Technofile: Why Do Kilns Stall? Phil Berneburg

Appears in the January 2015 issue of Ceramics Monthly.

It's late, you're tired, you've been at this for hours, your gas kiln is in reduction, you're only at cone 9, you want cone 10 flat, and nothing is moving. Why does this happen? And, what can you do to prevent it?What Goes Up, Must Come Down.

Defining the Terms

BTU: The British thermal unit (BTU) is a traditional unit of heat. It is the amount of heat needed to increase the temperature of one pound of water by one degree Fahrenheit. 

Conduction: The transfer or movement of heat though a solid or static (stationary) liquid.

Convection: The transfer or movement of heat in a liquid or gas by means of the flow of the liquid or gas.

Heatwork: The combined action of heat (as indicated by temperature) and time. Pyrometric cones respond to heat work, whereas pyrometers measure temperature.

Radiation: The transfer of heat by means of the emission of waves of energy (infrared/heat radiation) through space.

Reducing Atmosphere: The atmospheric condition that exists in kilns (primarily fuel-burning kilns) when there is insufficient oxygen present to burn all the fuel completely. The chemical reactions and coloration effects associated with a reducing atmosphere are the result of the presence of excess carbon monoxide.

What Goes Up, Must Come Down

Stalling is the term that is applied when the rate of temperature rise of a kiln slows down or even stops to the point where the temperature stays the same, or possibly even decreases a little, despite efforts to keep the temperature climbing. The underlying cause is a fundamental principle of the physics of heat transfer (heat flow). As an object is heated it is, at the same time, constantly losing heat by one or more of the three processes: conduction, convection, and/or radiation. However, as the temperature of the object increases with continued heating, the speed at which it loses heat also increases, i.e. the hotter it is, the faster it loses heat. This is due to the fact that the rate of heat loss is exponentially proportional to the difference in temperature between the heated object and the surroundings. So, as a kiln and its contents are heated, it also tends to lose heat—by conduction through the walls and radiation from the outside surfaces—at an increasing rate. The problem of stalling arises when the amount of heat being put into a kiln is matched by the amount of heat being lost.

The graph on page 56 illustrates this energy balance between the gains and losses that exists when stalling occurs. With a typically constant rate of heat input (BTU/hr) provided by the energy source,  (the upper, straight line in the graph), either from burning fuel or from electricity, as the speed of heat loss increases (the downward curved line at the bottom of the figure), the heat remaining in the kiln (as indicated by temperature) increases more slowly until the losses overtake the input, and the kiln temperature stops rising (the stall, shown by the middle curve in the figure).

It is a fairly common practice to adjust burners at the beginning of a gas firing, or to establish a regular stoking schedule for wood-fired kilns, to produce a steady temperature rise and then not significantly change the heating rate for most of the firing. Early in the firing, the heat being put into the kiln is more than enough to offset the relatively small losses. However, later in the firing, the losses may increase to the point where the input of heat is no longer enough to keep the temperature climbing. If additional heat could be put into the kiln faster, then the heat loss could once again be surpassed by the heat gain, and the temperature would resume rising. With fuel-fired kilns, however, the situation is complicated by several factors, including the need to maintain a reducing atmosphere in the kiln.

Gas Kilns and Reduction

The creation of a reducing atmosphere involves the incomplete burning of fuel. Operation of the kiln then requires maintaining a delicate balance between intentionally not burning the fuel efficiently (in order to create the reducing atmosphere) and burning enough fuel to produce heat to keep the temperature rising. With the speed of heat loss increasing with temperature, a point in the firing can be reached where heat is being lost almost as fast or faster than it is being added. This is the point at which stalling occurs. Conversely, if it were not necessary to maintain a reducing atmosphere, then preventing a stall would, in principle, just be a matter of increasing the gas and air flow or changing the stoking rate (or amount and type of fuel) in order to increase the amount of heat being put into the kiln.

A further complication arises because of the fact that kilns are fixed in design and limited in size. As the amount of fuel is increased, there may not be enough time for flame retention in the kiln (a long enough flame path) to reap the full benefit of the increased fuel. Only a portion of the additional heat produced is actually used to heat the kiln and the ware; the rest is lost up the chimney.

Electric Kilns

Why doesn’t stalling normally occur with electric kilns? In a commercially produced electric kiln, the elements have been designed (number of elements, element thickness and length) by the manufacturer to be capable of producing enough heat (in combination with a particular power supply) to overcome the increasing heat losses that occur with rising temperature, in order to reach the desired temperature. Furthermore, the kiln is not firing in reduction (wasting heat/fuel), and the kiln is fairly tightly sealed. Heat is still being lost from the kiln (and its contents) through the walls, but there is no exit flue or chimney. All the heat produced by the elements is initially available to heat the kiln and its contents. To increase the temperature of the kiln, we can simply turn up the power to the elements. In a sense, there is an excess capability to produce heat. Also, this heat (power) adjustment is commonly made more frequently throughout the firing with electric kilns than with fuel-fired kilns.

Stalling does occur in electric kilns when the elements cannot provide enough heat to more than offset the losses; however, this is generally the result of an equipment failure such as burned out or broken elements or aging elements (gradual increase in electrical resistance) or other component failure. In these situations, the kiln will take a lot longer to reach the desired temperature, or it may climb to a certain (lower) temperature and remain at that temperature without reaching the goal. The solution is routine kiln maintenance and/or kiln repair.

Similarly, gas or wood-fired kilns can stall due to equipment/design failure, such as if the burners or firebox have not been properly sized for the kiln, i.e. if they do not have adequate heating capability (BTUs/hour) for the size of the kiln.

Firing any type of kiln involves the competing processes of heating and cooling. In order for the temperature within a kiln to increase steadily, the amount of heat being put into the kiln has to be more than the total amount of heat being lost at any time. With an electric kiln, which has sufficient insulation and properly sized elements, this is fairly easily accomplished. A lot of heat is still being wasted or lost, but more heat can be created than is lost, so that a steady temperature increase can be maintained. With fuel-fired kilns, the need to maintain a reducing atmosphere, combined with incomplete burning due to possible burner inefficiency and the limited time/distance available for complete combustion in the kiln, creates the situation where, at higher temperatures, insufficient heat is generated within the kiln to overcome the losses. The kiln is effectively underpowered.

The Fix

How can stalling in fuel-fired kilns be overcome? Either the fuel and air supply have to be increased to produce heat at a faster rate, or the existing fuel has to be burned more efficiently (less reduction) to yield more heat. However, with fuel-fired kilns, depending upon the kiln design and size, there are practical limits as to how much the fuel can be increased to generate more heat without wasting most of the increase, unless controlled adjustments can be made to the fuel and air supplies.

the author Phil Berneburg is a professor of ceramic arts at Hood College in Frederick, Maryland, where he specializes in teaching the scientific background and technical problem-solving in the Certificate, MA and MFA programs. He is also a ceramic engineer and studio potter.