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firing schedule for LARGE & heavy works


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#1 sandi

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Posted 17 October 2012 - 10:04 PM

i'm using an earthenware , will fire @ 06 or 04 ( bisque / possibly 1x-fire). Pieces non-grogged /not paper clay ( due to fine surface finishing via rasping). Pieces 26" tall / varying thickness from 1"-3".
Any experience with such firings would be greatly appreciated. Using a rented digital skutt kiln. What firing schedule would you recommend ? Is it possible to NOT use the ramp/hold side and use instead the programed SLOW setting with a HOLD thru the water evaporation for 12 ???? or 24 ????? hours .
Please advise ..................sandra

#2 bciskepottery

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Posted 17 October 2012 - 10:25 PM

Does the digital controller have a Preheat option? If so, I'd suggest a Slow Bisque with a Preheat of a minimum of 12 hours due to the 3" thickness; you could go longer to be on the safe side. The Preheat fires up to 200 degrees F and holds it for the amount of time programmed; that allows water and moisture to be released. Preheat with lid cracked open slightly or top peep out. After the preheat period, check with a mirror to see if steam is still escaping; if so, extend the preheat (reprogram the firing). If not, drop the lid and let it fire.

Success will depend on the dryness of the forms (something 3" thick may feel dry but it's hard to tell what is inside) and going slow on the firing. I use to fire sculpture forms at a studio and they were always a crap-shoot. Some did better than others. You might also want to put some grog or cookies on the kiln shelf to help allow the forms to expand and contract easily and to get them some air circulation between the shelf and item . . . helps prevent cracking and uneven heating/cooling.

#3 justanassembler

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Posted 18 October 2012 - 01:10 AM

Does the digital controller have a Preheat option? If so, I'd suggest a Slow Bisque with a Preheat of a minimum of 12 hours due to the 3" thickness; you could go longer to be on the safe side. The Preheat fires up to 200 degrees F and holds it for the amount of time programmed; that allows water and moisture to be released. Preheat with lid cracked open slightly or top peep out. After the preheat period, check with a mirror to see if steam is still escaping; if so, extend the preheat (reprogram the firing). If not, drop the lid and let it fire.

Success will depend on the dryness of the forms (something 3" thick may feel dry but it's hard to tell what is inside) and going slow on the firing. I use to fire sculpture forms at a studio and they were always a crap-shoot. Some did better than others. You might also want to put some grog or cookies on the kiln shelf to help allow the forms to expand and contract easily and to get them some air circulation between the shelf and item . . . helps prevent cracking and uneven heating/cooling.


with something that thick, it might be wise to fire it on a waste slab and to slow down your climb through quartz inversion as well, the early "water smoking" period is important to treat with care, but equally so is this later portion of the firing.

#4 neilestrick

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Posted 18 October 2012 - 01:58 PM

Have you fired pieces like this before? Aside from the firing issues, I'm concerned about them being a grog-free terra cotta with thicknesses varying from 1-3". I think there's a really good chance they're going to crack regardless of how you fire them.

3" is REALLY thick. Do a Ramp/Hold. I'd give it a full 24 hour hour preheat and go half as fast as the slow program. You should be able to find the slow program steps in your manual.
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#5 sandi

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Posted 22 October 2012 - 09:49 PM

Have you fired pieces like this before? Aside from the firing issues, I'm concerned about them being a grog-free terra cotta with thicknesses varying from 1-3". I think there's a really good chance they're going to crack regardless of how you fire them.

3" is REALLY thick. Do a Ramp/Hold. I'd give it a full 24 hour hour preheat and go half as fast as the slow program. You should be able to find the slow program steps in your manual.


thanks for the input............The good news on these pieces are they have no back only a 'facade' .......as they will hang on a metal frame vertically in a wall niche..........( attached a photo ). In reality the thickness is likely to be mainly 1 inch to maybe 2 " in the end...............Yes i will fire them on a bed of grog vertically on end with support walls for each one.
You would concur with previous response to fire very slow thru quartz inversion as well then ..........so holding not only 24 hours @ 180 degrees but at what temp as well ?
( hope the image attaches - not evident ......) thanks much and hope to hear back from you .........sandra Attached File  IMG_0402.JPG   247.32KB   77 downloads

#6 sandi

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Posted 23 October 2012 - 12:07 AM

Does the digital controller have a Preheat option? If so, I'd suggest a Slow Bisque with a Preheat of a minimum of 12 hours due to the 3" thickness; you could go longer to be on the safe side. The Preheat fires up to 200 degrees F and holds it for the amount of time programmed; that allows water and moisture to be released. Preheat with lid cracked open slightly or top peep out. After the preheat period, check with a mirror to see if steam is still escaping; if so, extend the preheat (reprogram the firing). If not, drop the lid and let it fire.

