Fredrick Paget on mon 26 jun 06
Our neighborhood here is on top of a bed of red orange clay and when
ever I see digging going on I try to collect some.
The last firing I did in the electric kiln to cone 6, I put in a
couple of pieces (on waster saucers) to see what happens to it at
higher temps.
One piece softened up and bloated turning a dark chocolate color.
The other stayed firm and came out looking like purple sand clay from
Yixing. They were down at the bottom of the kiln and it was cone 7
down there.
Now if I can just remember where that sample came from!
I wonder why that sample survived. Sometimes I add some ball clay
when I am cleaning up the clay by making slip and straining it. Could
added ball clay raise the vitrification point?
--
From Fred Paget,
Marin County, CA, USA
fredrick@well.com
Charter Member Potters Council
Paul Herman on mon 26 jun 06
Hi Fred,
I think adding ball clay could very well raise the firing temp of
your local clay.
Ball clays are usually low in iron and are considered to be a
'secondary' kaolin, meaning it is kaolin transported by water.
Best,
Paul Herman
Great Basin Pottery
Doyle, California US
http://greatbasinpottery.com
On Jun 26, 2006, at 10:46 AM, Fredrick Paget wrote:
> I wonder why that sample survived. Sometimes I add some ball clay
> when I am cleaning up the clay by making slip and straining it. Could
> added ball clay raise the vitrification point?
> --
> From Fred Paget,
William Lucius on tue 27 jun 06
From your description - a red orange clay that bloated and turned a dark =
chocolate color - that is a classic response of a low fire earthenware =
to excessive heat. The clay is undoubtedly rich in iron (hence the red =
color) which acts as a powerful flux that melts the clay. The bloating =
happens when the generated gasses cannot penetrate the viscous clay =
body. If you break across the bloat you should see a honeycomb not =
unlike vesicular basalt. a lava which is also gas rich.=20
The sedimentary red clays favored by the prehistoric potters of the Four =
Corners area occur in the Chinle and Morrison Formations, which as far =
as I know do not extend into California. Although their maturation =
temperatures are variable, most cannot tolerate temperatures above cone =
04. I would guess that your clay is similar and as such is pretty =
useless at mid-range stoneware temperatures. Have you ever thought =
about exploring earthenware technologies like majolica? =20
William A. Lucius, Board President
Institute for Archaeological Ceramic Research
845 Hartford Drive
Boulder, CO 80305
iacr@msn.com
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