My pieces collapsed

[seancisse]

Hi, I've a problem. I made a piece with clay stoneware. When it dries = OK. When biscfire = OK. But when firing after glazing à 1220°C, the piece collapse, see pictures. Maybe the temperature is too high. But it's a long time I'm working this clay without problem. Whether I use earthware the piece doesn't collapse. But I want to use my glaze and I don't want to by glaze for earthware. What could I do ?

Thanks for advice

Sean

Collapsing.pdf

[Hulk]

Good question!

My guess is the clay softens just enough to overcome the form, where a cylinder or even an open shape - bowl - has enough structure to resist collapse, your four twisted ribbons of clay cannot, so.

Perhaps if the four arms had some arc, and no twists, they would stay upright.

You might try using a prop - a narrow cylinder - to hold the form upright, where the interface isn't glazed and has a bit of kiln wash or somewhat to prevent sticking. Even so, the twisted ribbons may still sag.

Good luck! I'm curious to know how your project comes out.

[Rae Reich]

Your collapsed piece is quite interesting as it is and demonstrates the nature of clay as it interacts with fire.
Romance!
Physics!
As you have discovered, you will need a different design to stand tall in high fire. Re-think your construction and make something that is structurally sound to show off your glaze. 

Edited October 5, 2023 by Rae Reich

[seancisse]

Thanks for your advices, I'll try something like a kind of fusible pillar in the center

Edited October 5, 2023 by seancisse

[Chilly]

How are you constructing the ribbons?  Rolled out flat, then bent and twisted?  If yes, then at stoneware temp the clay memory is kicking in and it wants to go back to how it was born - flat.

Might need to think a different way to create it.  Or build in a support.  Or fire to a lower temp/cone.  Try ^4 instead of ^6.

[seancisse]

Yes I've built my ribbons. Someone told me to use clay grog (or with grog) and may be it should work. I'll try with different size of grog.

[Callie Beller Diesel]

If the piece is collapsing that drastically, it’s not the size of the grog or the amount of it you’re using, it’s how much the clay that the temper is in is melting. You said this does not happen with earthenware, which is usually not fired to the point where it begins to melt as much.  You did not mention if you used a porcelain or a stoneware to make this piece, or what the product specifications of it are. If the clay is not meant to be fired to the same top temperature that your glaze is, you will have to change the clay to something that is. Or fire it with a support of some kind, much like you have it propped on in the first picture. The movement in the work in progress image has a lot of movement: using the kiln to create some more could be an interesting idea. 

 

[seancisse]

Right, it was stoneware.

[Callie Beller Diesel]

Can you let us know which one, and the glaze as well?

Most suppliers put their information online, and google translate is pretty good in the event the specifications aren’t already in English. More information would help us give the most practical answers.

[seancisse]

Hi Callie, distributed by a french supplier CERADEL. The clay is GT5700B (to be fired between 1250°C and 1280°C). The glaze is "vert pré" EG225 (to be fired between 1180°C and 1250°C).

I fired à 1220°C.

[PeterH]

I don't think that firing within the manufacturer's guidelines guarantees that your form won't slump.

Probably only an issue with forms outside the usually pottery shapes, as previously observed by Hulk.

On 10/4/2023 at 5:23 PM, Hulk said:

Good question!

My guess is the clay softens just enough to overcome the form, where a cylinder or even an open shape - bowl - has enough structure to resist collapse, your four twisted ribbons of clay cannot, so.

Perhaps if the four arms had some arc, and no twists, they would stay upright.

You might try using a prop - a narrow cylinder - to hold the form upright, where the interface isn't glazed and has a bit of kiln wash or somewhat to prevent sticking. Even so, the twisted ribbons may still sag.

Good luck! I'm curious to know how your project comes out.

Think of a sheet of printer paper. Hold it by the short edge and it flops about. Tape the long edges together and it forms quite a strong cylinder. Compound curves are probably even better at resisting deformation.

I suspect that during firing sheets of clay bend easily, unless the bending requires local stretching or compression of the sheet.

[Callie Beller Diesel]

Having a look at the glaze and clay information, it’s definitely not the clay being over fired. So +1 to what Peter said. The piece needs support, or a different design. 

[seancisse]

Thanks again for your comments, I'll see another shape or something else.

Keep you informed in case of success