Joyce Lee on fri 16 feb 07
Last week one of my rakued pieces was a leaf with a rainbow raku glaze
on white stoneware. Rather elegant looking, but still it was mostly a
shiny, somewhat metallic taupe color. Very acceptable for what it was.
However, along the narrow sides of the leaf, I glimpsed a little of
the anticipated color.
So... I put the large leaf in my next bisque firing, which was in my gas
Geil (first bisque in a gas kiln!) . The 08 cone tipped; the 06 was
straight up. The leaf was the only color in the kiln when opened. It
is a soft, light green..... all black was gone from the bottom. Still
very pretty, but not elegant.
Anybody care to hazard what happened since I'll probably do the same
thing again with other pieces...... and it would be confidence-building to
have some idea what to expect and why.
Thank you.
Joyce
In the Mojave anticipating NCECA and the Clayart Room... less than 4 weeks now!
Wowwowweewow.........
Marcia Selsor on sun 18 feb 07
Joyce,
This is the great thing about raku and how it teaches students about
different atmospheres in kilns.
You reoxidized the leaf in your bisque firing. The carbon burns out
at about 500 degrees and the copper went green in the oxidation
atmosphere.
Simple as that.
Marcia
Marcia Selsor
http://marciaselsor.com
Craig Clark on mon 19 feb 07
Joyce, I'm winging it here a bit since I don't completely understand
what you are after. First, the absence of the black is do to the second
firing. The blackened colour comes from the deposition of carbon in the
clay during the post fire reduction. This carbon was consumed in the
second firing. If you want a blackened color back you will need to
reduce the pot again.
The colors that you see are a result of the firing and post firing
reduction as well. I suspect that copper is the primary oxide you are
using for color. If this is the case the green that you see is what you
get when copper is pretty much in an oxidized state. If you see a copper
looking color then the oxide has been reduced. This is a very loose
explanation of what is going on but good enough for an basic
understanding of things. The variation that you see in the rainbow
quality has to do with different concentrations of carbon in the post
reduction firing. Reduction, once again very loosely, has to do with
carbons affinity for oxygen. When the glaze is molten, and the pot is
put into post firing reduction, the carbon is bonding with the oxygen in
copper oxide thereby "reducing" it. Ivor, Tom or a number of others can
give you a far more detailed explanation of this than I. Point is that
the more carbon, the more bonding with the oxygen, the more reduction,
the more pronounced copper look. Less reduction, more of a green look.
Thinking about the temperature to which you are firing, and
depending on what type of glaze you are using, it sounds like you may be
firing a bit low. If you want more action out of the glaze try taking it
up to 1900 F plus if it will fire that high. Once again this very much
depends on the type of glaze that you are using.
Hope this helps
Craig Dunn Clark
619 East 11 1/2 St
Houston, Texas 77008
(713)861-2083
mudman@hal-pc.org
When you opened the kiln door the molten glaze was in an oxidJoyce
Lee wrote:
> Last week one of my rakued pieces was a leaf with a rainbow raku glaze
> on white stoneware. Rather elegant looking, but still it was mostly a
> shiny, somewhat metallic taupe color. Very acceptable for what it was.
>
> However, along the narrow sides of the leaf, I glimpsed a little of
> the anticipated color.
> So... I put the large leaf in my next bisque firing, which was in my gas
> Geil (first bisque in a gas kiln!) . The 08 cone tipped; the 06 was
> straight up. The leaf was the only color in the kiln when opened. It
> is a soft, light green..... all black was gone from the bottom. Still
> very pretty, but not elegant.
>
> Anybody care to hazard what happened since I'll probably do the same
> thing again with other pieces...... and it would be
> confidence-building to
> have some idea what to expect and why.
>
> Thank you.
>
> Joyce
> In the Mojave anticipating NCECA and the Clayart Room... less than 4
> weeks now!
> Wowwowweewow.........
>
> ______________________________________________________________________________
>
> Send postings to clayart@lsv.ceramics.org
>
> You may look at the archives for the list or change your subscription
> settings from http://www.ceramics.org/clayart/
>
> Moderator of the list is Mel Jacobson who may be reached at
> melpots@pclink.com.
>
| |
|
