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sv: high fire vs. low?

updated sun 9 apr 00


Alisa and Claus Clausen on sat 8 apr 00

Hi Helvi,
It is important to have a proper fit between the clay body the glaze.
All of the technicalities involved in a good fit are covered in many good =
especially from Tom Buck and Ron Roy.

My tests were with a white body designated as broad range firing body, from
cone 06 to 6. Skeptical about such a range, I bisqued to 1000c and glaze =
earthernware glazes to 1020c. Consistently the glazes crazed and shivered.
Bad fit. The only good results I have had with this body was to bisque at =
and glaze fire with glazes between 1220 and 1250c.

Also at this low firing, the clay was extremely open and not vitrified.

On the other side of the spectrum, my supplier has fired every type
of earthernware clay designated for 1020c max. to 12220c. They made
coiled omega forms out of each clay and fired. Out of ca. 15 bodies, they
all withstood the high temps. Some of them bubbled, but only two bodies,
common red and blue clay (terracottas) actually slumped and melted.
This was only a temp. test, no glazing.

These are only some personal examples I have from my own records,(also some
trouble with engobes fitting the body and glaze, another chapter) but I am =
you can find a wealth of info. in the archives about expansion, fit, etc.
Once you make some specific tests, it is easier to narrow down your field
of variables.

Best regards,
Alisa in Denmark

-----Oprindelig meddelelse-----
Fra: Helvi Abatiell
Dato: 5. april 2000 22:07
Emne: High fire vs. Low?

=3E----------------------------Original message----------------------------
=3EOkay, I've gotten many great responses to my earlier questions (thank you
=3Eall) regarding low and high fire glaze temps but I have one more...
=3E What happens if you don't fire a clay to the temp suggested? What
=3Ehappens to a high fire clay that is fired less then that? Does it not
=3Evitrify? Does it contaminate if fired with a low fire glaze? Will =
=3Enot adheer properly? Why not?
=3E I'm sure all these questions have been answered before. I'm hoping =
=3Eare others out there with similar questions and I'm not just feeling =
=3Efor asking.
=3E Thanks for your help, you wise potting souls, you.
=3E Helvi :)