In a recent glaze firing at our community studio, something exploded and we wonder whether it might have been one or more of the bisqued "cookies" placed under a couple of pieces in case of possible glaze drips. These cookies have been used in multiple glaze firings - let's say perhaps 4 - 6, not sure. No thrown / glazed pieces blew up (as far as we know), though some were damaged, of course, from the bisque fragments embedded in them. Someone suggested clay fatigue as a possibility, saying that on occasion a ceramic baking dish used for some years can explode in the oven (and wouldn't THAT be a mess!).

Can anyone offer ideas about this? Thanks.

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This is an interesting question.

Please also post it on this more active forum:

http://ceramicartsdaily.org/community/forum/22-in-the-studio/

Thanks,

John255

Thank you, John.

John Forstall said:

This is an interesting question.

Please also post it on this more active forum:

http://ceramicartsdaily.org/community/forum/22-in-the-studio/

Thanks,

John255

Any wet bisque can easily explode, if it is not dried first with a several hour pre-heat at 200F or below.

Did someone "wash" debris off one of the thick cookies before they loaded the kiln? That's what caused the explosion.

Our studio experienced this type of explosion with an freshly glazed "Genie Bottle" in an older kiln with a kiln-sitter - so no real option for a pre-heat of any kind.

The potter wanted the kiln firing delayed until he was finished glazing his bottle inside and out. The next day revealed pieces of "Genie Bottle" had became embedded in the walls of the kiln and stuck to every other glazed piece - on multiple shelves, so the wet bisque became quite a number of explosions. I wish I taken photos.

Any ceramic which is repeatedly heated and cooled, in multiple ^6 firings, will inevitably develop cracks. I guess you could call that "clay fatigue"  linguistically modeled after metal-fatigue, but it doesn't have any "explosive" potential.

You had wet bisque in your kiln.

You're especially vulnerable to wet bisque explosions with a fast firing, and most people fire bisqued pieces at a faster speed than greenware. See the article below titled "Speed is not your friend".

http://www.dogwoodce...your-friend.htm

The Dogwood article suggests that drying clay or bisque in a kiln can be bad for the kiln. I don't believe this at all, nor have I ever seen any glaze oddities resulting from using a pre-heat.

When we've run out of test tiles, I've rolled-out fresh clay into 1/4" thick tiles and placed them in the kiln to bisque with a 6 hour pre-heat. The next day we had test tiles we could use without any fragmentation of the clay or explosions.

The Dogwood article also suggests fast drying greenware in a kiln preheat can cause salt migration to the surface, making it difficult for glaze to adhere. I have yet to experience this. Is putting fresh clay into the clay with a preheat and ideal way to fire? Of course not, but it works better than you might guess if the clay is thin enough.

If you have a kiln-sitter, instead of a computer controller, potters have been known to use a match stick in place of a cone. But you'll have to remain seated next to the kiln with a thermometer to have any control over the temperature of your preheat.

The CeramicArtsDaily.org forum seems pretty dead.

I've asked a question on that forum but haven't received any answers. Maybe it's just slow time of the year.


John Forstall said:

This is an interesting question.

Please also post it on this more active forum:

http://ceramicartsdaily.org/community/forum/22-in-the-studio/

Thanks,

John255

Is a 4 hour pre-heat expensive? No.

Working with a Cress kiln engineer, we calculated that a six hour pre-heat costs less than a penny in a Cress E-23 operating in a 65 F ambient temperature.

But using the SLOW firing speed, instead of MEDIUM, just about doubles our $5 ^6 glaze-firing cost.

The pre-programmed SLOW firing speed in the Cress customized Bartlett V6-7F-700 controller, customized for Cress, offers little practical advantage over MEDIUM-SLOW, as the ramp speeds through the critical 1,000F to 1,100F temperature range are both 100 degrees F per hour.

