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Cordierite and Mullite

This is part of a series of tips on refractory materials.

Click here to see the introductory page.

Cordierite and mullite are naturally occurring minerals that are commonly used to make kiln shelves and related kiln furniture items.  Of the two, cordierite is the most common; mullite is rarely found in nature and is usually made synthetically and sold as a white powder.

Most often, kiln shelves are made from a mixture of cordierite and mullite powder, which is mixed with clay and other ceramic materials, shaped, then heated to form kiln shelves.  Mullite shelves are sturdy, rigid, and long lasting, but they can break if dropped and may crack or cause air bubbles if heated when not dried thoroughly. 

Really large mullite shelves are generally not manufactured because they're very heavy and they also tend to warp as the size increases. If you break your shelves, don't discard them; instead, use a tile saw to cut them into strips to use as dam material. 

Interesting tidbit:  In its natural state cordierite appears bluish when viewed from one direction and yellowish gray when viewed from the other.  The stone is sometimes called dichroite, which is a Greek word, meaning "two colored rock."  This same Greek work is also the root of "dichroic."

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Copyright 2005 Brad Walker.  All rights reserved.

This list compiled from numerous sources, including the Warm Glass board, and manufacturer's materials.

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