Cordierite/Mullite vs. pizza stones or tiles

Description of the materials

Cordierite refractory shelves are generally combined with mullite to achieve low expansion rates.  These are most often manufactured as solid slabs, although there is an extruded version with hollow channels along the length, given the trade name corelite.

Cordierite is magnesium, iron and aluminium in a cyclosilicate form (or rings of tetrahedra).  It is named after its discoverer, Louis Cordier, who identified it in 1813.

cordierite/mullite shelves

Mullite is combined with cordierite in small amounts to increase strength and reduce the amount of expansion. It does this through the formation of needle shapes that interlock and resist thermal shock. It also provides mechanical strength.

Mullite was first described in 1924 and named for an occurrence on the Isle of Mull, Scotland, although it occurs elsewhere, usually in conjunction with volcanic deposits.   

Pizza Stones and Tiles

Pizza stones are a variant of baking stones where the food is placed on (sometimes heated) stones.  Baking stones are a variation on hot stone cooking, one of the oldest cooking techniques. The stones are normally unglazed tiles of varying thicknesses.  What is said of pizza stones also applies to tiles.

Characteristics

Pizza stones  

Ceramic tiles and pizza stones are essentially the same things.  Some tiles may be thinner, especially if they are not large. In both cases, the ceramic is a poor heat conductor and the thermal mass means care needs to be taken in rapid heating and cooling of tiles and of baking stones. These are dry pressed which give a coarser surface texture than cast shelves.  All these ceramics are generally fired at about 1100C, so they can withstand kiln forming temperatures.  They are adequate as small shelves, but will deform over larger areas over time.

Cordierite-Mullite kiln shelves and furniture.

This formulation of materials has an extremely low coefficient of thermal expansion that explains the outstanding thermal shock resistance of these kiln furniture materials. They are also strong although heavy. Cordierite/mullite shelves are sintered, to allow the mullite needles to form, and fired at 1400C+, higher than tiles.

This material can be cast, dry pressed or extruded.  Cast shelves are the cheapest of the methods and provides a smooth surface.  These are used for kilnforming glass, and low temperature ceramic firing. 

Dry pressed shelves have a higher temperature resistance than cast. For this reason, these are often marketed as ceramic shelves, even though the cast shelves are fine for smaller areas.  These are more expensive than the cast shelves.

Corelite, a brand name for extruded shelves with hollow channels, is often used where larger shelves are required, as the weight is less than the solid cordierite. Extruded shelves are ground smooth after forming.

pizza stones

Preparation

Pizza Stones and Tiles

Due to the thermal mass of pizza stones and the material's property as a poor heat conductor, care must be taken when firing.  Firing quickly can break the stone or tile.  The stone or tile should be fired slowly to just under the boiling point and soaked for a couple of hours to eliminate any dampness in the material.  This probably should be done each time kiln wash is applied.  Because it is porous, a baking stone or tile will absorb any liquid applied, including detergent. They should be cleaned with a dry brush and then plain water if further cleaning is necessary.

Pizza stones and tiles should be checked for having straight and level surfaces. It is not a priority for these to have flat surfaces as for glass and ceramics shelves.  If by placing a straight edge on the surface you can see slivers of light, the shelf needs to be smoothed.  You can do this by grinding two of the proposed shelves together with a bit of coarse grit between.  This best done wet to avoid the dust getting into the air.

Cordierite

Cordierite/mullite shelves do not need this level of preparation, unless they have been stored outside.  It is possible to kiln wash and air dry for a few hours before placing glass on the shelf and firing.  This difference is the low rate of expansion (CoLE 19, if you are interested).

corelite shelves

Corelite

The extruded corelite shelves are made with cordierite/mullite, but are more delicate due to the hollow channels along their length.  They should be fired slowly to just under the boiling point of water - to eliminate the moisture - then continue the slow rise to at least 260C/500F to avoid the crystobalite inversion.  It should be fired to 540C with a pause before going to the top temperature.  The shelf should be supported at 30cm intervals under the shelf to minimise breakage.  The whole surface of the shelf should be filled rather than having just one heavy piece; again this is to minimise breakage.

Revised 23.2.25

Care of Ceramic Kiln Shelves

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.