A standard Bartlett V6-7F-700 offers only a choice of a SLOW-BISQUE (which is equivalent to a Cress MEDIUM-SLOW), or a FAST-BISQUE (which is equivalent to a Cress MEDIUM speed at 150 degrees per hour through the critical 1,000F to 1,100F temperature range).

The SLOW firing speed on the Cress customized version can be used as a very expensive glaze firing for extremely thick assembled bisqued pieces.

Hi, Norm -

Thanks for taking the time to respond. The cookies aren't thick - perhaps 1/4" - and weren't wet (had been used a number of times.

Any ceramic which is repeatedly heated and cooled, in multiple ^6 firings, will inevitably develop cracks. I guess you could call that "clay fatigue"  linguistically modeled after metal-fatigue, but it doesn't have any "explosive" potential.    ----That makes sense to me.

This was a glaze - not a bisque - firing, so there wasn't any wet bisque in the kiln. And we do a medium speed glaze firing (and always a slow on bisque). The monitor who loaded the kiln said she remembered that one potter's piece had been re-glazed; we've learned since then that dipping & pouring (rather than brushing) a new glaze over a previous one could lead to such explosion. Do you have any experience with this?

Sorry to hear of the "genie bottle" incident!

Meredith

    

 

Bisqued clay is just like a sponge.  If it was not, glaze would slide-off when you apply it.

When you apply a glaze it is usually about half water by weight. This means your bisque is wet.

Clays, Bentonite in particular, can swell forming a crust on the surface which prevents the glaze and wet bisque body from drying out fully. It will feel bone-dry to your hand, but the bisqued ware is still filled with water from the glaze.

This why garbage landfills are lined with several inches of clay or bentonite. Any toxic liquid leaking from the garbage swells the clay/bentonite preventing the liquid from continuing down to the water table.

If your kiln is not designed to do a pre-heat, leave your glazed pieces in the sun or put them in your kitchen oven on warm.

A piece which has been previously glaze-fired can be just as vulnerable as bisque if the clay didn't completely densify. Washing the piece and applying glaze can allow water into the bisque through areas which are not glazed.

Fired glaze is even less permeable than clay. So water that gets into the glaze-fired bisque is going to require an even longer pre-heat to evaporate fully.

This is an illustration of a man sprinkling glaze (clay and bentonite) into his lake to prevent water from leaking out of his pond.

http://www.sturgismaterials.com/wp-content/uploads/2012/12/sprinkle.jpg

If glaze (clay and bentonite) can keep his lake wet, just imagine how effective it is at keeping your bisque wet!

http://www.sturgismaterials.com/products-view/sodium-bentonite/

Had any of the shelves been exposed to water?  Shelves that have soaked up enough water & then fired before being dried out have been known to explode.  Usually though it takes part of the kiln with it!  jhp

Also one thing I do quite often is to set greenware on top of the kiln before I start firing it.  It's a good way to dry it out w/o having to do a pre-heat.  By the time the firing is done, the greenware is completely dry.  jhp



Jeff Poulter said:

Had any of the shelves been exposed to water?  Shelves that have soaked up enough water & then fired before being dried out have been known to explode.  Usually though it takes part of the kiln with it!  jhp

Whoops! No, no water exposure & the shelves & kiln are fine (I vacuumed the kiln.)



Jeff Poulter said:

Also one thing I do quite often is to set greenware on top of the kiln before I start firing it.  It's a good way to dry it out w/o having to do a pre-heat.  By the time the firing is done, the greenware is completely dry.  jhp

Thanks!

That Dogwood article is interesting. I think just opposite is actually true in regards to salt migration during fast drying. The slower it dries, the more likely you are to get salt migrating to the surface. We see this in carbon trap Shino glazes that have soda ash in them. If you want the slat to migrate to the surface for optimal carbon trapping, you have to dry them slowly, or at least at normal room temperature rates. Drying them in the kiln will not allow the salt to come to the surface.

If something blew up in the kiln, it was wet. Pieces that are fired too many times simply crack. They don't explode.

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