This is the longest it has taken me to get a kiln loaded, we worked on it off and on for more than a week, but it is ready now and I'll fire tomorrow. Below is a picture of it before the fire, 2380*F will make a difference, melt the glaze and mature the clay.
I'm getting ready to do a glaze firing, loading the kiln and glazing the last few pots. It's exciting, the anticipation of the firing and seeing new pots, the good and the bad, it all makes for an interesting life.
While teaching at San Diego State University I had the opportunity to fire the soda kiln. I liked the results enough to use that kiln design to build one at my own studio. SDSU’S kiln was built as a class project by ceramics professor Richard Burkett and his students. Richard had the original plans and generously shared them with me. The kiln is a sprung arch, down draft with about 25 cubic feet of stacking space and about 50 feet total interior space. This was a good size for my studio, using two 14”x28” shelves per level, however the same general design may be scaled up or down to build a larger or smaller kiln.
I started construction by making a level cement slab for the kiln to sit on. Then a base of cinder block was put into place and the various levels of the floor were laid. Two important structural points to note are the layer of insulating soft brick in the sub-floor, and the trough located next to the walls on the burner port sides, which collects the soda not volatilized into the kiln’s atmosphere.
The kiln’s interior walls are constructed of hard brick. The exterior walls are mainly soft brick with some hard brick at the points of stress and ware as in the door-jamb. Hard brick was also used in the chimney area. I laid the interior and the exterior walls at the same time, that is, a row of hard brick and then the exterior row of soft brick. Of course the joints for the two walls are staggered to prevent heat loss.
Clay slurry mortar was used to keep everything level in all directions. Checking level, square and plump regularly is important especially for the door. I measured the door width and plumb with each new course of brick laid. The door jam requires a lot of extra effort to build since it means cutting more bricks. However, I felt it was worth the additional time because it makes such a good door seal when it is done. I allowed for six burner ports and eight soda ports. I USED TENAX MORTAR TO HOLD THE FIRST FEW LAYERS OF BRICK IN PLACE, TO JUST ABOVE THE BURNER PORTS. The chimney area is only one brick thick and rises to the top of the kiln and then is completed with a stainless steel stack 10’ tall.
The arch has a rise of 1 ½” per foot of span. This is not a very high arch so the brick must be laid accurately in order to hold its position. I made an arch form from plywood and 2x4”s with a masonite skin. Once the walls are completed the arch is held in place with 2x4 legs. Wood wedges are placed under each leg. When the arch is completed and the angle iron is in place the wedges are removed from the bottom of the legs and the wood arch from drops away from the brick arch. At this point the standing arch is a thing of beauty and seems to defy gravity. The filler brick are put in place to eliminate the gap between the back and front walls up to the curved arch. I used a grinder to shape the interior brick to a perfectly fit the curve of the arch. The exterior brick are just placed against the ends of the arch and no cutting is required. I used about 3 inches of safety fiber on the top of the arch. The arch is only 4 ½” thick so the insulation is needed to prevent excess heat loss through the roof. I had a 4’x 7’ piece of aluminum cut and curved it over the insulated arch and screwed it to a piece of box iron that I had welded to the structural angle iron. This forms a beautiful and effective covering.
As I mentioned the stack is made from stainless steel. Put together by a sheet metal factory, I had the stack rolled and seamed from two five foot pieces. This had several advantages: the total cost of materials and fabrication was less than using brick; it was easier to install; and it creates a better draft. I cut brick to fit around the bottom of the stack and used Tenax mortar to hold them in place. The length of the stack was supported by angle iron.
The burner system of this kiln is interesting because it defies conventional wisdom, but it works. I made the Btu requirement calculations and found that nearly 1,000,000 BTUs were needed to fire it. SDSU’s kiln was being fired on just four small natural gas venturi burners rated at about 75,000 BTUs each, far short of the 1,000,000 Btu’s needed. At first thought the university had significantly higher gas pressure which would increase the heat output, but I found that they had only 6” of gas pressure, about the same as at my studio. I then took one of the burners apart and found that the orifice had been removed creating a larger opening for the introduction of the gas. The result is a very powerful burner at a low-cost. I used six of these burners on my kiln to make sure that I had adequate power to fire to cone 10 in a reasonable time. Using these burners, I complete my firings in about nine hours and do not even turn the gas to full pressure. It is very important to have sufficient volume of gas to fire with these small burners. I use 1 ½” pipe as noted in the plans. The cost for firing with natural gas in reduction to cone 10 was about $80. THERE ARE NO SAFTY DEVICES ON THIS SYSTEM AND YOU SHOULD DETERMINE THE CORRECT ADDITIONS TO MAKE THE SET-UP SAFE FOR YOUR SITUATION.
Building a kiln is a lot of work and considerable expense (about $6,000) but the rewards are worth the effort.
Slips for the soda fire:
Fake Avery Slip
Nepheline Syenite 24
Calcined Kaolin 30
Newmen Red Clay 3
Apply to dampened bisque ware, use thin. Will fire to a nice warm orange, use more or less red clay to alter color
Orange Soda Slip
#6 Tile Clay 61
Calcined Kaolin 10
Nepheline Syenite 12
Apply to greenware, use thin
Other things to be covered
Leveling slurry 1 fine sand / 1 fine grog / 2 gold art clay / few drops sodium silicate. Apply thinly to damp brick. Use Tenax or similar strong mortar to just above burner ports.