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Single Fire A Sculpture

firing sculpture

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#1 Tenyoh

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Posted 24 May 2014 - 01:21 PM

Hello ceramic artists,

 

I made a figurative sculpture (21-inch tall, divided into 3 sections that will be glued together after firing) out of paper clay (made of red stoneware that matures at the range of cone 05 to 6). I would like to fire it to cone 5 because the clay turns to an earthly brown tone at the temperature. I am not planning to glaze it. The ceramic studio that I have been using has Skutt Automatic Kilns. My question is:

 

If I single-fire the sculpture to cone 5 at the speed "slow," would I risk it too much? I'm thinking to do a single firing mainly because I want to avoid unnecessary loading and unloading of the sculpture.

 

My potter friends are suggesting other approaches, and I am getting confused. What will be the best approach to minimize the chance of cracking the piece to bring out the earthly clay tone? Thank you in advance for your suggestions.

 

 

 

 



#2 Min

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Posted 24 May 2014 - 05:13 PM

Hi and welcome.

 

Might be an idea to give some more info re sculpture thickness and if there is grog or sand in the body. A picture would help give an idea of how tricky it would be to fire. Sometimes a slab of the same type clay used underneath the piece(s) is a big help so it shrinks at the same rate as the sculpture, other times a thin layer of grog or silica sand or coils of clay is enough. 

 

Probably be wise to fire it slowly up to approx 190F and hold it there, depends on how thick it is as to how long, but at least for several hours. The preset slow ramp isn't going to be slow enough or hold at below boiling temp long enough, you will need to put in a firing program. Sometimes the fumes from a bisque will discolour glazes, would there be glazed pieces in the same kiln firing? If you do it slow I don't see a problem with single firing unglazed work.

 

BTW you said that the clay "matures at the range of cone 05 to 6", not possible. At ^05 it will be underfired and porous if it can go to ^6 and not be overfired.



#3 Stellaria

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Posted 24 May 2014 - 05:53 PM

*shrug* the clay body that I use for my milk-fired pieces says ^04-8 on the box, when all the other clays from the same supplier have a much narrower range. It's definitely not fully vitrified at 04, but that's what the supplier information says.

#4 Benzine

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Posted 24 May 2014 - 06:40 PM

Stellaria, I would imagine that Cone range, simply indicates the clay's useful range. At the low end, it is strengthened enough, that it's not overly fragile, so it would be OK for non-functional wares, or even Raku firing, which only go up to around 04. The high end, is where it would need to be, for functional ware.

Tenyoh, I am always very cautious with even my small sculptures. Slow drying time with a bag, then more without, then a slow firing schedule. Sculptures aren't like my wheel thrown wares, where I can throw another in a matter of minutes. A loss of a sculpture, during a firing would be devestating.
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#5 Colby Charpentier

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Posted 25 May 2014 - 12:12 AM

I have to agree with Benzine, fire to the top end of the clay's range if you want a mature body. For a sculptural object that will remain indoors, it's not too big of an issue, though.

 

Also, Min is suggesting a preheat to help dry out your piece. Many people commonly use a preheat in their bisque cycle to ensure moisture is driven out of the work before pop, pop, pop... The single firing to c5 will be fine, just make sure you use a decent preheat. I use at least a 10 hour preheat for the community studio to make sure there aren't issues, and I actually just ran a firing with a 30 hour preheat as I was running behind on a show... (The work went in wet) Either way, you'll be fine using the easy-fire option or whatever it's called.

 

At the end of the day, you know you want the c5 surface, so go ahead and do the single firing, just make sure the work is dry and the ramp is slow. Or look up a ramp/hold program if you would rather do that. Just don't have hesitations about single-firing when you don't have to worry about glazing.



#6 Tenyoh

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Posted 25 May 2014 - 12:41 AM

Thank you very very much for the answers.

 

Min, the thickness of the sculpture is roughly between 1/2" to 3/4". The clay has some grog in it, but I don't know the percentage. The package or the manufacture's website does not indicate it. The package does say, however, the maturity range from cone 05 to 6. It has been 3 weeks since I completed the piece. (The climate is dry here in South Dakota, and I think it is ready to be fired.) So far, there is no crack. The piece is big enough that it can be fired by itself without other glazed items. I'm attaching a photo of the sculpture to this post. (Hope you will be able to see it without any problems.)

 

The kilns that the ceramic studio have are automatic ones. I'm not sure if we can manually set it to hold at 190ºF. When I set the speed to "slow," it takes about 16 hours to reach cone 5. I have fired my other sculptures to cone 5 (after bisque-firing and with other glazed pieces), and they came out well. Sorry I'm not so familiar with the kilns. I can go to take a look at the manual to see what features are available on the kilns.

 

Benzine, thank you for your understanding. It took me 3 weeks to complete the piece, and yes, I want to be very careful with it.

