Refractory"clay" is usually rather aluminous, for instance, in
crucibles. Fireclays run at 25-45% alumina, 70-50% silica; high
alumina fireclays at 50-90% alumina, 10-45% silica - in both cases
the other major oxides should be below about 5%. "Armouring"
furnace walls with quartz has often be used as way of making them
more refractory. However, these silica-rich systems are going to be
neither mineralogically nor granulometrically "clay". Refractory
systems at the silica-rich end would include the use of alluvial silts
(often derived from bolder clays in examples I've examined) where
the silica content can easily exceed 80%. The problem with these
is that they don't harden on firing very well, but become friable
(remember the cracks in the St Fagan's furnace?!). The use of
ordinary clays in the furnace wall, coupled with a more refractory
lining can overcome this (e.g. the bricks at Laxton, covered with a
sandy veneer). 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.
Tim
> Dear all,
> could anyone please tell me: what properties make a clay refractory?
>
> Many thanks, Irene
Dr Tim Young
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