Pottery and Ceramics Tools
Kiln Temperature Distribution
The heat distribution in a kiln can be affected by many different factors including:
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The design of the kiln,
During a firing, hot air rises and cool air falls. As a result, some kilns will fire cooler at the bottom than the top. The kilns that are designed today, generally have improved uniformity, but it is not uncommon to see a full cone difference from top to bottom in the kiln and a half cone difference on a single shelf in a large kiln.
How Heat MovesIn the early stages of firing, warmed air circulates through the kiln. Later the hot kiln elements radiate direct heat onto the pottery or ceramics. Hot air moves through the pieces transferring heat by convection to the pieces. Then it is conducted to the inside of the piece.
Firing TemperaturesAt higher temperatures, most of the heat is transferred by radiation from hot elements or hot gas. Kilns fired to lower cone numbers tend to be less uniform than kilns fired to higher temperatures. This is because radiation is more effective in transferring heat.
Load Makes a Difference
With a heavy load, it is more difficult to evenly distribute heat through the kiln. When a kiln has a moderate to light load, low heat can better circulate and radiant heat can reach more of the pieces to reduce hot and cold spots.
Kiln Characteristics, How Big is the KilnFor larger kilns, there is more space and usually more pieces to heat. It takes longer for heat to be transferred. It is not unusual to see even a two cone difference from top to bottom in a large kiln. Remember, it is not cost effective to fully eliminate top to bottom differences.
What Is Element ConditionOlder elements in one part of the kin will affect heat distribution in that section. At full heat, all the elements should glow red. Elements that do not are likely aging and should be replaced. Check your kiln manual. Some kiln manufacturers recommend replacing all the elements at the same time for best results.
Controlling the FiringOne of the easiest ways to improve uniformity is to adjust the switch settings on the kiln to send more heat to cooler areas. As an example, for a kiln that is cooler in the bottom, turn the bottom switches on earlier than the other switches, or turn them on higher than the other switches. This allows the bottom to heat up faster than the rest of the kiln so it doesn't have to catch up later.
If an automatic controller is used, the firing rate can be slowed up to let the heat get distributed throughout the kiln. Setting a hold or soak time just below the top temperature is also a good way to reduce temperature distribution problems and to help the pottery and ceramics mature more evenly.
Kiln Venting SystemsDowndraft kiln venting systems help solve temperature uniformity problems. A small amount of air is pulled in the top the kiln, circulated throughout the firing chamber and exhausted out the bottom of the kiln. As the hot air moves through the firing chamber, it evens out temperatures, often as much as two cone numbers.
Checking UniformityThe most reliable way to check heat distribution in your kiln is to use a series of witness cones on each shelf. By examining them after each firing, you can see how firing conditions, including uniformity, affect fired results. Adjustments in switching and loading can be evaluated for their effect on heat distribution.
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