nikom chimnok on sat 21 nov 98
When warming a gas burning down-draft kiln, is it customary to warm the kiln
with the door open, or closed?Where I work, they warm the kiln (4' wide)
with the door open and perhaps only 6 of 32 burners lit, until the pyrometer
hits 500 degrees celsius. This takes about 6 hours. It's hard to say what
the temperature on the floor is, but the stack is still cold at this point,
and the bottom shelves in front of the kiln can be touched without burning
your hand. The latest kilnload was of tall pots, up around a meter, and they
all cracked on the bottom. My assessment of the situation is that the bottom
of the kiln probably went from less than 100 up to 600 degrees within half
an hour or so, and this is what caused the cracks.
It seems to me it would be better to close the door, light one
burner, and perhaps stick a burner into the stack to assure draft. This
would tend to even out the top and bottom of the kiln. Since we have clay
that both cracks and explodes easily, I would try to hold the temperature
around 100 degrees for 2 or 3 hours, then light more burners to raise the
temperature.
I'm curious to hear what anyone else might have to say about these
two approaches. It is clear that the usual approach doesn't work, but the
people I work with are desperately conservative, and are not willing to try
any other way, fearing the results would be even worse than they are now.
Any suggestions would be appreciated.
Nikom
koratpot@loxinfo.co.th
In Thailand, where it is now winter, which is much like summer where you live.
Louis Katz on sun 22 nov 98
Sawasdii Nikhom,
I have never started a gas kiln with the door open. I would be even less likely
to do this with a downdraft as I want the chimney to warm up as soon as possible
and the open door is unlikely to help.
I would also close the damper down some during the warm up stage as this will
promote a more even temperature in the kiln and allow the bottom to dry out. Wha
you want to do is minimize the cold air coming into the kiln. The cold air
displaces any warm air that might warming the bottom of the kiln.
Although true dull red heat is below quartz inversion, you don't see dull red
well enough in daylight to see it until right near quartz inversion. You MUST
fire slow until a good bit After you see dull red heat on the bottom of the kiln
This is because insides of large pots and floors of pots may not have yet reache
quartz inversion.
This practice of slow firing through quartz inversion may also help your kiln
shelves and kiln bricks last longer.
IMHO,
Sawasdii
Louie?
--
Louis Katz
lkatz@falcon.tamucc.edu
NCECA Director At Large
Texas A&M-CC Division of Visual and Performing Arts Webmaster (512) 994-5987
Earl Brunner on sun 22 nov 98
My down draft drives me dingy when I'm trying to bisque in it. It's a small Gei
fiber. when I light ONLY the pilots, it will go to 800-1000 degrees F. And it
passes boiling WAY to fast for thicker peices. I have to leave the door open to
keep the temperature within limits. Still on my last bisque I popped a chunk of
the side of one of the larger peices. With the bigger stuff I'm learning that
trying to save a little extra gas just isn't worth it. I need to soak longer.
kiln only has four burners, for your problem your own suggestion certainly soun
reasonable.
Good Luck.
Earl Brunner
nikom chimnok wrote:
> ----------------------------Original message----------------------------
> When warming a gas burning down-draft kiln, is it customary to warm the kiln
> with the door open, or closed?Where I work, they warm the kiln (4' wide)
> with the door open and perhaps only 6 of 32 burners lit, until the pyrometer
> hits 500 degrees celsius. This takes about 6 hours. It's hard to say what
> the temperature on the floor is, but the stack is still cold at this point,
> and the bottom shelves in front of the kiln can be touched without burning
> your hand. The latest kilnload was of tall pots, up around a meter, and they
> all cracked on the bottom. My assessment of the situation is that the bottom
> of the kiln probably went from less than 100 up to 600 degrees within half
> an hour or so, and this is what caused the cracks.
> It seems to me it would be better to close the door, light one
> burner, and perhaps stick a burner into the stack to assure draft. This
> would tend to even out the top and bottom of the kiln. Since we have clay
> that both cracks and explodes easily, I would try to hold the temperature
> around 100 degrees for 2 or 3 hours, then light more burners to raise the
> temperature.
> I'm curious to hear what anyone else might have to say about these
> two approaches. It is clear that the usual approach doesn't work, but the
> people I work with are desperately conservative, and are not willing to try
> any other way, fearing the results would be even worse than they are now.
> Any suggestions would be appreciated.
> Nikom
> koratpot@loxinfo.co.th
> In Thailand, where it is now winter, which is much like summer where you live.
Vince Pitelka on sun 22 nov 98
>When warming a gas burning down-draft kiln, is it customary to warm the kiln
>with the door open, or closed?Where I work, they warm the kiln (4' wide)
>with the door open and perhaps only 6 of 32 burners lit, until the pyrometer
>hits 500 degrees celsius. This takes about 6 hours. It's hard to say what
>the temperature on the floor is, but the stack is still cold at this point,
> It seems to me it would be better to close the door, light one
>burner, and perhaps stick a burner into the stack to assure draft. This
>would tend to even out the top and bottom of the kiln. Since we have clay
>that both cracks and explodes easily, I would try to hold the temperature
>around 100 degrees for 2 or 3 hours, then light more burners to raise the
>temperature.
Nikom -
I think your assessment of the situation is absolutely correct. In the
preheat, the bottom of the kiln simply is not heating adequately with the
door open. For adequate preheating, the heat must be confined within the
kiln. I think the stack draft will take care of itself once you confine the
heat, so there shouldn't be any need for a burner in the stack. The only
exception to this is if weather conditions cause a reverse stack draft on a
downdraft kiln. Then it is often necessary to use a burner or a small fire
in the base of the stack to get the draft going properly.
We have an old Norman updraft with eight burners under the floor, and we
preheat with them all on very low, with the door closed, and with the damper
open several inches. Frankly, I have never heard of anyone preheating a gas
kiln with the door open.
Good luck -
- Vince
Vince Pitelka - vpitelka@DeKalb.net
Home 615/597-5376, work 615/597-6801, fax 615/597-6803
Appalachian Center for Crafts
Tennessee Technological University
1560 Craft Center Drive, Smithville TN 37166
Stephen Mills on mon 23 nov 98
In message , nikom chimnok writes
< It seems to me it would be better to close the door, light one
burner, and perhaps stick a burner into the stack to assure draft.>
Nikom,
This is exactly what I have always done, it does what you suggest it
will do, warms the stack, gets the draught going and ensures even warmup
of the wares within.
Steve
--
Steve Mills
Bath
UK
home e-mail: stevemills@mudslinger.demon.co.uk
work e-mail: stevemills@bathpotters.demon.co.uk
own website: http://www.mudslinger.demon.co.uk
BPS website: http://www.bathpotters.demon.co.uk
Geoff Walker on mon 23 nov 98
Nikom,
In my opinion, you are perfectly correct in your assumption that leaving
the door open for warming is the reason the pots at the bottom explode.
When I first started pottery, the kilns in the workshop were warmed that
way and there were quite a few losses unless they steaming went on for
at least 10 hours. I will never allow any openings (even spy holes) in
gas kilns in the early stages of firing.
I hope you have luck in persuading your workmates to at least try with
the kiln clamped and maybe only two burners going in the early stages.
To my mind, this would not only stop the explosions, but save on gas as
well.
Best of luck,
Geoff Walker.
www.cronulla-pot.com.au
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