Can I bisque fire twice?

[Byrd]

Im new to ceramics. I recently purchased a used Duncan kiln and did my first bisque fire today.  The instructions in the owners manual stated that bisque ware should be fired in low for two hours, then turned up to high for the remainder of the firing. I aimed for cone 04/05 for this firing.

After firing at low for two hours, I turned the kiln up to high. An hour or so later I checked my self supporting cone and it had melted, and the kiln sitter had shut the kiln off.

I was reading online that a bisque firing can take anywhere from 8-12 hours. Mine only lasted 3.5 hours from start to finish. Looks like I reached the goal temp way too fast. Is it necessary try to bisque fire again? Or can I move on the a glaze fire, since the pieces did technically reach out goal temp? Not sure how to move forward with these pieces. 

I’ve never done a bisque firing, but it seems that in the future I should have a more gradual step up to goal temp. 
 

thank you!

 

 

[Babs]

What temp did it reach after the initial 2 hours?

It will be interesting to see how your pots fared.

[Hulk]

Hi Byrd,

Welcome to the Forum!

Good question.
Making it through a fast bisque with no damage, that's a win - if the ware went in damp, a quick ramp up past 212°F could be problematic (pro BOOM atic)!
From there, thinner ware should be happier with a short bisque than thicker ware, same for "cleaner" clay, for one of the objectives is to burn out organics/combustibles, which takes longer for thicker ware, and can take longer for "dirtier" clay.

If the wares are thick, if you're using red, brown, or black clay, a more thorough bisque might help - testing will tell, hard to say...

My typical bisque runs about eight hours, however, I'm ramping up to about 200°F the night before, then skipping the initial warm up (to drive off moisture) the next day. I hold at about 1500°F for at least half an hour, which I've found helps burn out stubborn gassy bits, direct kiln vent running throughout - ample oxygen for the burn out.

See second paragraph:
Bisque (digitalfire.com)

Edited November 26, 2023 by Hulk
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[Byrd]

I’m unsure how hot it got after the two hours on low. I’m assuming not very high. But then the instructions said to turn it up to high after the initial two hours on low, so it reached cone 05 very quickly. 

I emptied the kiln today, and all the pieces are fine! Nothing broke and melted. I’m just unsure if they fired completely. 

[neilestrick]

@ByrdIf the cone bent, then it reached that cone. The potential problem is that it didn't spend enough time at high heat to adequately burn out all the organic matter, which could cause issues with your glazes. If you're using a white clay body, then it'll probably be fine. If you're using a brown body then it's more likely to be an issue.

HERE is a thread with a lot of good information about firing Duncan kilns and how to slow them down. If you use the search bar you'll find several other Duncan threads that may be of use to you.

[Babs]

Do you have a pyrometer to log your kiln progress through firing? 1 cone is not usual practice. A bank of 3 cones one below target cone , target cone and one above the target. A bank of cones on each shelf to see how the inside of  your kiln is behaving.

Good luck.

[Pyewackette]

@Byrd Don't use the search engine on the site.  For some bizarre reason it limits your results to stuff less than 2 years old.  Use google and limit the search to ceramicartsdaily.  Something like

Quote

Duncan kiln site:ceramicartsdaily.org

 

[Byrd]

Thank you for the thoughtful responses! 

I’m using a brown body clay, so I think it may be beneficial to cook off some of the extra organics. I will look at the Duncan thread (thank you @neilestrick !!) I plan to get a pyrometer and start using a cone pack to track my kilns temp throughout a firing more closely.
 

@Hulk  Thank you for the link to the bisque info! 

[Earthy Sam]

Hello, I found your response very helpful. My question would be, if my electric kiln, manual. Shut off with its timer after 6 hrs. But the cone 04 didn’t bend. Can I re-fire or should I go ahead and glaze my pieces? 

I’m not sure what temperature it reached. This was my first time firing it. 
 

 

thanks you kindly

 

On 11/26/2023 at 10:03 AM, Hulk said:

Hi Byrd,

Welcome to the Forum!

Good question.
Making it through a fast bisque with no damage, that's a win - if the ware went in damp, a quick ramp up past 212°F could be problematic (pro BOOM atic)!
From there, thinner ware should be happier with a short bisque than thicker ware, same for "cleaner" clay, for one of the objectives is to burn out organics/combustibles, which takes longer for thicker ware, and can take longer for "dirtier" clay.

If the wares are thick, if you're using red, brown, or black clay, a more thorough bisque might help - testing will tell, hard to say...

My typical bisque runs about eight hours, however, I'm ramping up to about 200°F the night before, then skipping the initial warm up (to drive off moisture) the next day. I hold at about 1500°F for at least half an hour, which I've found helps burn out stubborn gassy bits, direct kiln vent running throughout - ample oxygen for the burn out.

See second paragraph:
Bisque (digitalfire.com)

 

[neilestrick]

6 hours ago, Earthy Sam said:

Hello, I found your response very helpful. My question would be, if my electric kiln, manual. Shut off with its timer after 6 hrs. But the cone 04 didn’t bend. Can I re-fire or should I go ahead and glaze my pieces? 

I’m not sure what temperature it reached. This was my first time firing it. 
 

I'd re-fiire them since you don't know how hot they got. The time does not control how long a firing takes to get to temp, it just shuts it down when it reaches 0. You'll need to do a firing with the timer set too long (like 14 hours), see how long it actually takes, and from then on set the timer about 1/2 hour longer than the expected firing time. It's just a backup in case the cone doesn't melt properly.