Marget and Peter Lippincott on tue 4 feb 97
I have been having frequent breakage of my 3/4" silicon carbide kiln
shelves in my gas kiln. These shelves are new and often break upon
first firing. I have used 12 of them in my kiln and 8 have already
broken. The last batch was the biggest disaster. I fired 7 shelves of
tiles, raw glazed, and the top five broke, 4 new and one of the old ones
that had been through 6-10 firings. This firing was to ^6 ox, but most
of the firings have been ^10 red. On this last firing, thinking the
problem might be damp shelves, I kindled the kiln on one pilot for 24
hours. I checked with the distributor and he said that his other
clients have not experienced problems. I called RonKovic at the Univ of
Illinois where the same shelves are used and he says even student abuse
has not fazed them. What's up?? I am wondering if there can be
something in my natural gas that causes a problem.
Peter Lippincott
Erin Hayes on wed 5 feb 97
Peter, how are you storing your shelves? Standing them on end on a board
with wooden spacers in between each shelf is pretty healthy. If you are
storing them flat they could be cracking, especially if any are sitting on
top of each other. Sometimes this will occur as you are chipping a shelf
free of glaze drips if you don't have the shelve on an even, wooden surface.
Of course you are probably not doing this, but I have seen it happen in grad
school, and I always suspect it first. Others will have possible
explanations which may fit your situation more closely.
Good Luck!
Erin.
Elca Branman on wed 5 feb 97
Don't be insulted by this question, but are you stacking your shelves so
that each stilt is under another stilt?If you are not, then you are
putting great strain on your shelves..Elca
Branman Potters elcab1@juno.com
in Stone Ridge ,N.Y.
in the Hudson Valley
LOWELL BAKER on wed 5 feb 97
I have never known just water to cause such a problem in silicone
carbide shelves. Are you allowing the shelves to get wet and freeze?
I still can't imagine that yuou would have this problem.
If the old and new batch of shelves were shipped at the same time I
would guess that they had been handled very roughly in shipping and
that they were cracked before firing. do you have any unfired
shelves that you can examine very carefully for hair line cracks.
sounds like the crate was dropped.
Lowell
The University of Alabama
Alabama Clay Conference coming soon....
Kenneth D Westfall on wed 5 feb 97
Peter
I would help if you could include how you have been posting the
shelves. Are you using four post or three,and do you use wade clay
between the post and the shelves? How big of a kiln post are you using
and how fast are you cooling the kiln.? At what temp. are you opening
the kiln? Do you have any idea when the shelves are failing, before red
heat, after Quart inversion, after red heat, or after you turn off the
kiln.? Also do you leave the damper open after turning off the kiln?
Kenneth
Kirby Benson on thu 6 feb 97
Peter,
If you are placing your posts correctly (bearing load, one above the other
on the way up) and you are not laying them flat on top of each other when
you store them and you are not going up too fast or down too fast with your
kiln cycle I would suspect the they were dropped during shipping and have
been cracked, although you may not be able to detect until they are under
the stress of heat.
Kirby Benson
Las Cruces, NM
kbenson@zianet.com
Vince Pitelka on thu 6 feb 97
>Peter, how are you storing your shelves? Standing them on end on a board
>with wooden spacers in between each shelf is pretty healthy. If you are
>storing them flat they could be cracking, especially if any are sitting on
>top of each other. Sometimes this will occur as you are chipping a shelf
>free of glaze drips if you don't have the shelve on an even, wooden surface.
A wooden surface is far to rigid to support shelves properly during
cleaning. A pad of foam rubber works OK, but my favorite surface to set
shelves on while cleaning is a bag of sand, with the shelf nestled against
the sandbag so that it is absolutely evenly supported. The sandbag absorbs
shock, cushioning the entire shelf while you chip away the glaze.
Regarding the problem with cracked shelves, in production in California
years ago, I purchased forty new 14x28x3/4 sil. car. shelves. Six years
later, when I shut down the business to return to school, at least three
quarters of the shelves were cracked. Since I used ferro kiln posts for
floating set, the shelves were supporting only the pots upon them, and not
any additional kiln furniture and shelves. So even if they were cracked 2/3
of the way through they still worked fine, but when I sold them I got only
peanuts. I know why they cracked. I fired too fast. I used to keep the
burners on low for fifteen minutes, and then crank them up. These were
pretty mellow tube-burners, but they still fired the kiln to cone ten in
seven hours. If I had slowed down the firing until above red heat, I could
have avoided the cracking problems. This is a real downside to silicon
carbide shelves in ordinary firings.
- Vince
Vince Pitelka - vpitelka@Dekalb.Net
Phone - home 615/597-5376, work 615/597-6801
Appalachian Center for Crafts
1560 Craft Center Drive, Smithville TN 37166
Marget and Peter Lippincott on thu 6 feb 97
Thanks to all who have responded: Elca, Kenneth,Erin, Lowell
Here are my answers to those questions
I am three posting the shelves, two back corners and front middle
I use no wadding, posts vary from 1.5"sq to 2.5"sq
I cool the kiln as slowly as I can with damper and burner ports closed.
It takes 12-16 hrs I open it up at 150F
I'm not sure when the shelves fail. The tile in the last load was
warped where it bridged a break, so I guess it is safe to say that they
are breaking sometime before the kiln has cooled enough to where the
tile wouldn't warp. I watch the kiln a lot during the last stages of
firing, look through the peep hole, etc. and I have not noticed them
broken or heard them breaking, so if I am not missing something it must
be happening early during cooling.
I store the shelves, which I transported myself from the dealer, flat up
on two 2x4s. There is cardboard interleaved between each shelf which is
the way they came from the dealer. The are stored in a dry place in my
pottery. I have always handled them very carefully. I will inspect
them for cracks. I still have 12 that I have never fired.
Any more ideas?
Peter
Richard Gralnik on fri 7 feb 97
Peter,
One thing that may be contributing to the problem is
At 07:07 AM 2/6/97 EST, you wrote:
>I am three posting the shelves, two back corners and front middle
If this means
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then you've got a fairly wide unsupported span in the middle of the shelf.
You might want to try
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instead to support the middle of the shelf.
Richard
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