Waltraud Koestler on sun 2 nov 08
Hello all,
a friend of mine asked me for help.
She works with Raku.
She has the problem that the colour of the glaze turns to black when temperature for taking the piece out of the kiln is reached.
She works with a small top loading Raku kiln fired with gas.
She fires up to 950° C.
She opens the kiln in order to check if the pieces/glaze reached the proper state so that it can be taken out of the kiln for post reduction.
At the first glance the glaze looks ok to be taken out of the kiln. However after a few seconds the colour starts turning to black.
This happens not in every firing so that the glaze could not be the reason for that.
What happens there?
Could that be caused by the weather condition e.g. inversion weather condition?
Or could that be caused by under or overfiring?
Or can that be caused by a sudden heavy reduction?
I can't help her because I don't have any knowledge about Raku firing.
Could someone help?
Best regards
Waltraud
- Germany-
--
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Steve Mills on mon 3 nov 08
Waltraud,
We've had this happen with glazes that contain any form of cobalt -- they s=
eem to have a finite life of anything from 12 to 18 months. In all cases th=
e glaze concerned is very simple and has a high-ish frit content coupled wi=
th a bit of clay and the cobalt and displays the same symptoms you describe=
.. It's almost as if the glaze is like a Wine; keep it too long and it goes =
"off"!!=20
We've not yet found a cure, so after a glaze has passed the 12 month date w=
e make a new batch and keep it in reserve for when the old batch goes bad.
Steve
Bath
UK
--- On Sat, 11/1/08, Waltraud Koestler wrote:
From: Waltraud Koestler
Subject: Raku firing - glaze colour turns to black
To: CLAYART@LSV.CERAMICS.ORG
Date: Saturday, November 1, 2008, 11:14 PM
Hello all,
a friend of mine asked me for help.
She works with Raku.
She has the problem that the colour of the glaze turns to black when
temperature for taking the piece out of the kiln is reached.
She works with a small top loading Raku kiln fired with gas.
She fires up to 950=B0 C.
She opens the kiln in order to check if the pieces/glaze reached the proper
state so that it can be taken out of the kiln for post reduction.
At the first glance the glaze looks ok to be taken out of the kiln. However
after a few seconds the colour starts turning to black.
This happens not in every firing so that the glaze could not be the reason =
for
that.
What happens there?
Could that be caused by the weather condition e.g. inversion weather condit=
ion?
Or could that be caused by under or overfiring?
Or can that be caused by a sudden heavy reduction?
I can't help her because I don't have any knowledge about Raku firing.
Could someone help?
Best regards
Waltraud
- Germany-
--
GMX Download-Spiele: Preizsturz! Alle Puzzle-Spiele Deluxe =FCber 60% billi=
ger.
http://games.entertainment.gmx.net/de/entertainment/games/download/puzzle/i=
ndex.html
=0A=0A=0A
Paul Gerhold on tue 4 nov 08
Dear Waltraud,
You don't exactly say when the glaze goes black. I assume that the black
color is after the smoking. What may be happening is that she is firing based
on getting to a temperature on a pyrometer. A pyrometer temp is not a
particularly good way to judge the actual temperature or maturity of the glaze,
Rate of temp rise in the kiln, shape of the piece (closed or open) and thickness
all influence the glaze maturity. If the glaze is particularly touchy this
can be a big problem.
What I have found is that if real control over the Raku process is required
use a large cone with each Raku piece. Firings will be much more accurate
and predictable and variations in firing can be reduced.
If more help is needed you will probably have to supply the glaze formula as
well as firing particulars.
I should mention that I have not found that air temp or humidity really
makes much difference in firing results. More critical is the temp of firing and
the temp at which the reduction occurs.
Paul
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Ivor and Olive Lewis on wed 5 nov 08
Dear Waltraud Koestler,
Without information about the glaze that is on these pots I would
only be guessing,.....except that if these are glazed pots and the
temperature of the glaze is about 1000 deg C they will show only the
colour of radiant heat, red heat. As these pots cool luminosity
declines and the surface changes to black because far less energy is
being radiated to the viewer.
The other possibility is that the glaze contains an excess of Copper
oxide and may not give anticipated iridescence, remaining black
I think we need more information.
Best regards,
Ivor Lewis.
Redhill,
South Australia.
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