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| Mullite kiln shelves credit: IPS Ceramics |
The most popular and easily available ceramic shelves are made
from Mullite, Cordierite, and CoreLite. Other hard specialist kiln shelves are
available. They are made of other materials. Shelves are also made from other
materials such as refractory fibre board, vermiculite, and fire-resistant
ceiling tiles. This concentrates on the care of ceramic shelves.
Composition and Characteristics
This table gives
some information about the characteristics of the materials involved in these
shelves.
|
Name |
Thermal Shock Resistance |
Brittle |
Strength |
Composition |
|
CoreLite |
Low |
Yes |
Moderate |
Ceramic with a high silica content |
|
Cordierite |
High |
Yes |
Strong, but heavy |
Magnesium, iron, aluminium oxide, silica |
|
Mullite |
High |
Yes |
Strong. but heavy |
Silica, Aluminium oxide |
CoreLite is a trade name for an extruded ceramic
shelf. It is strong, but brittle. It is subject to thermal shock below 540ºC/1000ºF.
This suggests the ceramic has a high silica content as the quartz inversion is at
573°C/1063°F,
where the ceramic has a sudden expansion on heating and an equal contraction on
cooling. The cooling rate at this temperature is normally slow enough to avoid breakage.
 |
| credit: Clay Planet |
cordierite - composed of magnesium, iron,
aluminium oxide, and silica. hard, brittle, and with low expansion
characteristics.
 |
| credit: refractorykilnfurniture.com |
Mullite –composed largely of silica and
aluminium oxide. It is strong, brittle, and has good thermal shock resistance.
Care
There is enough information from considering the composition
of these shelves to indicate they are all brittle and have differing vulnerabilities.
These have implications for storage, use and cleaning.
Storage
If storing vertically, take care to avoid setting down on hard
surfaces. If they are in a rack, have a separate slot for each shelf. This
avoids friction between shelves and possible surface scratches. The most useful
material for these racks is wood, or harder materials covered with wood. These
racks can be horizontal or vertical.
If it is not possible to have a separate rack for each shelf,
do not lean them on each other. Shelves leaning against others or against hard
surfaces can become scratched. Provide a cushion against scratches such as
cardboard, or thin plywood.
When moving the shelves, avoid setting them down on their corners,
or bumping the shelf anywhere against hard structures.
Use
Reduce firing speeds to less than 220ºC/430ºF per hour up to
540ºC/1005ºF, especially for CoreLite shelves. Cordierite and Mullite shelves
are not as sensitive, but still can be broken by fast firing rates in this temperature region.
Cover a large portion of the shelf at each firing to avoid
uneven heating of the shelf. It is best to evenly distribute moulds and other
things that shade the heat from the shelf around the shelf to help avoid thermal
shock breaks.
If you cannot or do not want to cover the whole shelf, elevate
the mould(s). This helps to keep the whole shelf at the same temperature when only
small parts of shelf are covered. It does not seem to matter so much when flat
glass is in contact with the shelf. But continue to observe the moderate ramp
rates below 540ºC/1005ºF.
It is even more important to elevate damp or heavy moulds
from the shelf. These kinds of moulds shade the heat from the shelf immediately
below them while the rest of the shelf heats rapidly. This difference in
expansion over parts of the shelf becomes too great for the shelf to resist.
Another thing to avoid is cutting fibre or shelf paper on
top of the shelf. It often creates long shallow scratches in the shelf. These
can be the source of bubbles, but more often, flaws on the back of the fired
pieces.
Cleaning
Care is needed to avoid mechanical damage during cleaning. Scraping
can create scratches in the shelf. These are difficult to remove or fill smoothly.
So, scraping needs to be done carefully.
Any sanding also needs to be done carefully. If you use
power tools, it is very easy to create shallow depressions that will be the
source of bubbles in future firings. It is slightly more time consuming to manually
sand the kiln wash with a sanding screen with or without a holder. But it preserves
the flatness of the surface.
If it is decided to wash the shelf primer off the shelf,
consider how difficult it is to wash a very persistent baked on substance. It
requires thorough scrubbing to remove all the hardened material. Power washers are not advised since the high water pressure can abrade the surface of the shelf. But if you do
decide on washing, you need to air dry for several days afterwards. Then kiln
dry slowly to just below boiling point of water. Soak at that point for several
hours, or until a mirror held above the open port does not fog up.
There is more information on removing kiln wash here and
here.
Summary
Ceramic kiln shelves are hard, but subject to scratches,
impact breaks, excess dampness, failure due to uneven temperatures, and to
rapid rises in temperature below 540ºC/1005ºF.
