Long cooling

[Des & Jan Howard]

Hank
I've been using the electric kiln to give pre-fired
shino pieces a long soak at 1040C (~1900F).
Each time extending the firing by 1/2 hour.
The last 2 firings, a 6 hour soak & a 6 1/2 hour
soak have produced some unpleasant results. Cracking
apart of some cups, to the extent of large crumbles.
Last firing a large thick dish cracked.
Each time the cracks follow the craze lines 99%.
Some pots do, some pots don't.
Any thoughts??
Des

--
Des & Jan Howard
Lue Pottery
Lue NSW
Australia
2850

02 6373 6419
www.luepottery.hwy.com.au
-32.656072 149.840624

[Hank Murrow]

On Oct 12, 2009, at 3:37 PM, Des & Jan Howard wrote:

>
> Hank
> I've been using the electric kiln to give pre-fired
> shino pieces a long soak at 1040C (~1900F).
> Each time extending the firing by 1/2 hour.
> The last 2 firings, a 6 hour soak & a 6 1/2 hour
> soak have produced some unpleasant results. Cracking
> apart of some cups, to the extent of large crumbles.
> Last firing a large thick dish cracked.
> Each time the cracks follow the craze lines 99%.
> Some pots do, some pots don't.
> Any thoughts??

Dear Des;

Is it possible that you are encouraging Cristobalite formation with
these post-firing soaks? I only experience such behavior with 'wild'
porcelains that have an excess of free silica, and the fix is to add
a small portion of Neph Sye to soak up the excess of silica.

Before I did the correction, some pieces would break apart in the
bottom of the cooling cycle(572F) into hundreds of small pieces, like
tempered glass when it goes.

Cheers, Hank

[Lee]

Maybe Hank. Mick Cassion worked with a clay body that had a lot of
silica in it and found that crash cooled resulted in less Cristobalite
formation. Euan also did this with his high silica hanjiki, half
porcelain.

>
> Dear Des;
>
> Is it possible that you are encouraging Cristobalite formation with
> these post-firing soaks? I only experience such behavior with 'wild'
> porcelains that have an excess of free silica, and the fix is to add
> a small portion of Neph Sye to soak up the excess of silica.
>
> Before I did the correction, some pieces would break apart in the
> bottom of the cooling cycle(572F) into hundreds of small pieces, like
> tempered glass when it goes.
>
> Cheers, Hank
>
> >
>

--
--
Lee Love in Minneapolis
http://mingeisota.blogspot.com/
"The intuitive mind is a sacred gift and the rational mind is a
faithful servant. We have created a society that honors the servant
and has forgotten the gift." -- Albert Einstein

[Louis Katz]

Shino glazes with lithium minerals in them may have all sorts of odd
things forming in slow cooling. I have seen the shatter routine in
shinos and have always attributed it to a high contraction glaze ontop
of a low contraction glaze. Could be some low expansion mineral
forming on cooling.
If the glaze is just a feldspar (Japanese style) shino than
cristobalite seems as good a guess as any, but each time through
quartz inversion you can only expect the cracks in the body to grow.
There are always cracks.

Louis

[Des & Jan Howard]

Hank
OK. Let's go through some variables.
Two bodies, both low iron, one a tight fine stoneware,
the other closer to a Shigaraki style gritty clay.
Glaze is a soda feldspar, high alumina type.
12 hour firing cycle in LPG fibre kiln. Damper is
closed on completion of cycle, burner port is left
open. Pretty much a crash cool. Glaze is finely crazed.
After long re-firing soak some pots, not all, crumble
on pressure, like an ice cream cone. Not shatter
spontaneously, just crush when squeezed.
Des

Hank Murrow wrote:
> Is it possible that you are encouraging Cristobalite formation with
> these post-firing soaks? I only experience such behavior with 'wild'
> porcelains that have an excess of free silica, and the fix is to add
> a small portion of Neph Sye to soak up the excess of silica.
>
> Before I did the correction, some pieces would break apart in the
> bottom of the cooling cycle(572F) into hundreds of small pieces, like
> tempered glass when it goes.

--

[Lee]

What is the silica content of the clay.

[Des & Jan Howard]

Lee
Lue grey stoneware
Na2O 0.3% Al2O3 27.8% SiO2 67.4%
K2O 2.1% TiO2 0.3%
MgO 0.5% Fe2O3 1.5%
CaO 0.0%
Volatiles:
Other 9.8%
Loss On Ignition: 9.0%

Three components 10:30:60
Hydrothermally altered rhyolite 10%
potash feldspar, silica, kaolinite
Na2O 0.2% Al2O3 17.8% SiO2 78.5%
K2O 1.7% Fe2O3 1.7%
Volatiles:
Other 7.6%
Loss On Ignition: 7.1%

White plastic kaolin 30%
potash feldspar, silica, kaolinite
Na2O 0.2% Al2O3 36.8% SiO2 59.9%
K2O 0.6% TiO2 1.1%
MgO 0.5% Fe2O3 0.8%
CaO 0.1%
Volatiles:
Other 13.7%
Loss On Ignition: 12.0%

Volcanic ash 60%
Quartz, illite, muscovite mica, dickite (same chemical
formula as kaolinite, different crystal structure)
Na2O 0.4% Al2O3 25.1% SiO2 69.2%
K2O 2.9% Fe2O3 1.7%
MgO 0.6%
CaO 0.0%
Volatiles:
Other 8.4%
Loss On Ignition: 7.7%

The grit clay is the same recipe with coarse rhyolite
added.
Des

Lee wrote:
> What is the silica content of the clay.

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