Nathan Miller on sun 16 mar 08
About a year and a half ago, I built a small woodfire kiln following plans
from Pottery Making Illustrated. For the maiden firing, I strung several
IFB onto 1/2" threaded rods. The rods softened during the firing and the
roof sagged. I inquired of this list regarding roof options and was given
some good feedback, good ideas, and good leads.
In the end, I decided to use three 12x24 standard mullite shelves overlain
with IFB. This approach has worked well so far, although I've recently
begun to run into a few problems. The shelves I used had already been
employed as ware shelves in a cone-10 reduction kiln at the local university
for a few years and were already pretty beat-up. I figured this made them
perfect candidates for this sort of thing. Last June, during the 4th or 5th
firing using this method, the shelf over the firebox collapsed into the
kiln. Remarkably, casualties from this were in the single digits. I
flopped an old boiler blanket (some sort of polyester coating with a
borosilicate fill) over the hole and was able to stoke for another hour
before it melted. So I got a new shelf. Two firings ago (last weekend),
the same thing happened, this time over the back of the kiln. That shelf
was already cracking and I suppose this really should have been a red flag.
At this point, I was faced with getting yet another shelf, which would give
me one of these used shelves and two brand new ones, or trying something
else. I decided that buying new shelves was probably a work-around and
didn't really solve anything. I went with fiberboard. The one place I know
in the Portland OR area that carries such material had 2'x3' pieces in 1"
and 1/2" thicknesses. I bought two of each.
Initially, I was very pleased. Closing up the roof was remarkably fast. I
had the two 1" boards first and the two 1/2" boards on top of them and
overlapping by a couple of inches. My early temperature rise was very fast
and I was up to 700 C in two hours. I attribute this to the absence of the
additional heat-sink from the shelves and far fewer chinks through which
heat can leak. After about the first hour, I noticed that the board over
the firebox was developing an upward bulge. This stuff is essentially
Kaowool and since that stuff can shrink during the initial use, I figured I
was seeing something like that. About 3 or so hours into the firing, I
noticed that the meeting edges of both boards, right over the middle of the
kiln (and unsupported) were starting to sag and the front board had
developed a crack on its underside. As the firing continued, both boards
sagged and I found myself plugging gaps with Superwool. I was able to
finish the firing after 8 hours. I'll be opening the kiln this afternoon.
It'll be interesting to see how much of this deformation was sagging and how
much was extreme expansion, since the 1/2" boards didn't seem to have
deformed at all.
So now it seems I have a bit of a conundrum. Do I go back to the mullite
shelf method, buying 2 new shelves so that all the roof shelves are new and
less likely to break? Do I try to find even thicker fiberboard that might
be less likely to sag? Other ideas?
-Nathan Miller
Thistillium Pottery
Newberg, OR
Paul Herman on sun 16 mar 08
Nathan,
By all means, it's time to cut your losses and build a brick arch. It
will last a lot longer than the methods you have tried so far.
Good luck,
Paul Herman
Great Basin Pottery
Doyle, California US
www.greatbasinpottery.com
On Mar 16, 2008, at 11:19 AM, Nathan Miller wrote:
> So now it seems I have a bit of a conundrum. Do I go back to the
> mullite
> shelf method, buying 2 new shelves so that all the roof shelves are
> new and
> less likely to break? Do I try to find even thicker fiberboard that
> might
> be less likely to sag? Other ideas?
>
> -Nathan Miller
> Thistillium Pottery
> Newberg, OR
Clayart SCtag on mon 17 mar 08
listen tp Paul,Nathan, the arch with proper bracing and a hard skew line
will make you a lot more satisfied
.
.Magaret
In a message dated 3/17/2008 12:19:32 A.M. Eastern Daylight Time,
potterpaul@FRONTIERNET.NET writes:
Nathan,
By all means, it's time to cut your losses and build a brick arch. It
will last a lot longer than the methods you have tried so far.
