Current Kiln Construction

When designing a kiln, I always start with the dimensions of the shelves that will be used and the refractories and metal that I have at my disposal. I use 24 inch wide fiber blanket and 2300'F fiber board for the hot face, forming overlapping walls that are 4 inches thick. This combination allows for the trailer transport of my kazegama to workshops due to its light weight and flexing ability. I used safety grate for the lid and floor, and #9-3/4"expanded metal in the walls. The safety grate is expanded metal on steroids and provides additional strength for the horizontal supports. It is all held together with 15 gauge nichrome wire and 2 inch ceramic buttons made of a cone 10 porcelain. The expanded metal comes in 48 inch widths that are sheared in half lengthwise. This gives the wall a 24 inch height along with any additional height from the size of the angle iron frame. 2" x 2" x 3/16" angle iron is the minimum sizing I use in kiln construction. My first Kazegama started out as a small test kiln of soft bricks and kiln shelves and then castables in the second kiln. Neither of these kilns withstood the chemical attack of the ash and were very inefficient. But don't let that stop you from using these refractories in a stationary kiln. Old kiln shelves can be used as a hot face for the kiln walls. This protects the wall behind it. I would stay away from hard bricks. I know of a couple of guys who built their Kazegama out of hard bricks, and it cost them an arm and a leg to fire their kiln. 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 that is equipped with storage space for all of the things needed for a firing. This includes 2 - one hundred pound propane tanks, lid raising hardware, bricks, shelves, tools, wood ash, wadding, etc. Trailers require a 60/40 weight distribution over the axel, with 60% of the traveling weight between the axel and the vehicle. With this in mind, the propane tanks, shelves, and bricks are carried in the front half of the trailer.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.