Figure 1. Three different pyrometric cones (guide, firing and guard) can be seen in two locations exiting the kiln. Comparing the bending angle of the firing cone (middle cone) will illustrate the difference in relative temperatures between the two locations.

One of the final steps for all brick and ceramic products is the firing or sintering process. Utilizing raw materials with rigid specifications, body preparation to exacting standards and closely controlled forming processes will only produce a quality finished product when the final process—firing—delivers the correct amount of heat-work to the product. Heat-work is the actual measurement of the combined effect of heat and time. It is therefore necessary to measure the uniformity and consistency of heat-work delivered during the firing process for meaningful quality control/assurance.

The pyrometric cones used today were developed in the late 1800s by Edward Orton Jr., Ph.D. Orton recognized the need for a method to determine that ceramic ware was fired correctly. The rapid and universal acceptance of pyrometric cones prompted most ceramic bodies and glazes to be identified by their firing cone number: cone 9 sanitaryware, cone 6 stoneware, etc. Later, the development of electronic temperature controllers simplified and improved the control of the firing process, but they could not replace the cones as a measure of the accumulative effect of time and temperature (heat-work) on the ware.

Both tunnel and periodic kilns are equipped with several thermocouples to monitor temperatures and provide the electronic temperature controllers with the information necessary to control the firing process. The thermocouples are usually mounted in the crown or side-wall of the kiln and do a great job of measuring temperature at a given point in time and space. However, fixed thermocouples do not actually measure the temperature in the ware setting and cannot sense and record the level of heat-work being delivered to the ware throughout the cross-section of the kiln. The thermocouple measures temperature at one point in space and time: the end of the thermocouple(s).

A couple of options—traveling thermocouples and pyrometric devices—are available for measuring the temperature within the ware setting. Due to the difficulty and expense of running traveling thermocouples, they are used infrequently in production kilns. The daily use of pyrometric devices is a simple and cost-effective system for monitoring and aiding in the control of the firing process.

Pyrometric Basics

Figure 2. It is best to number and record the location of each TempChek before it enters the kiln.  Relative temperatures can then be easily identified by location once they come out of the kiln.

Today’s pyrometric devices are closely controlled for repeatable and consistent performance. Pyrometric devices should be used primarily to provide quality assurance that the firing process is delivering consistent, uniform heat-work to the ware.

When using pyrometric cones, several series of three different cones (guide, firing and guard) are placed throughout the kiln to determine the overall distribution of heat throughout the firing chamber (see Figure 1). When cones are distributed throughout the kiln load, the cones register the amount of heat-work taking place in the various locations. Comparing the bending angle of the firing cones (middle cone) illustrates the difference in relative heat-work between the locations. This information can then be used to adjust the firing conditions or the load in the kiln to achieve a better firing process.

Enlarge this picture Figure 3. Design a placement diagram for the positioning of pyrometric devices within the setting of the brick in the kiln(s).

Best Practices

Figure 4. Measuring pyrometric devices is simplified using a customized gauge and an automated input device that eliminates typing the information into the computer.

Utilizing a simple three-step procedure can improve the control of the firing process. The first step is to establish a system for the monitoring of heat-work delivered by the firing process. Design a placement diagram for the positioning of pyrometric devices within the setting of the brick in the kiln(s) and establish a schedule for placing and retrieving the devices (see Figure 3). Convert pyrometric device measurements to temperatures and enter the information into a database. The use of a digital measuring device connected directly to a computer makes the conversion of shrinkage to measured temperature simple, and the data can be placed directly into a custom database (see Figure 4).

Enlarge this picture Figure 5. When used along with SPC software, pyrometric devices establish the upper and lower control limits and alert manufacturers when the firings are trending out of control.

Second, develop a database for heat-work measurements and display the data graphically. Utilize a SPC software program to display data in X-bar and R-bar control charts, as shown in Figures 5 and 6. These charts can indicate firing trends before a major problem develops, and they also serve as a benchmark for the firing process. When problems occur, the benchmark data provide a reference to determine if the kiln is or is not the likely cause of the problem. The information on the control charts can also indicate the normal variation (capabilities) of the kiln(s). A clear understanding of the firing process’ capabilities will prevent the tendency to over-control the kiln(s).

Lastly, correlate the fired properties of the brick to heat-work. All ceramic products have a firing range window that produces quality ware, though quality and the required properties—such as fired absorption, strength, shrinkage, density, color and so on—are defined differently for different products. If the ideal firing range of the brick has not been determined, it is probably worth the time and effort to conduct a series of designed experiments utilizing a gradient furnace to determine the firing range of critical parameters. Once the correlation of product properties to heat-work is established, the heat-work analysis serves as a sound nondestructive quality assurance tool.

Improved Processes

Enlarge this picture Figure 6. After inputting the information from the pyrometric devices into the SPC software, a historical chart showing the natural variation of the range of temperatures can be displayed.

Monitoring a kiln’s efficiency in uniformly delivering heat-work to the brick can be easily accomplished through the use of pyrometric devices. Quality assurance programs are enhanced by knowing how well the firing process delivers the proper heat-work to the product, and plotting the data provided by the pyrometric devices in SPC charts makes the information easily accessible and user friendly.

For more information regarding pyrometric devices, contact The Edward Orton Jr. Ceramic Foundation at 6991 Old 3C Highway, Westerville, OH 43082; (614) 895-2663; fax (614) 895-5610; e-mail info@ortonceramic.com; or visit www.ortonceramic.com.