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need cone 7 transparent glaze for terracotta

updated sat 10 oct 98

 

leslierussell@my-dejanews.com on wed 7 oct 98

I had a reputable ceramic shop make me a cone 7 transparent glaze for black
slip and terracotta. It has tiny bubbles on the black and SEVERE bubbles on
the terracotta. What happened? I would sincerely appreciate it if anyone
could give me a better recipe. I used to have a glaze that worked
beautifully and turned the terracotta a deep, earthy green. Thank you very
much. -- Leslie

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Donn Buchfinck on thu 8 oct 98

To begin with
I think that you might be able to fire an earthenware up to cone 7 but you
have just changed the definition of what earthenware is.

when I fire our earthenware, we bisque higher than the glaze firing, this
burnes out all the gasses beyond the cone we will be firing to.
we bisque to 04 then glaze to 06
the glaze comes out smooth

I would think that you are severly overfiring your terracotta, and it is
releasing large ammounts of gas, is there alot of goldart in your clay body,
goldart has a tendancy to have a lot of sulfur in it.

good luck

Donn Buchfinck

Vince Pitelka on thu 8 oct 98

>I had a reputable ceramic shop make me a cone 7 transparent glaze for black
>slip and terracotta. It has tiny bubbles on the black and SEVERE bubbles on
>the terracotta. What happened? I would sincerely appreciate it if anyone
>could give me a better recipe. I used to have a glaze that worked
>beautifully and turned the terracotta a deep, earthy green. Thank you very
>much. -- Leslie

Leslie -
Terrracotta bodies are usually by definition earthenware bodies, with a
maximum firing temperature of cone 1 or 2. If this is in fact a red
stoneware body suitable for cone 7, then the problem is probably in the
glaze, and you should complain to the shop that made this glaze for you.
But if this is a true terracotta body, cone 7 is WAY too high, so it is no
wonder you are getting bubbles in your glaze, as the fluxes in the body
(depending on the claybody recipe) will volatize fairly aggressively at that
temperature.
Good luck -
- Vince

amy parker on fri 9 oct 98

Leslie -
I have found that bisque firing longer & slower cut down on bubbling in
my glazes. It seemed that the more impurities in the clay body, the
more time the body took for them to burn out. This could be part of
the problem. I have found that "transparent" in this temp range seems
to be somewhat fickle, also. Anywhere I get my glazes the least bit thick,
they are milky - caused by the tiny bubbles. I'm still searching for
the perfect glaze! (Aren't we all?)


>
amy parker Lithonia, GA
amyp@sd-software.com