Ray Carlton on wed 14 jan 98
hi all just a quickie for those who have medium sized brick kilns [30-75cuft]
do you call it reduction if there is flame showing at the spyhole or do you
need to see it from the flue as well..i know there are many answers to this
question but i am interested in an average of responses..the question is
how do YOU judge your kiln to be reducing???
cheers :)
Ray Carlton
Mmpottery on thu 15 jan 98
The way I figure a kiln is reducing is when there is flame coming out of the
top and bottom spy holes. I think if the flame is coming out the stack it is
too much and a waste of fuel.
Michelle McCurdy
Parachute, Colorado
JockyC on thu 15 jan 98
Although I have heard it said that kilns are either reducing or not, I would
not be happy with just a flame at the bunghole of my wood kiln; I would be
happier with a flame at the exit flue - but not really happy till I see a good
two foot flame of dragon's breath at the top of the chimney.
John Christie
Leslie Norton on thu 15 jan 98
If you have incomplete combustion in the flue area, you are just wasting
fuel. The idea of reduction is to not completely burn all the fuel you put
into the kiln, this forces the oxygen to be pulled from the clay or the
glaze materials to be used for combustion. So you only need to reduce the
amount of air to the burner to the point that combustion is incomplete.
-Leslie
Gavin Stairs on fri 16 jan 98
Think about what the flames are telling you.
If there is flame coming out of the spy, then the flow is outward, and no
air is going in. If no flame, then there is probably air entering the kiln
through the port. That means at least local oxidation.
If the flame is colorless or pale straw, that means that there is no soot
being burned. Large, cool molecules and carbon soot are what make a flame
luminous, bright yellow or red. So the more the flame looks like a candle
or a stove fire, all red and fiery, the more it is reducing.
The flue gases coming out of a reducing kiln must have excess fuel gas or
CO. This means that, if they are still hot enough when they reach the top
of the flue, they should burn. If the chimney is tall, and they cool down
too much, they may not re-ignite, in which case you may see smoke instead.
You should be able to ignite the smoke with a torch.
So a kiln in heavy reduction should show luminous flames coming out of
every port, as well as a flame or smoke at the top of the chimney.
However, as has been previously discussed on this list, reduction effects
are delicate, and do not need a great deal of reduction, unless you are
intending to smoke the pots. Some of the variability prized by potters
comes from the delicate differences in the reduction/oxidation atmosphere
in the kiln: Perhaps you really do want a sucking peep hole at some point.
So the degree of reduction is a matter of judgement and experience. What
effect do you want? It seems to me what the masters are telling us is that
you must become expert at correlating your memory of how the kiln looked
(smelt, tasted, sounded) with the results you get. But at the extremes, a
fuly reducing kiln belches smoke and nice, juicy fire, while an oxidizing
kiln sucks air, and emits only thin smoke out the flue. A neutral fire,
that rarest of beasts, is just on the borderline between the two. There is
no such thing in the firebox of a wood kiln, or in any fuel fired kiln in
which the flames extend into the ware chambers. A burning flame is a
mixture of oxygen and unburnt fuel in the process of combining, and
incomplete mixing will produce a flame front curling around the interface
between oxygen rich flows and fuel rich flows. What we call a flame, in
other words. So in the flame, you will get sometimes oxidation and
sometimes reduction, governed only by the peculiar gods of fluid flow and
chaos.
And just in case this isn't obvious, the most efficient fire is neutral,
and a heavily reducing fire in which there is no reignition at the top of
the flue is emitting particulates and unburnt fuel into the air, which is
pollution. So, a reduction kiln should have a stack short enough to emit
hot gas which reignites on leaving the kiln. And any kiln should be
operated in neutral or slight oxidation during the heating phase of the
operation, except when necessary to reduce the ware.
Gavin
Gavin Stairs
Toronto, Canada
Vince Pitelka on fri 16 jan 98
>If you have incomplete combustion in the flue area, you are just wasting
>fuel. The idea of reduction is to not completely burn all the fuel you put
>into the kiln, this forces the oxygen to be pulled from the clay or the
>glaze materials to be used for combustion. So you only need to reduce the
>amount of air to the burner to the point that combustion is incomplete.
