Orton Foundation Firing Tips: Improving Kiln Temperature Uniformity

Temperature distribution in a kiln can be affected by many different factors including:

· The design of the kiln

· How hot the kiln is being fired

· The load in the kiln

· The rate of heating

· If an automatic vent system is used

· How the kiln switches are adjusted

· If a hold (soak) is used

· The age and condition of the heating elements and kiln

Throughout the firing, hot air rises and cool air falls. As a result, some kilns will fire cooler at the bottom than the top. Today's redesigned kilns 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 very large kiln.

How Heat Moves

In the early stages of firing, warmed air circulates through the kiln. Later the hot kiln elements radiate direct heat onto the ware. Hot air moves through the ware transferring heat by convection to the ware. Then it is conducted to the inside of the ware.

Firing Temperatures

At 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 (^016 vs. ^6). 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. Firing slowly will help the most. When a kiln has a moderate to light load, low heat can better circulate and radiant heat can reach more of the ware to reduce hot and cold spots.

Kiln Characteristics:

How Big Is The Kiln?

For larger kilns, there is more space (and usually more ware) to heat. It takes longer for heat to be transferred. It is not unusual to see even a two cone difference (top to bottom) in a large kiln. Again, fire slowly.

While kiln manufacturers help compensate for this problem, it is not cost effective to fully eliminate top to bottom differences.

What Is Your Element Condition?

Aging elements in a part of the kiln 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 Firing

One 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 more/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 permit the heat to distribute throughout the kiln. Setting a hold time just below the top temperature is also a good way to reduce temperature distribution problems and to help ware mature more evenly.

Kiln Venting Systems

Downdraft kiln venting systems help solve temperature uniformity problems. A small amount of air is pulled in the top of 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 by as much as two cone numbers.

Checking Uniformity

The 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, loading, etc. can be evaluated for their effect on heat distribution.

How Is Heat Transferred

Heat moves through the kiln from hotter to cooler zones by: convection, conduction and radiation.

Convection:

Convection is the first step in the heating process in the kiln. Air is heated as it passes across the warming kiln elements. As the hot air rises and cool air falls, air currents are created which circulate hot air to cooler places in the kiln. This heat is transferred to the ware, shelves, etc.

The kiln will not be uniform in temperature at this early stage of firing unless the hot air is pushed through the kiln by mechanical means. Low cone firings such as 022 and 021 depend more heavily on convection for heat transfer.

The most common type of convection we are familiar with is wind chill. The cool air passes across the face and pulls heat from our warmer body, which lowers our skin temperature.

Conduction:

When heat moves through a solid, it is conducted. An example would be heat moving through the handle of a saucepan. This is a slow way to heat, but the handle will eventually get hot.

In a kiln, conduction moves heat from the inside to the outside of the kiln and from the outside to the inside of the ware. Conduction is the main way we get uniform heating in the kiln. This is a slow process and if we fire too fast, the inside of our ware will receive too little heat and not fire properly.

Radiation:

At the beginning of the firing, the elements are the hottest part of the kiln. The heat from the elements radiates out, like the the sun warming us on a cool day. Eventually the firebrick and the ware will also get hot and will radiate heat as well.

As the temperature increases, more and more of the heat is transferred by radiation from the heating elements. For uniform heating, it is important that all surfaces of the ware be exposed to heating elements.

Temperature Uniformity

At low temperatures, heat moves primarily by convection. This is not a very efficient method. The hot gases and some radiation heat the surface of the ware, where it is conducted into the ware. Sufficient time is needed to move heat to and into the ware.

Unless we move the heat through the kiln by mechanical means, the hot air will naturally rise to near the top of the kiln and the cool air will fall to near the bottom. We may see up to a 2 cone difference from top to bottom in the kiln at low cone values.

Soaking or holding the kiln at temperature can help equalize heat, but truly uniform conditions will not naturally occur until higher temperatures, where radiation is more effective.

Even Heating Ware

Heat moves through ware from the outside to the inside. It is important to try to uniformly heat all the surfaces of the ware. Loading of the kiln can help even heating. Also, fire slower to allow heat time to penetrate inside the ware.

If the top of a piece is heated, but the bottom is touching a cool shelf, the ware may crack or warp as one surface of a piece is heated more rapidly then the other. This is why it is important to use plate and tile setters for large flat objects which may warp.

Nesting bisque will prevent the inside pieces of ware from heating at all, and will restrict heat moving into the inside pieces, or the inside of the bottom piece. This is why it is better to invert or box bisque items.

It is also important to make sure that larger pieces of ware don't block smaller ones from the heat source and that ware is kept at least 1" away from the hot elements.

Ware placed too close to the elements may distort or colors may burn off or shift because they have received too much localized heat.

If ware is not placed so it can evenly receive the heat, some surfaces may end up cooler than others and the piece may crack, warp or bloat.

Reprinted with permission from Orton Foundation

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