Sue Leabu on mon 29 sep 03
I'm trying to understand why the anagama I've been firing for the last
three years always stalls out when the barometer is falling. We just
finished a five-day firing late last night. We started the fire-off at 8:00
yesterday morning and didn't get done until 11:00 PM. It was rainy all day.
We had to nurse it along and finally got cone 10 down in the tail, but it
was very slow going. Friday night, we spent 6 hours (in the rain) firing
cone 6 in the front. At midnight, it was at 3:00. 45 minutes later when the
sky cleared, cone 6 and 7 dropped completely and 8 was halfway gone.
(Ooops! Had to put the brakes on at that point!)
This kiln has run this same pattern every firing when it rains. Is that
typical? Is there something we can do to manage it better? One person new
to the firing group suggested a taller chimney. Could that be a cure?
Will appreciate any thoughts, ideas, suggestions! TIA!
Sue
Kalamazoo, MI
Paul Herman on mon 29 sep 03
Hello Sue,
I'm sure barometric pressure effects the draft.
And wet air/wood will affect the fire, causing more intense reduction
(from what I've read). I'm not sure exactly how it works. Owen Rye
(Australia) is experimenting with water added to woodfirings, He has put
water pipes under the floor, and can pour some in when he wants.
Does anyone on the list know if water "comes apart", freeing the
Hydrogen from the Oxygen, and if so, at what temperature?
A taller or fatter chimney would give you more draft, I'm sure. I like
fat chimneys. Is the kiln under a shed, and is the wood also?
Grand feu,
Paul Herman
Great Basin Pottery
423-725 Scott Road
Doyle, California 96109 US
potter@psln.com
----------
>From: Sue Leabu
> I'm trying to understand why the anagama I've been firing for the last
> three years always stalls out when the barometer is falling. We just
> finished a five-day firing late last night. We started the fire-off at 8:00
> yesterday morning and didn't get done until 11:00 PM. It was rainy all day.
> We had to nurse it along and finally got cone 10 down in the tail, but it
> was very slow going. Friday night, we spent 6 hours (in the rain) firing
> cone 6 in the front. At midnight, it was at 3:00. 45 minutes later when the
> sky cleared, cone 6 and 7 dropped completely and 8 was halfway gone.
> (Ooops! Had to put the brakes on at that point!)
>
> This kiln has run this same pattern every firing when it rains. Is that
> typical? Is there something we can do to manage it better? One person new
> to the firing group suggested a taller chimney. Could that be a cure?
>
> Will appreciate any thoughts, ideas, suggestions! TIA!
>
> Sue
> Kalamazoo, MI
Hank Murrow on mon 29 sep 03
Dear Sue;
Is it possible that rain is getting into the foundations of the kiln
and sucking up energy from the kiln in turning to steam? If so, you may
have to wait for drier weather or drain the water away from the kiln.
You did not say if it behaved this way with the lowered barometer, yet
without the rain.
That said, many Japanese potters fire only during rainy weather, and
others spray water into the kiln to improve the glazes.
On Monday, September 29, 2003, at 10:30 AM, Sue Leabu wrote:
> I'm trying to understand why the anagama I've been firing for the last
> three years always stalls out when the barometer is falling. We just
> finished a five-day firing late last night. We started the fire-off at
> 8:00
> yesterday morning and didn't get done until 11:00 PM. It was rainy all
> day.
> We had to nurse it along and finally got cone 10 down in the tail, but
> it
> was very slow going. Friday night, we spent 6 hours (in the rain)
> firing
> cone 6 in the front. At midnight, it was at 3:00. 45 minutes later
> when the
> sky cleared, cone 6 and 7 dropped completely and 8 was halfway gone.
> (Ooops! Had to put the brakes on at that point!)
>
> This kiln has run this same pattern every firing when it rains. Is that
> typical? Is there something we can do to manage it better? One person
> new
> to the firing group suggested a taller chimney. Could that be a cure?
iandol on tue 30 sep 03
Dear Sue Leabu,=20
A falling barometer tells you that the density of the air in your =
location has decreased. This means that less air enters you kiln to =
replace a similar volume displaced by buoyancy. The cause because the =
reduced differential between the density of cold air and hot exhaust =
gas.
To overcome the stall you need to get more air to the fuel. Seems to =
call for a larger firemouth, bigger grate area and a larger entry into =
the flue system. But I may be all together wrong!!
Best regards,
Ivor Lewis. Redhill, South Australia.
Who likes to fire on a rainy day, maybe tomorrow if the forecast holds =
firm.
steve harrison on tue 30 sep 03
Hi Sue in Kalamazoo,
From your brief description, it sounds like the flue and/ or chimney is
a little on the tight side.
Long firing isn't necessarily a bad thing, only when you want to go
faster and can't.
That said, differences in barometric pressure certainly do affect the
draught that is created via the chimney. When the pressure drops 10% it
is like the chimney is 10% smaller, or you have closed the damper a
little.
The solution is to have a chimney that is large enough in both diameter
and height. In this way you can choose to fire at the speed you decide,
not the weather.
What would be ideal for you at this point is too pull the damper out
another few inches and continue firing whatever the weather, at a rate
that suits you.
just adding extra height to the chimney isn't always as rewarding as it
might sound. It will depend on the cross-sectional area of the chimney
and the size and number of flue holes.
Certainly the easiest thing to do is to add a couple of feet on to the
chimney and try firing again. If that is not enough to solve the
problem, then it might be a rebuild of the flue system in total.
Keep in mind that this is my best bet going on your brief description.
But seeing that you say it fires this way every time it rains, it
sounds probable.
Have you noticed if it stalls when there is a drop in pressure without
rain. Say, at the approach of a thunder storm. You can often feel the
pressure drop with out a barometer. You might get thunder and dry
lightning, but no rain. and then it lifts. Has this ever happened?
