| Answers from the CM Technical Staff
| Q: I am firing to cone 6 in a 7-cubic-foot Olympic gas kiln. The kiln has five burners through the floor of the kiln and a port in the lid which is dampered with a piece of kiln shelf. I start reducing at cone 014 but I never get complete reduction throughout the kiln. Generally, I get partial to whole reduction on individual pieces but not on the whole load. When I am not striving for reduction, I still get reduction in unexpected (and sometimes unwanted) places. I record the usual areas of reduction achieved after each firing. This changes somewhat with each load and drastically changes after cleaning out the burners. Celadons and copper greens are not options, due to partial reduction creating really unpleasant results. |
A: Firing any gas kiln can be a challenge, but you can produce great firings with your gas updraft kiln. The fact that you are having problems indicates there is either something wrong with the burners, the arrangement of the kiln shelves, the gas pressure, or the adjustment of the damper during firing.
For authoritative information on your kiln, I called Rob Haugen, president of Olympic Kilns. He said your five-burner kiln is at least twenty years old. More recent versions of that kiln have been made with a three-burner design, but the things you need to check are the same. Here are Haugen’s suggestions for your kiln:
First, check the inside diameter of the orifice in each burner on your kiln. It is possible that one or more orifices may have been changed in the past if the fuel was changed, and that the right orifice may not be currently installed in one or more burners. The correct orifice size is 5/64 of an inch inside diameter for liquid propane and 3/32 of an inch for natural gas. The easiest way to check the size is with the correct size drill bit. If the bit fits tightly in the hole, that is the size.
The kiln shelves for a gas kiln should be smaller in diameter than the shelves for an electric kiln. The correct shelf size for a 23-inch gas kiln is 19 inches in diameter. The bottom shelf should be a minimum of 3 inches above the floor of your kiln. (Haugen likes stacking an updraft kiln using half shelves, so that they may be staggered and adjusted, depending on what experience shows produces the best results.)
Next, check the gas pressure to your kiln burners. This is critical. If the pressure is too high, the gas will ignite too far up into the kiln. The correct gas pressure for an Olympic gas kiln is 11 inches of water column pressure for liquid propane and 7 inches of water column pressure for natural gas.
Finally, as you obviously know, the adjustment of the damper determines the amount of reduction within the kiln. The damper position controls the amount of secondary air that enters the kiln to burn with the fuel. To reduce the kiln atmosphere, you want to damper the top hole and watch for flame to appear out of the peep holes.
Dave Finkelnburg, CM Technical Editor
Rob Haugen, President, Olympic Kilns
e-mail technical questions to editorial@ceramicsmonthly.org
Keep up with what's happening in the world of ceramics: Subscribe
|