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glaze defect

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I work at a community center that does some ceramics programs with kids. The teacher's here usually do their own firings for each of their classes. One of the teachers who is new said she doesn't know how to use the control panel on the kiln, and some of us there have been trying to help her learn how.

 

Anyway, this particular teacher just went away for vacation, but had recently fired some low-fire glaze work (cone 05) and left the work in the kiln, I happened to be using the kiln next and so was the one there to unload it. On unloading the work, I noticed that most all the work had glazes with milky appearance. I know that this can sometimes come from venting issues and underfiring. However upon reviewing the controller, I saw that the glaze work had been fired using the slow bisque program (04; approx 18 hour cycle)...the 05 glaze cycle is faster, so does that mean they could be over fired, not under fired? I know that no one touched the controller or changed the program since she last fired the kiln and she had send an email to our supervisor asking me to check the controller because she had trouble changing it from the bisque program to glaze program (the controller is working fine). So I'm wondering if anyone can help me figure out what could have happened to the glaze? The classes use basic commercial under glazes with a clear glaze. The colors appeared dull and the clear had turned the color of milk. I have fired glaze work in this kiln many times and ha never had a problem. Another issue was that the kiln was only loaded halfway. The kilns we use are Skutt's with enviro vent.

 

Any ideas / help on figuring out what could have happened is greatly appreciated...Thanks!

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I agree. The milky glaze is usually from a heavy application of clear. I just unloaded a load when one student had applied clear way too think in some spots. She said so as soon as she saw her piece. She had tried to remove some of the excess glaze prior to the firing, but it was still think is some areas...and milky. The rest of the kiln was fine.

Marcia

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I agree. The milky glaze is usually from a heavy application of clear. I just unloaded a load when one student had applied clear way too think in some spots. She said so as soon as she saw her piece. She had tried to remove some of the excess glaze prior to the firing, but it was still think is some areas...and milky. The rest of the kiln was fine.

Marcia

 

 

That's what is baffling me, because the milkyness was on every piece that had an application of clear (there were a couple pieces that used just a plain color glaze). If it were a couple pieces I would assume it's the application, but this happened to just about all of them, so I assume it has to be the firing. Also, I know it's not the glaze, because myself and others have used it in other firings and it's come out fine. Any ideas? I'm stumped.

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Guest JBaymore

If the CONSISTENCY of the liquid glaze suspension had been allowed to be thicker than what the (likely) inexperienced people applying it were used to, and they "applied it as usual", then the application thickness COULD have been thicker than "normal".

 

Pictures would be a big help.

 

best,

 

..............john

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