Joan McMullen on mon 14 apr 03
I am new to the list and I have a problem. I have purchased porcelain slip
from a private supplier -not one of the major ones -and when fired it has
black spots or inclusions in it. These are not apparent in the greenware
and are sometimes on the surface of the bisque or deep within it - they are
sizeable enough to ruin a piece. I have discussed this with the supplier
many times and he replaced the slip this past weekend with supposedly good
product. It is better but still contains these black things. Does anyone
have idea what this could be and how I can get rid of it?
Thanks for any help.
Joan
John Rodgers on mon 14 apr 03
There are any number of "black things" from any number of sources that
could be in porcelain. However, in my experience the one that is
consistently present and most common is lignite. It really shows up in
porcelain casting slip, and if to much water is added to the slip the
lignite will separate and float to the surface.
Never had any lignite cause me any firing problems though. Always burned
out and left no trace, either mineral or physical trace.
John Rodgers
Birmingham, AL
Joan McMullen wrote:
>I am new to the list and I have a problem. I have purchased porcelain slip
>from a private supplier -not one of the major ones -and when fired it has
>black spots or inclusions in it. These are not apparent in the greenware
>and are sometimes on the surface of the bisque or deep within it - they are
>sizeable enough to ruin a piece. I have discussed this with the supplier
>many times and he replaced the slip this past weekend with supposedly good
>product. It is better but still contains these black things. Does anyone
>have idea what this could be and how I can get rid of it?
>Thanks for any help.
>Joan
>
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Joe Cooper on mon 14 apr 03
My wife's great aunt was really into china painting about twenty years ago,
and she cringed when I even talked about earthy pottery. She said that
firing anything other than porcelain in a porcelain-firing kiln simply
ruined that kiln for firing porcelain, that it would leave a residue in the
kiln which would put "horrendous tiny black spots" on her beautiful
porcelain from that time forward. So I just kept my mouth shut and "ruined"
my own kiln. ;)
Joe
in green, warm, Missouri... where the rivers run.
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Snail Scott on tue 15 apr 03
At 10:48 AM 4/14/03 -0400, you wrote:
>I am new to the list and I have a problem. I have purchased porcelain slip
>from a private supplier -not one of the major ones -and when fired it has
>black spots or inclusions in it...
Does the jar have a metal lid with corrosion on it?
This can drop little iron specks into the slip even if
the original mix was clean. Seen this happen a few times.
-Snail
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