Kerstin Eckstein wrote
>Finally, refractories do not shrink or bloat on heating, as this leads to
cracking and crumbling,<
For many very satisfactory clays this is not true in their "raw" state. They
are mixed with a large proportion of high melting point grog, sometimes the
same clay that has already been fired at high temperature, to avoid
shrinkage and cracking. Very often the final form has to be carefully fired
again to become stable before use.
It is also not true of silica and semi-silica brick which were the high
temperature refractories of the industrial revolution. The silica has a
transformation with a large volume change while heating or cooling. On
cooling, particularly after a long period at high temperature, the
contraction causes spalling of the working surface. It can destroy the
furnace structure completely on cooling as usually happened with
regenerative coke ovens. Every effort was made to keep the refractory above
the transformation temperature for the whole of the life of the refractory.
This involved someone working though weekends, Christmas and other public
holidays.
Peter Hutchison.
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