Leslie -
This could be a bit misleading for those trying to learn how to reduce a
fuel kiln. Having a reduction atmosphere throughout the kiln is difficult
to gauge unless you can see a flame at the flue. And for everyone's
benefit, this does not mean that top of the chimney!! It simply means the
point where the flue exits the kiln.
If you simply "reduce the amount of air to the burner to the point that
combustion is incomplete", it is very conceivable that combustion might be
completed long before the resulting atmosphere circulates through the kiln.
Reducing the air makes a longer flame, and sufficiently reducing the air
creates a reducing atmosphere throughout the kiln. As I mentioned in an
earlier post, when you look in the damper slot of a downdraft kiln, if you
have adequate reduction you should be able to see flames in the flue gases
on the pressure side of the damper (the side facing the kiln). And as
mentioned, I like to include a small spy-hole in the flue at this point
(just before the damper) specifically for this purpose.
- 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
Leslie Norton on sat 17 jan 98
Vince,
You are right, my comment could be a bit misleading. The idea of reducing
the amount of air to the burner to the point that combustion is incomplete
is correct, but as you say the incomplete combustion must exist throughout
the kiln. A peep hole at the base of the chimney (for a down or cross draft
kiln) is a very great idea. If you are working without an oxygen probe
(which I assume most of us are) you need to learn how to "read" your kiln.
One method of adjusting reduction is to cut back the air on the burner till
you see smoke, then add just enough air till you don't. This will create a
very heavy reduction but may be just right for some people.
There was one kiln I use to fire that all you had to do to reduce it was
close the damper a couple of inchs, and create a very slight back pressure
at the peep holes. This created beautiful reduction glazes every firing.
I worked in a dinnerware factory for a while and they would stack X number
of plates, bowls and cups on each shelf and each layer was stacked the same
way each time. They had three variations or arrangements of ware in the
kiln depending on whether they needed extra cups and mugs or if they were
doing server bowls and things. There was a very exact firing schedule with
body and glaze reduction. Some of the kiln people fired "by the numbers"
not taking into consideration the load or the atmospheric conditions. On a
rainy, overcast, low pressure kind of day they would get different results
then if it was a sunny, bright, high pressure kind of day.
So two other factors in reduction is how the kiln is loaded and what kind of
day it is.
The point is, each kiln is different, so study the flames and keep a log of
what you are seeing and doing. You need to become "intimate with the fire",
understand it so that you can change the "rules" when the situation changes.
-Leslie
"Technique and skills must be absorbed and wrapped up and put away to
become such an integral part of yourself that they will be revealed in your
work without your thought"
Shoji Hamada
> -----Original Message-----
> From: Vince Pitelka [SMTP:vpitelka@Dekalb.Net]
> Sent: Friday, January 16, 1998 6:03 AM
> To: Multiple recipients of list CLAYART
> Subject: Re: reduction ?? yes or no
>
> ----------------------------Original message----------------------------
> >If you have incomplete combustion in the flue area, you are just wasting
> >fuel. The idea of reduction is to not completely burn all the fuel you
> put
> >into the kiln, this forces the oxygen to be pulled from the clay or the
> >glaze materials to be used for combustion. So you only need to reduce
> the
> >amount of air to the burner to the point that combustion is incomplete.
>
> Leslie -
> This could be a bit misleading for those trying to learn how to reduce a
> fuel kiln. Having a reduction atmosphere throughout the kiln is difficult
> to gauge unless you can see a flame at the flue. And for everyone's
> benefit, this does not mean that top of the chimney!! It simply means the
> point where the flue exits the kiln.
>
> If you simply "reduce the amount of air to the burner to the point that
> combustion is incomplete", it is very conceivable that combustion might be
> completed long before the resulting atmosphere circulates through the
> kiln.
> Reducing the air makes a longer flame, and sufficiently reducing the air
> creates a reducing atmosphere throughout the kiln. As I mentioned in an
> earlier post, when you look in the damper slot of a downdraft kiln, if you
> have adequate reduction you should be able to see flames in the flue gases
> on the pressure side of the damper (the side facing the kiln). And as
> mentioned, I like to include a small spy-hole in the flue at this point
> (just before the damper) specifically for this purpose.
> - 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
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