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eutectics - addendum

updated tue 14 jul 98


Stairs interlog on mon 13 jul 98

Hi again,

Just a short expansion on melting points.

When you start out with a granular mixture of components, they are
initially separated one from the other, and therefore each one has its own
melting characteristic according to the properties of that material alone.
However, every grain contacts another grain, and where they touch something
different can happen. Only two different materials can touch as solid
granules at a single point (this is a statement of statistical probability,
not topology). Where they do, atoms of the different grains can diffuse
across the boundary, with the result that at a temperature not far from the
eutectic temperature of those two materials together, they can melt
together, forming a liquid droplet into which more material can dissolve
and mix. By this means, we can predict the melting temperature of some
(most?) powder mixtures: it will be the lowest biphase eutectic
temperature of all the constituents. So, in Ron Roy's mixture of CaO,
Al2O3 and SiO2, it will be the eutectic temperature of the CaO-SiO2 system.
These temperatures are usually available from sources like the ACerS phase
diagram series, but these are resources beyond the reach of most potters.

Note however that if we start not with the pure oxides but with the
practical materials that Ron outlined, e.g., Wolastonite - 50.0, EPK -
33.5, Silica - 16.5, the lowest biphase eutectic will be that of perhaps
Wolastonite - EPK, which is no doubt smaller than that of CaO -SiO2. Pure
materials melt at higher temperatures, generally.