Firing thick sculpted clay

[Peter D Maas]

A friend of mine is firing sculptured gnomes--about 24" tall, with thick areas of up to 2", and having them explode.  After thorough drying at 180 deg F, at what rate should she heat the kiln?  The clay is a standard stoneware with no added grog.

[Mark C.]

They go thru a few critical stages in temps-water boils at 212 degrees and after 1,000 degrees its all gone so that range should be as slow as you can -I think a 8 hours to all day candle is not unheard of. 8 hours for sure.How long was the 180 degree deal? Thick clay holds moisture a long time and like to give it up with exploding. I would overnight the candle then slow as you can until over 800 degrees. 2 inches  is really thick and should be a clay body that has lots of open space in it like grog or sand. All this will help.You cannot go to slow only to fast. Gnomes be gone otherwise.

[neilestrick]

Why so thick? At 2" you'll not only have explosion problems, but also cracking in cooling. Get them down to 3/4" at most and they'll be a lot easier to fire.

[glazenerd]

15 hours ago, Peter D Maas said:

A friend of mine is firing sculptured gnomes--about 24" tall, with thick areas of up to 2", and having them explode.  After thorough drying at 180 deg F, at what rate should she heat the kiln?  The clay is a standard stoneware with no added grog.

Peter:

In the numerous studies done on the effects of heat work- one in perticular is applicable to your question. It found that it can take up to 30 minutes for the ambient kiln temperature to penetrate a 1/2" clay wall. In your question: ambient temperature has to penetrate a 2" clay wall: which means there is always going to be a lag between the exterior and interior wall temperatures. As Mark noted: 10-12 hours of candling  is well within reason to ensure moisture is driven off. Personally, I would not climb more than 108F an hour during the whole process. White stoneware has 15-25% kaolin typically: which will make this piece susceptible to daunting during quartz inversion (1064F/563C)  Normally, the kiln is slowed down from 1000-1100F to allow time for expansion during inversion. In this case: because it has 2" walls- I would climb at 60F an hour from 1000 up to 1200F- which allows time for heat to penetrate extra wall thickness. Finally, you have to deal with the size and weight: you have to allow free movement of the piece during firing. This piece needs to be placed on a bed of sand or alumina hydrate to eliminate surface drag. 

T

[CactusPots]

Would there be issues with cooling as well?  I assume these are fired in a standard electric kiln, so reaching top temp and cooling could be pretty fast.

Also, Peter doesn't state that the entire piece is 2", so I'm further assuming some parts are considerably thinner.  This also can create issues in heating and cooling.

You could solve the exploding part and still have an unworkable design due to cracking, I think.

[Sorcery]

I recently became convinced firing anything over an inch thick is a fools errand.

It takes less time and energy to make things thinner, and even thickness.

Sorce

[Benzine]

On 7/30/2020 at 10:55 AM, Sorcery said:

I recently became convinced firing anything over an inch thick is a fools errand.

It takes less time and energy to make things thinner, and even thickness.

Sorce

I tell my students that anything over an inch thick, isn't going in the kiln.

I don't even show them the hollowing out method.  Instead, I show them how to use slabs and coils to create sculptures, since it will give them a uniform thickness, and avert cracks and explosions.

 

[Dragonfly Pottery]

It seems to me that the sculptures were not sufficiently dried prior to putting into the kiln for firing.  Such a piece (2" thick) would take many months to dry well prior to firing, and it is unlikely that any manipulation of the firing parameters could adequately compensate for a lack of sufficient drying time.