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The gradient furnace is a special horizontal tube furnace with a known, reproducible, linear temperature gradient along a monitored zone. Figure 1 is a schematic of the furnace showing an extruded clay bar lying on the hearth, with monitoring thermocouples overhead. The hottest temperature is at one end of the 12-in.-long zone, the overhead monitoring thermocouples are placed at 2 in. intervals, and the temperature drop of approximately 10°C per inch is surprisingly linear.
The thermocouples are monitored and recorded manually or via a PC so the operator knows the temperature or amount of heatwork at every point along the 12-in. zone. The operator measures or analyzes each sample and plots the ceramic property as a function of its position inside the furnace.
Efficient TestingFor R&D applications, the gradient furnace quickly "shotguns" the effects of varying batch constituents on the desired fired properties of a body over a wide temperature range. New body formulations or substitute raw materials analysis can be performed for a wide range of temperatures so that the user can quickly identify the specific critical temperature range.
For QC applications, the furnace can fire samples of the standard mix against a mix made from a batch with another lot of the critical component, or even with another vendor's product, so the QC department can see the effect before the revised batch is released to production.
Testing CapabilitiesFigure 2 is a photograph of dry pressed discs in a special D-tube hearth that were fired at the same time in a gradient furnace. The color variation from hotter to cooler is clearly visible. Four standard measurements were made on each disc: percent linear change, percent cold water absorption, specific gravity and bulk density. The results from the measurements were plotted vs. temperature in Figure 3.
The boat of raw material in Figure 4 shows not only the color change as a function of temperature, but also an apparent shrinkage and particle size reduction of the raw material. If desired, the user can take samples at 1-in. increments along the length of the boat and perform a surface area analysis at approximately 5°C increments to determine where and how these properties change over the course of the firing.
Figure 5 is a photograph of a glass bar that was soaked for 24 hours in the gradient furnace. The glass bar was observed under a 10x microscope to determine the temperature at which vertical plane crystals occur. Due to the known thermal gradient that formed the bar, the liquidus temperature can be identified within ±1°C per ASTM test protocol.
Saving Time-and MoneyThe most compelling benefit of a gradient furnace is that it enables users to make batch or firing mistakes before moving the product to the production kiln. The total cost of the gradient furnace can be less than a single lot of lost product that might result from surprise changes in raw materials, and far less costly than the daily rate for a production kiln shutdown.
For additional details regarding gradient furnaces, contact the Edward Orton Jr. Ceramic Foundation, 6991 Old 3C Highway, Westerville, OH 43082-9026; (614) 818-1331; fax (614) 895-5610; e-mail firstname.lastname@example.org; or visit www.ortonceramic.com.
Editor's note: This article is based on a paper presented at the Ceramic Manufacturers Association (CerMA) conference held May 2007 in Pittsburgh, Pa. Visit www.cerma.org for the downloadable PowerPoint presentation.