Jeff Tsai on fri 18 jan 02
Hiya,
hoping someone can help me. When I burnish my pottery, I usually use a smooth porcelain stone. When I burnish the clay surface directly with just a touch of water, the finish is great, but when I apply a terra sig made of ball clay, soda ash, and copper carbonate, whereever the stone comes to a stop before reversing direction a small little flake or crack occurs in the finish. My sig is made using the method described by Vince on the digitalfire website and has a specific gravity of around 1.15. I've tried burnishing slowly, burnishing prior to applying the sig and then again after applying the sig, and varying the kinds of stones I use. But these cracks just keeping coming. Burnishing with a plastic bag doesn't cause this to occur by the way, but the finish isn't exactly what I want. Any advice?
My second question has to do with clay porosity limits. I know how to measure a fired piece of clays porosity, but I don't know what a safe limit for earthenware is if I want a clay earthenware pot to hold water for a long period of time. I know stoneware sometimes has porosity around 1-2% after being fired, but what about earthenware. Does it make a difference if I don't glaze the surface that is in contact with water? If it is unglazed, does the porosity have to be zero to hold liquid safely?
-jeffrey T.
(getting sick and tired of explaining to people that they can't put water and flowers in my pots.)
vince pitelka on sat 19 jan 02
Jeff wrote:
"hoping someone can help me. When I burnish my pottery, I usually use a
smooth porcelain stone. When I burnish the clay surface directly with just a
touch of water, the finish is great, but when I apply a terra sig made of
ball clay, soda ash, and copper carbonate, whereever the stone comes to a
stop before reversing direction a small little flake or crack occurs in the
finish. My sig is made using the method described by Vince on the
digitalfire website and has a specific gravity of around 1.15. I've tried
burnishing slowly, burnishing prior to applying the sig and then again after
applying the sig, and varying the kinds of stones I use. But these cracks
just keeping coming. Burnishing with a plastic bag doesn't cause this to
occur by the way, but the finish isn't exactly what I want. Any advice?"
Jeff -
Burnishing with water has never worked well for me. Apply as many layers of
terra sig as you want to the bone-dry pot, let it soak in for a few minutes,
and then apply a thin skim coat of crisco, olive oil, or lard, and let that
soak in completely. Then do your burnishing. And as you burnish, stop
periodically and use the plastic grocery bag material over your fingertip to
rub the surface. Burnishing of course compresses and flattens the surface,
but I have found that the plastic still gives the best final shine. When
burnishing, I usually switch back and forth between the stone and the
plastic, checking to see what kind of shine I really have. But if you do
much of this, you will know a lot more than I do about it, because
burnishing is not my thing at all. I teach a basic version as part of my
classes and my "Ancient Clay" workshops, but that's it.
Good luck -
- Vince
Vince Pitelka
Appalachian Center for Crafts
Tennessee Technological University
1560 Craft Center Drive, Smithville TN 37166
Home - vpitelka@dtccom.net
615/597-5376
Work - wpitelka@tntech.edu
615/597-6801 ext. 111, fax 615/597-6803
http://www.craftcenter.tntech.edu/
John Hesselberth on sat 19 jan 02
Hi Jeffrey,
Earthenware normally has 10-15% water absorption after firing--that's the
nature of the beast. It will not hold water unless encased in a very
durable, well-fitting and properly fired glaze. I'm sure those things exist
at earthenware temperatures, but they are much more difficult to find than
they are at stoneware temperatures. They are probably all in the glossy
family of glazes. If you are determined to stay with earthenware then focus
on finding a good glossy glaze that fits your clay.
Stoneware/porcelain that has 1-2% absorption will normally hold water with
or without glaze.
Regards,
John
on 1/18/02 10:57 PM, Jeff Tsai at DemJeffHQ@AOL.COM wrote:
> My second question has to do with clay porosity limits. I know how to measure
> a fired piece of clays porosity, but I don't know what a safe limit for
> earthenware is if I want a clay earthenware pot to hold water for a long
> period of time. I know stoneware sometimes has porosity around 1-2% after
> being fired, but what about earthenware. Does it make a difference if I don't
> glaze the surface that is in contact with water? If it is unglazed, does the
> porosity have to be zero to hold liquid safely?
