CINDI ANDERSON on sun 20 may 01
Hi
I'm stumped. Over many firings, different clay bodies, different commercial glazes, my bowls almost always have blister / crater things (often sharp) and my vases, mugs, etc never do. Even in the same load with the same glaze. The only thing different about my bowls is they are trimmed and therefore usually a little thinner. The problem is not pinholes where the trimming occured. It happens on the inside as well. Any ideas?
Fire oxidation, 07 bisque, cone 5-6 glaze.
Thanks
Cindi
Fremont, CA
Wade Blocker on mon 21 may 01
Cindi,
Do you pour your glazes? Perhaps there is too much glaze on the bottom of
your bowls. Possibly the cratering is the result of overfiring. Try to fire
to cone 5 and see if that is the cure. Mia in sunny ABQ
iandol on tue 22 may 01
Dear Cindi Anderson,
Have a look in Hamer's dictionary on this one. He suggests that the =
glaze is not mature and has not had time to settle down. This =
presupposes that you are not over firing and getting the bloats.
Another source of gas might be from your clay, through not destroying =
all the volatile salts. This could be cured by raising the bisque by one =
or two cones.
Best regards,
Ivor.
John Hesselberth on wed 23 may 01
I agree with Ivor on this one; although it doesn't seem to match all the
data. I think your bowls are underfired. When a glaze is melting it
sometimes will bubble quite a bit as it off-gasses. Then it settles down and
smoothes out. When I ran into this problem on some glazes I was developing
a while back I was able to solve it by increasing the lower melting fluxes
(Na/K)--in other words lowering the melting temperature of the glaze. I can
envision how the inside of a bowl might not see as high a temperature. If
you could see down inside the mouth of a narrow necked bottle you might see
the problem in there also. I would run a test at one cone higher with a 20
minute soak at the end and see if it goes away.
I would also recommend the same thing to Bob. Even though your cones are
right beside the problem pieces it could be that your glaze is really a cone
11 or 12 glaze. It just sounds like you have a cold spot there.
I definitely don't think it is an overfiring problem as a couple people have
suggested; although I guess nothing should be ruled out at this point.
Regards,
John
Web site: http://www.frogpondpottery.com Email: john@frogpondpottery.com
"The life so short, the craft so long to learn." Chaucer's translation of
Hippocrates, 5th cent. B.C.
> From: iandol
> Reply-To: Ceramic Arts Discussion List
> Date: Tue, 22 May 2001 15:51:15 +0930
> To: CLAYART@LSV.CERAMICS.ORG
> Subject: Blisters / Craters on bowls only
>
> Have a look in Hamer's dictionary on this one. He suggests that the glaze is
> not mature and has not had time to settle down. This presupposes that you are
> not over firing and getting the bloats.
> Another source of gas might be from your clay, through not destroying all the
> volatile salts. This could be cured by raising the bisque by one or two cones.
> Best regards,
Michael Wendt on thu 24 may 01
To All:
If blisters or craters happen on the inside of the bowl only, check for
dust. I have found the number one cause of one sided defects in glazes on
my pots is dust. Since bowls are usually stored upright, dust collects
there. Very little dust is needed to cause terrible problems and a lot of
the dust is the fine ash given off during bisque firing from the burn out of
organic matter found in clay.
Regards,
Michael Wendt wendtpot@lewiston.com
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