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friend's wood kiln - colors

updated thu 15 apr 99


John Baymore on wed 14 apr 99


Hi, a friend (honest...really a friend) has just fired his modified
aborigami for the third time and is thinking about letting it oxidize at
the end to brighten the glazes and flashing. ............ He has been
firing up to temp in a reduction atmosphere, stuffing the fire box at the
end, closing up the fire box and then stoking the back chamber for a while
longer (hours).



Being a long time four chamber noborigama owner and fire-person (five if
you count the stackable section in the dogi), I'll share a thought or two
on firing these beasts.

There are numerous ways to approach the firing of a multi chamber climbing
kiln. Some of the approaches are personal firing decisions and can be
varied at will, and some are based on the design of the kiln itself, and
are not so easy to vary.

Some kilns designs require that the main firebox is continued to be
intermittently stoked throughout the entire firing even when the sucessive
chambers are being side stoked. This is similar to firing certain types of
anagamas when you get to side stoking to bring the rear of the kiln up to
temperature. It doesn't sound like this is the design your friend is

On some kilns the main firebox is stoked until a certain point (sometimes
through the completion of chamber number 1) and then stoking ceases there
and continues on the side stokes of chamber 1 or 2 and so on. This sounds
like your friend's kiln.

In this type of kiln, most of the primary air ports are closed up on the
main firebox at the changeover point. But not completely. Some air must
flow through to combust the HUGE amount of fuel remaining in the coal bed.
If this is not done and the chamber directly connected to the main firebox
(chamber 1) was completed by firing the main firebox or a small amount of
side stoking just after switching from the main firebox, chamber 1 will now
be cooling in a STRONG reducing atmosphere. This reduction cooling will
greatly affect the color of the glazes and the claybody.

Stuffing the firebox with fresh wood and closing it up completely pretty
much assures this condition in chamber 1, and if the dogi is big enough,
the firing in number 2 is short enough, and some coals are left in chamber
2 also, the entire kiln may be cooling in reduction.

If the main firebox (and or whole kiln) is not designed as well as it could
be, it is also possible that the coal bed in the main firebox becomes
excessively large. This is an indication of either insufficient air flow
OR of poor distribution and mixing of an adequate airflow, or a combination
of both. This excessive amount of coals accumulating can contribute to
reduction during the cooling phase too, if you can't get adequate burn-off
in a reasonable time with an appropriate amount of openings.
This reduction cooling is frequently used in anagama firing...... not so
often in noborigama firing of glazed wares. (Good for shino carbon trap.)
It is often coupled with stoking charcoal directly onto the pots. If this
effect is NOT what your friend wants, he needs to carefully regulate the
flow of air across/through the remaining coalbed in the main firebox so
that the coals are burned off in a reasonable time and so that the first
chamber cools in oxidation conditions.

This flow ALSO has to supply a significant portion of the primary air for
the combustion now taking place in chamber 2, depending on the design of
the firebox in chamber 2. It is interesting to note that at this
changeover point, the first chamber can often INCREASE in temperature....
so watch the overshoot relative to the final firing cone=21 There is a LOT
of potential energy in the remaining coal bed.

One of the benefits of a multichamber kiln is the ability to preheat the
primary air for each successive chamber by drawing it through the prior
chamber(s) and dogi. To do this well requires really careful management of
the air openings, draft, and stoking pattern. In a sense, you are moving
the SAME HEAT up the hill to each sucessive chamber. The cooling of one
chamber is used to heat the next. If you don't do this, you lose a good
portion of the potential efficiency of these designs. This pre-heating of
air and heat zone moving comes in addition to the fact that the second and
sucessive chambers are firing on waste heat coming off the firing of number
1. In a way, these kilns are truly the precursors to the continious
tunnel kilns common in 20th century industry.

(An anecdote here on this fire management stuff in a climbing kiln
........... In a workshop setting about 18 years ago I was (frankly) not
paying close enough attention to things for a short while. (Add this to
the dumb mistakes thread =3Cg=3E.) Participants were stoking chamber 2 on =
noborigama after changeover from stoking the main firebox to complete
chamber 1.

I asked one participant to check the cones in number 2. He did and said he
couldn't really see them. I had another participant check. She couldn't
see them either. Nothing. So I went and checked, expecting that the cones
were just difficult to see. They WERE difficult to see......... cause they
were ALL FLAT. The chamber had jumped from about cone 04 to 10 OBLITERATED
in about 45 minutes=21=21=21=21=21 Needless to say the glazes in that =
chamber were
not what we had hoped =3Cwg=3E. The preheating of primary air, and moving =
heat zone uphill is a powerful tool that can get away from you if you let

Lesson learned =3Cg=3E.)

It takes a suprisingly small number of square inches of holes left open
into the main firebox to accomplish this proper airflow .... because at
this point in the firing the differential pressure created across those
ports by the draft in the kiln is great (if the kiln is well designed)....
so a lot of air goes through a small orifice. The goal is to let enough
air in to burn off the coals in the main firebox at a rate that doesn't let
TOO MUCH cold air flow through chamber 1 and cause too rapid cooling and/or
dunting, yet provides the primary for chamber 2 also. Mouseholes that let
air in UNDER the coal bed are a great help here.

This burning off of the coals contributes not only potential reduction to
the chamber on cooling .... but is also heat input and can prolong the
cooling cycle in the chamber .....nice for glazes that depend on
microcrystaline surface effects. If the microcrystals grow in an
atmosphere that is also reducing and they contain iron compounds, those
crystals will be very DARK in coloration.

On my kiln, at the changeover point, I leave the main firebox primary air's
at the same level of openness they were for the stoking there for a
considerable period of time while the main firebox =22clears=22. This means
that all appreciable pieces of wood on the grates have burned off to coals
in the ashpit. While this is going on, light stoking of chamber 2 starts.
As the main firebox conflageration starts to die down, the primary airs
there are slowly closed down to match the decreasing level of combustion.
The intensity of things in chamber 2 grows at the same time the main
firebox is quieting down. They run in sync with each other.

These primary air ports (and mouseholes) on the main firebox are never
FULLY closed off and sealed until the end of the firing of the entire kiln.
A couple of square inches of holes are open (spread out across numerous P
air ports and mouseholes) until the very end. There is a huge amount of
heat energy stored in the refractories (and coal bed) of the dogi (main
firebox) if it is sized and designed correctly...... this heat is
transfered to the air flowing through it, and keeps the first chamber from
cooling too rapidly while the others are still firing.

If you shut down ANY chamber with a lot of coals still in the firebox and
cut off the air supply, you get reduction during the cooling phase. This
can SIGNIFICANTLY darken the colors produced by iron compounds toward the
grey/black range.... not the bright iron red that is typical of oxidation
cooling. Affects flashing marks too.

So rather than think of =22oxidizing=22 at the end of, or during, the UP =
have your friend think about what is happening to the ware during the
cooling cycle in the chamber. This has more effect than you think, and
sounds like the first thing I would try to lighten up the coloration he is
getting (based on your description of process).

Unlike a gas kiln, you don't just shut a noborigama wood kiln firing off
suddenly. No =22turn the valve, shut the damper, go to bed=22 abruptness.
There is a little time at the end where you SLOWLY shut down the kiln.....
gradually sealing off the air flow as you watch the wood and the coal bed
in the last chamber dwindle in size. You monitor the ending and make
adjustments as you rake up the debris of wood chips around the kiln, wet
down the surrounding area, pile up unused bundles of wood, and so on.

It is a gentle, soft ending to an intimate, personal process. As it should



John Baymore
River Bend Pottery
22 Riverbend Way
Wilton, NH 03086 USA