Matt MacIntire on mon 17 dec 01
I could use some advice please...
1. In our wood kiln tenmoku glazes ALWAYS come out looking like tea dust
glazes. What might be causing this?
I've been firing in a wood kiln with some friends. The kiln is very simliar
to the design of the Peg Udall kiln in Jack Troy's book. No matter what
tenmoku glaze formula I try it ALWAYS comes out as a tea dust type of glaze.
I never get the traditional black, breaking to reddish where thin. I always
get a good black base, but with LOTS of little yellow-green specks all over.
The result is an impressive tea dust glaze, but I really miss the tenmoku
effects.
I've read Currie's excellent information about tenmoku glazes and tea dust
style glazes, but we seem to be getting tea dust in a wide variety of base
glazes. We get the same resuls on a variety of clay bodies.
I haven't done any systematic testing yet. Before I did, I want to ask for
guidance from the group about what sort of base glaze might work best.
FWIW... This is a hard brick kiln that takes a few days to cool. After
reaching cone 10, when we decide to stop stoking, we let the fire burn off
the grate and then close up the kiln and completely open the passive damper
in the chimney. I haven't monitored the cooling rate, but intend to next
time. I note that when we close up the kiln, there is always a big pile of
coals inside the firebox, which is inside the kiln. I imagine that this
maintains a reducing atmosphere for some considerable time as the kiln is
cooling slowly.
My speculation is that there isn't any reoxidising of the iron where it has
recrystalized out of the glaze solution in thin spots. I also presume that
this hard brick kiln cools slowly enough to allow these yellow-green
crystals form extensively.
If I took the wise approach I would try not to fight the kiln, but
learn what it does naturally. So far, I know we can get good tea dust
glazes. This kiln does not get much ash build up since it fires fairly
quickly and has a strong draft. Since this is a group setting, and since
it is not my kiln, it will be hard to implement any big changes in the
firing process itself. I wish we fired more often, but we only fire a few
times a year, so progress for me is fairly slow. Next time I intend to do a
great deal of testing.
So... Let me ask a second question:
2. In a slow cooling wood fired kiln that will probably not reoxidize too
much, what types of glazes would you expect to work especially well? (and
on what sort of body - if that is relevant)
Thanks in advance for your help.
Matt
Craig Martell on mon 17 dec 01
Hello Matt:
I was thinking that it might be fly ash that's making the teadust. You
could fire some small pots with tenmoku in saggars to protect them from the
ash and see if this makes a difference. I would imagine that your wood
kiln has a slow cool too and this is always a good crystal making scenario.
The teadust effect is from crystal growth and these are a product of
Magnesia and silica or Magnesia, Calcium, and silica. So, you want to
avoid magnesia in tenmokus. These glazes are mainly potash feldspar,
whiting, clay, and silica with iron for color.
regards, Craig Martell in Oregon
Dave Finkelnburg on mon 17 dec 01
Matt,
Regarding the tea dust, you have nailed a principal contributor, the
slow cooling. Since you can't change that, you might investigate the
titanium content of your glaze ingredients, and test the same glaze made
with ingredients which are as low in titanium as possible. The titanium is
probably what's giving you the lovely surface crystals! :-)
Good potting,
Dave Finkelnburg in Idaho
James Bowen on tue 18 dec 01
Cardew reminds us that the iron content of the body
certainly plays a part and Currie claims underfiring also is
a major contributor. We have had a lot of teadust effect in
the past especially those pots glazed with Temoku over Ohata
Kaki. Also our firings were heavily reduced in a slow
cooling (70 degrees/hr) kiln. Add to that we underfired the
glazes (seldom reaching cone 9). the last two firings we
were much more efficient. Thanks to the digital thermometer.
We reached cone 10 very flat throughout. Moderate reduction.
White clay instead of the usual buff. No teadust.
