mel jacobson on wed 28 feb 07
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 kiln shelf material in a
top loading 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(r) kiln shelves were introduced to the market by Saint-Gobain
Ceramics. Advancer(r) 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(r)
to other types of silicon carbide kiln shelves. The uniqueness of the
Advancer(r) 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(r) kiln shelves when compared to other types of
silicon carbide kiln shelves. As a result, the electrical conductivity value
of an Advancer(r) 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(r) kiln shelf and their increased thermal mass make
them more prone to thermal gradient failure. Furthermore, traditional
nitride bonded kiln shelves are not as strong as an Advancer(r) 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 LO-MASS(r) kiln shelves like
Advancer(r).
Generally speaking, the range of temperatures and typical even heating
cycles characteristic of top loading electric kiln applications are
considered mild for Advancer(r) kiln shelves. The relatively lower operating
temperatures in these small kilns (Advancer(r) kiln shelves are rated for
continuous use up to 2650oF), 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(r) 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 is detrimental to the use of
Advancer(r) 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.
Marshall Browne
Smith-Sharpe Fire Brick Supply
marshall@ssfbs.com
612-331-1345
from: mel/minnetonka.mn.usa
website: http://www.visi.com/~melpots/
Clayart page link: http://www.visi.com/~melpots/clayart.html
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