Kiln Design / Hootagama
www.garyhootman.com
(download the msword file)
Brief history in kiln design:
Kiln
design has been a slow and evolving process.
Its advances are tied directly to the needs of
the makers of ceramics and the clay they were
using. I think we sometimes forget that ceramics
was an absolute necessity to development from
hunters and gatherers to the social farming community.
As they grew in size and rootedness, so did the
needs for production pottery to improve kiln
design and their efficiencies. This grew
into community within a community. There was
a division of crafts, i.e. clay gathers, pottery
makers, decorators, and kiln fireman and builders.
Nomadic
tribes in Africa and the early Americas fired with kilns made up of broken
shards on top of pots covered with wood. More permanent communities developed
bricks making walls combined with shards on top. Still others began to separate
the fire box from the wares. We see this in the developing areas of Europe
and the Mediterranean who used up-draft (bottle kilns) and later down-draft
kilns. Pacific Rim areas of the world developed bank kilns, the cross draft
style, where the slope of a hill was an important factor in the firing. Later
they developed bricks, where half of the kiln was built in the ground and covered
with a dome. This idea was later evolved into the chamber kiln when the need
for a separation of the fire box and ware was developed. In general, the universal
concept was that wood fired kilns were used as the finishers of clay products
throughout the world.
It wasn’t
until the 19th century when great changes were made in the way pottery was
being produced due to the industrial age. In America, change was rapid compared
to most of the world. Firing with wood all but disappeared with other cost
effective ways of firing including coal, oil, natural gas, even later electricity.
Better materials to build kilns were also being developed. Not for potters,
but for industries, foundries, glass factories, etc. Some wood
firing is still being done by the southern folk potters in their ground hog
kilns. The artisan pottery shop was all but gone here in the states. Small
art schools picked up the teaching of ceramics. Colleges also taught ceramics
in their home economic classes or through engineering courses as it related
to industry. (Remember clay was a craft and not an art form).
Worlds War
II changed the ways things were in our education system. There was the
development of the GI bill for returning vets to go to college and the invention
of the MFA program. Where ceramics were moved from the home economic class
to a real art form, but not all colleges made this move early on. What
we have now is referred to as the first generation of ceramic artists/ teachers. America’s
answer to the apprenticeship had many other places around the globe that we
had lost. Armed with a degree and a very basic knowledge about making
and firing kilns, there wasn’t much out there in the way of materials
to teach with. Bernard Leech and Shoji Hamada made tours of the states doing
workshops about making pottery. They were a big influence in this first
generation of potters. This was passed on to the second generation. This is
when wood fire kilns and movement took roots in this country. It was these
young potters fresh out of college that wanted to know more. Some went to the
source, two schools of thought; one followed that of Leech (English) where
people went to study and other of Hamada (Japanese) way of pottery. These
people traveled abroad to see how the traditional potters worked before the
industrial revolution. Books flowed from all of this movement to know more
about kilns and to educate. One was Kilns by Daniel Rhodes in
1968 followed shortly by The Kiln Book by Fred Olsen in 1973. But as
good as these books were, they really did not tell us how to fire these wood
fire kilns.
Even with schools being armed with these new
books, schools were not jumping into the mix
with wood firing. A lot of it had to do with
amount of effort and work involved with this
art form. It was Olsen’s book and the Fast
Freddie down draft kiln, I think, made the most
immediate impact to wood firing and teaching
of it in schools. Demonstrating with the building
materials we have today, wood firing didn’t
have to be this big time consuming event to produce
a finished pot. By the early to mid 70’s,
many of these second generation potters were
coming back to the states. After apprenticing
in Japan and England, they brought with them
varying degrees of knowledge setting up one person
potteries or getting teaching positions. It was
from this group of young potters to name a few,
Paul Chaleff, Peter Callas, John Neely and Randy
Johnston, The coming onboard of recognized, established
artists, such as Peter Voulkos, Ken Ferguson,
and David Shaner established a strong force in
wood firing in the making of American ceramics.
The first wood fire workshop in this country
was held in 1982. It was determined that there
were no more than 100 kilns, of varying designs,
in the states. Still there were only a
few schools that had wood kilns. Most of
them were built without instructions how to fire.
John Neely’s, Train Kiln design helped
potters and schools teach wood firing both as
a tool and an aesthetic choice of what you can
get from wood firing. This kiln can give you
the look of 5 or 6 day anagama firing, in 40
to 50 hours, or a soft glaze firing in 10 to
12 hours depending on building materials.
