Selecting Silicon Carbide Shelves for use in Electric Kilns

 

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By Marshall Browne and Mike Arbini

There is currently a lot of misinformation circulating throughout the pottery market regarding the use of nitride bonded silicon carbide kiln shelves in top loading electric kilns. The two most widely discussed topics are electrical conductivity and thermal shock.

The pottery market has historically avoided the use of silicon carbide kiln shelves in top loading electric kilns due to the electrical conductivity of silicon carbide. While a portion of this belief is correct, it should be noted that there are many types of silicon carbide kiln shelves. The differences are mainly due to formulation, method of manufacture and bond type. These differences can also have an affect on the degree of electrical conductivity. For example, recrystallized silicon carbide is often used as heating elements (hot rods, glow bars, etc.) in other types of electric kilns. This is obviously a very conductive type of silicon carbide material and would be considered dangerous when used as a shelf material in an electric kiln. The danger could exist when a loose element is in contact with the top kiln shelf; the operator opens the kiln while in operation and touches the kiln shelf resulting in an electrical shock (i.e., a lack of common sense and the alignment of several variables). Traditional nitride bonded kiln shelves (there are many on the market like the type being introduced from China) have more electrical resistance than a recrystallized silicon carbide due to the type of bond, but their electrical conductivity values are still considered to be potentially unsafe.

Advancer® kiln shelves were introduced to the market by Saint-Gobain Ceramics. Advancer is a patented advanced silicon nitride bonded silicon carbide. Before its introduction to the pottery market, Saint-Gobain Ceramics conducted tests comparing the electrical properties of Advancer to other types of silicon carbide kiln shelves. The uniqueness of the Advancer material comes from its formulation, type of bond and especially the glassy oxide layer that is manufactured on the surface of the product. It is this glassy oxide layer that dramatically increases the electrical resistance of Advancer kiln shelves when compared to other types of silicon carbide kiln shelves. As a result, the electrical conductivity value of an Advancer kiln shelf is low enough to be considered safe for use in top loading electric kilns.

Regarding thermal shock, different types of silicon carbide kiln shelves (including their size and geometry) are more or less susceptible to thermal shock. While it is generally true that the higher thermal expansion of silicon carbide kiln shelves make them more susceptible to thermal shock than cordierite kiln shelves, this is only one factor among many complex variables that can contribute to thermal shock. It is thermal gradient (significant temperature differences across the shelf) more often than not that contributes to thermal stress failure of silicon carbide kiln shelves. Traditional nitride bonded silicon carbide kiln shelves are typically thicker than an Advancer kiln shelf and their increased in thermal mass make them more prone to thermal gradient failure. Furthermore, traditional nitride bonded kiln shelves are not as strong as an Advancer® kiln shelf, making them more susceptible to failure at lower thermal stresses. It is also worth noting that silicon carbide kiln shelves can actually be cycled faster than cordierite kiln shelves because of their higher thermal conductivity. This is particularly true of true lo-mass shelves like Advancer.

Generally speaking, the range of temperatures and typical even heating cycle characteristic of top loading electric kiln applications are considered mild for Advancer kiln shelves. The relatively lower operating temperatures (Advancer shelves are rated for continuous use up to 2700 degrees F) in these small kilns along with their small and well insulated firing chambers are considered to be confined and uniform when compared to larger industrial kiln applications where Advancer kiln furniture is more widely used.

It should also be noted that there is no evidence that the use of negative pressure, under-kiln venting systems are detrimental to the use of Advancer shelves in electric kilns. These systems, when properly installed and maintained do not create conditions within the kiln that can lead to thermal gradients as discussed above.

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