Tim Young said:
"Many of our UK clays are not very refractory, either
through a high CaO content (e.g. many of the Jurassic clays), or
high K2O, or both."
On the other hand, a lot of modern refractories are very rich in CaO. High
melting points can be achieved, as Tim states, in the SiO2-rich corner of
systems, but equally on the other end with CaO or MgO.
The reason for these seemingly inconsistent statements such as: CaO lowers
the melting point, or: it increases it, are the principles in complex
systems. Most systems have ranges of composition where the melting points
are quite low. For instance SiO2 melts at ca. 1600, CaO at ca. 2500. Between
37 and 57% CaO the melting points are under 1450 degree. Thus CaO can indeed
lower the melting point although in a lot of cases it is doing exactly the
opposite. The behavoir of a single compound is always dependant on the
overall composition of the system. This is very unfortunate, as it becomes
impossible to give a short and simple answer to a short and simple question,
and we constantly have to meditate about all these dubious phase diagrams
which none of us completely understands :-).
There are a few elements or compounds which are known to lower the melting
points in most systems, which are mainly the alkaline group metals,
hydroxide, and iron, and some other elements or compounds which tend to
increase the melting points in most systems such as the oxides of magnesium,
calcium or aluminium.
Furthermore, every further compound in a system, even in traces, generally
lowers the melting point. Refractories should thus have simple compositions
rich in either CaO/MgO, or SiO2, or Al2O3, little alkaline group metals,
hydroxide and iron. Dave Killick has already said that.
Finally, refractories do not shrink or bloat on heating, as this leads to
cracking and crumbling, and they do not react with the materials possibly
coming in touch with them during use, as SiO2 is doing with lead oxide,
forming a substance with a low melting point.
Regards, Kerstin
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Kerstin Eckstein
TU Bergakademie Freiberg
Professur Archaeometallurgie
Gustav-Zeuner-Str. 5
D-09596 Freiberg
Tel: +49-3731-39-3156
Fax: +49-3731-39-2489
http://www.am.tu-freiberg.de
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