Michele Reed on thu 1 apr 04
I have a 25 year old second hand, 12 cubic feet California kiln with 4
venturi burners. I want to fire to cone 10 in a climbing reduction. The
kiln walls are only 2 1/2 inches thick and I am having problems with slow
cooling. I welcome any advice regarding firing down after reaching
temperature, and any opinions on adding a layer of insulating fire brick
to the inside of the kiln. Many thanks. Michele Reed in Vermont
Dave Finkelnburg on thu 1 apr 04
Michele,
I also have a second-hand gas kiln with only 2.5-inch IFB walls that I
fire in reduction. The temperature drops like a rock when I shut it off.
The good news is I don't have to wait long to unload it!
You do not say what the "problems with slow
> cooling." are, so I'm going to assume your kiln cools too fast.
What I have learned about this, from Mayor Mel Jacobson, AKA "The Tick
King," on this list, is to fire to the end point you want, then crash cool
(shut off the burners, leave the damper and burner ports open) to 2,000 F or
a bit less, then relight the burners and hold there or allow the temperature
to creep down toward 1,750 F. John Hesselberth has also done a lot of
testing of slow cooling. You may want to search the archives for posts by
these two on slow cooling.
The duration of a slow cool is entirely up to you. Once the kiln drops
below 1,600 F, though, I'd let it cool as fast as it wants on its own, with
the damper and ports shut.
Some things to consider. First, 2,000F may be arbitrary. The reason
for dropping rapidly to below 2,000F is based on the knowledge that
crystobalite forms most rapidly from free silica above 2,000F. However, I
have not seen good, authoritative data on where crystobalite really forms,
and have seen some sources that indicate it forms above 1,650F (which
2,000+ certainly is). I'm not sure whether you get entirely out of the
crystobalite zone below 2,000F. For that matter, if you have a good clay
body, you're not going to make much crystobalite anyway. I don't mean to be
wishy-washy about this but I haven't seen anything definitive that dictates
just what temperature is critical for starting the firing down.
Second, slow cooling is intended to allow glaze "development." That
means crystal growth on the glaze surface. I have not done draw trials to
see where my glazes freeze, but I understand they are molten enough for
crystallization down to around 1,650F at least. So to be safe I like to
soak between 1,750 and 1,950F.
Finally, not all glazes benefit from a slow cool! In particular, glazes
with rutile or titanium dioxide will become more and more opaque the slower
you cool them. Don't slow cool those glazes if you want a glossy, mostly
translucent look. Do some testing to confirm the surface you want before
firing down with a whole kiln load of pots.
If you add IFB to your kiln, you will save fuel during your firings
because you won't lose so much heat out through the kiln walls.
It is impossible to say if the added IFB will give the slow cooling
effects you want for your glazes. That's so subjective, so dependent on
what you as the artist want. However, you will get a slower cooling rate
for sure with more insulation.
I hope this helps a little.
Dave Finkelnburg in Idaho
----- Original Message -----
From: "Michele Reed"
Sent: Thursday, April 01, 2004 8:46 AM
Subject: slow cooling kiln
> I have a 25 year old second hand, 12 cubic feet California kiln with 4
> venturi burners. I want to fire to cone 10 in a climbing reduction. The
> kiln walls are only 2 1/2 inches thick and I am having problems with slow
> cooling. I welcome any advice regarding firing down after reaching
> temperature, and any opinions on adding a layer of insulating fire brick
> to the inside of the kiln. Many thanks. Michele Reed in Vermont
Ivor and Olive Lewis on fri 2 apr 04
Dear Michele Reed,
Solve your dilemma by reversing the rate of imput of heat used in
getting the kiln to fire to temperature. You may have to reverse the
rotation of each burner controller by slightly more than the original
rotation used when going up. For example, I rotate the knob on my
gauge 1/4 turn per hour going up. So would revers the rotation and
adjust by 1/3 to 1/2 turn when firing down. With developing skill you
should be able to make adjustments so that you accurately control the
rate of cooling of your kiln
You will need to keep a close eye on your pyrometer and log
temperature and time and draw the graphs as you go. I would take
reading every 15 mins. The way the graph behaves will guide you. If it
is steep turn the knob less next time, if too shallow turn it more.
Best regards,
Ivor Lewis. Redhill, South Australia
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