PPP: Controlling Your Firing Process
Kiln control has undergone many enhancements in the past decade, with a change from manual control to electronics that automatically regulate the firing. Existing kilns can be upgraded by adding a controller.
Electronic controllers offer many advantages for firing kilns. They provide a convenient and consistent method of control and can ensure temperature uniformity in electric kilns equipped with zone control. They regulate heat into the kiln through a program entered by the user and stored in the electronics. Temperature is measured, and if it is lower than the programmed (set point) temperature, heat is added.
Controllers can heat kilns for drying or carbon burn out, or can soak heat into the middle of the kiln load. They can slow cooling to avoid cracking of thick pieces. Preset programs can make cone firings simple. A controller can even start an electric kiln when it is convenient and, when a downdraft venting system is used, the kiln can remain closed throughout the firing cycle. Combined with tried-and-true monitoring tools such as thermocouples and cones, today’s controllers can ensure better control and more consistent firings.
Controlling TimeAs ware is heated, oils, binders and organics burn out at lower temperatures. Both oxygen (from the air) and time are needed for gas to pass into the body and react with the carbon. If the ware is heated too rapidly early in the firing, the carbon in the clay body can char and become difficult to remove. Once charring occurs, higher temperatures above 1400?F are required to burn the carbon out properly. Carbon left in the body weakens the fired part and promotes cracking. It can also oxidize during glaze firings, causing bubbles or blisters.
The time needed to remove all the carbon depends on the thickness of the piece, its pore structure and the firing temperature. Sawdust burns faster than graphite. It is a good idea to test fire pieces that are of the same thickness as the ware and break them open to see if any darkening (black coring) occurs due to poor carbon burnout. If black coring is evident, slow down the heating rate in the early stages of the firing.
Other reactions occur during firing as the clay changes and the body shrinks. Any sulfur is released around 860?F, chemically combined water comes off the clay between 890 and 1110?F, and minerals such as carbonates decompose from 1020 to 1560?F. All of these changes are irreversible and cause the clay body to permanently change. It takes time for these reactions to run their course, so it is important not to fire too quickly through this temperature range. Controllers can help ensure that the firing progresses at the appropriate rate.
Above 1600?F, sintering of particles occurs, followed by the formation of a glass phase as certain minerals melt. This provides the strength needed in the final ware. These reactions are driven primarily by temperature and can proceed as rapidly as heat can be provided, thus enabling the firing to go faster. Pores are eliminated as the body continues to shrink. If the body is fired to a temperature that is too high, gas in the closed pores expands and causes bloating and blistering in the body. Reduction will cause color changes and increase the rate of body maturation.
Cooling too fast can cause cracking of the ware if stress builds up due to expansion differences or because of a volume change when quartz (silica) transforms around 1100?F. Controllers allow you to slow down the cooling of the kiln, thereby keeping problems such as cracking from occurring.
Controlling TemperatureTo obtain consistent firings, it is important to accurately control the temperature in the kiln, as well as any temperature changes. Temperature is measured with a thermocouple, usually located near the kiln wall. A thermocouple consists of two different metals welded together into a bead at one end. Heating the bead produces an electric signal used by the controller to determine temperature.
Thermocouple performance depends on the type, size and protection of the bead. Thinner wires, such as those found in Type K thermocouples, give a more accurate signal but may not last as long. Type K thermocouples oxidize and drift (the signal changes) with time, indicating a lower temperature than actually exists. This can cause overfiring of the ware. A Type S (platinum) thermocouple drifts less and lasts longer but is more expensive. For high temperatures (above Cone 4), Type S is usually the best choice. Some thermocouples are encased in a metal or ceramic protection tube to extend their life.
Placement of the thermocouple in the kiln is also important. The bead should be located where it is not affected by heating elements, hot gas from the burner, or cold drafts. Temperatures measured at the wall of the kiln are usually different from temperatures in the middle. However, the change in temperature will usually be consistent in both locations. Temperature differences can be minimized through proper loading of the kiln and through the uniform distribution of hot gases within the load.
Uniform temperatures are desired but can be difficult to obtain. In fuel-fired kilns, hot gas must be uniformly moved through the kiln load. Passing hot gas down through the load is the best way to balance gas flows, especially when passage sizes vary. Hot gas gives up its heat as it contacts cooler ware and kiln furniture. Some heat is also transferred by radiation from water vapor and carbon dioxide gas molecules. To change the flow of gas, one must change the pressure across the passages. Gas flows through the passages with the least resistance, so it will travel through a wide passage before a narrow one. Obviously, both kiln design and placement of ware will affect hot gas flow and the uniformity of temperature. Since it is impractical to place thermocouples throughout the load, cones can be used to determine differences in temperature.
While controllers are best for inputting heat into the kiln, pyrometric cones can be used to measure the distribution of heat within the kiln load. They can determine when a firing is complete, if enough heat was available or if a temperature difference exists in the kiln, and they can also help diagnose firing problems—something controllers do not do. Cones are invaluable, which is why bodies and glazes are often referred to by their cone number (e.g., Cone 5 stoneware). Some potters rely on cones to manually end the firing when they observe the firing cone bend. The bent cone can even be converted to a temperature using available charts or cone calculation software.
Controlling AtmosphereAtmosphere control is especially important in fuel-fired kilns and is accomplished by regulating the amount of fuel and air in the kiln. Reduction occurs when not enough air is available to support complete combustion of the fuel, and carbon monoxide and hydrogen gases are formed.
Atmosphere affects colors, the burning of carbon, and properties of the glass formed during firing. Water vapor and reducing gases, such as carbon monoxide, have the greatest effect by lowering the viscosity of glass to allow lower maturation temperatures. When metal oxides are reduced, their color changes; iron oxide, for example, changes from brown to black in a reducing atmosphere.
It is also important to maintain control of the kiln atmosphere to obtain consistent firings. Often potters initially keep the atmosphere oxidizing for carbon burnout and then make it reducing during maturation to fix certain colors and improve ware properties.
Control of reduction firings is usually done in gas-fired kilns with natural draft burners by adjusting the damper, which reduces the flow of air into the burner until there is excess fuel. An oxygen probe is useful to set the oxygen level, sometimes automatically with a controller. This provides more consistency between reduction firings and prevents the formation of soot caused by too much reduction. Reduction is sometimes done in electric kilns by adding alcohol or other carbon material near the end of the firing.