If the kiln is starved fro draught you may notice a difference between
day and night or summer/winter. Does it fire easier at night or when
the temperature is colder?
There are so many things.
I have written 2 books on wood firing that may interest you.
'Australian Woodfiring' deals with the the technology of wood firing in
some depth, as well as its history since the second world war as
applied by artist potters. The focus is on Australia, as the name
suggests, however, the technology is universal.
'Laid Back Wood Firing' is a monograph that deals specifically with the
use of the 'Bourry' firebox. Although it too covers a lot of ground in
the way that it analyses kiln design and firing procedures. It has a
particular section on chimneys and flues.
In both books, I deal with the balance between the chamber size and the
draught required to fire it up to stoneware temperatures. These
chapters may be of interest to you.
more info is available at Ian Currie's web site;
http://ian.currie.to/sh/Steve_Harrisons_books.html
Best wishes
Steve Harrison
Hot & Sticky Pty Ltd
5 Railway Pde
Balmoral Village
NSW 2571
Australia
On Tuesday, September 30, 2003, at 03:30 AM, Sue Leabu wrote:
> I'm trying to understand why the anagama I've been firing for the last
> three years always stalls out when the barometer is falling. We just
> finished a five-day firing late last night. We started the fire-off at
> 8:00
> yesterday morning and didn't get done until 11:00 PM. It was rainy all
> day.
> We had to nurse it along and finally got cone 10 down in the tail, but
> it
> was very slow going. Friday night, we spent 6 hours (in the rain)
> firing
> cone 6 in the front. At midnight, it was at 3:00. 45 minutes later
> when the
> sky cleared, cone 6 and 7 dropped completely and 8 was halfway gone.
> (Ooops! Had to put the brakes on at that point!)
>
> This kiln has run this same pattern every firing when it rains. Is that
> typical? Is there something we can do to manage it better? One person
> new
> to the firing group suggested a taller chimney. Could that be a cure?
>
> Will appreciate any thoughts, ideas, suggestions! TIA!
>
> Sue
> Kalamazoo, MI
>
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>
>
http://ian.currie.to/sh/Steve_Harrisons_books.html
Sue Leabu on tue 30 sep 03
Thanks to all for the responses!
I don't think groundwater is an issue. The kiln is built in an abandoned
gravel pit, on the highest ground around. Drainage is not a problem because
of the sandy soil. There is a shed over the whole thing, with a very
large "living room" on the front. All the wood is dry and either stacked in
the shed or tarped. We are burning seasoned slab wood for the most part.
Our anagama is not a totally traditional design. It has about 450 cubic
feet of stacking space, but has a very broad front. The person who designed
the kiln was very interested in firebox pots, and so he created a
(relatively) huge firebox in the front. Interior at the front of the kiln
is about 8 feet across and 6-1/2 feet tall. The front rank of shelves start
about 2 feet from the front doors. From there it widens out a bit, but then
starts tapering in both in width and height. The interior dimensions at the
exit flue is roughly 3' x 3'. Total interior length is about 22'. There are
three stoke doors across the front, and two stoke ports on each side, with
secondary air vents built into the floor under all the side stoke ports.
Probably the best visual description I can give is that it looks like a
whale with the head cut off right at the gills.
There is always a noticable decrease in the velocity when the weather
starts to change with rain approaching. When the rain starts, so does the
stall. I have not noticed any difference in velocity in hot or cold
weather, night or day firing, windy or calm conditions, only in changing
barometric pressure.
I fire by sound and color (and cones, of course), but don't bother with
watching the pyrometer once we have fired the low cone, usually 04. The
damper is pretty responsive. I'm a fairly conservative damper-puller, but
the changes are noticable with only 1/2 inch or so change. Through this
last fire-off, we made about an 1-1/2" change in damper position over the
course of the day. Probably could have pulled it more, but didn't want to
completely lose the reduction atmosphere in the tail zone where the shinos
are. I was also concerned with dropping cone 13 in the front, preferring to
nurse 12 down slowly.
My other question (I know this is getting lengthy!) is is there an
effective way we could be using the secondary air vents during the stalls?
To date, the only situation we have used them for is to melt down coal
piles if the side ports have been over-stoked. (This kiln is owned by a
community art center that offers Anagama classes. We have quite a few
newbies ever firing who sometimes get a bit exhuberant in side stoking.)If
we were to open them up, would a damper change also be needed?
At some firing, I really want to try adding water to these vents (L-shaped
pipes) to see the "aqua-gama" effect on the glazes. At this point, it's an
untried experiment.
Steve, I am very interested in your books, and will contact you off-list.
Thanks!
Sue
Kalamazoo, MI
Sue Leabu on tue 30 sep 03
On Mon, 29 Sep 2003 19:22:40 -0700, Hank Murrow wrote:
>That said, many Japanese potters fire only during rainy weather, and
>others spray water into the kiln to improve the glazes.
Yes, I find this most intriguing, and want to try it at some point.
Funny thing, our new fire master was telling me throughout this firing
about her anagama in Montana that would race in a rainstorm. They loved
firing in rain! And always planned the fire-off at daybreak to take
advantage of the morning dew.
Sue
Kalamazoo, MI
Snail Scott on wed 1 oct 03
At 01:30 PM 9/29/03 -0400, you wrote:
>I'm trying to understand why the anagama I've been firing for the last
>three years always stalls out when the barometer is falling...
I've known two kilns that did this. They weren't mine to
mess with, but looking at them, I suspected an insufficient
chimney on one (a downdraft) and inadequate secondary air
intake on the other (an updraft). Atmospheric pressure can
make a huge difference in a kiln that is 'borderline' in
design, even if it's adequate when the atmospheric pressure
is high.
-Snail
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