Good luck,
Paul Herman
**************It's Tax Time! Get tips, forms, and advice on AOL Money &
Finance. (http://money.aol.com/tax?NCID=aolprf00030000000001)
Steve Mills on mon 17 mar 08
Nathan,
Here in the UK I/We use second hand shelves to cover our wood kiln, they're 11/4 inches thick, and so far have held up to a fair number of cone 10 firings.
Last October at a workshop I was involved in we used second-hand (Japanese) Silicon Carbide shelves in a sandwich with 1 inch fibre in between, no problem, and a friend who has the same design Kiln uses that system as well, again, no problem.
The span of the roof is 27 inches.
How thick were your roof shelves?
Steve
Bath
UK
Nathan Miller wrote: About a year and a half ago, I built a small woodfire kiln following plans
from Pottery Making Illustrated. For the maiden firing, I strung several
IFB onto 1/2" threaded rods. The rods softened during the firing and the
roof sagged. I inquired of this list regarding roof options and was given
some good feedback, good ideas, and good leads.
In the end, I decided to use three 12x24 standard mullite shelves overlain
with IFB. This approach has worked well so far, although I've recently
begun to run into a few problems. The shelves I used had already been
employed as ware shelves in a cone-10 reduction kiln at the local university
for a few years and were already pretty beat-up. I figured this made them
perfect candidates for this sort of thing. Last June, during the 4th or 5th
firing using this method, the shelf over the firebox collapsed into the
kiln. Remarkably, casualties from this were in the single digits. I
flopped an old boiler blanket (some sort of polyester coating with a
borosilicate fill) over the hole and was able to stoke for another hour
before it melted. So I got a new shelf. Two firings ago (last weekend),
the same thing happened, this time over the back of the kiln. That shelf
was already cracking and I suppose this really should have been a red flag.
At this point, I was faced with getting yet another shelf, which would give
me one of these used shelves and two brand new ones, or trying something
else. I decided that buying new shelves was probably a work-around and
didn't really solve anything. I went with fiberboard. The one place I know
in the Portland OR area that carries such material had 2'x3' pieces in 1"
and 1/2" thicknesses. I bought two of each.
Initially, I was very pleased. Closing up the roof was remarkably fast. I
had the two 1" boards first and the two 1/2" boards on top of them and
overlapping by a couple of inches. My early temperature rise was very fast
and I was up to 700 C in two hours. I attribute this to the absence of the
additional heat-sink from the shelves and far fewer chinks through which
heat can leak. After about the first hour, I noticed that the board over
the firebox was developing an upward bulge. This stuff is essentially
Kaowool and since that stuff can shrink during the initial use, I figured I
was seeing something like that. About 3 or so hours into the firing, I
noticed that the meeting edges of both boards, right over the middle of the
kiln (and unsupported) were starting to sag and the front board had
developed a crack on its underside. As the firing continued, both boards
sagged and I found myself plugging gaps with Superwool. I was able to
finish the firing after 8 hours. I'll be opening the kiln this afternoon.
It'll be interesting to see how much of this deformation was sagging and how
much was extreme expansion, since the 1/2" boards didn't seem to have
deformed at all.
So now it seems I have a bit of a conundrum. Do I go back to the mullite
shelf method, buying 2 new shelves so that all the roof shelves are new and
less likely to break? Do I try to find even thicker fiberboard that might
be less likely to sag? Other ideas?
-Nathan Miller
Thistillium Pottery
Newberg, OR
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shane mickey on mon 17 mar 08
nathan
try getting some insulating castable and making a frame the size of your =
entire roof,
or just large enough for your ware chamber, pour the castable into the =
mold and
you will have a nice roof, you could even go as far as putting a slight =
arch to it. if you=20
need more details email me off list.
shane micke
kiln building and desing services
Paul Haigh on mon 17 mar 08
Disclaimer: I don't know what the heck I'm talking about.
Is there a reason that you didn't go with a sprung arch? Was this some sort of MFT hybrid thingy?
It sounds like the money that you will spend, and the time and effort that you will lose (with aborted firings, ruined glazes with bits of this stuff on pots) with these sorts of fixes could be spent on a sprung arch that is proven and rock solid.
Paul Haigh
Londonderry, NH
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