Richard Burkett on thu 8 may 97
Veronica Shelford asks:
>I would be
>curious to understand the ways in which glaze and clay differ, which would
>make it possible to work out usable calculations on the one and not the
>other. Does anyone have anything to offer on this subject, or is there
>somewhere I can look it up (comprehensible to someone without a degree in
>the subject)?
Basically, the reason one can't easily calculate thermal expansion for
clay bodies is a problem that exists in some glazes, too. Anytime
crystalline materials exist in the glassy matrix (as in most matte glazes
and all clay bodies - which are mostly crystalline with some glass),
theoretical calculations of thermal expansion based on the usual thermal
expansion coefficients for glass are often far from the actual measured
thermal expansion.
Glass and glossy glazes are non-crystalline - more of a supercooled liquid
in nature than the highly ordered molecular structure of a crystalline
solid.
Crystalline materials may have non-linear thermal expansion (the big jumps
in expansion for crystoballite, for instance) and often have different
thermal expansions in each of the different crystal axes. All this makes
it very difficult (or impossible in a studio situation) to predict the
actual thermal expansion from a simple molecular analysis. If we had
intricate crystallographic data on the clay body it might be possible, but
it's usually easier to determine the thermal expansion of clay bodies
experimentally.
A machine called a dilatometer can be used to measure thermal expansion.
Some studio potters use a carefully formulated set of glazes to test clay
bodies and determine the likely glaze fit. Jim Robinson had a good article
on this in Studio Potter magazine a few years back.
Glass and gloss glazes have little or no crystalline component and behave
somewhat more predictably. The theoretical calculation of thermal
expansion for these glazes is more useful, but still theoretical.
Richard
Richard Burkett - School of Art, SDSU, San Diego, CA 92182-4805
E-mail: richard.burkett@sdsu.edu <-> Voice mail: (619) 594-6201
Home Page: http://rohan.sdsu.edu/dept/rburkett/www/burkett.html
CeramicsWeb: http://apple.sdsu.edu/ceramicsweb/index.html
HyperGlaze@aol.com & http://members.aol.com/hyperglaze/
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