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Meissen Chymistry

Roald Hoffmann

The Right Stuff

Porcelain is a ceramic material. Once it was simple to define ceramics as inorganic, refractory, porous, brittle, and insulating. All parts of this definition have frayed at the edges: It’s fun to open a ceramics text, see the authors struggle for a definition at the outset—and then take it all back. There are ceramic superconductors, and brittle is not the word for the stuff of turbine blades. Does one need a definition? Yes: It may be essential for good science, as in the defining moments of thermodynamics. No: It may be merely a refuge for people who want their world clean and neat, this not that. A way the world refuses to be.

Perhaps transformation by heat, if not fire, remains the defining essence of ceramics. The chemical and physical changes in the kiln are certainly complex. Porcelain is a high-temperature–fired ceramic with recognizable, if fuzzily defined, properties of whiteness, hardness and resonance—that ringing tone when struck. Its traditional components varied, as there was not one Chinese porcelain but many: Longquan Celadon, Jingdezhen-ware, the products of the Dehua kilns. But the fine, white clay called kaolin was essential. Other fusible materials were added: the mineral sericite (a type of mica called petuntse) by the Chinese, alabaster by the Böttger workshop. The bulk of kaolin is kaolinite, a layered, hydrated aluminosilicate with the nominal formula Al2O3 ·2SiO2 ·2H2O. Heating expels water, then some silica, which may form its high-temperature form, cristobalite. The remainder of the aluminosilicate exists as mullite, 3Al2O3 ·2SiO2. The special qualities of porcelain derive from the development of fine needle-like crystals of mullite, cemented by glassy silica.

The sequence of transformation on firing porcelain is more complicated than this summary. Yet, as in so many things in this world, complexity (or is it our partial understanding?) is absolutely no barrier to reproducibility, whether the porcelain is a fine Chinese export or one's toilet bowl.

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