Marni Turkel on sat 20 sep 97
I don't understand it. I am having trouble with a glaze base on a
particular shape of piece. It is a commercial mat glaze base that I have
used for several years. I have had a pit or crater every once in awhile in
the past, but the current batch is giving me real problems. On shallow,
wide-bottomed pieces I am getting flaws that range from pinholes to craters
that are ONLY on the inside bottom of the piece, the walls and underneath
are perfect. The glaze is beautiful on all other shapes. It is not on every
low wide piece, but it is on many, 50 to 75% at present. One of the pieces
out of the kiln today (15 by 11, 3" high) had a 4" diameter circle of glaze
in the center that looked as if boiling sugar water had been quick frozen,
under an 8 power lens it looked like froth; the rest of the piece was
beautiful. Most of the problem is not concentrated as this one was, nor is
it usually centered, but sprinkled over the bottom, sometimes a few craters
on an 8" diameter bottom, sometimes 1 or 2 per square inch. Refiring
intensifies the problem dramatically, causing craters to show up where
there had been none before. That makes me think it is not underfiring that
is the problem. I have tried firing them in all parts of the kiln, I have
had successes and failures in all parts. I don't normally fire this type of
piece in the bottom of the kiln (which fires cooler) because I worry about
it sweating. I have put a problem piece on the cool bottom shelf in the
kiln today: pits or sweating, it is a second either way. I'll know
tomorrow.
The pieces are cast, they are footed and glazed underneath. They are
bisqued fired to c08 and oxidation fired to c5 in electric kilns with kiln
sitters, but not computer controls. The problem may show up on one piece
and not on the piece right next to it. Both pieces having been cast and
bisqued together, dipped at the same time. It is also showing up on bisque
ware that has been around the studio for several months (different clay
batches, wasn't having trouble then with these shapes). I have tried
bisqueing the pieces stilted and empty in case the heat was uneven, didn't
matter. I have screened the glaze through a 150 mesh screen instead of 100
mesh in case there were some contamination, didn't matter. I've tried
applying the glaze thinner, didn't matter. I have one shape that is 10"
square and sits flat on its unglazed bottom (no foot ring) that I have not
had this problem on (yet).
Does anyone have any ideas on why it would be only one part of one type of
shape? Any suggestions?
Thank you,
Marni Turkel
Stony Point Ceramic Design
Santa Rosa, California
Marni Turkel
Stony Point Ceramic Design
Santa Rosa, California
David Hendley on sun 21 sep 97
Marni,
This is just an educated guess,
but I think the glaze is too fluid.
I runs down to the bottom of the piece and makes
a boiling lake of glaze.
Stiffen up the glaze by adding some clay
or lower the firing temperature.
I've had the same problem at ^10 with a glaze,
and, like you, re-firing just made it worse.
Adding about 5% EPK fixed it.
David Hendley
Maydelle, Texas
At 09:56 AM 9/20/97 EDT, you wrote:
>----------------------------Original message----------------------------
>I don't understand it. I am having trouble with a glaze base on a
>particular shape of piece. It is a commercial mat glaze base that I have
>used for several years. I have had a pit or crater every once in awhile in
>the past, but the current batch is giving me real problems. On shallow,
>wide-bottomed pieces I am getting flaws that range from pinholes to craters
>that are ONLY on the inside bottom of the piece, the walls and underneath
>are perfect. The glaze is beautiful on all other shapes. It is not on every
>low wide piece, but it is on many, 50 to 75% at present. One of the pieces
>out of the kiln today (15 by 11, 3" high) had a 4" diameter circle of glaze
>in the center that looked as if boiling sugar water had been quick frozen,
>under an 8 power lens it looked like froth; the rest of the piece was
>beautiful. Most of the problem is not concentrated as this one was, nor is
>it usually centered, but sprinkled over the bottom, sometimes a few craters
>on an 8" diameter bottom, sometimes 1 or 2 per square inch. Refiring
>intensifies the problem dramatically, causing craters to show up where
>there had been none before. That makes me think it is not underfiring that
>is the problem. I have tried firing them in all parts of the kiln, I have
>had successes and failures in all parts. I don't normally fire this type of
>piece in the bottom of the kiln (which fires cooler) because I worry about
>it sweating. I have put a problem piece on the cool bottom shelf in the
>kiln today: pits or sweating, it is a second either way. I'll know
>tomorrow.
>
>The pieces are cast, they are footed and glazed underneath. They are
>bisqued fired to c08 and oxidation fired to c5 in electric kilns with kiln
>sitters, but not computer controls. The problem may show up on one piece
>and not on the piece right next to it. Both pieces having been cast and
>bisqued together, dipped at the same time. It is also showing up on bisque
>ware that has been around the studio for several months (different clay
>batches, wasn't having trouble then with these shapes). I have tried
>bisqueing the pieces stilted and empty in case the heat was uneven, didn't
>matter. I have screened the glaze through a 150 mesh screen instead of 100
>mesh in case there were some contamination, didn't matter. I've tried
>applying the glaze thinner, didn't matter. I have one shape that is 10"
>square and sits flat on its unglazed bottom (no foot ring) that I have not
>had this problem on (yet).
>
>Does anyone have any ideas on why it would be only one part of one type of
>shape? Any suggestions?
>
>Thank you,
>Marni Turkel
>Stony Point Ceramic Design
>Santa Rosa, California
>
>Marni Turkel
>Stony Point Ceramic Design
>Santa Rosa, California
>
>
David Hendley
Maydelle, Texas
See David Hendley's Pottery Page at
http://www.sosis.com/hendley/david/
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