May Luk on sat 8 mar 03
I am trying to fire terra cotta at cone 7. The information I was given was 2
parts stoneware clay and one part terra cotta. I made the mistake of mixing
1 part stoneware clay and 2 parts terra cotta.
I bisqued a small pinch pot 2" (D) x 2"(H) x 1/8" (thickness) together with
the glaze firing just to see if the clay would hold up at cone 7. It appears
to be alright.
Before I realized my clay mixing mistake, I slab built 3 containers (8" high
max / 6 mm thick) and they are bisqued at 1,000 C. I'm going to glaze it
with a commercial leadless transparent glaze. Should I take some precautions
while I put them in the kiln? Will bloating and slumping affect the
neighbouring pots?
T.I.A.
May
Snail Scott on sun 9 mar 03
At 07:56 PM 3/8/03 +0100, you wrote:
>...The information I was given was 2
>parts stoneware clay and one part terra cotta. I made the mistake of mixing
>1 part stoneware clay and 2 parts terra cotta...
>Should I take some precautions
>while I put them in the kiln? Will bloating and slumping affect the
>neighbouring pots?
Both 'stoneware' and 'terra cotta' are really
general terms, so it's tough to predict what
will happen to your specific stuff. To be safe,
though, just fire one of the suspect pots, not
all of them. If it works out, you can fire them
later. If not, you won't have wasted firing space
on them. I'd put it on a waste slab and/or paint
the bottom with alumina wax, in case it vitrifies
enough to fuse to the shelf. You might also keep
other work out of range of any potential warping.
Bloating won't hurt anything but the specific pot,
though.
-Snail
Vince Pitelka on mon 17 mar 03
May -
Sounds like you've got a winner. But just as a technicality (and this may
have already been pointed out), there is no such thing as midrange
terracotta. Once you get above cone 2 or 3, what you have is a midrange red
stoneware. Terracotta is lowfire only.
If you want to use your recipe for functional work, then you will want to do
an absorption test and also a freeze-thaw test. You can find plenty of
information on those in the archives, but feel free to email me any
questions.
Alumina wax is just wax resist or hot wax with some alumina mixed in it to
decrease the chance of wares sticking to the kiln shelves, especially with
porcelain claybodies.
Best wishes -
- Vince
Vince Pitelka
Appalachian Center for Craft
Tennessee Technological University
1560 Craft Center Drive, Smithville TN 37166
Home - vpitelka@dtccom.net
615/597-5376
Office - wpitelka@tntech.edu
615/597-6801 x111, FAX 615/597-6803
http://iweb.tntech.edu/wpitelka/
May Luk on mon 17 mar 03
With the test, I had fired one small container with lid (4" h) past cone 7
(though I punched in 1240 C top temp in the controller, the #7 cone is in a
yoga child position, the 28 min soak might have been to much) The pot
appears to be okay with no sign of bloating. The clay has a nice Khaki
colour. I will try to fire my other 2 bigger pots. If they are okay, does it
mean that my clay mixture is alright? Is there any other type of simple test
that I can perform to ensure that my "recipe" is sound?
Also; What is alumina wax?
T.I.A.
May
on 3/9/03 5:56 pm, Snail Scott at snail@MINDSPRING.COM wrote:
> At 07:56 PM 3/8/03 +0100, May wrote:
>> ...The information I was given was 2
>> parts stoneware clay and one part terra cotta. I made the mistake of mixing
>> 1 part stoneware clay and 2 parts terra cotta...
>> Should I take some precautions
>> while I put them in the kiln? Will bloating and slumping affect the
>> neighbouring pots?
>
>
> Both 'stoneware' and 'terra cotta' are really
> general terms, so it's tough to predict what
> will happen to your specific stuff. To be safe,
> though, just fire one of the suspect pots, not
> all of them. If it works out, you can fire them
> later. If not, you won't have wasted firing space
> on them. I'd put it on a waste slab and/or paint
> the bottom with alumina wax, in case it vitrifies
> enough to fuse to the shelf. You might also keep
> other work out of range of any potential warping.
> Bloating won't hurt anything but the specific pot,
> though.
>
> -Snail
>
May Luk on tue 18 mar 03
Hi Vince;
Thanks for the pointer.
Regarding the alumina wax, which one is used; alumina hydrate or calcined
alumina? What is the suggested proportion or consistency?
