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.............Kazegama / Wind Kiln by Steve Davis

Since 1995, I have been pursuing a wood fired aesthetic in my ceramic work. I live in an urban area of Southern California where it is difficult to fire with wood because of the smoke and fire dangers. In 1997, I set out to build an alternative kiln that would yield anagama results in a gas fired kiln without the use of wood. The challenge was finding a way to deliver wood ash to the ceramic surfaces in the kiln.

I had an old raku kiln comprised of loose soft bricks and a blower burner. With these I constructed a small test kiln similar in shape to an anagama. The kiln was fired with propane and achieved maturity after six hours. At that time, I fed a small amount of screened wood ash through the air intake of the blower burner and then heat soaked the kiln for twenty minutes. Upon opening the kiln the following morning, I found the ash had been well distributed throughout the kiln. The results showed great promise and inspired me to build several variations of the kiln. The kiln obtains wood fired results through the use of blower burners that disperse screened wood ash throughout the kiln and onto the surface of wares. There are no firebox or charcoal bed surfaces in the traditional sense, but subtle flashings to heavy ash flows, carbon trapping, and dry, crusty, melon surfaces are achieved. The kiln usually fires for 6 hours versus the several days required for a wood firing and can be fired virtually anywhere.

I named the kiln Kazegama, (which is Japanese for "wind kiln") after asking my step dad, Sueo Serisawa, for suggestions. Sueo was a Japanese born, American painter who led tours through the six ancient kiln sites in Japan during the 1960s and 70s. He was acquainted with leaders of the Mingei movement such as Dr. Soetsu Yanagi, Bernard Leach, Kanjiro Kawai, and Shoji Hamada and was very a close friend of Hiroshi Kawai. He had a strong influence on my aesthetics through his own personal work as well as the ceramics he and my mother brought back from their travels to Japan. Sueo came up with the name Kazegama. Kazegama had a good ring to it and the word gama lent homage to the anagama.

Steve Davis and Sueo Serisawa
Dr. Soetsu Yanagi, Bernard Leach, Shoji Hamada

Current Kiln Construction

Any kiln that employs wood or sodium sources to generate atmospheric influences on wares, will ultimately succumb to chemical attack from those sources. To shield the walls of my Kazegama from chemical attack of wood ash, I mount a layer of 1" thick Corelite kiln shelves against the hot face of the lid and kiln walls.

When designing a kiln, the dimensions are established by totalling the width and depth dimensions of the shelves that the wares are placed on. I then add a 2" gap between the shelf edges and the back wall and side walls, 1" for vertically placed Corelite shelves that form the hot face of the walls, 4" walls of fiber blanket or 4.5" walls of K-23 bricks, and an expanded metal and angle iron frame. There is an 18" spacing between the edge of the front shelves and the inside surface of the front wall where the blower burners are positioned. This 18" spacing allows for the expansion and flow of combustion gases and wood ash where they enter the kiln. All joints where the edges of fiber meet have overlapping joints to prevent heat leakage. Using fiber blanket allows for the trailer transport of my kazegama due to its light weight and flexing ability. I use 1/4" thick expanded metal for the lid and floor, and #9-3/4"expanded metal in the frame walls. The 1/4" thick expanded metal is much thicker than #9-3/4" expanded metal and provides additional strength for the floor and suspended refractories in the lid. The vertical corners and the lower edges of the metal frame are constructed with 2" angle iron and the wall's top edge and lid are made with 3" x 3" angle iron. All fiber blanket for the walls, floor, and lid are placed into the metal frame in a "wall paper" fashion and held in place with 2" handmade, porcelain buttons and 15 gauge nichrome wire. The fiber blanket walls and roof are covered with 1" Corelite kiln shelves. These are held in place with 9 gauge nichrome wire. The floor is covered with dense, used 1" kiln shelves. The #9-3/4" expanded metal for the side walls comes in 48" widths that are sheared in half lengthwise. This gives the kiln wall a 24 inch" height along with any additional height from the angle iron frame.

