Tony Hansen on mon 14 apr 03
Glaze at low fire has a poor interface with the body
(it is not glued on well). Combine this with poor
thermal expansion match, and voila, your problem.
The body probably has too much talc for this glaze.
The talc increases thermal expansion to prevent
crazing but it this case it is too high and you
have the opposite problem, shivering.
One possible solution would be to mix 10%
nepheline syenite into either glaze. That will
increase the thermal expansion of the glaze but
it won\'t melt quite as well either.
In future I would consider firing to at least 03
to get much better strength and chip resistance and
at least a little bit of a clay-glaze interface for
functional ware.
This page has some info about using your own low
fire glaze and adjusting it to optimize fit with
the body.
http://digitalfire.ab.ca/cermat/education/226.php
> I have made some special order personalized goblets which I have made many
> times before. I underglazed them and single fired them to cone ^06 (which I
> have also done many times) with Spectrum clear glaze. I have not used that
> brand before. About a day after they came out of the kiln, glaze started
> pooping off the rim. I smoothed the places down, reglazed them and refired.
> Same thing happened again. I reglazed them with a little Duncan clear that I
> had left. Popped off again. I have fired them three times and the glaze
> continues to pop off. Does anyone have any suggestions, or do I need to
> trash these and make new ones? Any help will be appreciated.
> Marcia in Chattanooga where we have had a beautiful spring.
========
Tony Hansen
Pheetie@AOL.COM on mon 14 apr 03
I have made some special order personalized goblets which I have made many
times before. I underglazed them and single fired them to cone ^06 (which I
have also done many times) with Spectrum clear glaze. I have not used that
brand before. About a day after they came out of the kiln, glaze started
pooping off the rim. I smoothed the places down, reglazed them and refired.
Same thing happened again. I reglazed them with a little Duncan clear that I
had left. Popped off again. I have fired them three times and the glaze
continues to pop off. Does anyone have any suggestions, or do I need to
trash these and make new ones? Any help will be appreciated.
Marcia in Chattanooga where we have had a beautiful spring.
Ron Roy on tue 15 apr 03
Hi Marcia,
The problem is shivering - when a glaze does not contract as much on
cooling as the body - winds up too big for the clay - it does not do any
good to refire - they will always not fit properly.
You need a body that contracts less on cooling or a glaze that contracts
more on cooling.
And it may just be the underglaze in between the two.
Every time we try something new we should understand - this can happen -
and test new clays and glazes and slips before going into production.
All clay bodies are different, all glazes are different and all slips are
different - to expect anything else is unreasonable.
RR
>I have made some special order personalized goblets which I have made many
>times before. I underglazed them and single fired them to cone ^06 (which I
>have also done many times) with Spectrum clear glaze. I have not used that
>brand before. About a day after they came out of the kiln, glaze started
>pooping off the rim. I smoothed the places down, reglazed them and refired.
>Same thing happened again. I reglazed them with a little Duncan clear that I
>had left. Popped off again. I have fired them three times and the glaze
>continues to pop off. Does anyone have any suggestions, or do I need to
>trash these and make new ones? Any help will be appreciated.
>Marcia in Chattanooga where we have had a beautiful spring.
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
Fax: 613-475-3513
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