So, you have made the move from vermiculite to a kiln and your workpiece is still cracking and you have already tried several annealing profiles to no avail! Do you really know what the temperature is inside your kiln? Maybe it is time to calibrate your temperature reading device or replace the thermocouple. This article explores some of the methods involved with this decision making matrix.

Fig. 1 Infinite switch from EvenHeat Kiln

Fig. 2 Watlow F4 controller with display

Before we can discuss calibration let us review controllers and temperature measuring devices. Many kilns are equipped with a simple infinite switch (fig. 1), while others come equipped with a thermocouple and controller (fig 2). An infinite switch allows the user to set the heat from low to high and everywhere in-between, hence the name "Infinite." This switch differs from a "Low-Medium-High" switch, which provides only three heat settings. The infinite switch uses a piece of bimetal; two dissimilar metals joined together. Each metal expands at a different rate when heated, causing physical movement through the process of heating and cooling. The main switch contacts open and close, controlling the power applied to the heating elements.

Unplug the Kiln. Remove all wires from the switch terminals. (Remember to label them!)

1. Set the infinite switch on high and place your ohmmeter on the L2, H2, P and H1 terminals to check for continuity.
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3. 1. If there is no continuity between L1 and H1 or between L2 and H2, the infinite switch is defective.
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   3. If there is no continuity between P and H1, the indicator light circuit is defective
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Fig 3

When using an infinite switch a mechanical pyrometer (fig 3) and thermocouple are often used to measure the actual kiln temperature. Controllers not only provide temperature readings but can often be programmed for “ramp and come soak” cycles, that is you can control has fast they go up and down as well as how long they will remain at any given temperature.

Let’s consider what can go wrong in the calibration of a kiln. Typically, when an infinite switch malfunctions, the element can heat but possibly to only 1/2 heat, or it may remain on high in every position. Other possible problems are that the pyrometer adjustment can drift with time or the thermocouple can fail. When using a controller, problems could include thermocouple failure, the use of improper electrical wire or connections can loosen over time.

It has been our experience at GA that infinite switches can vary immensely over time. If we calibrate the switch and make a “mark” to set to, that mark becomes very unreliable after just a few months. Upon opening the switch we find a lot of corrosion and burnt contacts. For this reason we always use the infinite switch in combination with a temperature measuring device.

When GA acquires a new mechanical pyrometer we find that the temperature reading can be 200 degrees off right out of the box. We have also found that throughout the year the mechanism can be jarred or accidentally reset to a new “room temperature” such as zero. For this reason we recalibrate our pyrometers every 6 months.

Controllers present their own unique problems. Controllers have to receive information from the kiln regarding the temperature. Thermocouples are the most common device used to measure the kiln temperature. There are many types of thermocouples, such as K, N, and R among others. Each one of these is made with different metals and exhibit different properties. When connecting the thermocouple to the controller each one must be done so with its specific type of wire. If you use wire purchased from the hardware store to connect the two together you will have errors in the read-outs. For instance when we use a type K wire with a type N thermocouple we experience a reading off by 126 degrees F.

The thermocouples themselves are also very interesting. They work by wearing out; they wear out faster at higher temperatures. As they wear out they generate electricity which is read by the meter and translated into a temperature reading. In theory a thermocouple either works or it doesn’t. As it wears out it should provide an accurate reading. It has been our experience that this oversimplifies real life. Without going into the dozen or more reasons that we have documented, let’s just say that it is not uncommon to have a perfectly normal reading yet when we look into the kiln it is a bright white or a very dull orange, both of which indicate that the thermocouple is sending a false signal. Fault diagnostics occasionally lead you to a loose connection, a frayed or melted wire, but generally it is a failed thermocouple.

Since there are several possible sources that generate false information it is important to have a method to verify your readout and if discovered to be incorrect a methodology for diagnosing and fixing the problem.