Success will depend on the dryness of the forms (something 3" thick may feel dry but it's hard to tell what is inside) and going slow on the firing. I use to fire sculpture forms at a studio and they were always a crap-shoot. Some did better than others. You might also want to put some grog or cookies on the kiln shelf to help allow the forms to expand and contract easily and to get them some air circulation between the shelf and item . . . helps prevent cracking and uneven heating/cooling.


great to be getting responses here.........So taking Neil's recommendations into account I will use the ramp/hold side - and preheat thru the h20 evaporation period - for at least 24 hrs. then. I was going to settle each piece ( 3 sections) into a bed of grog - would you recommend a clay cookie instead ?
Do any of you have a recommended ramp/hold schedule ? This firing can afford to be l o n g and v e r y slow - it is a commission i can't afford to have explode in the kiln .............thanks all .

#7 perkolator

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Posted 23 October 2012 - 04:54 PM

in our studio we make mainly large-scale sculpture and it's not uncommon for pieces to get into the 2"+ thickness with questionable wetness at the core, since students all build differently. our studio body is a stoneware body with around 15% grog (20m and 35m blend) in it to facilitate with the large scale and questionable thicknesses. we also mainly once-fire to ^04 for these large pieces and sometimes there will be pieces going into the kiln that are not yet bone dry. i use a very conservative firing schedule and it's very seldom that i blow items up in the kiln.

since you say your clay body has no grog in it, i'd say that right there will be the most difficult thing to overcome and there is a high chance your piece may crack somewhere. the piece could even crack just from the uneven wall thickness you mentioned, since the different thicknesses will pull the clay unevenly as it shrinks. i would recommend a very slow climb as well as down-firing this piece to prevent cooling cracks if your kiln cools fast.

i would definitely NOT recommend using the cone fire mode on the Skutt. even on it's slowest setting, it's still way too fast for large scale work and is more appropriate for pottery and things that are "thin" without a lot of mass. i would definitely use ramp/hold and manually program your schedule. it will definitely help to have previously made work like this before when figuring out a schedule.

if this were a piece being fired in our studio, i'd probably try a program like this but i cannot guarantee safety of your piece since i've never really fired work this thick when made from an un-grogged clay body:

5-6 ramps:
1- 50*/hr to 185, hold 7-8hrs
2- 40*/hr to 195, hold 8-9hrs
3- 30*/hr to 205, hold 9-10hrs
4- 50-60*/hr to 450*, no hold to possibly a few hours hold
5- 100-150*/hr to 1941*, hold 5-15min if glazed, off, cool on its own.
down-fire
6- 150*/hr down-fire to around 1200-1000*, kiln off and let cool on its own.

the first 3 ramps purge out the water slowly, especially with each segment climbing slower than the previous to help draw it out even longer. i use this first 3 ramps for pretty much everything greenware, but vary the hold times depending on thickness and wetness. the 4th ramp is the start of the actual firing and I usually like to go 100*/hr to 450*, then blast off at 200*/hr to ^04 -- but for a piece that's super thick i'd definitely slow it down to somewhere near what i mentioned in the schedule above. the hold time at 450* is kinda up to you - it's definitely over the boiling point of water so if steam were to escape it would have already blown up, but it's mostly to help get the core of the thick work up to the same temp as the outside of the piece before continuing the firing since thick work takes time to get energy absorbed to the core. the main firing ramp, #5, i'd probably go slow like recommended and would stay at the conservative side if you have the time. down-firing is up to you, but i've seen plenty of pieces crack when cooling, especially if it's not fired in a packed kiln that's going to cool really slow.

that schedule will get you "close". perhaps some others can edit it or provide an alternative, but that's where i'd start. i'm all about experimenting but sounds like you won't want to be going this route since you mention it's a commission.

good luck.