The kiln shown here has been in use since 2001. It has gone through many improvements, and a major rebuild in 2006. The kiln is bolted onto a trailer and used to have storage spaces for all of the things needed for a firing. In 2008, I shortened the trailer by four feet and eliminated the storage areas. I now store the blower burner assembly, ash screens, bricks, shelves, tools, wood ash, and wadding, etc. in the kiln. The lid is lowered and bolted into place for travel. Two - one hundred pound propane tanks and lid raising hardware are stored on the outside of the trailer in a secure manner. Trailers require a 60/40 weight distribution over the axel, with 60% of the traveling weight between the axel and the towing vehicle.

First firing of this kiln in 2001, Idyllwild, California
2006 rebuild
60% of trailer weight is between the axel and hitch
Bricks and supplies are stored in containers
The fiber board will last between 20-40 firings or more, depending on its exposure to heat and ash. Some boards have a "skin" on one side, which is the side that should face the interior of the kiln. This skin does a good job of repelling the ash. The porcelain buttons and nichrome wire are placed every six inches into the safety grate for the roof, and every 12 inches in the side walls. The fiber board is fired before installation to at least cone 5. This is done to pre-shrink the panels and remove the organic binders. The fiber board will easily warp if not propped and supported by fire bricks. They are best fired on edge, not in a horizontal position. I use a four inch wide length of one inch fiber to seal the space between the walls and the lid.
45 firings later from 2001-2006
Stripping kiln down for rebuild
I recycled as much of the old insulation as possible
2" blanket with 2" hot face of fiber board
Chimney flues, roof flues, and construction of the kiln lid

This kiln is a side draft kiln. Flame and ash move horizontally through the kiln just like a wood fired kiln. In order to distribute heat and ash evenly throughout the kiln, I place 2 flues in the back wall, not one flue. The flues are on the floor and exit into 2 chimneys that are lined with 2 inch fiber blanket. These openings are cut prior to installation with a rounded butcher knife. I simply rock the knife back and forth and press down through the fiber or board. One of the improvements to the kiln is a flue option that I now use. I placed 2 flues in the roof at the back of the kiln, about 15 inches from the back wall. The blower burners start firing in the lower of 2 positions, directly above the floor. The flame path moves under a layer of shelves, 9 inches above the floor shelves. With the chimney flues covered, the flue gases will exit along the back wall, over the wares in the upper, rear section of the kiln, and out the 2 roof flues. This provides more heat to the upper rear of the kiln which used to fire cold and is a much more efficient use of the propane. The lid on my kiln uses 3"x 3"x 1/4" angle. Safety grate is welded to this frame and 1-1/2 inch angle iron supports are welded into the frame or grate to support the lifting cable and the bricks that form the flues. I have built an angle iron frame that 4 - 9" bricks rest into, forming a flat, durable flue housing. There is a transition from the fiber in the lid, to this brick flue. Porcelain buttons that hold the roof in place, have their nichrome wires fed through a small hole in the inside lip of the angle iron frame, where the fiber is pulled up tight into the angle iron frame. A 1 inch gasket of fiber blanket is place between the fiber, the inside edge of the angle iron, and the bricks that seat into the frame. The size of each flue hole in the fiber lid is about 3 inches narrower than the inside edges of the angle iron frame. This is where the fiber seal is sized and fitted. The distance between the flue opening and the edge of the angle iron frame, along with the fiber gasket, makes for a good, sealed transition from fiber to brick.