Hootagama;
The
most important thing to know about kiln design,
in conjunction with wood firing, is that you
can’t have fired enough times in different
styles of kilns. There really is no substitute
for experience. You must know what you
personally want for your work and from the kiln
before beginning to build. I know from building
kilns for people that they tend to build the
wrong kiln for their needs. Their reasons
being: so and so has a kiln and I like the work
he/she gets out of it, or that’s the kiln
people will come and help me fire; I need a big
kiln; and they all want an Anagama style kiln
with stoke holes. Looking at the work most people
are making, they weren’t looking at their
personal needs.
The first
two kilns I built at my studio were a large Anagama (280 cubic ft stacking
space) and a small tube (8 ft long with back wall, dumping into a centenary
kiln). Looking at the Hootagama drawing I have provided shows what I truly
need for a kiln.
All that I have been writing here in this paper
led me to this design.
I made a
list of things I felt were holding my ideas back from being made due to the
kilns I had and fired.
- Big enough to handle large scale work.
- Weight would not be a constant getting work
in & out.
- Ease of loading, not having to crawl in a
kiln or squeeze trough a tiny door way.
- Not running around the kiln to back stoke.
- One person can fire alone during 6 hour shift.
- All wood easily assessable, places for it
to be stored.
- Directional look to most work easy to read
a piece from front to back.
- Easy to fire (doesn’t choke up, fires
evenly, etc.) no real experience needed
to fire the kiln.
- Hold enough work to fire 7 to 10 days get
some juice looks to the large work
The hardest thing for me to get away from was
what the kiln should look like, even saying that
it had to look a certain way. I narrowed
my focus in wood firing to wanting only work
that comes out of the first seven feet of an
Anagama kiln. Armed with the list I have
put together, I built the Hootagama Kiln. I
had no idea what the kiln was going to look like. I
drew tons of sketches that I translated for you
in these drawings. I began with wanting
a kiln that had six feet of stacking space. The
amount and scale of my work would by necessity
require a wide and tall kiln. Then the
question was how am I going to heat a wider space
evenly. Remembering the large chamber kilns
I have seen in Japan and Korea and the large
beehive kilns here, they all had more than one
firebox or had several places in which to pitch
the wood to create even heat distribution. I
would have two distinct fireboxes.
I wanted the kiln arch to be self supporting,
so I knew I would use a catenary style of arch. The
height was determined by what I thought would
be the largest piece of work I would make (seven
feet). The width was determined by having
an idea of the floor to ceiling height.
I started out with a width of nine feet to have
a barrel vault. It would have to be 54
inches tall in the center or the arch. I
wanted to be safe so I added two more inches,
pushing the arch into the realm of a parabola
(the shortest possible centenary) to get the
height I wanted. I would build straight
walls to spring the arch on.
The bricks I used to build the Hootagama are
9 x 4.5 x 3, 70% alumina. From this information
I could determine the height of the straight
walls that would support the catenary. The
height was determined by the height of the bottom
of my firebox doors, 27”. The
27 inches added to the height of my catenary
which is 56” gave me floor to crown height
I was looking for.
I knew I wanted the floor to be at ground level
because I wanted to be able to get heavy pieces
into the kiln by myself. Since 1991,
I have built all of my kilns directly on the
ground. I needed a tunnel system for my
two fireboxes below the floor height. These
would have to be built before I could begin laying
the straight walls or the floor. This
is where I begin to build the kiln. But
I still needed to know how I would place the
fireboxes. I would determine this by the
width of the door entering the kiln. From
the work I have made in the past and the ease
of splitting bricks, I came up with 32 inches
for the width of the door. The door would
be in the center of the nine foot width. I
also took into consideration work placed in the
sides of the kiln between the firebox and the
wall which I thought I would need 12 inches.
This gave me the placement of where the tunnels
would be placed for the kiln. The width
of the grate system was made from past experience
and the bricks that I had available were 18 x
6 x 3. I wanted a full brick support on
each end of the 18 so the internal opening of
the tunnel would be 9 inches wide and I made
the height of the lower firebox 12 inches. The
depth of the grate system is a total of six feet. Twenty-eight
inches from the inside front wall plus 12 inches
to the first step gives me a firebox depth of
40 inches. There would be seven grate openings
of one inch.
I then laid the floor brick to the nine foot
width of the kiln incorporating the grate system
to the first step. Between the brick and
the ground, I put a layer of silica sand as a
separator and leveler. I was then able
to begin building my straight walls using good
brick laying techniques of stretcher course three
to four in height followed by a header course
and so on. The depth of the kiln was determined
again by past experience.
I wanted six feet of stacking space from the backside of the firebox
to the back wall. The back wall is to help force the flame to the center of
the kiln and in doing so helps to even the temperature in the back. Behind
the back wall is a dead space of approximately 18 inches to the back of the
kiln. This dead space allows for two things to happen: 1) help burn gases before
exiting the kiln; and 2) to pull the flame down in a down draft direction to
exit the kiln which also helps in evening the temperature of the kiln from
top to bottom.
