Lee Love on wed 18 jun 08
On Wed, Jun 18, 2008 at 9:42 AM, Antoinette Badenhorst
wrote:
> ...and while you mention that Ivor; Paul I hope I do not underestimate your >
>knowledge here, but for the record; The main difference between stoneware and >porcelain is that stoneware contains more clay, which makes it easy to shape.
I have always found porcelain to be just as easy to use as
stoneware. Traditional Asian methods really help. I use a Gyubera
(cow's tounge rib) only on porcelain. You can see one in use here:
http://www.japanpotterytools.com/howto.php?page=uThrowing
Egote are great for vases, bottles and tokuri in any clay. you
can see some of Chris Hendley's magnificent here, including egote:
http://hominid.net/toolpage2.htm
Today, I threw Continental Clay's wood fire porcelain, Super White
and 2 Parts Super white with 1 part Helmer's. Also additions of neph
sye gravel and crushed Custer rocks in Super White.
--
Lee Love in Minneapolis
http://mashikopots.blogspot.com/
http://claycraft.blogspot.com/
"We are such stuff as dreams are made on, and our little life is
rounded with a sleep." --PROSPERO Tempest Shakespeare
Antoinette Badenhorst on wed 18 jun 08
...and while you mention that Ivor; Paul I hope I do not underestimate your knowledge here, but for the record; The main difference between stoneware and porcelain is that stoneware contains more clay, which makes it easy to shape. It's sticky(when wet) and stronger(when bone dry). On the contrary porcelain contains very little clay(in comparison with other clays); a very traditional recipe has 25 percent of each of the following: grolleg, kaolin, silica and feldspar. There are even simpler recipes. It is often too dry, too quickly or too wet, too soon. So one has to develop a feel for it. Porcelain's best qualities comes out if it is thin, which is harder to do since the clay has very little green strength. Thinner also makes it harder to glaze, since it needs just the right touch. Take these characteristic( which causes more breakages than average under the most experience porcelain artists) and add expensive materials, firing etc to it and you understand why it is such an outst
anding ceramic matter. Since the glass content of porcelain is so high and there is lesser clay to hold it together, porcelain "moves" in the kiln. Easy to slump or deform. So one has to know his sit and stand when designing and firing porcelain. It is also interesting how one learns to predict and use the movement of porcelain in the firing process.
On the other hand a porcelainous stoneware is simply a white stoneware clay that is harder to work with( because it has not tooth and that aims to be as white as it can be to produce bright colors. It imitates porcelain ( unsuccessfully I have to say). It is not a replacement for porcelain, but for the inexperienced potter it might be a nice bridge if you want to get into true porcelain. It fires much easier and (mostly) without trouble.
There you are: a mouth full; if old news to you, maybe a beginner will benefit from this.
Antoinette Badenhorst
www.clayandcanvas.com
www.studiopottery.co.uk
-------------- Original message ----------------------
From: Ivor and Olive Lewis
> Dear Paul Gerhold,
>
> I am surprised none of the contributors responding to Antoinette has
> referred you to the standard authors on matters aesthetic, technical
> and scientific regarding the meaning of the term "Porcelain". Are the
> words of Rhodes, Cardew, Hamer and Leach, to name but a few, to be
> disregard in our effort to reach a consensus of opinions as to the
> meaning of the terms we use ?
> Best regards,
> Ivor Lewis.
> Redhill,
> South Australia.
>
>
> <> common
> definition for porcelain what do we do with porcelaineous stoneware?
> How about porcelaineous earthenware.>>
Tim See on thu 19 jun 08
Antoinette Badenhorst, I understand you have some
experience and a great bit of acquired knowledge but
some of things you have written recently have been
quite opposing to my personal experiences, and to have
a balanced dialog here I figured I would include some
of what I have encountered.