Lid flues on left and chimney flues (not used) on right Bricks are placed in angle iron frame to form flue
Starter hole before final shaving of flue opening Fiber board is held up into fiber seal & bricks to form flue
Old lid after 45 firings New lid after 5 firings-note erosion

Installation of burner ports

To create holes for the burner ports, I place the burners into position and trace the burner tip positions with a felt pen. I then cut out the expanded metal mesh where the burner ports are marked with dikes. The best tool for cutting clean holes in fiber products is a cheap hole saw. However, if the hole saw diameter does not match that of the burner port tube, you can use the midsection of a soda can, and wrap it around the tube. Then twist and push the tube through the wall with the aluminum soda can for the cutting edge. I use extruded ceramic tubes in the burner ports because this reduces erosion to the burner ports caused by falling ash. These extruded tubes are placed at least 1 inch into the kiln and are flush with the outside of the kiln. Placing the tubes one inch into the kiln wall, keeps the ash off the inside wall where the ash can erode the surface. Small holes are drilled at the edge of the tubes to hold them in place with 15 gauge nichrome wire. Be sure to use leather gloves to protect your hands. Most importantly, always wear a face mask when working with fiber products.

Cutting the burner port with a soda can in 2001
4 Burners with trailer pilot and burner port tubes
There are 2 levels for the burners and shelves
Burner port tubes placed well into kiln to keep ash off wall

Notes on fiber blanket

One note about framing the kiln; steel exposed to heat needs to be at least 3/16 inch thick. The one inch fiber seal between the lid and walls will compress after one or two firings, possibly exposing the frame to high temperatures. Keep in mind that heat exposure also exists in the areas around the burner ports, flues, and corners where the fiber layers should be overlapped. Fiber products that are commonly sold at ceramic supply stores are only rated to 2300'F. A refractory that is fired above it's service temperature is going to shrink. Just going up a hundred degrees over the service temperature, causes a 24 inch fiber blanket wall to shrink to 23 inches. I force a 24 inch panel of fiber into a 23 inch space to compensate for this shrinkage when designing the dimensions of the kiln walls and the floor depth. Be sure to over-pack the fiber in areas of concern.

Lifting the kiln lid

Lifting the lid is achieved with a hand winch. The winch is mounted on the side of the kiln. There is a horizontal cross bar with square tubes on opposite ends that catch the tops of two vertical poles. These poles drop into square tubing on both sides of the kiln at the mid section of the kiln. A cable is run from the winch through 2 pulley wheels, up, across, and down to where it is attached to the lid support. A removable bolt is used to secure the cable end to the lid support. The lid has 2 oversized square tubes on the side of the frame that the vertical poles travel through as the lid is raised and lowered. The lid just floats on the cable as it is raised. Any pitching of the lid can be counter balanced with bricks, This is basically a guillotine set up and very dangerous. I drilled a hole in each vertical pole where the lid comes to a rest. I place screw drivers in these holes and then lower the lid onto the screw drivers. This takes pressure off the cable and protects kiln loaders from a nasty headache. The cross bar and 2 vertical poles are removed and stored in the trailer when transporting the kiln.

Hand winch welded to frame
Pulley system
Cable mounted to lid with a bolt through cable loop
Lid is counterbalanced with a kiln shelf
Screw driver supporting weight of lid for safety
4 eye hooks attach lid to hooks on the frame
Constructing the burner system

The burner system is the heart of the Kazegama. It is comprised of 4 burners welded to a gas manifold. Each burner has its own ball valve for control of the propane gas and a Dayton #4C440 blower motor for the air supply. This blower motor has an air flap which opens and closes to adjust the air/fuel ratio. The blower is mounted to a pipe flange, and that flange is mounted to a 18 inch pipe. 6 inches from the flange, a pipe fitting is welded on the 18 inch pipe. There is a ¼ inch hole in the pipe where the pipe fitting is welded. There is no insert or orifice. A trailer pilot is positioned just under the tip of all 4 burners. It is connected to the manifold with a small gas cock which is adjustable. This is a 36" long, 3/8" stainless steel tube that is perforated down its length, and has a small air notch and orifice at one end.

4 ball valves and 1 main gas valve shown at top
Early manifold

Safety system

This trailer pilot is what I use as a safety system for this kiln. It works well for me, but is not as good as a solenoid safety system . With blower burners, it is quite easy to blow the flame right off the tip of the burner. If you don't have an ignition source to combust the propane, you can have a serious explosion on your hands. You must always maintain an ignition source for any combustion system. These burners do not use flame retention nozzles because they would clog up rather quickly with wood ash. There is a tremendous amount of heat that comes off the burner wall. So I have welded a bracket onto the 4 burner tubes where heat shields are easy placed or removed. They are remnants of fiber board and do a great job of keeping the burner valves and myself cool.

