MARGINALIA
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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