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eutectics, phase diagrams and the melt space--ivor's details

updated thu 15 feb 07


Dave Finkelnburg on wed 14 feb 07

Thanks for making the important point below and
adding more, important, detail. You are absolutely
I can also see some readers asking, "So? How do I
use this information?"
It's my opinion, as opposed to a fact! :-) that
the precise location of particular eutectics is not of
much value for making studio glazes. At the same
time, understanding these fine points can be very
powerful. For example, this detail helps explain how
a person can compound two glazes which, according to
glaze calculation, are chemically identical, and then
find that on the same clay body, fired side by side in
the same kiln, the glazes don't look the same. One
may have compounds which melt at a different
temperature and thus follow a different melting path
than the other.
My point is simply that this sort of very technical
discussion has to relate to studio art or else one has
to ask, why discuss it on Clayart? I confess
sometimes I lose sight of that, so I'm really writing
this to myself. With any art, the results are what
counts. All this science is valuable...provided it
helps us get results.
Good potting,
Dave Finkelnburg, who confesses to frequently
getting overly excited about geeky glaze technology...

From: Ivor and Olive Lewis
Taking Dave's example of two eutectic points from the
K2O-Al2O3-SiO2 System. The eutectic point at 710 deg C
must be compounded from Quartz (SiO2), Potassium
Tetra-Silicate (K2O.4SiO2) and Potash felspar
(K2O.Al2O3.SiO2). The other, at 695 deg C is
compounded from Potassium Tetra-Silicate (K2O.4SiO2),
Potassium Di-Silicate (K2O.2SiO2) and Potash Felspar
(K2O.Al2O3.SiO2). The differences are slight in terms
of Oxides, about 1% Al2O3, 8% Na2O and 7% SiO2.

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