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Isculpt

Firing ranges for low fire clay

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I usually work in terra cotta or other low fire clays. When there is a firing range given for a clay, I understand that the high end of the range will provide the most strength in the finished, fired piece (although I'm told that it won't become vitreous if it's lowfire clay), but what, specifically, is the lower number telling me? Will there be a significant difference in the strength of the piece if I fire it at the high end of the range or the low end? And is a low fire clay a poor choice for a necklace pendant? Will it be fragile no matter how high it's fired?

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I usually work in terra cotta or other low fire clays. When there is a firing range given for a clay, I understand that the high end of the range will provide the most strength in the finished, fired piece (although I'm told that it won't become vitreous if it's lowfire clay), but what, specifically, is the lower number telling me? Will there be a significant difference in the strength of the piece if I fire it at the high end of the range or the low end? And is a low fire clay a poor choice for a necklace pendant? Will it be fragile no matter how high it's fired?

 

 

Depends on how accurate the given range is. Amazingly, most clay producers don't even do MOR test on their clays and some advertize clays as having a "maturity" range from something like cone 3 to cone 10 which is total BS. It is either way underfired at cone 3 or way overfired at cone 10. I've always assumed firing any clay as high as you can without it beginning to deform will result in that clay's strongest state, but John B. here says that isn't true, that a clay can begin to lose strength from overfiring before it deforms. But, still, it's a pretty sure bet that if your clay's maturity range is from cone 04 to cone 01 that it will be stronger at cone 01 than at cone 04.

 

Clay strength depends of the composition of the clay, lack of additives like sand and grog, and being fired to full maturity, not how high it is fired. Pete Pinnell's MOR tests even showed earthenware fired to maturity was stronger than the stonewares and porcelains fired to maturity at cone 10 that he tested. Rob Roy later questioned the accuracy of the earthenware part of the test. In short, if your terra cotta is grog- and sand-free and fired to maturity, it should be as strong as most other clays.

 

Jim

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For jewelry, a lot of potters make choices based on color results ... the same red clay looks extremely different at all the various firing temps. They stop when they like the color.

As to its being fragile ... its jewelry so it's supposed to be treated with care ... I have glass jewelry, fragile beads, raku pendants ... nothing broken yet.

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For jewelry, a lot of potters make choices based on color results ... the same red clay looks extremely different at all the various firing temps. They stop when they like the color.

As to its being fragile ... its jewelry so it's supposed to be treated with care ... I have glass jewelry, fragile beads, raku pendants ... nothing broken yet.

 

 

Good point. I like Lizella Red which goes through a whole range of reds from a pale orange around cone 1 getting darker until it is a dark brown at its maturity at cone 8. I like it best around cone 4 but usually fire it to cone 6. For about a century this clay was used to make flower pots from shot glass-size to pots big enough for a kid to hide in and they were shipped all over the country packed in a little pine straw. They were fired to about cone 010.

 

Jim

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Well, Jim, I hope Pete Pinnell is right about the strength of low fire clays, since I got deeply into the jewelry thing before I considered the question of final strength of the fired clay! And Chris, you have a point about the inherent fragility of jewelry. When I sold my woodcarvings, whether jewelry or sculpture, I sent them out with a promise of "lifetime free repairs". Given that I've just finished repairing a 25 year-old wood sculpture -- gratis, you'd think I'd let that whole worrying-about-durability thing go! Now that I'm working in something far more breakable than wood, I believe I'll have to.

;) Jayne

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