion.

YES, it is better to have no air bubbles - the pot's integrity is universally better with no air pockets.

NO, the air pocket did not cause the explosion - the moisture in it did and a little more time spent ramping temperatures up at the early stage of firing should prevent the explosion.

Lastly - a note on eliminating explosions without breaking the bank. There are three considerations to bisque firing without explosions:

Atmosphere; pot construction; firing schedule (specifically before boiling point).

Atmosphere determines how much moisture is in your pots when they go into the kiln - no matter how many weeks they sat in the sun, if you're lucky enough to live and pot in Cancun, the pots will usually have a significant amount of moisture in them. If you're humidity is at 30%, then you have pretty dry pots going in and can usually fire more quickly.

Pot construction - 'know thyself (or your students/colleagues)' - if you have beginner pots going into the kiln, assume they are thick and have moisture nestled deep inside. Fire more slowly by default.

The weight of pots is critical - I've seen MANY beautiful forms that weighed a ton - these are the heartbreaking pieces we often see, looks GREAT, but was very thick and exploded. Fire slower to account for these pots.

Firing schedule is EVERYTHING - we can double the time below water boiling point and eliminate ALL explosions, but this is not a rational approach and can cost more money than it saves. The first thing I do when a studio is having issues with explosions is to know what the current firing schedule is, whether a digital controller, or a 'recipe' for manual firing.

If the studio has multiple explosions on different shelves in each firing, I assume the early stage of firing is way too fast. I confirm that pots feel dry when they go in and if so will add 2 hours to the early stage of firing (below boiling point). My stopping point varies depending on the kiln - for most, I ramp up to 190F, then do 20 minutes at 200 in case the controller or pyrometer are off. I also ramp slower through the 212 F point.

If the studio has an occasional explosion, I only add one hour to the early stage, again, assuming all pots feel bone-dry when they go in.These minor changes cost very little money ($1 to $10 depending on kiln size and energy cost) and usually eliminate all explosions.

From my years of experience firing in group studios, I learned a couple of things - one is that the value of a firing to a studio is much greater if nothing explodes than the extra costs to extend the firing a few hours. For individual potters, same applies. The loss of one $20 pot pays for the extra firing time twice over. Secondly, I learned that tiny changes in firing schedules make huge differences in firing success.

May all (ceramists) see this post. Comments and corrections are always welcome, as are tips and tricks.

CME would like to thank Andrew for his willingness to share this information to all ceramists.

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Causes for Explosions in Kiln During Firing Process

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