The first step of my stacking space is three
courses high and 12 inches deep followed by the
second step of two courses high and 24 inches
deep and the third course is two courses high
and 48 inches deep. All of the floor brick
were laid separately from the walls and fairly
loose to allow for expansion and contraction.
My side walls from front to back are nine inches
wide. In the front of the kiln, I stopped
laying courses at nine or 27 inches. The
front walls were interlocked with the side walls
up to this height of 27 inches in the front and
6 inches in the back. This was carried
all the way around the kiln creating a level
surface to center the catenary arch form on.
The leading edge of the arch form, I set back
nine inches from the front edge of front the
wall. I then laid the arch brick using
stretcher courses, 4.5 inches thick. When
I go the 12th course, I measured in 48 inches
from the left side of the kiln where I would
leave an opening 6 x 4.5 that I would use to
cast a hole for the thermal coupler. Because
of the odd dimension, I used castable as the
key. I could not interlock the front and
back walls with the catenary so they were built
separately from it.
On the back wall of the kiln, I built in the
exit flue so that I had a 54 inch opening for
the inside of the chimney. The back wall
and the chimney were laid interlocking with nine
inch walls. The back wall of the kiln
is laid with three openings starting at the floor,
three courses high, each opening was designed
to be 9 x 9 with two 9 x 9 walls separating these
openings centered in the nine foot width. The
rest of the chimney was laid until I reached
a height of four courses from the inside edge
of the crown of the catenary. In the center
of the back of the kiln I would leave out two
header bricks and then continue to lay the rest
of the chimney and the back of the kiln. This
6 x 4.5 opening, I am using as a method of preheating
the chimney early on in the firing. The
total height of the chimney from the ground to
the top is nine feet. The chimney’s
internal dimensions from the floor to the top
of the catenary plus two courses is 13.5 x 54. From
this point up three feet, I corbelled in for
the next five courses to an opening of 9 x 54. I
use some broken shelves for a damper system.
The purpose of the back wall is to push the flames
to the middle of the kiln and down. This
allows the kiln to function as a cross draft
kiln. Once the flame gets to the dead space
behind the back wall, the kiln then becomes a
down draft kiln.
The front wall was built 27 inches high because
I wanted a comfortable height to pitch wood into
the firebox. I built the doors with an
opening of 9 x 9 centered over the grates. I
used eight 18 x 6 x 3 bricks as aprons and lentils
for the door openings centered within the nine
inch wall. I could use these overhangs
on the outside as a place to attach the swinging
doors. I continued to lay courses until
I came within three courses from the top of the
catenary.
My plan was to have a free standing heat-sealed
door. This would be laid 36 inches on the
inside of the front wall and the door is 13.5
thick and 31.5 inches between the walls. These
brick are laid on a stretchers and headers throughout
construction of the door which interlock with
the last three courses into the catenary creating
a heat seal at the top.
On the outside of the kiln, I built two walls
of cement block 44 inches tall and 14 feet long
and 48 inches away from each side the kiln and
one and one half blocks from the front of the
kiln forward to create a corner for stability. This
48 inch dead space was filled with clay compacted
as I went. I covered the kiln arch with
a layer of clay no less than 12 inches. I
also buried the chimney in clay all except the
last three feet so that the catenary and the
chimney were filled with the same depth of clay.
I also built
a roof out of recycled telephone poles and used tin as the roofing material
large enough to cover the kiln and standing area in front of the kiln and for
wood storage that would accommodate approximately six cords of wood.
Other
things you might find of interest:
L & R Specialties, Nixa, Missouri, (phone
number 877-454-3914, ask for Jerry) has three
of my wood fired clay bodies:
- Wood fire light – nice in the front
of the kiln
- Wood fire new dark – nice anywhere
in kiln
- Wood fire black – nice anywhere in
kiln
Available only in 1,000 lb batches
Hootman’s Nuka Glaze:
Fired to cone 10 to 11. At cone 12 runs
some and at cone 9 looks a little under fired.
6.5 lb wood ash (mixture oak, elm and hickory,
screened through 60 mesh, not washed)
6.5 lb Custer Feldspar
9 lb burnt rice hull ash (screen through 60 mesh,
not washed)
Hootman’s Shino Glaze:
Fired to cone 9 through 12 ok but not too thick
at higher temps.
5 lb Spodumene
5 lb Unimin Ball Clay (401 Yellow
Banks)
1.5 lb EPK Kaolin
5 lb Nepheline Syenite
5 lb Custer Feldspar
1 lb borax
.25 lb Rutile , this could lead to some
interesting results it has for me |
