Marcia Selsor on sun 9 may 99
Dear Nikom and Dean,
I enjoyed Nikom's response. I thought I'd add to it.
In Agost,Spain where the primary product of the local potters was a
botijo-a white porrous jug used as a thermos-water cooler, the clay was
a mixture of local clays and salt-in the clay body. Back in the '80s the
mule cart operator dug the clay and delivered it to the potteries. He
brought gray and tan clays to the pottery factories and the clay mixer
man would agitate/dissolve the clay in a basin of water. The mixture was
drained into other basins where excess water was removed and the clay
set up.
After much more manual labor of moving the clay into drying beds, the
clay was put into a mixer (not a pugger but more of a dough mixer)
and salt was added. The clay then was stored until the women assistants
came for it. They pugged in for the potters whose job was throwing only.
The pots were fired in the three story kilns(literally three stories
tall) starting out "poco a poco" for two days then increased the fire
for three more days. Testing on the top of the kiln where pieces were
drawn from the center hole in the dome. Pots were broken to see if the
core was pink or not. If it was pink, they continued firing for two more
hours and tested again. The potters said the salt made the water taste
better.
I did some experiments with this clay and fired it for what I considered
slow in a small eletric kiln. It always blew up. The clay had an
incredible dry strength. These botijos were stacked solid up to twleve
feet high using a ladder leaning against the loaded pots. It was amazing
to me. There are few potters left in town but they still make botijos.
Raw clay is brought in by trucks now. They still mix the clay in basins
but with machines.
Marcia Selsor in Montana where snow is predicted for tomorrow and the
lilacs are starting to bloom
but soon going back to Agost with some clayarters
nikom chimnok wrote:
>
> ----------------------------Original message----------------------------
> Hello Dean,
> I figured Clayart would explode with responses to your queries, so I
> didn't bother to respond the first day. But nobody answered, so I'll tell
> you what I know.
>
> First, how fast you can fire has most to do with your claybody. You
> can design a clay which will fire fast, but you might be stuck with one
> which will only fire slow. Several materials are the friends of the fast
> firer: talc, wollastonite, grog, and kaolin. The enemies are ball clay and
> very plastic red clays. Small particles sizes promote plasticity, and at the
> same time allow water to migrate out of the pot slowly. Large particle size
> and inert ingredients allow fast water migration and fast firing. I've seen
> the clay formula for a local company that fires tiles in a continuous kiln,
> from room temperature to 1200 degrees C to cool in one hour. The tiles are
> pressed and contain very little clay.
> The first danger of fast firing is explosion. If the water in the
> clay boils and the steam can't escape, the pot will explode. Grog is the
> best insurance against explosion. The second danger is cracking. Here's
> where talc and wollastonite help.
> The clay mix is extremely important. Our local clay is normally
> once-fired for two days and cooled for two, and still explodes and cracks
> 50% and more. But with 60% local clay plus additions of ingredients
> mentioned above, I can once-fire it with 100% success in 6 hours--that's
> from the time of lighting the fire to cone 04. No candling.
> Thickness is also important. The thicker it gets, the longer you
> have to fire. This is easy to understand--it's the same problem of the
> turkey being burnt on the outside but raw inside. With clay the result is
> not inedibility, but explosions and cracking.
>
> As to Fred Olsen's fast firing, I think he is not generally talking
> about bisque. It's a pretty unusual claybody you can bisque in 4 hours. The
> only time I've fired an Olsen kiln was in college, where we bisqued in an
> electric kiln for about 12 hours, then glazed with wood in 6. It's much
> easier to do a fast glaze firing than a fast bisque firing, because you
> don't have the water to contend with. I'm not commenting here on the quality
> of the glaze, only the possibility of breaking the pot.
>
> At present we once-fire glazed pots about the size you're talking
> about to 1230 C in around 16 hours. The clay contains 20% grog, and never
> explodes. We start it slow, proceed at 50 degrees C per hour to 650, then
> put the pedal to the metal and finish as fast as the kiln will go. Above
> 650, a rise of 300 degees per hour doesn't hurt a thing. We do get the odd
> crack, but it is considered cheaper to put up with this than to buy the
> ingredients to stop it.
>
> I hope this discussion is helpful to you.
> Regards,
> Nikom, in Thailand, where the mangoes are now so plentiful they're
> practically giving them away.
> ****************************************************************************
> ******
> At 14:06 4/5/99 EDT, you wrote:
> >----------------------------Original message----------------------------
> (snip)
> >Until now I haven't had a kiln that could fire really fast. Now I can see
> >the possibility of going from 300F to 2250F in about four hours. Since I've
> >never fired this fast I'm wondering if it's wise to risk a kiln load of pots
> >to find out if it can be done.
> (snip)
> Bisqueing might be a factor, too. In
> >'The Kiln Book' Fred Olsen describes fast firing his wood kiln from
> >400-2300F in 4 hours but he's probably firing bisque..
> (snip)
> >How fast can you go? How fast do you go? I'd like to hear from anybody that
> >has explored high speed firing. What are the dangers? What are the
> >parameters?
> >
> >Dean in Beautiful, Expensive Kauai
> >
> >
--
Marcia Selsor
selsor@imt.net
http://www.imt.net/~mjbmls
http://www.imt.net/~mjbmls/spain99.html
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