Ivor and Olive Lewis on sat 31 jan 04
Dear Friends,
This seems to be an issue where we are discussing a phenomenon without
considering the nature of the system in which it is happening.
Unless the actual composition of the clays in question are known any
opinions are just that.
So we are no nearer the cause and hence, a solution to the problem.
However, there is one point which concerns me, related to the nature
of some plastic porcelain bodies.
If the Kaolin used in the mixture is on the less than plastic end of
the scale, workability of the clay body is improved by adding some
Bentonite. As you are all aware, the relationship between Water and
the Montmorillonite group of minerals, of which Bentonite is a member,
differs markedly from the relationship between Water and members of
the Kaolinite group. The most prominent feature of Bentonite being
that free water collects between the T-O-T molecular sheets which
allows solutes to remain suspended in solution. These remain in situ
and probably concentrate within Bentonite particles when the material
dries. Though it may be a very minor fraction there could be
sufficient Sulphate, Chloride and Carbonate residues from chemicals
added to assist flocculation to cause outgassing as temperature rises
close to vitrification.
Just a thought.
Best regards,
Ivor Lewis. Redhill, South Australia.
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