Craig Martell on fri 10 jan 97
In a message dated 97-01-09 15:02:28 EST, Dano wrote:
<< I am firing at the same temperature, but in the cooler parts of
the kiln it seems to work better. >>
Hi: When asking advice for glaze problems, it is always helpful to provide
info about temperature and atmosphere. What cone do you fire to and what
type of kiln do you use? Do you oxidize or reduce?
It would seem that the bubbles and blisters are due to overfiring, if the
glaze is better in cooler areas of the kiln. Neph Sy can cause gassing from
Sodium at higher temps but I can't say if this is the problem, not knowing
the cone you fire to. There is a lot of Neph Sy in the glaze, have you
purchased a new supply of Neph lately? Perhaps the analysis has changed a
bit.
Sorry I can't help more, Craig Martell-Oregon
Craig Martell on mon 13 jan 97
Hi again Dano: After some more thought and reading, I think there is a good
chance that the problem is coming from the neph sy in the glaze. Spars
containing a predominance of sodium are unstable and can cause problems when
the soda volatilizes at higher temps. Neph Sy contains more sodium than any
of the other available spars, and might be the worst offender, although this
problem is not REAL common, IMHO.
As you have stated that the problem is either absent or lesser in cooler
parts of the kiln, you might want to lower your end point slightly and soak
the kiln to achieve the melt you want. I know this can affect other glazes
and may not be paractical. It is also possible that the problem is stemming
from the claybody and a higher bisque is needed to clear the clay of
carbonaceous material. If you can detect odors coming from the bisque kiln
when you have reached temperature, you may want to soak the bisque until all
odors are gone. I have no knowledge at all about the claybody you are using.
Perhaps the supplier can help. Test the glaze on porcelain tiles to
determine whether or not the body is at fault. Porcelains don't have the
carbonaceous content that stoneware clays do and should tell you a lot.
If firing adjustments don't work, your next step would be to alter the glaze.
Looking at your receipe, it would appear that this glaze was formulated for
a strong alkaline color response, and a matt texture from the high barium
content. This looks like the classic "copper blue" formula. Alumina is kept
very low in these glazes because it will inhibit the color development. The
silica in your glaze is also way low. As you probably already know, this
ain't a good glaze for food. You might eliminate the bubbles by exchanging
some or all of the neph sy for straight soda feldspar, which is less fusible
and volatile. Soda spar will also contribute more silica to the glaze. I
wouldn't try potash spars if you want to keep the color response. You may
also try adding a bit more alumina to the glaze to stabilize it and bring the
maturation temp up a little. You mentioned that you reduce only lightly.
Barium Carb. will not break down and release the carbonate with temp alone,
it needs reduction to do the job. Above about 1650 F., after you burn off
carbonaceous stuff, reduce moderately to aid in the release of the carbonate
from the Barium. If this is not done, Barium acts more as an opacifier and
you don't get the full benefit of the Baria in the glaze. Let me know how
things go! I hope I've helped a little.
Craig Martell-in sunny frigid Oregon
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