Warped rims on my porcelain sculptures

[Bernardita Cossio]

Hi everyone!

I have been struggling with warping rims on my porcelain pieces. I was wondering if you could give me some advice on this matter. To provide some context, I make porcelain pieces using handmade leather molds. The challenge I am facing is keeping the rim in a circular shape, as it warps during the glaze firing process. I am considering firing the pieces at cone 6 to minimize the warping. However, I am also concerned that firing them upside down could cause the body's weight to bend the walls. 

 

 Do you have any suggestions or methods I could use to avoid this issue?

[Rae Reich]

A thicker rim will hold its shape better, if you can work that into the design/construction.

Q: is it strictly necessary to have a perfectly round rim? The intriguing soft appearance of your sculpture does not, to me, require a perfectly circular rim.

[Min]

The concave then convex side design might also be contributing to it being pulled out of round. 

[PeterH]

21 hours ago, Bernardita Cossio said:

I have been struggling with warping rims on my porcelain pieces. I was wondering if you could give me some advice on this matter. To provide some context, I make porcelain pieces using handmade leather molds. The challenge I am facing is keeping the rim in a circular shape, as it warps during the glaze firing process. I am considering firing the pieces at cone 6 to minimize the warping. However, I am also concerned that firing them upside down could cause the body's weight to bend the walls. 

 Do you have any suggestions or methods I could use to avoid this issue?

Yes, but it may introduce more problems than it solves.

To my untutored eye it looks like firing with a sitter would probably solve your slumping issues.

... but would introduce glazing issues.

The classic bone china solution uses a supported high-fire bisque and an unsupported lower-fire glaze, which creates its own problems.

Bone China https://digitalfire.com/glossary/bone+china
The process is completely different than what a potter would do: Bisque fire, glaze, high fire. Bone china is bisque fired to high fire and then glazed at a very low temperature. Since the porcelain has zero porosity, getting a glaze to stick and dry on it is not easy, the process needed goes well beyond what a normal potter would be willing to do.

PS

Balancing slumping and decoration has a long history. Robert Tichane expressed his surprise when he finally realised that the ancient Chinese cup he regularly drank from had a guilded rim to disguise the lack of glaze on the rim (a result of firing the cup rim-down to minimise slumping). 

I'm having difficulty visualising your "handmade leather molds". I assume that they are semi-flexible press-moulds, but am unsure how you extract the bowl from the mould.

Edited March 12 by PeterH

[neilestrick]

Unless you make it considerably thicker, porcelain is going to warp with a form like that. 

[Hyn Patty]

I second Peter's suggestions on this one.  I bisque fire all of my porcelains and bone china before I glaze (if I didn't acquire it already bisqued).  But then I also tend to use airbrushed underglazes with clear glazes over top so getting the glaze to stick has never been an issue for me.  It's not brush applied.  As said though, that presents other issues like a ventilated and filtered spray booth and suitable P100 type mask due to spraying silica, etc.    But with this form I think you are going to get the distortion without proper supports even in your bisque fire without glaze.  I'm not as knowledgeable about pottery though.  I work with sculpture so it's a slightly different beast.

Edited March 31 by Hyn Patty

[Jarman Porcelain]

I made a body of sculptures out of porcelain paper clay that were somewhat similar. I was using a translucent cone 6 porcelain and firing to cone 8. To prevent them from collapsing in the kiln I made a rudimentary saggar that I placed them into, then filled the void, as well as the piece, with silica sand. This went about half way up the side, I stabilized the "handles" at the top by rolling up some fiber blanket into a tight tube and running through each loop. For more complex shapes you can use kanthal wire to bind rolls to each other. Make the saggar (maybe armature is a better word?) out of cone 10 raku clay or fire clay so you can reuse them forever. 

Good luck, those pieces look great already.

[Kelly in AK]

Support for the pyroplastic clay seems the name of the game.