Best Regards
May
UK
on 3/18/03 6:04 am, Vince Pitelka at vpitelka@ATT.NET wrote:
> May -
> Sounds like you've got a winner. But just as a technicality (and this may
> have already been pointed out), there is no such thing as midrange
> terracotta. Once you get above cone 2 or 3, what you have is a midrange red
> stoneware. Terracotta is lowfire only.
>
> If you want to use your recipe for functional work, then you will want to do
> an absorption test and also a freeze-thaw test. You can find plenty of
> information on those in the archives, but feel free to email me any
> questions.
>
> Alumina wax is just wax resist or hot wax with some alumina mixed in it to
> decrease the chance of wares sticking to the kiln shelves, especially with
> porcelain claybodies.
> Best wishes -
> - Vince
>
Vince Pitelka on tue 18 mar 03
> Regarding the alumina wax, which one is used; alumina hydrate or calcined
> alumina? What is the suggested proportion or consistency?
May -
It doesn't matter which you use. By the time it gets to a temperature that
matters it will be oxide. As to quantity, I seem to remember using about a
level teaspoon of alumina to a cup of wax. It doesn't take much.
And if you ever need to refire a high-fired porcelain piece, don't use
alumina wax. Sponge off the foot or the contact area on the foot and/or the
lid/seat, and brush on a thin coat of alumina and water. On a bisque-fired
piece the wax soaks into the clay, leaving the alumina on the surface where
it belongs. On a previously fired piece, if you use alumina wax, the wax
can melt and flow, carrying the alumina into or onto the adjacent glazed
areas, making a real mess.
Best wishes -
- Vince
Vince Pitelka
Appalachian Center for Craft
Tennessee Technological University
1560 Craft Center Drive, Smithville TN 37166
Home - vpitelka@dtccom.net
615/597-5376
Office - wpitelka@tntech.edu
615/597-6801 x111, FAX 615/597-6803
http://iweb.tntech.edu/wpitelka/
May Luk on wed 19 mar 03
Hi Vince;
I'm going a bit off course from the original thread. I'm a bit test-crazy.
I had recently mixed some David Leach "Porcelain" clay (1250 C). The pinched
pots are waiting to be bisqued. Since it's not high fired, should I still
give it an alumina wax treatment at glaze firing because it would behave
differently from normal stoneware type clay?
Thank you in advance for all the valuable information.
Much appreciated
Best Regards
May
on 3/19/03 3:00 am, Vince Pitelka at vpitelka@DTCCOM.NET wrote:
>> Regarding the alumina wax, which one is used; alumina hydrate or calcined
>> alumina? What is the suggested proportion or consistency?
>
> May -
> It doesn't matter which you use. By the time it gets to a temperature that
> matters it will be oxide. As to quantity, I seem to remember using about a
> level teaspoon of alumina to a cup of wax. It doesn't take much.
>
> And if you ever need to refire a high-fired porcelain piece, don't use
> alumina wax. Sponge off the foot or the contact area on the foot and/or the
> lid/seat, and brush on a thin coat of alumina and water. On a bisque-fired
> piece the wax soaks into the clay, leaving the alumina on the surface where
> it belongs. On a previously fired piece, if you use alumina wax, the wax
> can melt and flow, carrying the alumina into or onto the adjacent glazed
> areas, making a real mess.
> Best wishes -
> - Vince
>
Snail Scott on wed 19 mar 03
At 09:48 AM 3/19/03 +0100, you wrote:
>I had recently mixed some David Leach "Porcelain" clay (1250 C). The pinched
>pots are waiting to be bisqued. Since it's not high fired, should I still
>give it an alumina wax treatment at glaze firing because it would behave
>differently from normal stoneware type clay?
You don't need alumina during bisque; you need
it for the high fire, when the clay will vitrify
and want to stick to the shelf.
-Snail
Vince Pitelka on wed 19 mar 03
> I had recently mixed some David Leach "Porcelain" clay (1250 C). The
pinched
> pots are waiting to be bisqued. Since it's not high fired, should I still
> give it an alumina wax treatment at glaze firing because it would behave
> differently from normal stoneware type clay?
May -
You only need to use the alumina wax on true thoroughly vitrified high-fire
porcelain bodies in the glaze firing. It becomes so glassy that it can
aggressively stick to itself or to the shelf, and the alumina wax prevents
that.
- Vince
Vince Pitelka
Appalachian Center for Craft
Tennessee Technological University
1560 Craft Center Drive, Smithville TN 37166
Home - vpitelka@dtccom.net
615/597-5376
Office - wpitelka@tntech.edu
615/597-6801 x111, FAX 615/597-6803
http://iweb.tntech.edu/wpitelka/
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