supporting unstable forms in the kiln?

[youpital]

Hello,

currently I am busy with making a porcelain, slip cast lamp cap and need some advice for firing it because of the unstable form. I have read some posts about using setters, silver sand or even custom kiln stilts but I just feel lost in the absence of guides and forums talking about it. The problem with the lamp cap is that it is cone shaped but with a slid from the point of the cone, all the way trough the outer rim. When bone dry, it can stand on its own but is very fragile but when fired in the kiln it just warps and puddles in a flat lillypad shape (which is obviously not what I want it to do)

 

I want to make something that holds it shape when being fired in the kiln. And hope that someone has some advice.

kind regards,

[Kelly in AK]

It sounds like it’s being over fired. If it’s a low wide cone, perhaps fire it point side up with just a lump of clay underneath supporting the center. A picture might help people understand your problem better. 

[PeterH]

Over-firing sounds a likely cause, but it could be that the highly unstable form may be the issue†. (If I understand your description.)

Are you glazing, and if so at what temperature? Bone china often needs sitters, and  typically has a high bisque firing (so it doesn't stick to the sitters) and a low glaze firing (so it doesn't slump without the sitters). Are you painting yourself into a similar corner?

† If a cup-shape slumps you get a distorted cup. If a bent sheet slumps a flat sheet must be a real possibility.

Edited July 8, 2023 by PeterH

[neilestrick]

Post a picture of the lamp. What slip are you using?

[youpital]

Hello again, I also think that I over fired it the first time but even if I would fire the shape, it will probably deform a lot 

here are some pictures to clarify how the shape looks, I know that most of the time these forma are unstable but id love to experiment with it!

And for the slip im using a premixed witgert g52 mixture

Edited July 9, 2023 by youpital

[youpital]

13 hours ago, PeterH said:

Over-firing sounds a likely cause, but it could be that the highly unstable form may be the issue†. (If I understand your description.)

Are you glazing, and if so at what temperature? Bone china often needs sitters, and  typically has a high bisque firing (so it doesn't stick to the sitters) and a low glaze firing (so it doesn't slump without the sitters). Are you painting yourself into a similar corner?

† If a cup-shape slumps you get a distorted cup. If a bent sheet slumps a flat sheet must be a real possibility.

Right now I am looking at bisque firing the piece, I haven't got any further than that. But I read about setters used for bone china but cant find more information. Ideally I would like to make my own setter or just in general "support" so that i could bisque fire the porcelain and glaze the inside of the cone later.

[Callie Beller Diesel]

With industrial bone china, setters or cranks would be made to fully support rims or other points that were prone to sagging. They also make more efficient use of your kiln space. The full support versions were basically a bowl/plate that the other bowl/plate would be fired in. The support is just made of a refractory material that doesn’t shift during the firing.

If you’re not going into mass production yet with this design or are only wanting to making a few, you can build a pile of refractory sand in your kiln and gently push the piece into it, so the sand acts as that full support. I seem to recall that Witgert has a wide selection of different grogs/chamotte, and they might have something specific for using this way, or they could possibly make better suggestions. 

 

[neilestrick]

Yeah that's about the most unstable form you could try to make with porcelain! In bisque it should hold shape okay, but glaze firing is going to be an issue without a lot of support. Without the slot you could probably get it to work, but with that slot pretty much all structural integrity is gone.

[Jeff Longtin]

Have you thought of firing the piece upside down? (for the bisque anyway) Build up a mound of sand, as Callie suggests, and then push the upside down cone shape into it. It won't help you in the glaze firing but at least you can reduce distortion in the bisque.

[youpital]

On 7/12/2023 at 3:11 AM, Jeff Longtin said:

Have you thought of firing the piece upside down? (for the bisque anyway) Build up a mound of sand, as Callie suggests, and then push the upside down cone shape into it. It won't help you in the glaze firing but at least you can reduce distortion in the bisque.

I have contacted Witgert for this matter and they would send me a sample of fine kilnproof sand for this exact reason. However I am still researching how I can produce my own "profiled setter", but it takes a but more effort than I could have imagined :,) Right now I'm trying to reimagine a normal store bought setter that can be stapled/placed above each other in the kiln to I can stack more of those lamps in the kiln. 

I am thinking of two solutions; one, is a way to stack profiled (the cone shape) kiln shelves with a layer of the sand so that the piece stays in tact, the other solution would be just making a normal cone shaped profiled setter to support the object. 

I am only looking at bisque firing the piece for now so I do not worry about the glaze

Does anyone have a guide/experience on how to make profiled setters for bone china?

 

[youpital]

I have found some imaging of which I could base the second solution from my last post.

I would change the form so the cone would be fired upside down and the bottom would also be supported.

Any thoughts?

 

[PeterH]

59 minutes ago, youpital said:

Does anyone have a guide/experience on how to make profiled setters for bone china?

There is a little bit on one-use setters for bone china in
Tempting Transparency
https://ceramicartsnetwork.org/pottery-making-illustrated/pottery-making-illustrated-article/Tempting-Transparency#

 

Ideas on the body to use for multi-use setters
 

Bone China AntiWarp Setter test molds
https://digitalfire.com/picture/3156
... Tony Hansen uses a "kiln shelf" body for his setter L3693H1

 

Edited July 14, 2023 by PeterH

[PeterH]

27 minutes ago, youpital said:

I have found some imaging of which I could base the second solution from my last post.

I would change the form so the cone would be fired upside down and the bottom would also be supported.

Any thoughts?

Fine if you didn't have the slot and the form almost supported itself. But IMHO the area near the slot is going to droop unless it is supported over ~100% of its surface.

