Vince Pitelka on sat 30 jul 05
Craig DC wrote -
"Just to piggy back on what Phil has said.....the percentage of alumina is
too low if you want to get any real life out your bricks. The salt will make
realativly short work of them. I helped design and build a coupla salt over
ten years ago and we ended up putting Kruzite 70's in the 80 cubic foot one.
It is still working fine and the bricks are holding up quite well. The
higher alumina fire brick will cost more but they last a lot more time."
Now I'm getting confused. Earlier in this same thread, Hank Murrow wrote
about a beloved salt kiln at University of Oregon that was built with
high-silica bricks. The salt formed a glaze on the inside surface of the
bricks, and then the kiln just kept working and working for many years.
Back in the early 70s I helped Mike Selfridge build some salt kilns at
Gorilla Gulch in McKinleyville, CA, and we used regular high-duty firebrick
recycled from sawmill boiler fireboxes. As in Hank's descriptiong, those
bricks built up a nice salt glaze on the inside, and then endured years of
salt firings.
At the Craft Center, our experience with Kruzite 70s (70% alumina) is
exactly opposite Craig's. We got a very large amount of them for free when
APGreen sold out to Harbison Walker and was emptying out warehouses. This
was when we were going through the infamous funding crisis in 1996-98, and
we had no money to build new kilns. Entirely with donated materials, we
built a 60 cubic foot salt kiln and our 200 cubic foot "Hoggama" wood kiln.
The latter is holding up fine, but the salt kiln lasted a little over three
years. Somewhere I do have pictures of what that kiln looked like when we
demolished it, and it was indeed a frightening sight. In our experience,
the high alumina content in the Kruzite 70s resists the deposition of sodium
vapors, and thus do not build up a protective glaze on the surface. The
bricks are extremely refractory, and are porous enough that the sodium
vapors penetrate the body of the brick. After enough firings, they start
swelling up and moving around, and they spall badly on the inside surface.
There's a couple of possibilities here. Perhaps there was some discrepancy
in quality in Kruzites towards the end of APGreen, and perhaps Craig got a
better batch of bricks. Also Craig, how heavily has your salt kiln been
used? At the time, our salt kiln was our most popular high-fire kiln, and
was probably fired several hundred times during its three-plus-year life.
If your kiln has not been fired that much, then I think I can predict what
is going to happen to it in the coming years.
After our experience with the Kruzites, I sent out enquiries about
refractories for salt kilns, and received replies from Jack Troy, Brad
Schweiger, Wil Shynkarunk, John Neely, and some others experts on salt
firing. The concensus was that for salt and soda kilns, you should avoid
high-alumina brick, and that for the money, ordinary high-duty firebrick
like APGreen Empires or Clippers, and high-duty castable like APGreen Mizzou
are the best choices. We used new standard high-duty brick on the current
salt kiln, and it seems to be holding up well for the hard use it gets.
I should point out that we did have some trouble with a batch of Mizzou
castable used around the burner ports and flue on the current kiln, but I
attribute that to poor practice in mixing the castable. It was a hard
lesson to learn. We mixed the Mizzou in a regular cement mixer, and I think
we added a little too much water in order to get it mixing properly in that
kind of mixer. Once cast, it several days for the cast areas to cure - much
longer than normal. The excess water produces a porous casting, susceptible
to attack from the salt. Unless you have access to a Simpson Muller-Mixer
or a horizontal-shaft cement mixer, castable refractory should always be
mixed with a hoe in a wheel barrow or mason's trough. We recently repaired
the damaged areas with Mizzou mixed properly, and the repair is holding up
just fine.
When I was hired at the Craft Center in 1994, the salt kiln had been built
out of salvaged interlocks donated by Alcoa, and some of those were pretty
low-duty. The inside of the fireboxes and burnerports melted away the full
thickness of one brick. We chiseled away the melted surface, built wood
forms, and cast new burner ports and inside walls on the fireboxes with
Mizzou. At that time we didn't have a cement mixer, so we mixed the Mizzou
in a wheelbarrow, and that repair survived at least another hundred salt
firings with little evidence of any deterioration. That kiln lasted a few
more years until we got the big donation of Kruzites.
Phil Rogers - if you read this, what's your slant on the above? What
refractories have given you the best performance in salt?
- Vince
Vince Pitelka
Appalachian Center for Craft, Tennessee Technological University
Smithville TN 37166, 615/597-6801 x111
vpitelka@dtccom.net, wpitelka@tntech.edu
http://iweb.tntech.edu/wpitelka/
http://www.tntech.edu/craftcenter/
Hank Murrow on sat 30 jul 05
On Jul 30, 2005, at 8:59 AM, Vince Pitelka wrote:
>
> Now I'm getting confused. Earlier in this same thread, Hank Murrow
> wrote
> about a beloved salt kiln at University of Oregon that was built with
> high-silica bricks. The salt formed a glaze on the inside surface of
> the
> bricks, and then the kiln just kept working and working for many years.
We did this on the advice of A.P.Green people at the yard where we got
them. They built their slat glazing sewer tile kilns out of them. The
kiln glazed to a lovely transparent green color and that was it until
many years later. Don't suppose that kiln would have liked being fired
real fast, though.
>
Cheers, Hank
www.murrow.biz/hank
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