Stay Centered
James Bowen
Boyero CO
jbowen43@yahoo.com
jbowen43@plains.net
Jon Pacini on tue 18 dec 01
Hi Matt--Holiday greetings to one and all-------fly ash is an interesting
concept. If they were landing on the surface I would expect to see it
predominately on one side of bottles and vases. I fired some paper cutouts
on glazed plates once upon a time and though the paper shrank considerably,
they did leave a pattern. I thought it was the Kaolin used to whiten the
paper left behind on the glaze surface effectively Matting it.
There's a number of minerals that tend to crystallize out in slow
cooling, zinc and titanium/rutile and the saturated iron itself being the
principal culprits. Since you can't effectively speed up the cooling you
might want to try to add a crystal inhibitor to your glaze, such as alumina
in the form of kaolin. Iron also likes to break red/brown in high alumina
glazes, an effect you are also looking for---just something else to add to
your list of tests to try----
Jon Pacini
Clay Manager
Laguna Clay Co
Lee Love on tue 18 dec 01
I've noticed in the Noborigama, that the temmoku like glaze we use (Kuro Ame)
will turn teadust if the oxidation chamber is unintendedly reduced. Try an
oxidation atmosphere.
I think I'll be building a two chamber woodfire kiln because I want to
do the traditional mino glazes (oribe) and nuka, nuka sage, kaki and temmokus.
My second chamber will fire oxidation for these glazes.
--
Lee Love
Mashiko JAPAN Ikiru@kami.com
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Ruth Ballou on tue 18 dec 01
Hi Matt,
Both oak and pine ash have substantial amounts of magnesia and in my
experience, this has been the material most responsible for tea dust. It
doesn't take much.The Fe2 experiment in Currie's book is probably the most
relevant one. On my test, it shows tea dust in the lower left quadrant. As
the silica and alumina increase, the tea dust disappears. Are you salting
as well? If yes, then any recipe you use is fluxed back toward the lower
left quadrant. Firing hotter will also move the effect and slow cooling
gives the pyroxene crystals more time to grow. I'd try a tenmoku from the
uppermost right quadrant of the Fe1 experiment ( the one without MgO) for
the best chance of avoiding tea dust.
If you like to see the tests, I can scan the tiles and send the pics as an
attachment. Given the recent virus activity, I don't want to send
attachments without warning.
Ruth Ballou
Paul Taylor on wed 19 dec 01
Dear matt
Your tea dust glaze is enviable; but if you do not want it I suggest you
make your tenmocu a little more fluid, or fire a little higher, or in a
neutral atmosphere, or try spraying a layer of clear glaze over the tenmocu.
or glazew thiner. All this will inhibit the forming of nuclei on the surface
of the glaze which prevents crystallization . all this is to try absorbing
all the iron into the glaze.
and/or Take draw trials and find out at what temperature the
crystallization is happening and fast cool at that temp.
Falling ash could also be setting off a reaction so fire a pot well
protected from ash, but not in a closed saggar as they tend not to let the
pot get as hot as the rest of the kiln.
If your kiln is indeed slow cooling the shinoes should work well. and if you
look up some of those japanese master potters sights you can see some glazes
that use the effects of crystallization (on slow cooling) decoratively. How
they do it exactly is their secret that would only work in their kiln any
way .
Thats not a lot of help. I have just told you to do the obvious and I expect
you have tried all those. so if you do find a solution will you tell me? I
think the importance of firing schedule are underrated and am very
interested in other peoples experiences . What is the cooling rate of this
kiln (roughly - i do not need an equation)
Regards from Paul Taylor
http://www.anu.ie/westportpottery
Alchemy is the proof that economics is not a science.
> From: Matt MacIntire
> Reply-To: Ceramic Arts Discussion List
> Date: Mon, 17 Dec 2001 10:00:49 -0500
> To: CLAYART@LSV.CERAMICS.ORG
> Subject: turning to dust
>
> I could use some advice please...
