iandol on thu 6 apr 00
------------------
Elise,Yes=21
It=92s not a question of thinking about the answer but of applying known =
ceramic
craft technology and science. A straw poll will not change the answer.
There is a lot of air voiced about the effects of rapid cooling of pottery
through various Quartz inversions which cause problems of crazing, shivering=
and
so on to our pottery. But little is ever said about cracking kilns early and
what the results of doing this are on the refractories of our kilns when =
they
crash cool.
Now I don=92t mind being told I am wrong about this one but here goes.
All refractories we seem to use are based on alumino silicate substances. =
The
higher the Pyrometric Cone Equivalent of the refractory, it=92s PCE, the =
higher
the percentage of Alumina and the lower its percentage of Silica. So RI 23 =
have
a PCE of cone 23 and high silica, RI 26 a PCE of cone 26 and less silica. I =
have
some hard bricks with a PCE of 37. If free silica starts to form, this will =
be
subject to thermal stress due to quartz inversions. I have seen one kiln =
where
this had happened. Half inch slabs spalled from bricks in the arch over a =
period
of time. I have no doubt, though I have no proof, that this also happens to
fibre because it embrittles over time.
So if kilns are crash cooled just because the pyrometer registers an air
temperature of less than two hundred Celsius and you think your pots are =
cold
enough to remove and you crack the door, remember that the refractories may =
be
well above that temperature in their interiors. . I almost forgot about the
effects of rapid contraction at the surface of the refractories which also
causes spalling. The shock of crash cooling may lead to disaster sometime in=
the
future.
All kilns are expensive to purchase, replace, maintain or rebrick. It pays =
to
treat them with respect. Curb your excitement. Let your kiln get dead cold
before you crack the seal. And have a bit of respect for your pots. Though a=
pot
may be below the danger point on the surface, it may be well above that
inversion point in the thickness of the clay. If this inversion happens =
because
of air quenching, the stress may exert itself much later when you are =
brewing
tea.
Ivor Lewis.
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