Kilns
are fired with anything that will give heat. Fuels from animal
dung to solar energy have been used successfully but the most
usual are wood, coal, oil, gas and electricity. The last is
certainly the easiest, although more expensive. Kilns probably
started as just fires lit under and around a heap of dry pots.
Holes in the ground were used to achieve higher temperatures. The
simple up-draught kiln, where the heat simply traveled up through
the ware and out through the top of the kiln, served the Western
potter for centuries, but in the East the down-draught kiln, which
allowed heat to build up as the flames were drawn down through the
work and out through the chimney at the back of the kiln, meant
that much higher temperatures could be reached. The technology of
today allows the use of alumina and silica spun into blankets or
compressed into boards to make kiln walls.
Oxidation and
Reduction
The
potter has the choice of two atmospheres, oxidation and reduction,
inside the kiln. The two kiln atmospheres have a crucial bearing
on the final appearance of the work. Oxidation means firing with
adequate air to allow good clean combustion. In reduction, the
kiln is starved of air by adjusting the air intakes and flues. If
the combustion is to continue, it must take oxygen from somewhere,
and so if it cannot draw it in from outside the kiln, it will take
the necessary oxygen from the oxygen in the clays and gases. This
results in iron-bearing stoneware clay bodies turning warmer and
speckled in a typically reduction-fired look. Electric kilns do
not take kindly to a reducing atmosphere, while wood and coal are
not easy to handle, especially at the high temperatures. It seems
sensible to settle for the very extensive range of effects that an
oxidizing atmosphere can give from an electric kiln, or use gas or
oil to achieve reduction.
Recording
Temperature
Electric
devices, such as pyrometers can be bought for measuring the
temperature inside the kiln. You can use specially made pyrometric
cones of compressed materials similar to those used in glaze
making to record the temperature in the kiln. They are made to
melt when they reach predictable temperatures. You can buy a whole
range of cones to suit any temperature requirement. Usually three
or four cones in a row are used, one designed to melt before the
temperature required, to give warning, one to melt at a hotter
temperature so we know we haven't over fired. As the temperature
may vary from one part of the kiln to another we often use a
couple of rows of cones set in different parts of the kiln in
front of the spy holes in the kiln door. This way we have warning
of under-or over-heating.
Lastly,
when the kiln has fired and starts to cool down, do not be in too
much of a hurry to open up and see results. In stoneware and
porcelain it helps to let the temperature drop very rapidly just
after the firing has ended by opening all the spy holes or
fire-box doors. When the kiln is really getting cool, that is the
time to avoid cold air getting in to crack the pots inside.
Complicated changes are taking place in the clay body from
approximately 600 ºC downwards, and the kiln should be left
undisturbed until below 200 ºC, when it can be opened up, a
little at a time.
Return to Home
Page
Return to
More About Clay
