Originally Posted on: Mar/03/2005 11:48
How to Fire that Big Old Downdraft Gas Kiln
In 2000, I started teaching an adult continuing education pottery class at Pioneer Craft House in Salt Lake City. I was immediately confronted with an amazing handbuilt downdraft gas kiln. It filled a quarter of the kiln buidling and loomed over the room. I was lucky enough to have Andrew Bird, the prior teacher, around to show me how to fire the thing. I am sure I would have blown up the school without him.
The grand old kiln has recently been removed from Craft House in exchange for new technology. But I am sure there are many teachers being confronted with an old school kiln for the first time, and I thought my firing instructions would help.
These instructions are for a kiln with a door on the front and a flu (heat exit) at the bottom rear of the kiln that exits to a chimney stack of some sort. If you have a kiln with a hole in the top of it, it is an updraft kiln and these instructions won't help much.
DISCLAIMER: Every kiln is different and has different requirements for a firing. This article expects you to know the basics of firing a kiln, escpecially safety proceedures needed for your particular kiln. If you do not know how to safely run your kiln, seek professional help. A local pottery would be a good place to ask for assistance.
STEP #1: OPEN EVERYTHING
A kiln inside a buidling needs air. Open all doors and windows. Open the damper, the kiln door and anything else that vents the kiln.
STEP #2: PILOT LIGHTS
Find the valve that releases fuel to the pilot lights. Light all pilot lights.
The Craft House kiln had four pilot lights, one in front of each burner. Each pilot light had a swtich that you held in the open position while lighting it. Once lit, you still had to hold the switch open for about 30 seconds to get it warmed up. If a pilot light went out for any reason the system was set to shut off all gas as a safety measure. When all four pilot lights were lit, an audible click occured as the main gas valve opened.
STEP #3: LIGHT THE BURNERS
The burners should have a seperate control valve for the gas. Slowly turn on the gas to light all burners. At this stage, leave the gas as low as possible while keeping the burners lit.
On our kiln each of the four burners had a knob to control the amount of gas being sent through the burner. Each knob was turned on slowly until the gas was lit by the pilot light. Occassionally, if the gas was too low, the flame would blow out with a frightning pop. These pops never did any harm other than cause me minor heart attacks.
STEP #4: WARM THE WORK
Especially for student work which is more likely to contain air bubbles and other such problems, allow the kiln to warm slowly for 30 min - 1 hour. Even on a low setting, however, you will find that the kiln reaches 500 degrees very quickly. Don't put your hand in the kiln after it has been lit. (Yes, I learned that the hard way.)
STEP #5: CLOSING THE KILN
The heat will create pressure in the kiln. If you suddenly close the kiln door, the back pressure will likely blow out your burners. Close the door with a two inch gap for 20 minutes, then a one inch gap for 20 minutes, then closed but not screwed tight. This allows the stack (chimney, smokestack, exit of some kind) to warm and begin to draw the air out properly.
If your burners blow out, keep the cursing to a minimum, then open the door completely before relighting the burners. Never turn on gas into a closed kiln because the gas can pool in the closed space and cause an explosion when lit.
Our kiln took about 40 minutes - 1 hour to close the door. On wet, wintery cold days, it took much longer and we often blew out the burners closing the door too early.
STEP #6: GET THE HEAT MOVING
Once the door can be tightly closed, turn up the gas a little. The idea is to force the heat to go up along the sides of the kiln and then back down through the work and out the flu at the bottom of the kiln. The damper should remain completely open at this stage to encourage the air to exit that way. Any peep holes in the upper half of the kiln should be plugged, to get the air to move down rather that escaping through the peep hole. Once the heat is moving to the lower area of the kiln, close the damper about half way, but make sure it is not causing reduction. If you smell a gassy smell, open the damper slightly until you are back in a neutral atmosphere.
The Craft House kiln had two peep holes in the door. One at the top and one at the bottom to allow us to check cone packs in the two different levels of the kiln. At this stage, we would plug the upper hole, but not the lower hole. When we felt significant pressure coming from the lower hole, we knew the heat was moving correctly and could partially close the damper.
The kiln can stay in this configuration for 2 to 4 hours until the start of color - which means the pots inside the kiln start to glow orange-red.
STEP #7: TURN UP THE HEAT
When color starts, the heat can be increased slightly again. As before, make sure to stay in a neutral atmosphere.
At Craft House, the slightest move of the burner valves changed the amount of gas significantly, especially since all four burners would be adjusted. We became experts at barely tapping the knob to adjust the gas.
STEP #8: CONE 06
At cone 06 I was taught to do a body reduction. We would close the damper until the pressure or flame from the lower peep hole was about two inches. Adjustment by smell also worked. A light gassy smell means light reduction. Body reduction takes about an hour. We would then reopen the damper until the kiln was back to a neutral atmosphere.
NOTE: I eventually learned that the Craft House kiln was built so tight it lightly reduced regardless of what we did. So I started skipping body reduction. I have since read articles in which potters oppose doing any body reduction. It can increase the chances of pinholes in the final glaze.
At this point, carefully monitor the eveness of the cones. If the top cone falls significantly sooner than the bottom cone, close the damper slightly to warm the lower part of the kiln better. However, beware of sending the kiln into reduction which will stop your temperature from increasing. It is a constant balancing act.
STEP #9: CONE 9 & 10
Glaze reduction is done at a variety of temperatures depending on the glaze. At craft house, we used something between 9 and 10. When cone 10 just started to think about bending, we would close the damper until we had medium reduction. We left it in reduction for 1 hour. At the end of reduction we would open the flu to a neutral atmosphere until cone 10 was down hard - but always for at least 15 minutes.
NOTE: Although this always worked for our glazes in the Craft House Kiln, I do not recommend glaze reduction at cone 10. It should be started sooner and be done more consistantly starting at cone 8. I hope this hot issue will be covered in more depth in other topics.
STEP #10: SHUT DOWN
Every potter has a point the cone bends that the potter calls "cone 10." We used "down hard" which means bending to the point the tip touched the cone pack clay. Usually the top cone would be down hard and the bottom cone would just be starting to bend. What ever you use, just be consistant so that your glaze can be consistant too.
Once temperature is reached, shut down the kiln by going backwards through the lighting process. Turn each burner off individually. Turn off the gas to the pilot lights. Turn off the main gas valve. Plug up the kiln completely including, close the damper and plug the peep holes. The tighter the kiln is sealed, the slower it will cool, and we want a slow cool.
STEP 11: OPENING
When the kiln is cool, open the door slowly. If the glaze starts to "ping" as the cold air hits it, you are opening too soon. As much as I love to hear a kiln sing, I would rather see good glazes without crazing.
Thats it. I hope this helps. One last note: The best thing I ever did was to place cone packs all over in the kiln one firing. I learned where the hot and cold spots were. From that point on, I packed the hot spots tightly with pots and the cold spots loosely. Since the hot spots had more work to do, which requires more heat, this evened out the heat thoughout the kiln.