[Min]

1 hour ago, youpital said:

However I am still researching how I can produce my own "profiled setter", but it takes a but more effort than I could have imagined :,)

Recipe below would probably work but it's going to take a huge amount of effort plus a kiln that can fire them to cone 10. Link is behind a paywall but you can access 3 free articles per month.

https://ceramicartsnetwork.org/pottery-making-illustrated/pottery-making-illustrated-article/In-the-Studio-Make-Your-Own-Kiln-Setters#

 

[youpital]

18 hours ago, PeterH said:

Ideas on the body to use for multi-use setters
 

Bone China AntiWarp Setter test molds
https://digitalfire.com/picture/3156
... Tony Hansen uses a "kiln shelf" body for his setter L3693H1

 

the setter in this example gives me one question, on the top right of the image the slip cast cup is perfectly fitting on the inner ring of the refractory setter, how does that work since the cup should still shrink in the kiln? wouldn't the cup tear from the shrinkage because of the setter? 

... Tony Hansen uses a "kiln shelf" body for his setter L3693H1

[PeterH]

38 minutes ago, youpital said:

the setter in this example gives me one question, on the top right of the image the slip cast cup is perfectly fitting on the inner ring of the refractory setter, how does that work since the cup should still shrink in the kiln? wouldn't the cup tear from the shrinkage because of the setter? 

A good question, and I'd like to know the answer.

Presumably - in this case - it doesn't cause problems, although it may depend on the nature of the body and the firing range.

Tony has always been very quick and professional  in responding on the very few occasions I have draw attention to minor errors in his pages: e.g. misleading typos and wording.

Maybe this is a case where you might want to take advantage of the $5 offer at the bottom of his web page.

Edited July 15, 2023 by PeterH

[youpital]

4 minutes ago, PeterH said:

Maybe this is a case where you might want to take advantage of the $5 offer at the bottom of his web page.

Thanks Peter, I will look into it. 

I will update when I have some more information.

[youpital]

Have just modeled a simple form of setter that could be stacked on top of each other and supports the whole cone.

 

                                    

[Callie Beller Diesel]

I think you’re onto the right path with that. Next step is to test it. 

[Min]

On 7/15/2023 at 3:38 AM, youpital said:

the setter in this example gives me one question, on the top right of the image the slip cast cup is perfectly fitting on the inner ring of the refractory setter, how does that work since the cup should still shrink in the kiln? wouldn't the cup tear from the shrinkage because of the setter? 

It will work if the cup was fired to maturity first and then a lower fire glaze applied and fired without the setter.

On 7/15/2023 at 5:47 AM, youpital said:

Have just modeled a simple form of setter that could be stacked on top of each other and supports the whole cone.

 

                                    

Even though there is very little shrinkage from greenware to bisque fired I would hazard a guess there is enough to crack this piece where the little nub in the middle is.   As the clay shrinks it will squeeze up against the nub, nub won't shrink, clay will and it will crack. 

Why are you only worried about the bisque of this form? Will you leave it unglazed or ?

If you have access to spray glazing the finished pieces I would use your setter design as above but flip the slack over and have the clay fit the inside of the setter, including the nub then fire to maturity first then do a low fire spray glaze and re-fire.

[PeterH]

7 minutes ago, Min said:

If you have access to spray glazing the finished pieces I would use your setter design as above but flip the slack over and have the clay fit the inside of the setter, including the nub then fire to maturity first then do a low fire spray glaze and re-fire.

I think that would give far better support, but handling the lip at the slot might be tricky.

[youpital]

52 minutes ago, Min said:

Why are you only worried about the bisque of this form? Will you leave it unglazed or ?

For now I will leave the piece unglazed at the top side, and only glaze the inside with a high gloss glaze, but I also don't want to think of glazing too much yet since the piece hasn't been bisque fired properly till this day.

 

 

54 minutes ago, Min said:

If you have access to spray glazing the finished pieces I would use your setter design as above but flip the slack over and have the clay fit the inside of the setter, including the nub then fire to maturity first then do a low fire spray glaze and re-fire.

This afternoon I thaught about the same thing, and already made some changes to the setter:

Still have a plaster mold of which I should be able to make setters easily. However I am not entirely sure if leaving the setter at the middle open like you can see in the pictures above, I don't think there would be big and notable side affects?

[Jeff Longtin]

Min has picked up on what I was suggesting. 

If you fire a cone form on TOP of a cone form you will create a stressful situation, for the clay, as it shrinks in the firing. (The clay will shrink around the cone and probably crack as a result.)

If you fire a cone shape with the cone pointing downward, in a setter that is also a downward cone, a bowl basically, then the cone will only sink deeper into the cone as it shrinks. As there is nothing inside the cone you are not putting stress on the clay. The likely hood is greater that it will retain its cone shape this way.

Leaving the middle of the cone support open allows the clay to shrink and sink without interference. If there is no hole then you run the risk of the cone bottoming out in the middle of the setter.

[PeterH]

How do you achieve the taper on the conical section, solid casting?

... and how thick is it at the rim?

[Kelly in AK]

9 hours ago, Jeff Longtin said:If you fire a cone form on TOP of a cone form you will create a stressful situation, for the clay, as it shrinks in the firing. (The clay will shrink around the cone and probably crack as a result.)

If you fire a cone shape with the cone pointing downward, in a setter that is also a downward cone, a bowl basically, then the cone will only sink deeper into the cone as it shrinks. As there is nothing inside the cone you are not putting stress on the clay. The likely hood is greater that it will retain its cone shape this way.

Pardon me if this has been addressed. Clay on the outside of the form will shrink on and crack, left to its own devices.  Clay on the inside will shrink away from the mold and survive. Clay fired on the outside of a sitter has a similar risk if tolerances haven’t been calculated.