Dan Johnston on mon 22 apr 96
Dear All:
I have been firing a Raku process using a glaze with a 80/20 base of Gerstley
Borate and Nephleine Syenite (oxides added for color). I have been firing to
cone 06 and reducing in a can for 1 hour. I have been putting on 2 coats
thin (at least I think it seems to be thin). The problem I am having is that
I am getting bubbles and blisters. WHY! Is the glaze wrong; too thick; too
hot of a kiln; too long reduction..... what am I doing wrong. I can sand off
the bubbles and refire and the second time it looks better, but I don't want
to fire twice. Any suggestions would be appreciated.
lurking no more
Dan Johnston
Tahlequah, OK
DLJ57@aol.com
Cynthia Hull on mon 22 apr 96
At 09:23 AM 4/22/96 EDT, you wrote:
>----------------------------Original message----------------------------
>Dear All:
>I have been firing a Raku process using a glaze with a 80/20 base of Gerstley
>Borate and Nephleine Syenite (oxides added for color). I have been firing to
>cone 06 and reducing in a can for 1 hour. I have been putting on 2 coats
>thin (at least I think it seems to be thin). The problem I am having is that
>I am getting bubbles and blisters. WHY! Is the glaze wrong; too thick; too
>hot of a kiln; too long reduction..... what am I doing wrong. I can sand off
>the bubbles and refire and the second time it looks better, but I don't want
>to fire twice. Any suggestions would be appreciated.
>
>lurking no more
>
>Dan Johnston
>Tahlequah, OK
>DLJ57@aol.com
>
Some of my raku glazes do this if I fire fast, and 80/20 is one of them. A
recent post on this problem on ^10 reduction explains why this is happening.
I have found that soaking at the end (in raku) is not as effective as just
slowing down the fire. Hey but I like those bubbles!
Cynthia
Marvin Bartel on mon 22 apr 96
At 09:23 AM 4/22/96 EDT, you wrote:
>----------------------------Original message----------------------------
>Dear All:
>I have been firing a Raku process using a glaze with a 80/20 base of Gerstley
>Borate and Nephleine Syenite (oxides added for color). I have been firing to
>cone 06 and reducing in a can for 1 hour. I have been putting on 2 coats
>thin (at least I think it seems to be thin). The problem I am having is that
>I am getting bubbles and blisters. WHY! Is the glaze wrong; too thick; too
>hot of a kiln; too long reduction..... what am I doing wrong. I can sand off
>the bubbles and refire and the second time it looks better, but I don't want
>to fire twice. Any suggestions would be appreciated.
>
>lurking no more
This Raku glaze bubbles for a while, but if you fire it hotter it will
smooth out. We never fire to a certain temperature or cone number when
firing Raku. We simply look at the glaze. When the glaze is smooth (no
longer bubbly) it is ready to take out. A slightly thinner application will
bubble less, but may not give the gloss you want. We only reduce for five
or ten minutes, but whatever works for you is fine.
>
>Dan Johnston
>Tahlequah, OK
>DLJ57@aol.com
>
++++++++++++++
Marvin Bartel, Art Dept
Goshen College, Goshen, IN 46526
marvinpb@goshen.edu
http://www:goshen.edu
++++++++++++++
.. . . on being influenced by a good idea . .
"Don't borrow it. Steal it. Make it your own."
from poet, Nick Lindsay
Jack Phillips on mon 22 apr 96
>----------------------------Original message----------------------------
>Dear All:
>I have been firing a Raku process using a glaze with a 80/20 base of Gerstley
>Borate and Nephleine Syenite (oxides added for color). I have been firing to
>cone 06 and reducing in a can for 1 hour. I have been putting on 2 coats
>thin (at least I think it seems to be thin). The problem I am having is that
>I am getting bubbles and blisters. ****
>
>Dan Johnston
>Tahlequah, OK
>DLJ57@aol.com
>
Dan,
In a general way, you are not fireing the glaze until it melts
and smoothes out. Check this. Watch the glaze go through the "ugly"
stage, ( I call it) then it will start laying down. It's a visual judgement
thing. When the piece "STARTS" looking like a red hot piece of ice, you are
close. The glaze, shortly after this will start to flow down the pot, if
you keep gaining in temp. So, Fireing several pieces in a row will
help you get a feel for the way you want your glaze to end up. One last
thing, you could also be fireing up to temp too fast. Here again, in a
general way, your first fireing of the day should take about 1.5 hours for
a piece say 10" tall, and at least an hour for each fireing after that, (if
you use a fiber top kiln ). Pushing the kiln too hard will burn out the
glaze, especially if you let the flame "lick" the pot, so give it a chance
to go through a steady but gradual gain. If you have any more questions,
feel free to email me at stonart@worldstar.com. Best of luck.
Jack Phillips
Portland, Or.
Jack Phillips
STONART Ceramic art
Portland, Oregon
Gary W. Wagoner on mon 22 apr 96
Re: the craters in 80/20 raku glaze.
It may well be that the glaze in question has not yet smoothed out entirely
from the initial melt, when most raku glazes will do some pretty vigorous
boiling. But it is also possible that the glaze is overfired and
volitalizing.In any case the behavior of the glaze can be carefully
observed during the firing, and you'll have the answer. Cones don't tell
you that much about glaze melt in raku; making sure the glazed piece is
clearly visible through the peephole and watching the glaze melt is much
more informative.There is a period of intense bubbling followed by a
levelling out of the glaze; catching the moment of complete smoothness
before the glaze begins to volatilize and bubble again is crucial. Getting
all the pots in the kiln to smooth out over their entire surface, neither
under nor overfired, is best accomplished with a steady, not too rapid rate
of climb, but I never spend the hour and a half someone recommended getting
to temperature. It's only during the glaze melt that rate of climb has any
effect I can think of on the quality of glaze surface; you might try
slowing the kiln a bit when you see that going on if you have problems
controlling the melt.
mgiles@Onramp.NET on tue 23 apr 96
I don't think your bubbly 80-20 raku glaze is overfired. We've had a
few pieces slip through into the cone ten firing. The glaze slides
right off of the pot, but it's still smooth and glassy.
Kristen
mgiles@onramp.net
http://rampages.onramp.net/~mgiles/
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