Craig Martell on fri 25 mar 05
Sincultura was asking:
> what I wanted to know was why single firing makes those
>big porcelain platters survive the firing without cracking. I feel
>I'm missing a piece of information when I read that more heat in the
>bisque helps the big ware stand the glaze firing and at the same
>time it would also survive the firing if you skip the bisque step
>all together=85 but like I said I'm new to matter discussed.
Hello Sincultura:
Most often what makes large pieces, and especially large porcelain pieces=20
crack is uneven cooling thru the quarz inversion. This will happen on the=
=20
way up when the kiln is heating and on the way down when the kiln is=20
cooling. Quatz will expand about 2% when it's at the inversion point and=20
above. Upon cooling, quatz will contract at the same percentage.
When raw clay bodies are being bisqued or single fired they usually=20
traverse the inversion point without failure unless the firing is radically=
=20
quick and the pieces are poorly made. Raw clay has a lose structure=20
because of the chemically bound water in the lattice. The term for the=20
chemically bound stuff is hyroxyls. This just refers to the hydrogen and=20
oxygen present. The hydroxyls collapse and the water is liberated=20
somewhere between 1300 and 1400 F. Kaolin and ball clays too will then=20
have a structure called metakaolin that's a tighter configuration than raw=
=20
kaolin. If the ware isn't fired to a high enough temperature to gain some=
=20
fired strength there's a much higher chance of cracking or "dunting" when=20
the ware passes thru the inversion point. Large flatware pieces are more=20
prone due to the size and form. When a piece takes up more room in the=20
kiln it is subjected to more of a temperature differential. Think of a=20
platter sitting on a hot shelf when cooling. The rim is closer to the edge=
=20
of the kiln and will cool quicker than the foot. The rim goes thru the=20
quartz inversion first and contracts and becomes smaller before the=20
foot. If the ware has little fired strength, the rim will crack and you=20
will see this extend toward the center of the piece. More fired strength,=
=20
less chance of cracking.
Single firing develops an even stronger piece than a hotter bisque. The=20
ware is fired to full maturity and has developed body glass and is as=20
strong as it will get so almost no chance of dunting.
This has nothing to do with big ware surviving a glaze fire. The crux is=20
surviving the quartz inversion during cooling. Either a hot bisque or a=20
full fusion single fire will give the ware enough stength to have a better=
=20
than good chance of surviving. Since I increased my bisque temp from 06 to=
=20
03, I've had no dunting at all and that's years of making pots. For me,=20
the proof was in the pudding. But, as mel and Kathi have said, different=20
clays and different types of ware have their own parameters.
regards, Craig Martell Hopewell, Oregon
sincultura13 on sun 27 mar 05
thanks Craig Martell and Ron Roy, I think I get it now. Since I only
low fire I had no reference as to why it works out that way... now I
know :)
Sincultura
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