A noborigama is a
highly sophisticated kiln that is the precursor of the modern continuous tunnel kilns used
in ceramic industry today. They are exceptionally efficient in
their use of heat energy for a periodic kiln . To understand
how the Japanese arrived at the design in about the 16th century,
you have to look at the types of kilns that preceded them.
The history of
pottery development really parallels the technological
development of kilns. As kilns became more sophisticated, they
allowed better quality control, more efficient use of both the
fuel and the potter's time, and higher temperatures. Better
control resulted in better economic realities for the potters.
Higher temperatures resulted in more durable wares and the
development of glazes. More time allowed the potters to explore
more decorating options.
In the Asian countries, early kilns were often made by digging a tunnel into a
sloping hillside of clay. At the bottom was a narrowed hole that
served the double purpose of acting as a door through which to
crawl and load the pots and also as the main stoke hole for the
wood fuel. Then this excavation widened out into a large chamber
to hold the pots. At the top, a vertical "chimney" was
dug upward to allow the heat, gases, and smoke to circulate through out the kiln and be vented into the atmosphere. The Japanese
name for this type of kiln is "anagama".
Once these primitive
kilns had been fired, a "skin" of fired hard clay
formed on the inside surface making the structures somewhat permanent. As you might imagine, building such a kiln in the
first place was a hazardous occupation, with collapse of the soft
ground always a possibility!
As the exact
proportions of these anagamas evolved, they eventually were able
to achieve high temperatures through excessively long firings and
consumption of huge amounts of wood. The early chambers fired
very unevenly, and many pots either had to be refired, or were
ruined. The firings were often a communal project involving whole
villages. A firing was a major undertaking, and was costly in
terms of human resources, firewood, and in badly fired pots.
In seeking to make
better pots and have better lives for themselves, potters
experimented with how to improve the firing of their kilns. For
most, a potter's life teetered on the edge of hand-to-mouth
survival, and any economic competitive edge was welcomed.
They found that if
they created temporary segments within the single large chamber
by piling up solid walls of saggars and allowing small flues between them, they could
even out the temperature in the chamber somewhat. They then made
small openings from the outside ground through which they could
drop firewood into each segmented area.
It was a small
technical step from this discovery to the construction of
permanent segments within the anagama-type structure with permanent stoke holes along the sides of the chamber. Clay
pillars were placed inside to create numerous distinct chambers
with flues leading out of one and into the next. At the same time
this change was taking place, the kilns tended to be constructed
half in the ground, and half above ground out of bricks made of
very pure clays. These kilns resembled a piece of bamboo cut in
half lengthwise with the nodes forming the end walls of the
chamber, and were called "split-bamboo" kilns.
Experimentation with
the exact configuration of these split-bamboo kilns showed the
potters that the shape of the chambers could make a difference in
how the temperature in the chamber was regulated and also in
reducing wood consumption. As new kilns were built, the chambers
slowly evolved into a more upright configuration with the curved
arches following the flame flow path rather than at right angles
to it.
Because the flames
were moving more slowly and circuitously through the large tall
chambers on their way to the chamber exit flues, they gave off
more of their heat energy in the chamber, and did so more evenly
throughout the chamber. This circulation pattern, called
crossdraft, was perfected to an art by the Asian kiln builders.
A modern noborigama
is a series of insulated crossdraft kiln chambers linked together
which are running up a sloping hillside. Each chamber has flues
leading into and out of it where the flames can pass through on
their way up the slope to the last chamber. The sloping kiln
itself creates a natural draft without the need
for a really tall chimney.
At the bottom of the
hillside, a large external firebox is attached to the
first chamber inlet flues. This firebox is used mainly to bring
the first chamber near to completion, and it is sized to also
allow the rest of the kiln to be heated by the waste heat coming
off of the first chamber firing. A large portion of the total
firing time for an anagama or noborigama is used in stoking this
large main firebox at the bottom end of the kiln.
At the top of the
kiln, the final chamber's exit flues can either just end at the
floor height of the chamber, or they can be ducted together into
a short vertical chimney. The former was much more common in
historical Japan, while short chimneys are certainly the rule in
the few of these kilns that are located here in the United
States.
A fire is started in
the main firebox and the hot gases and flames pass out of the
firebox into the first chamber. They pass through the stacking of
pottery on their way to the first chamber exit flues, and give
off heat energy to the pots and kiln walls on the way.
The gases exit the
first chamber colder than they entered it. They then enter
chamber two, where the ware itself will be far colder than the
ware in chamber one. So the gases exiting chamber number one can
still effectively give off remaining heat energy to that relatively cooler ware on their way through the
second chamber. This same process continues on through each successive chamber up the hillside.
