Pottery in Australia Vol 17 No 1 Autumn 1978

CERAMIC FIBRE KILNS ­
THE DAWN OF A NEW AGE
Joe Davis
At the outset it must be stated that the ceramic fibre kiln was not initiated by
people who were, first and foremost, ceramists.
Our many years of association with all aspects of ceramic and refractories
production had highlighted the technological shortcomings of established clay
firing procedures.
On the large-scale commercial level, and particularly in the heavy clay side
of ceramics production-that is the production of building bricks, tiles, sewer
pipes, etc., some attempt has been made to bring modern technology into play.
But the typical small traditional kiln used by the individual potter is, from the
technological viewpoint, at best, an anachronism left over from the Middle Ages.
To illustrate this, let us look really critically at the manner in which the
heating energy put into that unit is actually dissipated.
For tbis exercise we can take a fairly typical LPG-fired down-draft kiln of
about 10 cu ft setting with a 9 in overall wall thickness, using fire-brick on the
inside and insulating brick outside th is.
It is not unusual for a kiln of this capacity to use up to 150 Ibs of gas for a
stoneware firing. A well-packed glost kiln will on average have about 10 Ibs of
pottery per cubic foot of space, so that the kiln we are discussing will hold about
100 Ibs of ware.
The only reason we are putting heat into the kiln is to mature the clay and
glazes on this 100 Ibs of pottery. Consequently, it is true to say that in fact the
only really useful heat is that which is being absorbed by the pottery itself. When
we calculate this theoretical amount of heat, we find an almost unbelievable
situation.
The amount of heat required to raise the temperature of J 00 Jbs of pottery
to 1280°C is no more than that released by burning approximately 21bs of gas!
Before commenting further on this, it is necessary to investigate where the
heat from the other 148 Ibs of gas goes to. The kiln structure itself, consisting of
some 500 bricks, will weigh about 3000 Ibs. Only the inside face of the kiln
reaches top temperature, but it is quite straightforward to calculate an average
temperature for the whole structure. These calculations show that something over
30 Ibs of gas will be used to heat up the structure.
On basic design, this type of high temperature equipment will never have
an efficiency over 25-30% , that is, 25-30% of the total beat input is the
maximum that can be used for actual heating. This efficiency is an inherent
thermodynamic characteristic of all kilns. Because of it, the 32-35 Ibs of gas will
be but 25-30% of the total at best.
This accounts fo r the remaining 100 Ibs or so being used in the kiln.
So, summing up, we have a piece of equipment which uses 150lbs of gas each
time it is fired, yet we are obtaining a heating benefit from only 32-35 Ibs of gas,
and it is a cold hard fact that we are really only interested in the effects of a
mere 2 Ibs of this fuel!
If this proposition is now transferred to a solid or liquid fuel-fired kiln, we
find that the efficiency is reduced to about half that shown above, and we finish up
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