 

If single-firing does not work, I'm willing to fire the piece to cone 05. Check to see if there is any cracks. If no crack, fire it again to cone 5. But it will tie up the facility's kiln for 4 days, and it seems waste of electricity...

 

 

 

 

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#7 Tenyoh

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Posted 25 May 2014 - 12:52 AM

Colby, I just read your reply. Thank you very much.

 

I see that holding the kiln temp to 190º means "preheat." I know the studio's kiln does have the feature. How do you determine the length of preheat? Min wrote the it will be depend on the thickness of the sculpture. Do you know any article that covers this topic?



#8 Benzine

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Posted 25 May 2014 - 08:31 AM

Truly awesome sculpture. Do those deep lines, on the torso, go all the way through?

I don't let my students make anything an inch thick, or over, without hollowing it out, and I still will generally use a four hour pre-heat.
I also go slow, and have a small hold around 900 F, as this is where the chemically bonded water is driven off, and if this is forced out too quickly, especially in thick pieces, it can still cause cracks/ explosions.
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#9 Colby Charpentier

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Posted 25 May 2014 - 09:12 AM

Colby, I just read your reply. Thank you very much.

 

I see that holding the kiln temp to 190º means "preheat." I know the studio's kiln does have the feature. How do you determine the length of preheat? Min wrote the it will be depend on the thickness of the sculpture. Do you know any article that covers this topic?

 

I don't know of any article, it's generally always a judgement call. The main concern is interior spaces that cannot breathe. If everything is well ventilated, there's not too much of a problem. Don't worry about the use of electricity, especially on a preheat. An insulated kiln requires very little power to maintain temperature.

 

For your specific sculpture, try to dry it as much as possible prior to firing, if areas are rather thick, ~an inch or so, give it a good 2 weeks to dry. The pre-heat is just an extra step to help make sure everything is safe. A 10 hour pre-heat is excessive, but it helps ensure no mistakes are made. Understand that I can't give a definitive answer without evaluating the piece in person, but the questions you're asking show that you're being cautious, and that's good for the situation...



#10 Benzine

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Posted 25 May 2014 - 09:57 AM

Colby, I just read your reply. Thank you very much.
 
I see that holding the kiln temp to 190º means "preheat." I know the studio's kiln does have the feature. How do you determine the length of preheat? Min wrote the it will be depend on the thickness of the sculpture. Do you know any article that covers this topic?

 
I don't know of any article, it's generally always a judgement call. The main concern is interior spaces that cannot breathe. If everything is well ventilated, there's not too much of a problem. Don't worry about the use of electricity, especially on a preheat. An insulated kiln requires very little power to maintain temperature.
 
For your specific sculpture, try to dry it as much as possible prior to firing, if areas are rather thick, ~an inch or so, give it a good 2 weeks to dry. The pre-heat is just an extra step to help make sure everything is safe. A 10 hour pre-heat is excessive, but it helps ensure no mistakes are made. Understand that I can't give a definitive answer without evaluating the piece in person, but the questions you're asking show that you're being cautious, and that's good for the situation...

Very true on all fronts Colby. When I am preheating the kiln, I sometimes will go check to see if it is on, because I hadn't heard a relay click on for a while.

In terms of drying, once something seems fully air dried, I will put a fan on it for. while, just to be safe.
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#11 Tenyoh

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Posted 25 May 2014 - 10:33 AM

These posts are very informative to me. Thank you very much, Benzine and Colby.

 

What I learned from your posts is the importance of drying pieces extra throughly. I will pre-heat the kiln for 4 hours and hold it at 900ºF for an hour(?). Benzine, thank you for the explanation. It helped me understand the reasons behind it.

 

The lines and holes on the torso go all the way through. (The sculpture is designed to put a light inside.) Hope the design will help it ventilate the interior and prevent cracking.

 

I expect to fire it sometime next week. I will let you know how it turns out.



#12 Min

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Posted 25 May 2014 - 10:39 AM

Nice work! As Benzine and Colby have said go really slow and long for the preheat, 10 hrs like Colby said would be a good idea. If you have fired similar sculptures with similar thicknesses with the slow ramp it might be okay. Would be safer to put in your own ramp schedule so you know you are going slow enough through critical temps. The grog will help it from cracking, the lines that go through might be the weak spot.

 

If you don't have a slab to fire it on I would put a layer of grog or silica sand down on the shelf for it to sit on. Works like little ball bearings so the piece can move more freely as it shrinks in the kiln.

 

My comment about a clay being labeled at mature through ^05 - ^6, it was the word "mature" that I had a problem with. Just can't be mature at earthenware temps if it can go to stoneware temps. Doesn't matter at all for sculpture, just functional stuff.

 

Be nice to see your sculpture when finished and lit up.

 

edit: If I was firing this I would use this type of schedule:

 

ramp1 / 25F hr / to 190F / hold 10 hrs.

ramp2 / 75F hr / to 1300F / 0 hold

ramp3 / 250 hr / to 2170 / 10 minute hold * keep an eye on a witness cone to determine hold time to bring ^5 down.

kiln off.