#8 sandi

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Posted 24 October 2012 - 04:24 PM

in our studio we make mainly large-scale sculpture and it's not uncommon for pieces to get into the 2"+ thickness with questionable wetness at the core, since students all build differently. our studio body is a stoneware body with around 15% grog (20m and 35m blend) in it to facilitate with the large scale and questionable thicknesses. we also mainly once-fire to ^04 for these large pieces and sometimes there will be pieces going into the kiln that are not yet bone dry. i use a very conservative firing schedule and it's very seldom that i blow items up in the kiln.

since you say your clay body has no grog in it, i'd say that right there will be the most difficult thing to overcome and there is a high chance your piece may crack somewhere. the piece could even crack just from the uneven wall thickness you mentioned, since the different thicknesses will pull the clay unevenly as it shrinks. i would recommend a very slow climb as well as down-firing this piece to prevent cooling cracks if your kiln cools fast.

i would definitely NOT recommend using the cone fire mode on the Skutt. even on it's slowest setting, it's still way too fast for large scale work and is more appropriate for pottery and things that are "thin" without a lot of mass. i would definitely use ramp/hold and manually program your schedule. it will definitely help to have previously made work like this before when figuring out a schedule.

if this were a piece being fired in our studio, i'd probably try a program like this but i cannot guarantee safety of your piece since i've never really fired work this thick when made from an un-grogged clay body:

5-6 ramps:
1- 50*/hr to 185, hold 7-8hrs
2- 40*/hr to 195, hold 8-9hrs
3- 30*/hr to 205, hold 9-10hrs
4- 50-60*/hr to 450*, no hold to possibly a few hours hold
5- 100-150*/hr to 1941*, hold 5-15min if glazed, off, cool on its own.
down-fire
6- 150*/hr down-fire to around 1200-1000*, kiln off and let cool on its own.

the first 3 ramps purge out the water slowly, especially with each segment climbing slower than the previous to help draw it out even longer. i use this first 3 ramps for pretty much everything greenware, but vary the hold times depending on thickness and wetness. the 4th ramp is the start of the actual firing and I usually like to go 100*/hr to 450*, then blast off at 200*/hr to ^04 -- but for a piece that's super thick i'd definitely slow it down to somewhere near what i mentioned in the schedule above. the hold time at 450* is kinda up to you - it's definitely over the boiling point of water so if steam were to escape it would have already blown up, but it's mostly to help get the core of the thick work up to the same temp as the outside of the piece before continuing the firing since thick work takes time to get energy absorbed to the core. the main firing ramp, #5, i'd probably go slow like recommended and would stay at the conservative side if you have the time. down-firing is up to you, but i've seen plenty of pieces crack when cooling, especially if it's not fired in a packed kiln that's going to cool really slow.

that schedule will get you "close". perhaps some others can edit it or provide an alternative, but that's where i'd start. i'm all about experimenting but sounds like you won't want to be going this route since you mention it's a commission.

good luck.


thanks much for your input..........I called up Skutt and have also been working with Perry on a VERY slow firing schedule with at least 24 hrs. on the soaking / h20 evaporation/ candling segment........So another 3 weeks or so to wait now till pieces dry...........and THEN I'll be keeping ( everything) crossed ................( yes we are firing down as well with the schedule Perry suggested ........)

#9 Dinah

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Posted 26 October 2012 - 07:01 PM

Hi Sandi,
Perry at Skutt rocks. Follow his wise words. I know you'll have a successful firing. Let us know how you've done!
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#10 trina

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Posted 03 February 2013 - 08:26 PM

Hi there,

I would imagine your slow bisque programm you are talking about woild work just fine. I also wouldn't think that the spray in the pieces would harm your kiln. The only thing I would do is fire the objects on a good bed of kiln sand or make some coils just to place underneath so that they have a chance to move. I can see why you are apprehensive about firing though, your work is really beautiful and it would be a shame to lose it in the process. There is always going to be some risk.....

I am sure you will get quite a few responses to this question.

