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anne mossman

Bloating On Refired Porcelain

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I use Southern Ice porcelain, bisque fired then sanded then fired to cone 9 and sanded again, I don't use glazes. I have started making complex neriage coloured work, using the same clay body. I sometimes develop fine cracks that can successfully be patched and refired to cone 9 but the result is an all over tiny bloating of the surface. I have read a lot about bloating and the possibility it is due to over firing but don't really know what that means. I have twice fired thinner, slip cast work (from the same clay) without developing this problem. Could it mean I need to slow down the firing at one or more stages?

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Min    783

Bloating is from the body breaking down and out-gassing. Firing slower isn't going to solve it, firing to a lower temp the second time might. Are the slip cast pieces from the same body? If so it might just be luck that they didn't bloat or they were perhaps in a cooler part of your kiln.

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neilestrick    1,381

What type of kiln are you firing in?

 

It could also be that the colorants are having an effect. What are you using for color?

 

You may also want to look into working at cone 6. You can get cone 6 porcelain that will look just as good, your colorants will be more stable, and you'll save on firing costs.

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Thanks for the answers and questions.

 

The colourants are both body stains and oxides and I've been using them in the slip cast work for a long time so I'm very familiar with the issues so I've discounted that. Also the plain white body is bloating so it's the clay body not the additions causing the issue.

 

I fire in oxidation in a small 3.8cuft brick kiln made by a local Australian manufacturer, Tetlow. I'm happy with the kiln and it seems to fire fairly evenly.

 

The slip cast pieces were from the same body of clay, I make up the slip myself.

 

I'm beginning to orient towards changing the clay body to a cone 6 type.......it's just that change factor, moving from something you've been so familiar with for 8 years. 

 

I was hoping to resolve this issue that I have not come across before thinking it might be something fairly simple but it doesn't look like it is.

 

This is a pic of the work I'm waiting to refire but I think I may repair a couple of cracks with gold leaf instead of having it fire again and be all bloated.post-64110-0-25708400-1405039447_thumb.jpg

post-64110-0-25708400-1405039447_thumb.jpg

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neilestrick    1,381

I'm assuming it's an electric kiln? Have you checked the accuracy of the firing with actual cones on the shelf, or are you relying on the computer or sitter? I'd start with that. It could also be an issue with the clay, but that's not as likely. Kiln accuracy changes over time as the thermocouples wear out, and sitters can go out of calibration.

 

If you switch to cone 6 your element life will be almost double what it is now, your bricks will last longer, lower fuel bills, etc. If you're firing in oxidation there's very few good reasons to be working at cone 9. There should be little difference in the working quality of the cone 6 version of your porcelain. You'll have to run a few color tests, but other than that I think you'll find the transition to be fairly smooth since you're not working with glazes.

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Yes I'm almost convinced I should go to a cone 6 body so I'll start some experiments with the colourants in the next few weeks. But I'll also check the (electric) kiln with cones as well. Thanks for your input.

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