Trailer pilot and old heat shield(kiln shelf)
Trailer pilot held in place with welded hex nuts
New fiber board heat shields in metal holders
Heat shields are easily removed
Loading the Kazegama

Clay bodies, glazes, flashing slips, and wadding used in wood firing work equally well in the Kazegama. I place the kiln shelves three inches from the back wall so heat and ash can move through all sections to the back of the kiln. Wares are loaded in the same manner as in a wood firing. Loading shorter pieces towards the front of the kiln allows more ash to reach the back of the kiln. Pieces loaded closest to the burners will get the biggest hit of ash and heat. Pieces in the back are likely to exhibit flashing if their exposure to ash and heat is limited. The ash build up can be quite heavy, so wadding placement and sizing is important. Wadding should be placed inside of foot rings and a half inch thick to keep ash flows from fusing pots to the shelves. The formula for a simple wadding mix is 50/50 EPK kaolin and alumina hydrate by dry weight. Water is added until a moist clay mixture is achieved. The only drawback is that this wadding mix can leave a ugly white mark that in contrast to the character of the pot does not fit well aesthetically. Owen Rye
addresses this issue in the May 2000 (Log 2) issue of the Log Book.

Preparing to fire the Kazegama

Safety first. I always keep the area around the kiln organized with only the necessities. The propane tanks are kept in an area where they are easy to reach and shut off in an emergency, and where they are not likely to be knocked over. The propane tanks are at least 10 feet away from the kiln. The electric power cord is wire tied to the propane line so they are one unit for 4 feet from the burner manifold out. This makes it less likely that I will get tangled up in them. The ground is swept clean in the burner area because a lot of ash will find itself in this area, and I will sweep it up for use during the firing. The tools I keep handy during the firing are a crescent wench, leather gloves, welder's goggles, rod to clear the burner tubes of ash, an air compressor, 15 pounds of screened wood ash, dust pan, hand broom, water, and a face mask.

I keep the kiln area clean and organized for safety
Gas and electrical lines are wire tied together

Firing the Kazegama

Because four blower burners produce such a tremendous amount of heat, only one of the middle burners is used during the first 20 minutes of the firing. With the blower motor air flap in the closed position, the gas valve is adjusted for the lowest turn down possible. At this stage of the firing, physical water is driven off before steam forms at 212'F, which can cause wadding to blow up. After 20 minutes the second middle burner is turned on. Within an hour, the main gas valve is turned way down and all 4 burner valves are turned to wide open. Then I turn up the main gas valve to the desired pressure and adjust the 4 air flaps so they are all the same. All 4 burners are now controlled with the main valve. Each burner has a 9" post (target brick) in front of it. These posts serve as shelf supports and break up the flame and ash that enter the kiln. Over the next hour, red heat will show on these target bricks. At this time I give the gas a good bump with the main gas valve and adjust the air flaps to achieve the desired air/fuel ratio. A half hour later, I turn the gas valve and blower air flaps to wide open, and let it fly. I set the regulator pressure at a level that allows me to set the air flaps on the blowers to wide open and the main gas valve to full bore. The setting on the regulator is around six pounds of pressure. This eliminates the guess work about what the proper fuel/air ratio is in the bright light of day when it is hard to read the atmosphere of a kiln. The best method to figure out the proper air/fuel ratio and settings is to fire at night when it is easy to read flames coming from the flues. Another way is to read the color on specks in the walls of the burner ports. These nodules will glow bright orange when the kiln is at the most efficient air/fuel ratio. You can also look for the build up of carbon on the underside of dampers when the atmosphere is reducing. Depending on the density of the load, cone nine can be achieved in three hours.

