Pink Boy on mon 27 jan 03
Last Saturday I tried firing a couple of things in a
fire at Ocean Beach in San Francisco. Beautiful
night.
Most of the cups that I made spalled. I think because
I was in too much of a hurry warming them up beside
the fire.
Some of the smaller stuff, including beads worked out
fine. Fired to a nice bone white with streaks of
dark.
Two things learned.
1) Pots need to be heated very slowly before going
into the fire. Probably should place them two
feet from the fire and spend an hour walking them
into the fire.
2) Might also try baking then in the over to dry.
Last week was wet and rainy, so that might also cause
problems.
3) The Raku clay body is sweet, you can pull the
pots out of the coals while they are glowing orange.
PS: Anyone know how hot the coal get in a camp fire?
Mr Foo
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William Lucius on tue 28 jan 03
Spalling possibly means that you had air bubles in the walls of your cups or
even bits of organic material (you would not believe what stray birdseed
will do during firing). You might try to wedge the clay very well and build
the vessels so that no air is trapped.
It also might be the clay. Prehistoric Pueblo potters added over 30%
crushed rock to their clay in order to minimize both shrinkage and heat
shock. You might want to add some aplastic to your clay body.
Preheating is good, but an hour sounds a bit obsessive. Of course I have
never pitfired next to an ocean.
We have stuck a lot of pyrometer leads into pitfires and I seem to remember
that the usual temperature is between 700 and 800 degrees Centigrade, with
as much as a 100 degree variation from one part to another. With a stiff
breeze and the right fuel type it is possible to exceed 1000 degrees
Centigrade.
sounds to me like air bubbles
William A. Lucius, Board President
Institute for Archaeological Ceramic Research
845 Hartford Drive
Boulder, CO 80305
iacr@msn.com
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