ONLINE EXCLUSIVE: Measuring Heat Work

May 1, 2004
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No quality control program is complete without the proper tools to measure heat work inside the kiln

One of the final steps for all ceramic products is the firing or sintering process. Using raw materials with rigid specifications, preparing body formulations to exacting standards and employing closely controlled forming processes will only produce a quality finished product if the firing process delivers the correct amount of heat work to your products. Heat work is the actual measurement of the combined effect of heat and time. For meaningful quality control/assurance, you must be able to measure the uniformity and consistency of heat work delivered during the firing process.

Both tunnel and periodic kilns are equipped with several thermocouples to monitor temperatures and provide the electronic temperature controllers with the necessary information for controlling the firing process. The thermocouples are usually mounted in the crown or sidewall of the kiln and do a great job of measuring temperature at a given point in time and space. However, they are not actually measuring the temperature in the ware setting, and they cannot confirm the level of heat work being delivered to the ware throughout the cross-section of the kiln. As a result, quality-conscious ceramic manufacturers also run traveling thermocouples and/or place pyrometric devices within the setting. Although traveling thermocouples can sometimes be difficult to use, pyrometric devices are a simple and cost-effective way to monitor your firing process on a daily basis.

Figure 1. To accurately monitor heat work, you should first establish a schedule for placing and retrieving the pyrometric devices.
The pyrometric cones used today by both ceramic artists and industrial manufacturers were developed in the late 1800s by Edward Orton Jr., Ph.D. Orton recognized that ceramists needed a way to determine when their ware was fired correctly in order to develop the properties they required in their finished products. Thus, all ceramic products were eventually assigned a cone number to which they were to be fired (cone 9 for sanitaryware, cone 6 for stoneware, etc.) to ensure the maturity of the ware during the firing process. 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 cumulative effect of time and temperature (heat work) on the ceramic ware. Pyrometric shrinkage keys and process temperature control rings work in much the same way and can be used where space or operating conditions make the use of pyrometric cones impractical.

To maximize the use of pyrometric devices, you should first establish a system for monitoring the heat-work delivered by the firing process, then develop a database to track heat work measurements and display them in a user-friendly format. Finally, you'll need to correlate the heat work measurements to the fired properties of the ware. By using this simple three-step procedure, you can improve your control of the firing process-and your overall product quality.

Figure 2. A digital measuring device connected directly to a computer can simplify the conversion of shrinkage to the measured temperature.

Monitoring Heat Work

First, design a placement diagram for the location of pyrometric devices within the setting of the ware on your kiln(s), and establish a schedule for placing and retrieving the devices (see Figure 1). Next, convert the pyrometric device measurements to temperatures and enter these figures into a database. Using a digital measuring device connected directly to a computer makes the conversion of shrinkage to the measured temperature simple, and the data can be automatically transmitted to a custom database (see Figure 2).

Figure 3. A sample X-bar control chart.

Analyzing the Data

Use a simple statistical software program to display the data in X-bar and R-bar control charts (see Figures 3 and 4). These charts will indicate any trending of the firing process before it becomes a major problem, and they also serve as a benchmark for the firing process. When problems do occur, it is essential to know where the firing has been historically before you eliminate or indict your kiln as the source of product defects.

Figure 4. A sample R-bar control chart.
The information in the control charts can also indicate the normal variation capabilities of your kiln(s). A clear understanding of your firing process capabilities will prevent the tendency to over-control your kiln(s). These charts are also a valuable tool when establishing ISO protocols, since standard pyrometric cones are traceable to National Institute of Standards and Technology (NIST) standards.

Correlating the Data to Fired Properties

Ceramic products have a window of firing range that produces quality ware. Quality and required properties are defined differently for different products, and may include fired absorption, strength, shrinkage, density, color, etc. If you are not sure of the ideal firing range of your products, it is probably worth your time and effort to conduct a series of designed experiments to determine the firing range of the critical parameters.

Once the correlation of product properties to heat work is established, the heat work analysis serves as a sound, nondestructive quality control tool.

Enhancing Quality Control

Gaining an understanding of your product's firing range and monitoring your kiln's efficiency in delivering heat work uniformly to your products can be easily accomplished by using pyrometric devices such as standard pyrometric cones, pyrometric shrinkage keys or process temperature control rings. Plotting the data provided by the pyrometric devices in statistical process control charts makes the information easily accessible and user-friendly. Your quality assurance program will be enhanced through your knowledge of how well the firing process is delivering the proper heat work to your product.

For more information about measuring heat work during firing, contact Orton Ceramic Foundation, 6991 S. Old 3C Hwy., Westerville, OH 43082-9026; (614) 895-2663; fax (614) 895-5610; e-mail info@ortonceramic.com ; or visit http://www.ortonceramic.com .

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