Powerful Furnace for the Lab

The gradient furnace is a simple device that can thermally process a series of samples to demonstrate the effect of temperature on a number of ceramic properties.

Photo courtesy of Synsil Products Inc.

Figure 1. Schematic of a gradient furnace showing an extruded clay bar lying on the hearth with monitoring thermocouples overhead.

The most powerful QC and R&D tool in any ceramic lab is not a complex or sophisticated thermoanalytical instrument. Instead, it is the gradient furnace, a simple device that, in a single step, thermally processes a series of samples that can be examined or analyzed to demonstrate the effect of temperature on shrinkage, density, porosity, color, surface area, strength and other ceramic properties.

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 Testing

For 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.

Figure 2. Dry pressed discs in a D-tube hearth that were fired at the same time in a gradient furnace.

Testing Capabilities

Figure 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.

Figure 3. Results of percent linear change, percent cold water absorption, specific gravity and bulk density measurements.

Note the break in the data curves. After reviewing the results from the data curves on the left (the first firing), a second firing was performed on another set of green discs at a higher temperature. The discs were measured and plotted as before, and the second set of curves completed the study of the firing range. The furnace was able to generate a large amount of information in a very short time.

Figure 4. Different types of samples, including (from left) an extruded clay bar, a boat of raw material and a glazed substrate, can be tested in a gradient furnace.

The gradient furnace is not limited to samples of dry pressed discs. Figure 4 shows various types of samples: an extruded clay bar, a boat of raw material and a glazed substrate. In addition, actual measurements are not required to generate important data. For example, with the extruded clay bar, it is easy to see the effect of temperature on color, surface texture, shrinkage and over-firing (severe bloating). It is also simple to determine if the clay, or clay body, has a wide or narrow firing range from just one firing.

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. A glass bar that was soaked for 24 hours in the gradient furnace.

The glazed substrate shows the firing range and glass formation differences between a stripe of the standard glaze composition next to a stripe of a modified glaze batch with a substitute raw material. The user can visually examine the stripes of glazes to observe the differences in glass phase development, the firing range, or the presence or absence of glaze defects (bubbles, cracking, crazing, shivering, etc.). The user could also elevate the substrate at an slight angle, place drops of water at the elevated high-temperature end, and observe where the water drops cease to flow. This is an indication of a glass phase development boundary and the corresponding temperature.

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 Money

The 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 slevin@ortonceramic.com; 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.


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