T

#11 neilestrick

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Posted 03 February 2013 - 11:47 PM

Even bone dry pots need to go slowly to drive out that last bit of remaining moisture. I would give them a 12 hour preheat, then fire even slower than slow bisque. Do a custom program, altering the slow bisque to get it up to 20+ hours. I've fired pieces that thick for a customer on slow bisque and had cracking problems.
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#12 trina

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Posted 04 February 2013 - 02:45 AM

Hi I agree with Neil as well to a certain extent, you could build in some more time between segment 5 and 6. But call me crazy I would have just stuck the programm you were planning. Raku clay is very normally very forgiving. Just dont get too excited and open the kiln before it has totally cooled down. T. send pics

#13 perkolator

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Posted 07 February 2013 - 12:00 PM

"cone fire" on pretty much all the commercially made kilns are not really programmed for sculpture, which is almost always thicker and larger than what most ceramicists in the world are making - pottery/functional work. since our studio is 99% sculpture, rarely do we have success using cone fire mode on slow setting unless it's small things that don't have much mass to them. for your pieces, i would definitely recommend manual programming to fire slower than the slowest cone fire mode your kiln has.

#14 trina

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Posted 08 February 2013 - 02:40 PM

sounds good to me...can't wait to see the finished product. GOOD LUCK to you and i will try to point the kiln gods in your direction (not that i have any influence there whatsoever!) fingers crossed T

#15 trina

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Posted 10 February 2013 - 11:03 AM

Yippe that looks great! I just came online to have a whine about why we haven't seen some pics yet and lo and behold....

The warp is a bit of a drag, I personally don't think that you can re-fire it out... For a piece like that I would have fired it flat on some clay coils so that any movement happens on a flat surface. That may or may not have prevented the warp though. You can have greenware that looks totally flat but good old clay memory shows in the firing.

Good work, what are your plans for the lion?

T

#16 Marcia Selsor

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Posted 10 February 2013 - 01:19 PM

Really like your work. My architectural pieces are not as thick. You are safer with the drying out/pre-heat. I do fire my large pieces on 1/4" rolls of clay to help circulate the heat and allow steam to escape.

I did recently unwrap a piece while I was in France. It was a black clay that warping in drying. I was drying things too quickly in time for an exhibition and kiln time pressure. It is the tile of my avatar. I made supports for the 2"framed edge and the middle tile was on the coils. I have never seen this achieved in 45 years of working with clay. Just dumb luck. The piece was once fired to cone 5. It flattened out. It was thinner that your pieces. about 14 x 14"

Marcia

#17 trina

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Posted 12 February 2013 - 05:06 AM

well that is all looking great, next you are going to have built yourself a huge kiln for the really big pieces. T

#18 trina

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Posted 23 February 2013 - 04:01 PM

looks like its all paying off...great stuff T

#19 Benzine

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Posted 24 February 2013 - 10:17 AM

RDWolff, I really like your Deco pieces....Actually, I really like all your work, great details. Where does most of it end up? From some of the pictures, it appears some of it is for businesses.
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#20 Fireoflynn

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Posted 12 March 2013 - 03:02 PM

Thanks for the input, I have 3 pieces I'm going to take over shortly to fire, and see if I can figure out how the programming works on the controller. One of the models is the course red clay I used for the angel.
Just for reference, this is the angel panel which was fired to 05, she is 20" long and weighs 28#, this was fired with the Bartlett slow cone program, 13 hours and about 11 minutes total time, an Orton kiln vent was used through the firing and shut off about 45 minutes after the kiln turned off.

There are no cracks or warpage. The 3 pieces I will fire today are close in size, and about the same wall thicknesses, one is a little thinner actually as I holled it out a bit more extensively, it is the same red clay, while the other 2 pieces are the raku clay 04-10

I figured since this angel worked on the slow fire with no problems, I shouldn't need to do more than add maybe 3 hours in the program to slow it down a little, and maybe a short hold at the end to soak it better all the way through, the lioness model is a different story, but for now I'm going to fire some of these models that are about the size/weight/thickness of the angel.

I plan to fire to cone 1 today, as this red clay has a darker richer red at that temperature that I'm anxious to see on something larger than a sample.
I am thinking it might be best to put these in the kiln with the hollow backs facing towards the coils, just seems like it might be better though in practice it might not make abit of difference but my way of thinking on that is with the larger surface areas in the back compared to the front, the radiant heat will have an easier job reaching the inside, and it might heat the model a little more evenly that way too.
Feel free to chime in with comments on any of these aspects.


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The back side, the thickness of the walls average 1-1/4"


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The other 2 pieces to be fired today are this keystone, in raku clay, completed in 2008, and a monogram panel which is thinner walled than the angel panel, so it should do fine. It was completed last July so I know they are good and dry throughout. The last photo is a cherub keystone, it too is very hollowed out in the back and is thinner walled than the angel, so this one should be fine as well.

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