Water in wads is burned off in early stages with one burner
All propane systems require a regulator
Establishing gas, air, and flues settings is easier at night
Air flaps face down to keep ash away from motor bearings

Feeding screened wood ash

A cone nine witness cone placed on the top front shelf alerts me to start introducing screened wood ash. Mesquite, fruit tree and hardwood ashes work best and should be screened to remove large chunks which could damage the blowers. I place a bucket of fifteen pounds of screened wood ash between myself and the blower burners. Each blower has an air opening that is wide open and facing the floor. I start with the left burner and place a loose handful of wood ash up, underneath the air opening of the blower. The blower takes the ash as my hand moves in a circular motion. I feed ten handfuls of ash into the blower intake. The burner blows the ash into the kiln, which hits a target brick eighteen inches into the kiln. After ten handfuls of ash have been fed into the first burner, I move on to the other blowers. Ten handfuls of ash are fed into the four burners, three times. This brings thirty handfuls of ash to each burner port. Once the lower level of burner ports have received their ash, I pull the plugs in the upper ports, I turn off the gas, leave the air on, and bump the burner tips on the ground to empty the burner tubes of ash. I then move the burners up to a second level, place the port plugs into the lower ports, and start dispersing ash into the upper part of the kiln.

Screening wood ash through a window screen
Handfull of screened wood ash
30 handfulls of ash are fed into each burner port
Feeding the upper deck

Cleaning out the burners

During the feeding, ash will start to clog the burner tubes and will have to be removed. First, I turn off the gas. Then, with the air still on, I remove the burner assembly with the burner tips pointed downwards, and tap the tips on the ground. I return the burner assembly to its mounts, and turn the gas back on to its original setting. Since the kiln is well above red heat, combustion will occur. After all the ash has been delivered, I repeat this process to ensure the burners are cleared and firing properly. I use an air compressor to blow out the blowers and ports. At this point, the kiln is ready for the heat soaking of the ash. The kiln usually climbs in temperature during the feeding of the ash and can reach cone 10 or more. Therefore, the feeding of ash must start at no higher than cone nine. Heat soaking takes twenty minutes to an hour depending on the stacking and the movement of the flames. I observe the melt of the ash with welder's goggles. When the desired melt is achieved, or cone ten has been achieved in the back of the kiln, I turn off and remove the burner assembly, plug the burner ports, and close the dampers. I unload the kiln the following morning. The entire firing takes five to six hours and reaches temperatures of cone twelve or higher in the front of the kiln. Introducing ash at the end of the firing and not heat soaking this ash also creates crusty surfaces. Placing work on a bed of sintered ash can create dry, melon surfaces. Firing to lower temperatures can also produce drier, flashier surfaces. Many surfaces at lower temperatures will exhibit transitional surfaces and colors where the clay is starting to vitrify. Many wood fire kilns that produce orange flashings with heavy iron bodies are actually under-fired. When fired to vitrification, iron oxide produces brown colors.

Banging the burner tips to clear burners of ash
Placing upper plugs into lower ports
Placing burner manifold into upper deck
Clearing blowers with air gun
Scraping out ports to reduce build up of ash with rod
Blowing out the remaing ash before place plugs in ports
Process and Aesthetics

The firing process of a wood fired kiln or a kazegama is an integral part of the aesthetic and visual vocabulary of ceramic work by complimenting the form, gesture, and surface qualities through the natural coloring and layering of the ash. From very subtle blushes to heavy ash flows, wares from the Kazegama are remarkably similar to those fired in five day wood firings.

Workshop participants' pots
Various slip tests
Workshop participants' pots with unmelted ash
Workshop participants' pots
Good color development from mesquite ash and slips
Ash sprinkled on shoulders of paddled bottles
Tradition

The kilns we fire and the types of wares we produce spring from a long history dating back many centuries. Each of us stands on the shoulders of those who have come before us. Kiln designs have changed with the introduction of new technologies, refractories, fuels, safety demands, and regulations. With all of these changes, the overriding emphasis for me is aesthetics. The Kazegama is just a tool that brings its own visual vocabulary to the ongoing tradition of the wood-fired aesthetic.