by Marc Ward
Clay Times May 2003
As I’m writing this (March), it’s the time for spring-cleaning,.. at least here in the South where it’s in the high 60’s and the daffodils are blooming and I’m struggling to start dormant yard equipment. Fortunately our Northern readers won’t be jealous because they won’t read this until May. Anyway, the phone calls are already starting that concern one of the rites of spring. As folks get rid of stuff, other folks get stuff. In my business, that means people are getting old, dead electric kilns. This is just the time of year when these poor castoffs make their way out of basements and garages into the hands of those that want to build a raku kiln.
So, now what? You’ve had this old electric kiln come into your life and you need to convert it into a raku kiln. First of all, I suggest you gut the wiring and elements. It’s not that this material is in the way or will affect the firing, it’s just that this kiln may find its way into someone else’s life several spring’s down the road. Since you’ve been firing it with gas, you most likely will reduce in it and this will force some flame or very hot gases out through any available opening. Some of these openings lead back to where wiring is connected and to the insulation that surrounds it. This wiring area will now be very degraded and could cause a dangerous situation if this old soldier is called upon to perform an electric firing.
Your wiring is now removed and you’re wondering about all those grooves where the elements lived and worked. Quite a few people call me and say that they have filled in the grooves with castable refractories or kiln patch. To be honest, that seems like a waste of time to me. If you want to do this for aesthetic reasons, cool… but it won’t really change how the kiln fires in a gas raku firing. I guess people think it will make the gases exit smoother and make the kiln “tighter”. To me, this is like saying you aren’t going to hang your arm out of the window of your SUV because the extra air drag will decrease your gas mileage. This thing is, what it is.
It’s time to cut holes. You should have a burner port hole that is an inch wider than the head of your burner. Then you are going to keep that burner at least a half of an inch back from the outside of the burner port. Farther away is fine, closer is not. Now, cut a hole in the top of the kiln in the center. It should be at least the same size as the burner port hole. I actually would make it a bit larger… like 2” bigger than the burner head size (1” larger than the burner port). You can always damper it a little if you want to reduce. Do you put the burner port hole in the center of the bottom shooting up or on the side at floor level shooting across? If you are young and agile, put it underneath where it’s harder to reach. If, on the other hand you are a grandfather and overweight, like me, you certainly want it on the side where it is easier to reach. Bottom line; it doesn’t matter with raku kilns. You’re blasting away and the even distribution of heat will take care of itself. I wouldn’t even bother with a target brick of any kind.
The next issue is shelves. You want the flame to enter under your shelf. Whether your burner comes from the bottom and up or across, you need room under the shelf for the gases to expand and the flame to find life. Four and a half inches is minimum with plenty of open space. Don’t make a wall of bricks to support your shelf. Think more on the lines of posts holding up your shelf. Shelves also are one of the great failure points of this endeavor. You shouldn’t use the shelves that came with the electric kiln. Why? Because they were sized for your fingers to hold on to the shelf while you set them down into the kiln. They were not sized to allow gas flow around them. The shelves that come with the kiln are too large for a gas firing. You want a good 2” average from the edge of the shelf to the wall of the kiln. A small square shelf or a smaller circular shelf will do the trick. You’ve got two more concerns facing you.
An electric kiln takes more energy to heat than a fiber drum kiln. You may have to get a larger propane tank, a bigger burner or use two burners instead of one if you have natural gas. I’m not a proponent of the old “tank in a bucket of water” trip. This is the “Western Medicine” approach… you’re treating the symptoms, not the cause. Get a bigger tank or hook two together. The last issue is how you approach this type of kiln to remove your work. With drum or “top hat” kilns, a good portion of the heat stays in the kiln when you lift or remove it to get your ware. With the electric kiln/top loading variety, all the heat stays down in there with your work. It’s rising as you are leaning in. Be careful. You may need more protective clothing than you otherwise would need with a lift-off kind of kiln. Have fun…