Almost all updraft and some downdraft periodic kilns have the inherent problem of non-uniform temperature distribution inside the chamber. The procedure of “soaking” the kiln at the peak temperature was developed to even out the temperature inside kiln so that all pieces of ware would eventually achieve the same degree of firing (as is often demonstrated by pyrometric cones). We fire to a certain cone number to determine the degree of firing.

The end point of a cone depends on both time and temperature, so if the ware is fired to slightly different temperatures but is held at those temperatures long enough, the degree of firing will be the same. But sometimes, even with the use of a soak, the ware will come out warped, cracked, over-fired or under-fired. Often this is a complete mystery. Why did it happen?

The answer is usually non-uniform heating inside the load, which causes some parts of the load to fire differently than others.

Identifying the Problem

Following is a quick summary of some of the problems that can happen during firing:

• cracking from heating one side of the load faster than another
• cracking from heating too fast during part of the cycle, phase change, binder burnout, etc.
• cracking from cooling too fast through the annealing period
• cracking from cooling too fast—thermal contraction
• cracking from heating too fast—thermal expansion
• cracking from heating thick parts too fast
• over-firing
• under-firing

While most of these problems are a result of heating or cooling too quickly, they can also result from an imbalanced load. Inside any load are cool and hot spots. If you have carefully worked out a heating and cooling profile that works most of the time, the non-uniformity is being accommodated, but it is still there.

It is when you change the load that the trouble starts. Many potters have to fire a mixture of large and small pieces in the same kiln at the same time to meet production demands. Obviously, big, thick parts must be heated more slowly than small, thin parts. If the firing cycle is optimized for one size part, the other parts may be under- or over-fired, leading to losses from defects such as cracking or coring.

As the kiln heats, some parts of the kiln usually heat more slowly. If you have a firing profile that is a gentle, constant climb, the part of the kiln that lags behind is still behind when you reach the soak. In that case, soaking the kiln will take care of the problem. However, if you have a firing or cooling profile that changes speed at certain temperatures, as is common in many kilns, the cooler part of the load will suddenly start heating faster and may go too fast through a critical zone. A single heavy piece of ware inside a load of smaller, lighter ware will fire more slowly due to its greater weight. If the rate of the firing profile changes, it may affect this heavy ware much more than it affected the ware previously fired in that spot, causing increased defects.

Rearranging the Load

Potters that find themselves in this situation need not resign themselves to high losses. In many cases, simply moving some of the parts to a better location within the kiln can solve this problem.

In any kiln, the weight of the ware should be distributed to maximize the kiln’s firing pattern. One heavy piece that is equal in size to two smaller pieces can fire at the same rate if its weight is also about equal to the smaller pieces. However, a large, thin piece of ware may be over-fired if the smaller pieces of ware are each relatively heavy.

If only a few heavy pieces are being fired with a number of lighter pieces, the heavy pieces should be placed away from the control thermocouple. Heavy pieces placed near the control thermocouple will cause the burners to work harder, often resulting in over-firing of the lighter pieces. On the other hand, if the burners are working for the lighter pieces, the heavy ones may be under-fired.

Figure 1 shows a typical small shuttle kiln with burners firing across the bottom of the kiln. The control thermocouple is located near the top of the load. The bottom of the ware must get extremely hot for the heat to reach the thermocouple. If the right amount of heavy ware is placed in the bottom of the kiln, the ware will absorb a lot of the heat, and the burners will work harder to get the thermocouple to temperature. In many cases, this will result in both the light top ware and the heavy bottom ware getting the same heat treatment.

Too much weight on the bottom, however, can cause the bottom ware to overheat. If the heavy ware is placed on the top, the greater mass may cool the thermocouple slightly, also causing the bottom ware to overheat. If heavy ware is placed on top but is too far from the thermocouple, the load will likely be under-fired, as the thermocouple will sense a lighter load than is really there.

In this case, to prevent the ware from being under- or over-fired, the heavy ware should be placed on the bottom of the load, and only a few heavy pieces should be fired in a single load. If a large number of heavy pieces must be fired in the same load, the firing cycle will need to be adjusted accordingly.

Figure 2 represents another typical style of kiln, in which the burners are placed parallel to the load along the side. If only one control thermocouple is being used for the whole kiln, then the setting pattern described above still applies. If the control thermocouple is located near the bottom of the kiln, then the heavy ware should be placed at the top to ensure uniform heating, and vice versa. If the kiln contains more than one control thermocouple, the heat will tend to be more evenly distributed throughout the kiln, but the setting pattern will still need to be optimized to avoid product losses.

Finding the Right Pattern

Ideally, heavy pieces and light pieces would be fired in different cycles—but this is not always practical. When you must fire a mixed load, carefully examine both the burner pattern and the thermocouple placement, and then apply common sense in developing your setting pattern. Adjusting the setting arrangement involves both art and science, as well as a good measure of experience. Using an educated trial-and-error approach will usually solve problems of shape and density variations if you proceed logically and fully analyze the results. c