Janet H Walker on mon 26 apr 99
I've been reading a fascinating introductory text recently:
Paul Rado. An Introduction to the Technology of Pottery, 2nd ed.
Pergamon Press, 1988.
There are many eye-opening descriptions of industry practice in
there! I thought that the people wondering how many tiles they
could stack in a firing might be interested in the following nugget:
(p. 118)
"It was once thought that slow heating and slow cooling
rates were essential to prevent cracking of ware. However, it was
established that the cracking which did occur was not due to RAPID
heating and cooling but to UNEVEN heating and cooling which caused
stresses across the piece. Bone china cups could be biscuit fired
in a hover kiln in the incredibly short time of 7 minutes without
showing any sign of cracking.
Likewise, plates could be fired singly in a matter of
minutes; however, stacks of the same article required hours or even
days. The reason why this should be so has been elegantly
postulated by Holmes (1969): heat flow (conduction of heat) during
firing is the crucial factor; this can be calculated from the
thermal diffusivity (as opposed to thermal conductivity which would
only operate when steady temperatures have been reached).
Diffusivity determines the rate of temperature rise in the centre of
an article which is being heated at the surface.
The time required for centres to reach 99.9 percent of the
surface temperature is proportional to the square of the thickness
of the piece. Thus the time required for a slab 2 cm thick is 9.6
minutes, whereas for a slab 20 cm thick it is 96 minutes (1.6
hours); for a plate 6 mm thick it is only 52 seconds. It was shown
that the time required for the temperature to be equalized was over
300 times longer for a stack of six tiles than for a single tile.
heat in 1 hour and can be economically run for shifts as short as 8
hours.>
Transactions Journal of the British Ceramic Society, or something
like that.>
Enjoy.
Jan Walker
Cambridge MA USA
Arturo M Devitalis on wed 28 apr 99
FYI - John Glick said he had a 24 hour firing cycle, cone 11....his
layered glazes do thier best that way.
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