The easiest way to “calibrate” or verify your reading is to make a measurement with an independent system such as a multi-meter that has thermocouple capacity. If you don’t want to purchase such a device you can often rent one from your local hardware store, rental yard or ceramics supplier. You may also want to ask around among your friends, especially any within the construction trades for they may have such a meter. If they have the correct type of multi-meter without a thermocouple the probe can be purchased fairly cheaply.

It is also possible to use one of the newer handheld infrared pyrometer “gun” if it is calibrated. Be careful not to shoot the temperature of the element for it will be much hotter. The best bet is to shoot a brick on the side wall rather than a glass piece. When shooting glass the emissivity setting of the gun has to be set and the setting will be different for every color. For these reasons the infrared pyrometer is not a good choice for verifying your thermocouple readings.

A time honored method, used by many ceramicists, is to use the Orton Cone Method. Orton is a company that manufactures, among other things, pyrometric cones that have been used since 1896 to measure the performance of kilns. For a complete description of the process and cone charts you should visit the Center for Firing at the Orton Ceramic Foundation, www.ortonceramic.com/center.

When using the cone method you need three Self Supporting witness cones, not kiln sitter or Large Cones (the type you would stick into a blob of clay). If you were testing your kiln at 1050° F you would use 1 each of Cone Number 05, 04, 03, which are 1021°, 1046°, 1071° F respectively. This selection would “bracket” the temperature you believe the kiln should be, based on the thermocouple reading. After placing these into the kiln you would ramp up to temperature. The ramp rate during the last 120 minutes would need to be monitored and recorded if you do not have a controller. If you have a controller it would need to be programmed. For the temperatures of the cones listed above the ramp rate would be 15°C/hr, if ramping faster the cone temperature will shift upward in temperature due to the time it takes for the heat to soak into the cones (like cooking a turkey). As the cone heats the clay turns to glass and they weaken and bend over. If the kiln is really at 1050° F the first cone will be folded over and touching the floor, the middle cone will be bent over with the tip folded over to a “plane” parallel to the floor of the kiln extending from a “mark” clearly visible on the side of the cone. The higher temperature cone will be only slightly deformed (Fig 4.).

Fig. 4 Showing cones before and after.

Cone placement in the kiln is important for several reasons. The placement in the kiln can be used to determine the overall distribution of heat throughout the firing chamber. When distributed throughout the kiln load, the cones indicate what is happening in the various locations. This information can be very valuable when trying to strike or anneal a work-piece.

Location “is also important in allowing the cones to deform in the manner that they are supposed to bend. If the cones are located in an area where they can be hit by a draft for example, they can develop a "hardshell" if they are in the state of flux. This can affect the bending of the cone in a severe manner. If they are located too close to the heat source such as a burner or an element, the time and manner of deformation can also be affected” (Orton website, 2004).

If you discover that your current readouts are incorrect then the next step is to determine why. If you are using an infinite switch with no pyrometer you may want to invest is a simple system. Today, an electronic system costs about the same as the old analog types as shown in Figure 3. Small ramp/soak controllers with relays can be purchased and assembled for less than $250.

If you are using an analog pyrometer a simple adjustment of the screw in the front panel, which changes the starting point, may be the only adjustment necessary. It is amazing how many kilns are set to zero rather than room temperature when they are cold, this alone makes then off by 70 degrees when they are hot. If you set the needle to the correct value while hot and a retest on another day results in an improper reading; the thermocouple may be grounding to the kiln, have a loose connection or be in need of replacing.

The same holds true for a controller. If the thermocouple is providing an improper reading always investigate for worn through wires, loose connections within the plugs, shorts between the thermocouple and the kiln case or the use of improper thermocouple wire. With an electronic controller it is also a good idea to check the calibration offset, we once had an employee who inadvertently set the offset to 1000° and it took us 2 months of replacing parts to figure this one out. If these items are in order then replace the thermocouple. Remember there are many thermocouple types, so check the original documentation to determine which to use in your kiln.