Jesse Hofmann-Smith on tue 21 nov 06
This is my first post (woo hoo!)
I was a ceramics student at the U of Oregon. Fast forward a few years, and I've moved to portland,
working, and as of last week, I've been thinking a lot about pottery.
I've done a limited amount of raku work, and I'm pretty familiar with the basic concepts. I'm
curious, however, what you all have to say about a "high temperature" raku kiln. Here's my train of
thought: how about a kiln (I'm thinking that the walls and ceiling should lift off w/ a pully-cable
system) that instead of going to just bisquish temperature, got closer to the stoneware
temperature. Glazes, of course would have to have a different composition, and the body would
probably have to have a different composition (more grog?/ refractory material) to avoid complete
vitrification and subsequent cooling shattering.
Have any of you heard or done such experimentation? I'm guessing a lot of the copper effects that
raku is known for might not work on a hotter temperature scale, which is too bad. Any other
difficulties I might encounter? Would hard bricks stand up to a kiln being opened at such a hot
temperature? I think I could lift the kiln up, and then drop it back down in less than a minute (if
that), but I wouldn't want the furniture to shatter, etc...
How about making an afternoon out of it? Could I load in fresh bisqued pottery in a very hot kiln?
Would it shatter? Could I pre-heat it in a different kiln (or a different chamber of the same one)?
Jesse
Snail Scott on wed 22 nov 06
At 11:05 PM 11/21/2006 -0500, you wrote:
>I've done a limited amount of raku work, and I'm pretty familiar with the
basic concepts. I'm
>curious, however, what you all have to say about a "high temperature" raku
kiln...
OK, a couple of thoughts:
Kyanaite can do wonders for thermal shock,
and I have seen fairly vitrified stoneware
with kyanite survive the raku process.
Pulling from a hotter kiln will be a bitch.
Hardbrick of sufficient rating will probably
survive the shock, but it will almost
certainly shorten its lifespan. Why not use
a fiber kiln?
If you load bisque into a hot kiln, you need
to preheat it, whether it's going into a
hot kiln or a very hot kiln. It's the first
few hundred degrees that bite the worst.
If it's copper flash you want, 'bisque'
doesn't have to mean low-temp. All it
really means is 'unglazed'. You can bisque
to vitrification, then refire using
low-temp raku glazes like the copper luster
flash glazes. No need to create a stoneware
kiln that also allows for fast pulls. Just
fire the dang stuff bare, then glaze with
regular raku glazes (using glue in the glaze,
or heat, or whatever to make it stick) then
raku as usual in a raku kiln. Just use plenty
of kyanite in the clay! The glaze fit will
probably be bad, but that's normal for raku
anyway. Nore: you won't get any black smoke
effects on crazing or on exposed clay; a
vitrified body won't absorb it. If you want
that look, you'll have to get it by other
means.
Which brings us the actual question: why?
What do yo hope to accomplish if you do
achive a high-temperature raku process - and
is a 'high temperature raku kiln' really
what you are after, or is that just your
presumed prerequisite to the true intention:
To have vitrified functional ware? (If you
use raku glazes, their stability on foodware
is still variable.) For strength? That may
not be as big a difference as you hope,
though it may add some. To be frostproof?
Anyway - clarify your long-range intent, not
just the means you think you need; you may
not need that means after all.
-Snail
Bruce Girrell on wed 22 nov 06
Our kiln has a hinged front door (refractory ceramic fiber insulation) that
can be opened at any temp. I have, in fact, opened it at cone 10 just for
the heck of it. I think a front opener would be much easier to deal with
than a top hat, as you are envisioning.
What do you want to accomplish with this, though? That should drive how you
go about it.
I have heard of one potter who takes pieces up to the vitrification point
and then opens the kiln to manipulate their shape.
In terms of time, it is still going to take a lot of time to get up to high
temperature. Anything that you introduce at high temp will have to be
preheated in an auxiliary kiln, at least past the quartz inversion point. As
compounds in the glazes start to break down at high temperature they emit
gases and you will have to take that into account. The glaze could be blown
off the pot by the gas expansion, for example. Think about the boiling of
the glaze that you observe in a raku kiln. Now think of that happening
almost instantly as the glaze is subjected to the intense heat of your hi
temp kiln. So your preheat may have to be almost as hot as your final temp.
Many high temp effects occur during the cooldown portion of the firing. Body
color, for example, is created during the cooldown. A clay that has a nice
toasty brown color when fired normally will be a dull gray if pulled at peak
temperature.
So I have to ask again - What do you want to accomplish with this technique?
We may be able to answer better if we know what goal you have in mind.
Bruce Girrell
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