Raw and Processed Materials / Refractories

Just Say No (to Clay)!

February 24, 2012
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Alumina kiln furniture can be either pressed or cast into a variety of standard or specially designed shapes.

Clay has been the basis of ceramics for millennia. Archeologists have discovered clay ceramic figurines that are 30,000 years old. The oldest clay pottery discoveries are at least 10,000 years old, more or less in the same timeframe as humanity’s initial transformation from hunter/gatherer to farmer.

Clay, consisting of silica and alumina with many additional inorganic and organic substances (i.e., impurities), originates from fine particles of silt that have settled out of moving water and been deposited in tight packs at the bottom of riverbeds. At some point along the path of human evolution, someone discovered that, when heated in a wood-burning fire, the material transforms into a very useful solid that is highly stable and durable (albeit also porous in the more common forms). From that point on, clay-based ceramics have been an extremely important part of our evolution from cave dweller to an advanced civilization.


Clay Pros and Cons
As practical and versatile as clay is for traditional ceramic applications, in the world of high-temperature thermal processing, the use of ceramics that include even “high-purity” clays is, at best, a double-edged sword. Generally speaking, the elimination of clay from any aspect of the high-temperature firing process can improve production economics for manufacturers of technical ceramics (or other materials that require extreme thermal cycling and/or chemistry issues).

This is particularly true with respect to the kiln furniture used in very high-temperature firings. Residual impurities in the clay not only provide possible contaminants to the ware, but they also result in glassy phases in the ceramic matrix that can dramatically reduce the hot strength of the kiln furniture. Both of these issues are undesirable, so why is clay not something that should be avoided entirely? Because the use of clay provides significant benefits in the forming and sintering aspects of making the kiln furniture, or other complicated shapes.

High-alumina kiln furniture used in the elevated temperature cycles of the technical ceramics world is usually either pressed or cast, typically via thixotropic casting. (By kiln furniture, we are referring to the infrastructure ceramic shapes used to hold the ware through the firing cycle, in the form of plates, saggers, setters, rings and discs, contoured shapes such as groove plates, and many specialty designs tailored to given applications.)

Much of the pressed high-alumina kiln furniture on the market today already has either zero or very little clay. When forming more complex shapes, however, it often is necessary to form the piece by casting to achieve the desired geometry or to at least maintain uniform density throughout the various contours of the shapes.

When casting high-alumina kiln furniture, using some clay in the body (at a level usually less than 10% of the mix) is very advantageous for a variety or reasons. First, clay shrinks when it dries, which makes it much easier to remove parts from the molds with a satisfactory yield. Second, the plasticity and green strength of a clay-containing body is very good. And finally, some of clay’s impurities serve as flux agents during firing, which allows for sintering at lower temperatures.

 

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Saggers produced by thixotropic casting loaded on the kiln cars of a tunnel kiln.

Case in Point
The following describes a real-world situation that demonstrates how the elimination of clay from a cast part provided a significant improvement to firing economics. A technical ceramics manufacturer had a new order for a high-volume part, and they needed to achieve a certain kiln car loading to hit their economic targets on the firing costs. For a variety of reasons, it was established that using saggers was the best kiln furniture shape for the application.

However, the manufacturer was also looking to significantly reduce the standard base thickness of the sagger in order to attain the kiln car loading requirements. In addition, the kiln was fired to well over 1650°C. The sagger shape is too complex to press, but traditional cast saggers with such a thin base would deflect unacceptably. To add to the challenge, maintaining flatness was also a key requirement.

A completely non-clay thixotropic casting formulation in the 90% alumina family has been developed that overcomes the mold release issue intrinsic to non-clay mixes.* While production yields in the forming process are still not quite on par with clay mixes, scrap rates are no longer at a prohibitive level with this new formulation, resolving a key impediment.

In testing of the new formulation, the high temperature hot strength at 1650°C exceeds that of pressed plates of the same cross-sectional thickness. Since the degree of sagging at high temperatures in alumina/mullite bodies is exponential in relation to the part’s cross-sectional thickness, reducing the base thickness of the saggers to the thin profile that was needed to hit this manufacturer’s loading target would usually have resulted in a dramatic increase in sagging. However, with a non-clay formulation, the minimal impurities in the ceramic matrix and the good particle interlocking from the thixotropic casting combined to provide a kiln furniture alternative that achieved the manufacturer’s needs.


The Best Fit
When contemplating any new kiln furniture design in the world of high-temperature thermal processing, many factors—often in competition with one another—need to be considered. It is key that suppliers propose and provide many alternatives, and collaborate with the ceramic manufacturer to find the optimal solution for the given application.

Testing is often very helpful. The upfront time put in to optimizing a high-temperature kiln furniture set-up usually provides a very fast payback in terms of loading efficiency, reduced scrap rate and kiln furniture life. If another tool in the toolbox is a clay-free cast mix to use in more complicated shapes, then that is one more option to help achieve the optimal solution.

 

For more information, contact Sunrock Ceramics Co. at 2625 S. 21st Ave., Broadview, IL 60155; call (708) 344-7600; fax (708) 344-7636; email dthurman@sunrockceramics.com; or visit www.sunrockceramics.com.

*Developed by Sunrock Ceramics.
 

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