Paul on thu 11 sep 03
Is there a specific definition of porcelain? In my research so far i have
only found that porcelain is higher in feldspar and silica, and that the
absorption of the fired piece can be close to zero, whereas with stoneware
it should be 2%. At one point i thought porcelain just meant that it was
made with Grolleg kaolin but then i started seeing porcelain throwing and
casting recipes with no grolleg at all. So how does one define porcelain?
The reason i ask is because i want to have as much silica as i can in the
stoneware i use for good glaze fit, but to do so means i will need more
feldspar to dissolve crystoblite. Is it possible to have a good clay for
functional use (ovenware, etc) with the same amounts of spar and silica as a
porcelain, but with more durable clays like fireclay and goldart that are
not as difficult to work with grolleg, espcecially if whiteness is not
important? Any advice appriciated.
Paul B
Falmouth, ky
Ellie Blair on fri 12 sep 03
If whiteness isn't a concern why do you want to use porcelain? Porcelain=
is a kaolin based clay. Some will have grolleg and some will not depend=
ing on it's function. There are two forms of Kaolin or china clay, primar=
y kaolin which is suppose to be about 95% impurities free. Secondary kao=
lin has more impurities although it is more plastic. A good porcelain wi=
ll fire to at least ^10. Some as high as ^12. Most porcelains will matu=
re at 3280 degrees F giving you a very vitreous body. =20
Are you throwing this or handbuilding. What glaze fitting problems are y=
ou having?
Ellie Blair
Blair Pottery
Lawrence, Kansas 66047
----- Original Message -----
From: Paul
Sent: Thursday, September 11, 2003 9:09 PM
To: CLAYART@LSV.CERAMICS.ORG
Subject: DEFINITION OF STONEWARE VS PORCELAIN
Is there a specific definition of porcelain? In my research so far i have
only found that porcelain is higher in feldspar and silica, and that the
absorption of the fired piece can be close to zero, whereas with stonewar=
e
it should be 2%. At one point i thought porcelain just meant that it was
made with Grolleg kaolin but then i started seeing porcelain throwing and
casting recipes with no grolleg at all. So how does one define porcelain?
The reason i ask is because i want to have as much silica as i can in the
stoneware i use for good glaze fit, but to do so means i will need more
feldspar to dissolve crystoblite. Is it possible to have a good clay for
functional use (ovenware, etc) with the same amounts of spar and silica a=
s a
porcelain, but with more durable clays like fireclay and goldart that are
not as difficult to work with grolleg, espcecially if whiteness is not
important? Any advice appriciated.
Paul B
Falmouth, ky
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Paul Lewing on sat 13 sep 03
on 9/12/03 10:53 AM, Ellie Blair at blairea53@MSN.COM wrote:
> Is there a specific definition of porcelain?
Ah, Paul, as with so many things in ceramics, the answer is "It depends".
In this case, it depends on whom you ask. I'm having this forcefully
brought home to me because of a project I'm working on now, which involves
the history and uses of porcelain.
I "grew up" in the studio pottery culture, but now do tile, and I china
paint on it. The potters, the china painters, and the tile stores all use
the word porcelain, but they don't necessarily mean the same thing by the
word.
To a potter, porcelain is a high-fire (meaning- to them- cone 6 or higher)
white clay that is at least somewhat translucent. It will have a large
proportion of kaolin of one brand or another, but the clay content will not
necessarily be all kaolin. Many cone ten porcelain bodies have a
significant share of ball clay, but this will cut down the whiteness. The
rest of the body will generally be feldspar and silica in various
proportions.
China painters (who, incidentally, prefer to be called "porcelain artists"
even though they do not generally make things out of porcelain, they just
paint on it) refer to three kinds of porcelain. They are hard-paste,
soft-paste, and bone china. The clay content in all these is kaolin, but
the fluxes differ.
Hard-paste porcelain has feldspar and silica, and generally vitrifies at
about cone 5 or 6, so it's what a potter might grudgingly call porcelain.
As an aside, the first hard-paste porcelain produced in Europe, in Meissen
about 1710, had alabaster as the flux. After about the first ten years of
production there, they used feldspar and silica.
Soft-paste porcelain uses something more akin to glass as the flux.
Originally it was ground glass, now it's usually a frit. It matures about
cone 01-1.
Bone china uses bone ash as a flux, sometimes up to 50%. It matures
somewhere between hard-paste and soft-paste.
No potter would ever consider either of these latter to be true porcelain.
Then there's the tile industry, which produces a wide variety of "porcelain"
tile. Their definition of porcelain has to do only with the absorbency
rate. If it's below about 3%, they consider it porcelain, no matter what
its clay content or other characteristics. So it's perfectly possible to
buy "porcelain" tile that's not even white- even dark brown or grey.
Incidentally, to the tile industry, the opposite of porcelain is "bisque",
regardless of whether or not it has a glaze on it. To a china painter, the
opposite of porcelain is "ceramic" as in, "That pot's not porcelain, it's
ceramic". Of course, to a potter, bisque and ceramic mean something
different than that. And let's not go into what the tile industry refers to
as "mosaic".
There now. Isn't that a lot clearer?
Paul Lewing, Seattle
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