If the pieces were thicker than they are for this sculpture I would slow the cool down also as follows:

ramp4 / 9999 / 1300 / 0 hold

ramp5 / 150 / 1000 / 0 hold

kiln off.



#13 Tenyoh

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Posted 25 May 2014 - 11:11 AM

Min, thank you very much. OK, I will preheat the kiln for 10 hours to be safe. I think the studio has silica that someone donated. I will ask an instructor there if I could use it. Otherwise, I can slab-roll a large piece of clay. The diameter of the sculpture bottom is about 17". If I roll a slab as big as that, it may warp. I wonder if I make lots of cookies and put underneath it, will it work…?

 

Sorry, Min, I confused you with my use of term "mature." The clay box has a stamp that says ^05-^6. Although the box says so, I suppose it does not get fully vitrified at low-fire. I have not shipped my sculptures yet, but I worry they may break during the shipment unless they are vitrified. That is another reason I try to high-fire my sculpture whenever possible. 



#14 Min

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Posted 25 May 2014 - 11:22 AM

 I think the studio has silica that someone donated. I will ask an instructor there if I could use it. Otherwise, I can slab-roll a large piece of clay. The diameter of the sculpture bottom is about 17". If I roll a slab as big as that, it may warp. I wonder if I make lots of cookies and put underneath it, will it work…?

 

 

 

It's not silica but silica sand that works like ball bearings, like 50 mesh size. For something that large I would take the time to roll out some stabs of clay. It doesn't have to be 1 piece the the full 17", I would make several strips, all the same thickness and dry them between sheets of sheetrock so they stay as flat as possible. Make enough to span the entire 17" with a couple extra just in case. When they are dry then place them on the kiln shelf with the sculpture sitting on them. A bit of a gap is fine between the strips of slabs. Also, use the flattest kiln shelves you have.

 

ps, if you knew me you wouldn't confuse me with mature  :P



#15 Benzine

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Posted 25 May 2014 - 11:30 AM

I wouldn't worry too much about shipping something, that isn't vitrified. I've shipped Raku wares, and they survive just fine, despite their relatively weak nature.
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#16 bciskepottery

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Posted 25 May 2014 - 11:39 AM

Cookies or slats cut from a slab would help the sculpture move during the expansion/shrinking phases; they will also help on cooling as they will lift the sculpture off the kiln shelf (which tend to retain heat longer than the wares sitting on them) and allow air movement underneath. Make sure your slab/cookies/slats are also dry -- preferably the same dryness as the sculpture.

Your preheat will be important, as well as a slow build up of temperature, as others have pointed out. Check with your instructor about the Skutt Slow Bisque firing schedule, but take it up to Cone 5, and with your preheat. But slowing the cool down is also important -- if your Skutt is an automatic controller, there should be a way in the manual that describes how to program a slow cooling. Or check their website for instructions.

You said the sculpture was three pieces, to be assembled when done. Will all three be fired in place, or will the pieces be separate?

From your picture, there seems to be a potential for cracking lower right corner, where the goat lies. Not sure what you can do at this point, but the join there looks a bit iffy.

#17 Tenyoh

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Posted 25 May 2014 - 12:10 PM

I'm learning a lot from the exchange. Thank you very much.

 

I will roll lots' of cookies and dry them this week. Knowing all the benefit of the cookies now, I will start using them under all my sculptures.

 

Min, thank you for the schedule. I quickly browsed through the kiln manual on the manufacture's website. I should be able to set the ramp schedule.

 

bsiskepottery, the place where the goat's head is a separation. The female figure is 1 piece. The surrounding with the animals are in 2 pieces. I intend to assemble them in the kiln, so that if they warp, they will warp together. 

 

Benzine, I'm just worried about the potential breakage during the shipment. Thank you for reassurance, though. I try not to worry too much.



#18 Tenyoh

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Posted 25 May 2014 - 12:20 PM

I don't need to use paper clay to make cookies, right?, as long as I use the same red stoneware. The shrinkage of paper clay is about the same as the clay without cellulose fibers?



#19 Angie Days

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Posted 26 May 2014 - 03:14 PM

Hello, I do not think that you need to vitrify it to give strength to your sculpture.

I do ceramic sculpture and in the workshop that I belong to 21 inches is not considered large or risky.

If the grog is at least 30% you will have a good resistance. The paper fiber gives it even better resistance and makes it lighter and more porous, which in large size ceramics is good.

If there is no aesthetic reason to vitrify it I don't think it will be necessary, unless it is part of your concept.



#20 Tenyoh

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Posted 26 May 2014 - 04:56 PM

Thank you, Angie. I will contact the clay manufacturer and find out the grog content of all the clay they produce. The information should help me decide which clay to use depending of the design of my sculpture.







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