Hank Murrow on sun 31 aug 03
On Saturday, August 30, 2003, at 10:42 PM, Steven Goldate wrote:
> This is a question for any clay experts out there. I have a porcelain
> that slumps at
> the maturing temperature. I realize I could fire it lower, but what
> could I add to
> stiffen it, without losing too much of its other porcelain-related
> qualities? Calcined
> alumina? Some other ingredient?
Dear Steven;
I have been following this thread for some time without responding, but
now feel a few words may help to put the subject of translucent
porcelain into a wider picture.
The wares made around Jingdezhen, China beginning in the 12th century
were made of a new material the potters called 'bai tunzi', or little
white bricks, called that because after crushing and levigation the
material was dried into that form. It was also called 'cishi' or
porcelain stone. The material was extrusive (volcanic) in origin, and
was highly weathered, reducing the normal amounts of alkali found in
this material. It was also nearly devoid of iron and (important for
translucency) titania. The early pots from the Sung times were made
entirely of this volcanic material without any additions. The remaining
approximately 4% of alkali rendered the bodies translucent even as low
as C/8, as revealed by later analysis. Yet, they could stand as much as
c/11 also. The reason for this is the way in which the alkaline content
occurs in this material. It is in the clay lattice at the molecular
level. That is, each clay particle has atomic alkali embedded within
it, so the melting is very gradual and steady.
Now contrast this with your average western european 'kaolin and
feldspar' based body. First, the feldspar particles, fine as they are
(300+mesh) are HUGE compared to the clay particles. Compare the size of
a house (feldspar) to the size of a business card. In such bodies, when
the HUGE feldspar particle begin to melt, they take a lot of kaolin
with them, and very precipitously. I have been working with porcelain
for 40 years, and the only bodies that I have found both translucent
AND minimally slump-prone have been those made with Grolleg or Limoges,
or the Rhyolitic (volcanic) based ones made from material similar in
origin to the 'Cishi' of Jingdezhen.
Well, the Grolleg is easy to come by, and limoges is being brought in
(at great expense), but how to get some altered Rhyolite, you ask.
Well, Plastic Vitrox is one such material, though residual alkali is
down around 2%. However, Rhyolitic deposits are found along the entire
Western Cordillera, from Tierra del Fuego to the Alaskan ranges. So if
you want a really authentic porcelain material, all you have to do is
consult your geology maps for surface deposits of Rhyolitic tuff, and
find one located near a logging or wilderness road so you can get your
picup close by it. Also you'd look for a deposit at the surface so
weathering (to reduce the alkaline % some) would be likely. Here in
Oregon, I mine just such a deposit on Calf Ridge, 2000' above the North
Fork of the Umpqua River, east of Roseburg. As it happens, iron and
titanium have come in with the hot geyser alteration, so it is not
translucent. But this body is very vitrified at C/10 (rings like a
bell) and 20% limestone added to the body creates a wonderful iron
green. The body takes flyash in the anagama fire beautifully, and is a
rich mahogany color from reduction. By the way, as the original
deposits of bai-tunzi got deeper down in the earth and therefore less
weathered and therefore plastic, they began to add kaolin from the
area, so that nowadays there is about 30% kaolin in the Jingdezhen
porcelain still manufactured there today. On the other side of the
Pacific, my friend, David Stannard, mines several deposits near
Fairbanks, AK, which are white, translucent, and according to a Tea
expert who was just visiting from Tokyo, "are the real deal......just
like Jingdezhen!" So, porcelain deposits may be somewhere near you if
you live in the western US or along the Andes. An aside.........the
afore-mentioned Stannard is now in the process of getting a lease from
the gov'mint on a deposit of porcelain-stone which also has a hill
which is the best wind energy site near Fairbanks, where he plans to
install a wind generator to electrically produce Hydrogen from water
and store the H2 in tanks. The electricity will also run the machines
to process his p-stone, as well as a Limestone that occurs on the 20
mile road into the place.
So visualize this: a potter drives in, picking up the limestone on the
way, grinds the p-stone to 10 mesh size, ball mills that until plastic
(it is sericitic, and plate-like in form), dries it out to plastic
condition, fires it with the site generated hydrogen with the limestone
enriched body as glaze, and the result is white, translucent, and
having a gorgeous celadon glaze that fits so well you can bang two
pieces together so they ring like bells. I am talking Sung-quality
porcelain. Only hitch in this true story is that David is 74 and not in
great health. But can you imagine the stir it would create to see some
(unheralded, Mel) potter making his high temp porcelain wares from
AMBIENT ENERGY??!! Hey, someone's got to do it, or we're all obsolete
when the oil runs out in 25 years. I told you it was an aside. When he
gets his ducks in a row, I will let you know where to send your
'pottery missions' check.
Anyway, I hope this post adds some useful perspective on what some of
us mean when we say 'porcelain'.
Cheers, Hank in Eugene
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