Exploding vases in bisque firing - please help!

[Jemima]

Hi, 

I am extremely new to the ceramics community and I recently purchased an electric kiln. I have been making stoneware vases in the shape of bodies which in places are particularly thick (1.5 inches maybe at the base).

I had left the greenware for about a week before attempting a bisque fire. I had previously fired bowls in this kiln (cone 05) and none had cracked or had any issues.

Unfortunately when I came to open up my kiln filled with these vases many of them had either cracked in linear type chips or seemingly exploded completely (small shards shattered around). I have researched and apparently it can be an issue with moisture however they appeared to be bone dry from colour and tapping them.

I was wondering if anyone had any knowledge they could share with me over whether the issue is more one of thickness, temperature or potentially moisture content? I was in love with these vases and spent hours working on them and would be devastated if there was not a way around me making more of them. 

(apologies if this is not the correct place to put on this website- I have never used it before!)

Thank you in advance! 

 

[liambesaw]

Explosions are always associated with moisture.  One week for 1.5 inch thick is not enough time.  You would have to dry for a month to be safe.  

If you're going to do thick sculptures, a really handy clay to use is paper clay, which had small fibers of cellulose inside that wick moisture out.  You still have to dry them very well, but thicker parts will dry more evenly.  The issue is that the clay will not be water tight after firing because of the microscopic voids left when the paper burns out.

You can normally candle a kiln, which means run it just under 200 f, for several hours to play it safe before firing... But with thick pieces it doesn't really work to dry the pieces out because once the outside of a thick piece gets dry, the moisture traps itself on the inside a bit.  

Hope that helps

 

[Jemima]

34 minutes ago, liambesaw said:

Explosions are always associated with moisture.  One week for 1.5 inch thick is not enough time.  You would have to dry for a month to be safe.  

If you're going to do thick sculptures, a really handy clay to use is paper clay, which had small fibers of cellulose inside that wick moisture out.  You still have to dry them very well, but thicker parts will dry more evenly.  The issue is that the clay will not be water tight after firing because of the microscopic voids left when the paper burns out.

You can normally candle a kiln, which means run it just under 200 f, for several hours to play it safe before firing... But with thick pieces it doesn't really work to dry the pieces out because once the outside of a thick piece gets dry, the moisture traps itself on the inside a bit.  

Hope that helps

 

Thank you Liam that has been incredibly insightful  I will definitely try the paper clay out. Do you think with your expertise then that the stoneware after a months drying would then survive a firing? Or would I be fighting a losing battle? 

[neilestrick]

1 hour ago, Jemima said:

Hi, 

I am extremely new to the ceramics community and I recently purchased an electric kiln. I have been making stoneware vases in the shape of bodies which in places are particularly thick (1.5 inches maybe at the base).

I had left the greenware for about a week before attempting a bisque fire. I had previously fired bowls in this kiln (cone 05) and none had cracked or had any issues.

Unfortunately when I came to open up my kiln filled with these vases many of them had either cracked in linear type chips or seemingly exploded completely (small shards shattered around). I have researched and apparently it can be an issue with moisture however they appeared to be bone dry from colour and tapping them.

I was wondering if anyone had any knowledge they could share with me over whether the issue is more one of thickness, temperature or potentially moisture content? I was in love with these vases and spent hours working on them and would be devastated if there was not a way around me making more of them. 

(apologies if this is not the correct place to put on this website- I have never used it before!)

Thank you in advance! 

 

You could leave them to dry for a year and they would likely still explode if they're 1 1/2" thick. The problem is that even bone dry pots have moisture in them. You can't get a pot to 0% moisture sitting in a room with 50% humidity. So that last bit of moisture must be driven off/evaporated before it turns to steam and blows up the pot. So for starters, let them dry for a couple of weeks. Then do a good long preheat in the kiln, like 8 hours at 200F. Also fire slowly. Something that thick may also crack due to uneven expansion/contraction, so you may find that you need to fire down as well, so the piece can cool more evenly. Ideally, try to make them thinner, as it will save you a lot of headaches with firing.

[Chilly]

As everyone above has said.

The other option, with sculptures, is to make it thick (very thick is better), then make a plaster cast of it, and slip-cast a thin version, and fire that.

[Sorcery]

Glad I'm not the only one who thinks they'll always blow up that thick. 

Why so thick? You can pour hydraulic cement in the bottoms after for weight if needed.

Elevating them off the shelf can help too.

The temp difference between the top and bottom if left on the shelf will be too great.

Sorce

[Pres]

I often fired student pots upside down when bisque firing, as some student work was HEAVY, even though I would continuously caution about excess thickness and weight. That said, one of our common workarounds was to carefully hollow out the bottom of the form leaving a foot ring edge. This got rid of the excess weight but left the form intact. Another option when planning ahead is to use sculpture clay that is heavily grogged allowing for more even drying.

 

best,

Pres