Gayle Bair on thu 13 sep 07
I just had this discussion with Jon Pacini.
I also generally fire to a hot ^6.
I also make sure the functional pieces are on the hotter
shelves in my kiln.
With Jon's permission I'm including his answer to me.....
"Hi Gayle---- our tests here show that B-mix 5 has approx 2.5 % absorption
when fired to ^5. If it's under fired it is not very forgiving however and
the absorption rate will go up quite a bit at ^4 to 4%+ and if the glazes
craze, then it is prone to leaking.
The bad rap comes from "potters" who have kilns that don't fire very evenly
or who don't use cones in their computer controlled kilns---they press the
easy fire ^5 button and expect that the kiln is actually going to fire to
^5. When it doesn't, they blame the clay.
We do have reports of potters who are successful firing the clay to ^8, but
it's not something we regularly recommend. It's absorption drops to under 1%
and I'd expect it to get a bit warpy and brittle. It's like potters who use
porcelain for raku ---yea you can do it, but I wouldn't recommend that
technique to everybody.
so there you have it--- if I can help out in the future, don't hesitate to
get in touch--
Laguna Clay Co"
I hope this helps.
Bainbridge Island, WA
I've been using Cone 6 B Mix for about 8 years and this is the first
I've heard of it not being vitrified unless it reaches cone 7. I've
never had a problem using them in the microwave, oven, etc. I know clay
that is not vitrified will generally have problems in the microwave. I
use this clay and the new B Mix with grog for 95% of my pieces and my
students use the same clay. I do, however, fire to a hot cone 6.
Josh Berkus wrote:
>> I found that Cone 5 B
>> mix was not fully vitrified (leak-proof without glaze) until about
>> cone 7.
> Oh? That's bad news; I have 2 shelves of pots made with Cone 5 B-mix
> to be fired to Cone 6. I'm used to the Cone 10 B-Mix at 2325F, where it's
> hard, vitreous and chip-resistant, like porcelain.
> Should I be using a different white-body clay for Cone 6? Which? I
> using a grog-free white body stoneware or synthetic porcelain.
> The Fuzzy Chef
> San Francisco
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