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Callum Donovan-Grujicich

Low-Fire Underglaze on High-Fire Clay?

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I am planning on making a sculpture out of cone 6 porcelain. I would like to use underglaze on the sculpture, so I bought some. But upon further inspection, I realized that the maximum temperature for the underglazes is cone 04. Is it okay if I bisque fire the porcelain to cone 6, then apply the underglaze and fire to cone 04? The porcelain has a porosity of 1% when fired, if that helps.

Thanks,

Callum

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I would apply the underglaze to the leather hard piece and bisque normally and then high fire with a coat of clear glaze.  If you don't want glaze, you can decorate at leather hard and then fire straight to cone 6.

A lot of underglaze says cone 04 but is fine to cone 6.  Cone 10 is a different story. At cone 10 some underglazes lose color.

What brand and model are the underglazes you used?

Edited by liambesaw

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9 minutes ago, liambesaw said:

I would apply the underglaze to the leather hard piece and bisque normally and then high fire with a coat of clear glaze.  If you don't want glaze, you can decorate at leather hard and then fire straight to cone 6.

A lot of underglaze says cone 04 but is fine to cone 6.  Cone 10 is a different story. At cone 10 some underglazes lose color.

What brand and model are the underglazes you used?

They are the 500 series of Spectrum underglazes. Specifically a white one, a red one, a black one, and a brown one. The clear glaze also says to fire to cone 04, so is it okay to fire to cone 6 for this one as well?

Edited by Callum Donovan-Grujicich

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4 minutes ago, Callum Donovan-Grujicich said:

Okay. It would probably actually be better for me to glaze fire to cone 04, in terms of power use. Thanks for the help!

It’s sculpture so you can fire to whatever you want.  Cone 6 clay fired to cone 04 will only be sintered and not vitrified meaning it  wont really be melted and it will be quite porous. It’s sculpture you can fire anyway you like realizing that clay fired  to maturity will be stronger than clay not fired to maturity. 

The 500 series underglaze says their colors fired to 04 should match their chart and fired to cone 5/6 may change slightly. Test fire to find out how much a color may change.

I believe they also make  cone 5/6 clear glazes in the 1100 series. Sculptures often fire to as high a temperature as they can for strength without changing their desired color.

Last sculpture piece I can remember gas firing for someone because of its giant size had to go no hotter than cone 2 so the color of her sons army uniform remained the perfect green. The artist had taken the time to test fire her underglazes and above cone 2 they started to fade ever so slightly. We fired the cone 6 clay to cone 2 and she coated the whole sculpture with spray urethane to protect it for years  come.

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Yes some colours will fade out if fired higher. Reds can do this.

Low fired clay if not vitrified will run into probs if outside in weather. A gradual disintegration and poss. Cracking if experiencing frosts and frozen water...porosity, expansion.

We've all seen the crumbling terracotta flowerpots...

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Many underglaze colors will hold up just fine at cone 6. A few will burn out, a few will shift color. It all depends on the brand and the color. It's definitely worth testing what you have, though, just do it on a little waste pot and see what happens before proceeding with your sculpture.

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