In a sense, all of
the chambers after chamber one are firing on waste heat that
would have gone up the chimney if chamber one were a single
chamber kiln. This makes for extreme efficiency in the use of the
wood's heat energy! Because each successive chamber is cooler than
the one downstream of it, the gases can still give off more of
the remaining heat energy all the way up the hill.
When chamber one is
completed, stoking ceases on the main firebox, and wood is now
thrown into a smaller firebox located right near the inlet flues
of chamber number two. The air for combustion of the wood in
chamber two is now coming through the main firebox and chamber
one...... so the heat already stored there is being moved up the
hillside into chamber two as the combustion air is pre-heated.
This process is
repeated up the kiln until the last chamber is completed. At that
point, the kiln is sealed from any draft flow,
and the potters go home for a much needed rest while the kiln
cools slowly.
My kiln, named "Kawagama" (River
Kiln), was completed in 1981, taking a number of years of
part-time construction and available capital investment funds.
Apprentices Warren Smith and Martina Landry, both Massart BFA
graduates, helped to construct it. It has four stackable chambers
with an additional small stackable area located in the main
firebox, so technically this is a five chambered kiln.
The kiln sits about
50 feet from some babbling rapids on the Souhegan River. This
location, combined with the "river of fire" contained
within, is the genesis of the kiln's formal name.
It is constructed
out of a combination of hard refractory firebrick of
many grades, insulating firebrick of many
grades, and some backup of ceramic block insulation. The
round stoke hole linings for each chamber side stoke hole are
hand-thrown. The arches are all sprung catenary ones, sitting perched
up on vertical walls. The organic looking covering over the
backup insulation in place over the arches is a mixture of
fireclay and vermiculite rammed into place by hand.
This kiln was
designed to be a reasonable size for a one person studio
operation. Although large by most U.S. potters standards,
compared to the large climbing kilns in Japan, it is quite small.
Enough pots can be made to fill it in a reasonable amount of time
for the necessary turnaround of work. It takes about 1500 - 1700
lbs. of wet clay to make enough pots to fill it...... about
500-600 various sized pieces. Glazing for a firing takes about
three to four long days, and loading and bricking up the doors takes
two full days. John can, and has, fired it all by himself.
The firing takes
about a total of 36-38 hours, including a slow, 8-10 hour
overnight pre-heat on a small propane gas burner that brings the
main firebox and the first chamber up to about 350F. It then
takes about 26-28 hours of continuous stoking to bring all four
chambers to the completion of almost 2400 degrees F. Cooling is
normally about 48 hours, so the whole firing cycle, cold to cold,
takes about four days.
Very efficient,
Kawagama consumes only about a cord and-a-half to two cords of
pine and hemlock to fire it off. This seems like a lot to a
non-woodfirer, but if it were a single chamber kiln of the same
total interior size it would consume far, far more wood.
Wood for Kawagama is
cut to about three feet long and must be at least a year old and
very dry. A lot of attention is given to the wood supply, and
plastic tarps are repeatedly taken off and placed over the wood
as the weather patterns change to allow good drying.
The wood is finely
split with the largest diameter pieces ever used being similar in
diameter to a construction four-by-four. As stoking progresses,
the diameter of the wood selected goes down. At the end of stoking each chamber, the pieces are
less than one inch in diameter, and almost vaporize as they hit
the 2500-2600 degree F temperature in the fireboxes.
Softwood is the
choice wood for many types of kiln firings rather than hardwood.
This is because it releases its heat energy quickly. Hardwood,
which burns slower, is sometimes used in the early stages of the
firing when you are trying to hold the kiln back in temperature
climb.
Usually the last few
degrees when approaching 2400F in the general chambers are the
hardest to attain as heat loses become significant. Finely split
softwood at this point in the firing helps squeeze the last
little bit of performance out of the kiln. At this point, John
hand selects the smallest diameter and driest pieces of wood for
the final stoking of each chamber.
All of the wood used
for firing Kawagama is scrap wood; John does not cut trees down
specifically to fire the kiln. He gets off-cuts, mill slab,
edgings, and the like from a number of local wood product
companies. These mills would dispose of this wood by burning it
themselves or sending it to a landfill to be disposed of by
burning in open piles.
In this way, the waste wood being burned is used to produce
something useful. This is the way recycling is intended to work.
AVI Video File: 1:07 min. 2.6 Meg
© 2000 + 2001 +
2002 J. Baymore
all rights reserved
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A Journey to Mashiko, Japan |
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On Woodfiring |
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10/31/2005 02:26 PM
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