Neil Fallon on mon 18 jul 05
Has anyone ever had the bottom fall out of their kiln?
Mine has cracks in it and I have been concerned about that
happening so I don't leave it when it is hot
As usual, I'm coming to this conversation late. My apologies.
This weekend I was getting my old electric, round, kiln (no controller,
just a kiln setter and two manual switches) ready for a bisque fire. The
kiln sits on a metal stand. As I was sweeping around it, I noticed some
rusted metal on the ground underneath the kiln. I knelt down and looked up
at the bottom of the kiln. There is (or in this case, was) a piece of sheet
metal that was attached to the kiln bottom. The metal had rusted away in a
circular pattern creating a sizable "donut hole" in the metal. All that is
left is a small amount of metal on the sides where is attaches to the kiln's
sidewalls. It appears that is rusted from the inside out. What I mean is,
the rust began where the metal and refractory material of the kiln's bottom
touch, not where the metal stand and metal sheet touch. I will replace
this, but my question is why? The kiln has always been inside (garage) and
to my knowledge has never been wet. Is there something I can do when I
replace the metal to prevent this from happening again? I must add, this
kiln has to be 30 years old. This is the first problem I have had with it,
so I'm not really bummed about this small issue.
Neil Fallon
Rock Pond Pottery
William & Susan Schran User on mon 18 jul 05
On 7/18/05 7:24 AM, "Neil Fallon" wrote:
> It appears that is rusted from the inside out. What I mean is,
> the rust began where the metal and refractory material of the kiln's bottom
> touch, not where the metal stand and metal sheet touch. I will replace
> this, but my question is why? The kiln has always been inside (garage) and
> to my knowledge has never been wet.
Moisture is from the clay.
--
William "Bill" Schran
Fredericksburg, Virginia
Snail Scott on mon 18 jul 05
At 07:24 AM 7/18/2005 -0400, you wrote:
>...a piece of sheet
>metal that was attached to the kiln bottom. The metal had rusted away in a
>circular pattern creating a sizable "donut hole" in the metal...
>The kiln has always been inside (garage) and
>to my knowledge has never been wet. Is there something I can do when I
>replace the metal to prevent this from happening again?
Firing forces moisture out of the kiln and
through the bricks. That's where it came from -
humidity and the last remaining moisture in the
clay.
Personally, I'd ignore it. Not worth fixing.
It'll fire fine without it (and evidently has
been).
If you do replace it, it'll be good for another
quarter century, then (dang it!) you'll need
another piece of sheet metal.
-Snail
Vince Pitelka on mon 18 jul 05
Neil -
Regarding your question about the metal liner beneath your kiln rusting out
over time - during the "water smoking stage" (approximately 800-1500 degrees
F) of bisque-firing, various materials outgass from the clay, including
sulfur dioxide and water vapor from chemically combined water. On contact
with metal surfaces, this creates sulfuric acid, which is responsible for
the corrosion we normally see on non-stainless ferrous metal parts on
electric and gas kilns. The slight back-pressure in the kiln causes these
fumes to seep through the porous softbrick, and thus the corrosion from the
inside out on the non-stainless steel parts of the metal shell. I don't
know of anything you can really do to prevent it. I'd like to see kiln
manufacturers use stainless steel for all these parts. When you do the
repair, a good coat of high-heat paint on all non-stainless ferrous metal
surfaces during re-assembly will slow down the corrosion. Also, if you have
a downdraft kilnvent, that will draw off the vapors, and then the primary
corrosion over time will be on the kiln vent parts, rather than on the kiln
shell parts.
- 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/
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