>
> -jeffrey T.
> (getting sick and tired of explaining to people that they can't put water and
> flowers in my pots.)
Web sites: http://www.masteringglazes.com and http://www.frogpondpottery.com
Email: john@frogpondpottery.com
"The life so short, the craft so long to learn." Chaucer's translation of
Hippocrates, 5th cent. B.C.
Jennifer Firestone on sun 20 jan 02
Jeff,
I've had the same problem with folks wanting to use my saggar & raku pots
for water & flowers. They are usually happy when I tell them they can insert
a plastic cup, or even a sturdy plastic bag with care to hold the
water........a lot of folks just don't know anything about creative problem
solving.
be well,
Jennifer
www.FireKitten.com
----- Original Message -----
snip:
> Hiya,
--------
> hoping someone can help me.
> -jeffrey T.
> (getting sick and tired of explaining to people that they can't put water
and flowers in my pots.)
>
Anita M. Swan on sun 20 jan 02
I'm wondering if the copper carbonate is the problem. Do you ball mill it? Try a
chamois on slightly damp terra sig. I apply Vince's terra sig to bone dry ware,
building up several coats. When it is no longer shiny-wet I polish it with a
chamois. When it is completely dry I add a bit of vegetable oil and polish again -
I tried polishing between coats of terra sig, but it didn't get any shinier, so I
have stopped that, just polish the last coat and then use the oil. I really think
the chamois result is as good as a stone on terra sig and a lot easier!! I try not
to fire above ^08 for bisque, then pitfire and polish the finished piece with a bit
of floor wax. I have been thinking about a very lowfire glaze I could put inside
the pieces - something gray or black-brown so it will look ok with the pots. I
tell folks to use a 'liner' in the pot -- a (gasp!) plastic cup to prevent it from
getting wet. I have also heard of people using Thompson's Water Sealer or other
types of sealers inside - ick. I think that would compromise the artistic value of
the piece and I just don't like the idea of it in my pots, so I don't do it. I am
looking forward to the responses you get. Good luck! 'Nita
Jeff Tsai wrote:
Hiya,
hoping someone can help me. When I burnish my pottery, I usually use a smooth
porcelain
stone. When I burnish the clay surface directly with just a touch of water, the
finish is great,
but when I apply a terra sig made of ball clay, soda ash, and copper carbonate,
whereever
the stone comes to a stop before reversing direction a small little flake or
crack occurs in the
finish. My sig is made using the method described by Vince on the digitalfire
website and has
a specific gravity of around 1.15. I've tried burnishing slowly, burnishing prior
to applying
the sig and then again after applying the sig, and varying the kinds of stones I
use. But these
cracks just keeping coming. Burnishing with a plastic bag doesn't cause this to
occur by the
way, but the finish isn't exactly what I want. Any advice?
My second question has to do with clay porosity limits. I know how to measure a
fired piece
of clays porosity, but I don't know what a safe limit for earthenware is if I
want a clay
earthenware pot to hold water for a long period of time. I know stoneware
sometimes has
porosity around 1-2% after being fired, but what about earthenware. Does it make
a
difference if I don't glaze the surface that is in contact with water? If it is
unglazed, does the
porosity have to be zero to hold liquid safely?
-jeffrey T.
(getting sick and tired of explaining to people that they can't put water and
flowers in my
pots.)
__
Lee Love on mon 21 jan 02
----- Original Message -----
From: "Jennifer Firestone"
> Jeff,
> I've had the same problem with folks wanting to use my saggar & raku pots
> for water & flowers. They are usually happy when I tell them they can insert
> a plastic cup, or even a sturdy plastic bag with care to hold the
> water........
Jennifer,
Here in Japan, they cut bamboo to make water containers for such pots.
We cut a bunch for some of Sensei's old pots (the one I saw, I think was a
Yayoi pedestal jar.)
--
Lee In Mashiko Ikiru@kami.com
"The past is our definition. We may strive, with good reason, to escape it, or
to escape what is bad in it, but we will escape it only by adding something
better to it."
-Wendell Berry-
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