" The main difference between stoneware and porcelain
is that stoneware contains more clay, which makes it
easy to shape. It's sticky(when wet) and
stronger(when bone dry). On the contrary porcelain
contains very little clay(in comparison with other
clays)"
It struck me right away that this is not true with the
clays I work with now or with what I have worked with
in the past. flipping through a few books it seems
that if there is more clay in stoneware it is a
marginal in amount. The difference in clay content
from a cone 10 to a cone 04 clay is much more
dramatic. Clay content in terms of percentage clay/not
clay can effect the dry strength of the clay but I
have found it is more the case of what kind of clay is
it. As an observation I have found clay with a higher
rate of shrinkage has a higher dry strength. And a
clay body which contains a variety of particle sizes
to be strong either porcelain or stoneware. and that
comes down to what happens when the water leaves the
clay. Imagine a bowl filled with balls all equal in
density to that of water but of varying size. As the
water is drained those balls come together till all
the water is gone. The larger the balls in the
container the larger the gaps. If there were small
balls and medium balls those spaces would be filled.
Now if this container was filled with very small balls
it would become very dense. Therefore if this was a
clay-body containing small particles you can see how
it would pack together tighter and therefore become
stronger than a clay with larger particle sizes. Fire
clays have the larger structures to kaolins and ball
clays which are smaller. A clay body containing a
higher percentage of kaolins and ball clays to the
over all percentage end up being stronger.
"Porcelain's best qualities comes out if it is thin,
which is harder to do since the clay has very little
green strength. Thinner also makes it harder to glaze,
since it needs just the right touch. Take these
characteristic which causes more breakages than
average under the most experience porcelain artists"
One would guess from this statement that you make
bowls or open items. the translucency from porcelain
can only be seen when light can pass from behind the
piece to other side, as in a bowl. other items like
jars and plates and mugs and such too could have a
interior available to light. I would say its whiteness
and fine tooth are better qualities but thats my
opinion. (I throw vases and teapots and the such) Now
if any clay was pushed to the thinness that you push
the southern ice to gain sufficiently translucent, it
too would become brittle. I feel it is not the
weakness of the clay but a weakness in the physcial
structure of the pot. example a 4x4 pine will be
stronger than a pencil made of ash, even though pine
is a weaker wood.
"Since the glass content of porcelain is so high and
there is lesser clay to hold it together, porcelain
"moves" in the kiln. Easy to slump or deform. So one
has to know his sit and stand when designing and
firing porcelain. It is also interesting how one
learns to predict and use the movement of porcelain in
the firing process."
Again this has to do with the thinness of the work
along with the clay it is created with. anything will
slump if thin enough and fired to maturity. We have
stoneware that goes through pyroplastic deformation
and porcelains that never move. Shape does play a part
but it plays a part in all clays and does not need to
be a concern to only porcleain artists.
And I guess I would like to add a bit of my feeling on
this discussion of porcelains. I have always felt that
the use of porcelain has some kind of romance aspect
to it. that using it is mythical and secretive and all
sorts of horrible things happen when you use it. I
have had few issues with porcelain my students have
had some issues but they are consistent to the issues
they have with any other clay. And I have wondered is
it the strictly porcelain artists that attempt to
elevate their material to fulfill their ego, to charge
more, to become elite. And in doing so do others that
haven't used it perpetuate that feeling by repeating
it "porcelain is too had to use." I can see the cost
of it making people timid and less risky with its use.
But if you have a unlimited source of free porcelain
the abilities and piece attempted would be as
ambitious as those in other clays. In my opinion
porcelain is no different than other clays. Each well
thought and produced clay has its strengths and
weaknesses. Every clay has issues that are unique to
that clay.
I think the larger issue is what is attempted with
porcelain is more extreme to exploit a characteristic
of porcelain. Thinness in any ware will be difficult
and cause breakage but why made a raku piece paper
thin (well I can think of a few) why made a dinner
plate from stoneware paper thin? you wouldn't but if
you did there would be warping and cracking and
breakage.
To define it well it seems simple to me. Any clay body
that is free enough from iron and other impurities to
become white and translucent, would be a porcelain.
TiM SEE
Syracuse, NY
http://www.youtube.com/profile_videos?user=timseepots
http://www.etsy.com/shop.php?user_id=5467443
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