>
> 1. In our wood kiln tenmoku glazes ALWAYS come out looking like tea dust
> glazes. What might be causing this?
>
> I've been firing in a wood kiln with some friends. The kiln is very simliar
> to the design of the Peg Udall kiln in Jack Troy's book. No matter what
> tenmoku glaze formula I try it ALWAYS comes out as a tea dust type of glaze.
> I never get the traditional black, breaking to reddish where thin. I always
> get a good black base, but with LOTS of little yellow-green specks all over.
> The result is an impressive tea dust glaze, but I really miss the tenmoku
> effects.
>
> I've read Currie's excellent information about tenmoku glazes and tea dust
> style glazes, but we seem to be getting tea dust in a wide variety of base
> glazes. We get the same resuls on a variety of clay bodies.
>
> I haven't done any systematic testing yet. Before I did, I want to ask for
> guidance from the group about what sort of base glaze might work best.
>
> FWIW... This is a hard brick kiln that takes a few days to cool. After
> reaching cone 10, when we decide to stop stoking, we let the fire burn off
> the grate and then close up the kiln and completely open the passive damper
> in the chimney. I haven't monitored the cooling rate, but intend to next
> time. I note that when we close up the kiln, there is always a big pile of
> coals inside the firebox, which is inside the kiln. I imagine that this
> maintains a reducing atmosphere for some considerable time as the kiln is
> cooling slowly.
>
> My speculation is that there isn't any reoxidising of the iron where it has
> recrystalized out of the glaze solution in thin spots. I also presume that
> this hard brick kiln cools slowly enough to allow these yellow-green
> crystals form extensively.
>
>
>
> If I took the wise approach I would try not to fight the kiln, but
> learn what it does naturally. So far, I know we can get good tea dust
> glazes. This kiln does not get much ash build up since it fires fairly
> quickly and has a strong draft. Since this is a group setting, and since
> it is not my kiln, it will be hard to implement any big changes in the
> firing process itself. I wish we fired more often, but we only fire a few
> times a year, so progress for me is fairly slow. Next time I intend to do a
> great deal of testing.
>
> So... Let me ask a second question:
>
> 2. In a slow cooling wood fired kiln that will probably not reoxidize too
> much, what types of glazes would you expect to work especially well? (and
> on what sort of body - if that is relevant)
>
>
>
> Thanks in advance for your help.
>
>
> Matt
Ron Roy on sun 23 dec 01
It's magnesium - take any tenmoku glaze and add some - you will see the
green crystals that make tea dust.
Could it be true that some ash does not have any MgO - if so - use that
wood - other wise you have to sagger.
RR
>I could use some advice please...
>
>1. In our wood kiln tenmoku glazes ALWAYS come out looking like tea dust
>glazes. What might be causing this?
Ron Roy
RR# 4
15084 Little Lake Rd..
Brighton,
Ontario, Canada
KOK 1H0
Residence 613-475-9544
Studio 613-475-3715
Fax 613-475-3513
Wesley Rolley on sun 23 dec 01
For Ron Roy;
You wrote:
>It's magnesium - take any tenmoku glaze and add some - you will see the=
>green crystals that make tea dust.
This is in line with my accidental observations. I have gotten the tea =
dust effect using Rhodes 22 loaded with RIO, but not gotten the effect=20=
when using the Cone 10 Glossy glaze that you have posted on Digital Fire=
=20
with the same load of RIO. =20
However, there were some tea dust glazes posted here recently that=20
included nickel oxide. At least two people questioned the need for=20
nickel oxide without any response. Do you understand what the nickel=20=
oxide does in the glaze? Is it's effect similar to that of magnesium? =
Or is there something else at play here?=20
--=20
Wesley C. Rolley
wrolley@refpub.com
"I find I have a great lot to learn =96 or unlearn. I seem to know far =
too=20
much and this knowledge obscures the really significant facts, but I am =
getting on." -- Charles Rennie Mackintosh
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