Karin Hurt on sun 6 oct 02
On Thursday I fired my new 18: Olympic Raku kiln (on propane) for the first
time. I fired test tiles and a couple of turtles. All pieces turned out
wonderful and true to color. I knew this because I had fired some in class
where we have the same kiln only at 23".
On Friday I fired several items, I felt the kiln went up too fast despite my
warming it slowly, even on low I felt the flames were too high. The Pacman
glaze (pre-mixed) which is beautiful, turned a dull, ugly black. I fired
another piece in Pacman and it had some hint of the real thing.
80-20 Clear Crackle (pre-mixed) which crackled till the cows came home on the
test tile, had only a couple of large crackles.
Hiroshi White from Aardvark usually has nice, small crackles, in my firing?
Zip, nada, nothin'.
Essence Red (pre-mixed) which turned out beautiful in a class firing, was
the only one that turned out halfway decent.
Pieces that should have been shiny, and showed a beautiful luster in the test
tiles,were dull, what could have happened?
I used bunched up and newspaper in the garbage can. I left them in the can
for about 10 minutes (like we do in class), fired to 1900 degrees.
Any helpful hints?
Thanks for your input,
Karin, the gigantic Raku goof
Mudkitty on sun 6 oct 02
Dear Karin,
Others may have more complete answers to your questions, but hard experience tells me
that there may be one simple answer to many of the problems you note - timing - of
the move between the kiln and the reduction container.
For bright metallic colors and good dark blacks in the clay areas, a piece must be
moved quickly to the reduction container so that cooling and exposure to the air are
minimized.
For lots of crackle in white and 80/20 glazes, the piece can be cooled in the air for
a few seconds to a few minutes before putting it in the container.
My guess is that your first load had just a few small things with metallic glazes in
it, and that you were able to move them to reduction quickly and easily - short
exposure to air and little cooling = great colors.
Try firing fewer objects in each load - maybe only 3 or 4 until you get very good at
handling the transfer. Also, try mixing pieces in the firing - maybe one of metallic
glaze, and a couple of 80/20 or white. Then pull the metallic first, as fast as you
can - next pull the whites - they can safely crackle in the air a little.
Hope this helps - stay at it - firing raku is a bit of an art in itself!
Pat
mudkitty@att.net
In Asheville where it is a perfect sunny cool day....
Pottery by Dai on sun 6 oct 02
Hi, Karin - sounds like over-firing----you're burning the glaze right off
the pots . If you're copying the firing schedule that you have done
elsewhere (that worked), maybe their pyrometer isn't registering right.
1900 sounds to us like pretty hot! Our raku glazes do best between 1700 and
1800 F. Try another batch at a lower temp. You could even try re-glazing
the spoiled pieces and re-firing them (make sure they're very dry first).
Good luck!
Dai in Armstrong, BC
dai@potterybydai.com
potterybydai@telus.net
nightfire@telus.net
www.potterybydai.com
Take your work seriously---take yourself lightly. Unknown
Craig Clark on mon 7 oct 02
Karin, it sounds like you may have a coupla problems going
simultaneously here. First, keep in mind that the post reduction quality and
color of the glaze are influenced by the thickness of the glaze application,
the temperature to which you fire the glaze before removing it from the
kiln, the type of clay body (primarily iron content) of the clay body that
you are using, the speed at which you are able to get your pots into the
post firing reduction containers, the amount of reduction material in the
containers, the amount of flame action that you have in the containers, and
the intergrity of the reduction containers (do you actually get a decent
seal to prevent reoxidation.)
Specifically in the case of the clear crackle glaze it does not sound
like you gave the glaze enough time to cool and craze before you put it into
the reduction can. Did you hear any nice "tinking" sounds from the pot
berfore putting it into the can? Did you get any real flame action? That is
where the carbon deposition actually comes from. What did the glaze look
like when you took it out of the kiln? Was it actually smooth and glossy or
was it more of a matt? How do you know what temperature you actually fired
to was? Did you use a pyrometer or did you do it by sight? Have you tried
"burping" the cans a couple times to promote more crazing?
In the case of the glazes with the color problems it sounds as if you
may have either over fired, over reduced by letting the flame action happen
for to much time, or a combination of the two. Once again, did you closely
monitor the maturation of the glaze in the firing cycle. I would be hesitant
to suggest that the problem was the result of the kiln firing up to quickly
in that when I have done that the glossier glazes have ended up with a more
mottled quality. It
was a radical overfiring that resulted in the color literally being "burned"
out of the glaze. I suspect that the metallic oxides end up vaporizing as
the glaze begins to flow off of the pot from an overfiring.
Were the glazes that you used the same, not type but literally from the
same bucket, as the ones that you used in the school? Were they mixed and
sieved thoroughly before application? Were the glazes ever in the possesion
or use of anyone else who may have inadverdently contaminated them?
I'm unable to comment on the glazes that you have mentioned specifically
in that I have not used them. I mix all of my own.
Hope this helps a bit
If you have any questions contact me off list and I'll try to answer
Craig Dunn CLark
619 East 11 1/2 st
Houston, Texas 77008
(713)861-2083
mudman@hal-pc.org
----- Original Message -----
From: "Karin Hurt"
To:
Sent: Sunday, October 06, 2002 7:47 AM
Subject: Raku disaster
> On Thursday I fired my new 18: Olympic Raku kiln (on propane) for the
first
> time. I fired test tiles and a couple of turtles. All pieces turned out
> wonderful and true to color. I knew this because I had fired some in class
> where we have the same kiln only at 23".
>
> On Friday I fired several items, I felt the kiln went up too fast despite
my
> warming it slowly, even on low I felt the flames were too high. The Pacman
> glaze (pre-mixed) which is beautiful, turned a dull, ugly black. I fired
> another piece in Pacman and it had some hint of the real thing.
>
> 80-20 Clear Crackle (pre-mixed) which crackled till the cows came home on
the
> test tile, had only a couple of large crackles.
>
> Hiroshi White from Aardvark usually has nice, small crackles, in my
firing?
> Zip, nada, nothin'.
>
> Essence Red (pre-mixed) which turned out beautiful in a class firing, was
> the only one that turned out halfway decent.
>
> Pieces that should have been shiny, and showed a beautiful luster in the
test
> tiles,were dull, what could have happened?
>
> I used bunched up and newspaper in the garbage can. I left them in the can
> for about 10 minutes (like we do in class), fired to 1900 degrees.
>
> Any helpful hints?
> Thanks for your input,
>
> Karin, the gigantic Raku goof
>
>
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