iandol on wed 23 may 01
Dear Charley Farrero,
An interesting question with a simple and obvious answer, but one which =
provides some insights into the things we may ignore in a search for a =
quick fix.
As I understand it, calcining Kaolin or any other clay for that matter =
which is to be added to a glaze, engobe or slip is to reduce shrinkage =
by eliminating the hydroxyl groups from the molecular chemistry, the so =
called "chemical water". This occurs in the region of 700 to 800 =
Celsius, So the people who say put it through a bisque firing are on the =
mark.
But there may be other reasons. For example, sometimes Molochite is =
advised for some clay bodies. This is fired to something like 1400 =
Celsius which causes some significant chemical and structural changes to =
the kaolinite which makes it a very stable material for wares fired in =
the cone 10 to 14 region when it confers resistance to pyroplastic =
deformation.
Now in between those limits changes happen, which include the release of =
highly active Silica and the creation of Mullite. There are valid =
reasons for using Kaolin calcined in this range which would confer =
advantages to those who would choose to explore them, for example, the =
rapid solution of new free silica in a melting flux which might assist =
in reducing the maturing temperature of a glaze or Mullite crystals =
conferring hardness to a talc body.
Best regards,
Ivor Lewis. Redhill, South Australia
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