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passive dampers; was problems with new wood kiln

updated wed 27 feb 02


karen terpstra on mon 25 feb 02

Hi Brandon,
Passive and mechanical dampers are intrigal parts of the kiln. While I
believe that how you use them can be very kiln specific, I think there
are some basics. Some people fire just fine without passives but if you
have them and you fire for more than 18 hours, it makes the kiln more
versatile. I'll try to explain how we use them in our kiln. I hope
that someone else on the list with more experience and technical
knowledge than me can explain "passives" further!

In our kiln, we use them to help us control the heat and flame. If you
have access to Ceramics Monthly Nov. '01, you can see our kiln and the
passives on the side. There are two more on the opposite side and three
in the back---all at the same level. These are the ones we use the
most. They are each two brick openings and placed in the chimney about
even with the top of the chambers and at the top of the widest part of
the chimney. In the picture, you can see how our chimney tapers.

We also have three more passives in the back, about 2 courses up from
the floor. We don't use these much unless we are having trouble holding
the kiln back or need to adjust for a cold area. We set cones in the
back chamber and placed so that we can use these passive openings as spy
holes as well. We can keep an eye on the flame running through the kiln
and see if the kiln is firing evenly on each side etc.

Just below the top set of passives are the mechanical dampers. There
are 3 of them made from silicon carbide, encased in a steel frame and
covered with kawool. this save the life of the shelves. However, we
seldom use them during the firing except when we shut the kiln down
after firing, then we close them. In fact they are ususally not in the
kiln when we fire. We keep the openings plugged with bricks.

In the beginning of the firing (during candling) the bricks of the
passives are left in so that as much heat is pulled through the kiln and
this heats up both chambers and the chimney. Getting the lower part of
the chimney heated up is important to promote a strong draft or
"draught" through the kiln. Some people open the passives (take bricks
out) to keep heat in right away. This doesn't seem to work for us. We
want the chimney heated up too.

When the back chamber and lower part of the chimney are warm and lower
cones are starting to drop in the front chamber, we pull out a few
bricks from the passive openings to let a mix of cold air into the
chimney. This slows down the draught and holds more heat in the back of
the kiln. If we stall, we can put the bricks back in so that more heat,
longer flame start going through the entire kiln again.

Once we have cones dropping in both chambers, we start looking for cold
spots and try to even out the kiln. We need to be aware of the hottest
part of the flame which is not at the tip or at the end, but seems to be
in the middle of the flame (depending when and what temp we are at).
So, we use the passives to adjust where we need the hottest part of the
flame to be.

With some firings, a person will stand near the passives. When a big
stoke is fed into the firebox, the person has the bricks "in" so that
the flame really draughts through. Then that person will pull some
bricks out to keep the flame and heat from that stoke in the kiln. When
the flame starts to recede back, the person puts the bricks back in to
lengthen out the flame again and repeats with the next stoke...

When we soak, we are maintaining a temperature from cone 10-12. During
the soak the hot air rises and evens out the top part, corners, cold
spots, etc. While we are doing this we have that top row of passives
open (bricks out) so in theory, the heat stays in the kiln. If we have
low barometric pressure (as we often do) this screws everything up and
we have to start playing around more often with the passives just to
keep more flame and heat traveling through the kiln. There are always
variables it seems.

I feel like I'm babbling and not explaining this well. Hopefully
someone else can. Maybe start a discussion on dampers?

happy firings,
Karen Terpstra
La Crosse, WI

> Date: Fri, 22 Feb 2002 22:15:34 -0600
> From: Brandon Phillips
> Subject: Re: Problems with the new wood kiln
> I just unloaded the the underfired ware today, and to my shock found that we
> got a good cone 11 behind the bagwall in the middle. Actually about 20% of
> the pieces were definite diamonds in the rough. We are going to make some
> adjustments to the structure. We found some gaps to fill and we need to
> tighten the firebox up. The stack is 16 feet high, but the university here
> won't let us increase its height. It's only 25-30 cu. ft stacking space and
> I think that 16 ft should be plenty of height, even though more would
> probably be better. Chimeny is 18 X 12" with a kiln shelf damper, no
> passive damper. Where would the placement of a passive damper be and how
> large would it be? I read Jack Troy's book but it doesn't give any info on
> passive dampers. We are going to fire again on monday so wish us luck!
> Thanks to everyone that responded, its much appreciated!
> Brandon Phillips

Tony Ferguson on mon 25 feb 02

Check this guys work out! Beautiful!

Tony Ferguson
Stoneware, Porcelain, Raku
Photographic, Web Site, & Marketing Services for Artists
315 N. Lake Ave
Apt 401
Duluth, MN 55806

Donna Sparks at Almost Art on tue 26 feb 02

yes, please, more information on passive dampers, and ideally some
reference to written material/book etc. we have natural gas kilns, 20 and
30 cu ft, with only passive dampers (one each). we are making plans for
rebuilding at least one of them and wondering about the pros and cons of
passive versus mechanical damper.