The line of light under the lid is normal; the heat is contained safely inside the kiln.
The Light Around the Kiln Lid or Door
Reader Response: Top-loading kilns
Recent Q&As: When to replace elements in a recently purchased used kiln
A Kiln Story: The Snow Storm
THE LIGHT AROUND THE KILN LID OR DOOR
Some people worry about the line of light that appears around a kiln lid (or door) at high temperatures. They wonder if the light means heat is escaping.
The kiln expands as it heats. The larger the kiln, the greater the overall expansion of parts. Since the hot inner surface of the lid expands more than the cooler outer surface, the lid or door bows slightly toward the firing chamber. This causes a small gap, where light from the firing chamber is visible. The gap is more pronounced on the ends of an oval lid than on a round lid.
But unless the lid rises during firing (the subject of another Kiln Pointer), there is little heat loss from the gap under the lid. At high temperatures, the molecules in air are so far apart that they no longer transfer heat through convection. This is why heat does not pour out of a peephole when you remove the plug.
On some models, we add a gap between the lid (or door) and kiln at the hinge. As the kiln gets hot, the gap closes due to the expansion of the firing chamber.
A reader suggested that I elaborate on a Q&A from last week’s Kiln Pointer:
Q. What is the difference between loading ceramics in a front-loading kiln and a top-loading kiln?
A. Smaller, lighter pieces are usually loaded in the front of the kiln near the door or on the bottom. This is because these are the cooler areas of the kiln. Load heavier pieces near the back wall of a front-loading kiln.
The top and bottom of a top-loading kiln tend to be cooler than the center, because the lid and bottom absorb extra heat. In some kiln models, we compensate for this by placing cooler elements in the center section and hotter elements in the top and bottom.
Load smaller, lighter items in the top and bottom and heavier items in the center. Until you become familiar with the firing characteristics of your kiln, use witness cones on each shelf.
Q. I have recently purchased an old kiln. Should I check the elements with an ohmmeter and change them if the readings are low, or only change an element when it breaks? Most of my potter friends say not to change elements until they go out completely.
A. To check the elements, fire the kiln. If the firing time is normal, don’t worry about changing the elements. Though the elements are old, they might be okay especially if the kiln wasn't fired often.
If the elements are at the end of a wear cycle, then it makes sense to change them all when one breaks. If the elements are fairly new and one breaks, change only the broken one.
A KILN STORY: THE SNOW STORM
Last week Lisa Westheimer of West Orange, New Jersey wrote, “The snow storm caught many of us by surprise and came during an outdoor saggar fire workshop. The kiln was my garbage can filled with combustibles surrounding aluminum foil wrapped ware. Just after the kiln was lit (holding an umbrella over it to keep the snow from putting it out), the storm came and trees started falling one after the other, sounding like gun fire. The neighborhood became so unsafe my class hunkered down for the night.
“That garbage can kiln burned for six hours in the driving snow and yielded great results. The next day my students and their car full of fired ware headed back to New York City.”
“There's nothing wrong with having a plan. But missions are better. Missions survive when plans fail, and plans almost always fail.” -- Seth Godin
The question about used kilns reminds me of a 30-year-old Paragon A-82B that someone brought to the factory to be tested. The owner had purchased it at a garage sale. Other than a slight discoloration, the bricks looked new. I could tell the age of the kiln by the color of the switch knobs and the coat of glossy gray paint on top of the lid.
Hoovie, seeing the kiln at the inspection station, told me he wired switch boxes when the kiln was made. Pancho was walking by when he saw the kiln. He said he installed elements 30 years ago, and I said, "That kiln may have your elements." We still sell parts for the early kilns.
With best wishes,
Arnold Howard Paragon Industries, L.P. – Better Designed Kilns 2011 South Town East Blvd., Mesquite, Texas 75149-1122 Voice: 972-288-7557 & 800-876-4328 / Fax: 972-222-0646 / firstname.lastname@example.org / www.paragonweb.com / www.facebook.com/paragonkilns
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