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Kilns: Theory and Practice
by Frank Colson
In order to achieve the desired firing
result, a potter must have some understanding of the basic
types of fuel kilns. With gas kilns, the effect desired is
usually accomplished by means of a so-called "reduction
firing" rather than oxidizing a kiln during the firing cycle
- the norm with electric kilns. The term "reduction firing"
is commonly used in contemporary pottery shop talk, and very
simply implies that the oxygen in the atmosphere of a kiln
is reduced, thus starving the flame of its natural fuel.
Since there is little oxygen within the kiln on which the
flame can feed, it goes after particles of minerals and
chemicals within the clay and glazes of the pottery. The
result is that the iron impurities in the clay body come to
the surface and "bleed" on the face of the glaze. Iron
glazes turn to green (celadon), while copper-based glazes
turn red (reduction red, or "oxblood") ; the general effect
of the pottery is much different than that obtained with
oxidation firing in electric kilns. Some exceptions
are when organic materials are placed into an electric kiln
to burn depriving the space of oxygen to create a reduction
atomosphere. This, of course, is very hard on the
heating elements!
Since reduction firing requires what most
industrial gas servicemen refer to as a "dirty" flame, it is
easy to understand why this firing method is more of an art
than a set of procedures. Each kiln has its own distinctive
firing characteristics. Furthermore, each potter must
discover his/her particular kiln's traits and learn to work
with them in order to have successful firings. Success
cannot be achieved with one firing alone; ten firings of the
same kiln may only begin to bring the consistent control
that the potter desires.
Most kilns come under two general categories
- updraft and downdraft - and each type has its own set of
advantages and disadvantages for reduction firing. Almost
all fuel-fired kilns built today are designed so as to allow
ample flame circulation within and around the stacking
chamber. But this was not the case some years ago when
"flashing", which occurs due to a flame lick, was not
considered an aesthetic attribute on a pot. Because of this,
kilns were built with complete baffle chambers around the
entire stacking area so as to prevent flames from even
remotely approaching the ware. In some cases, kiln
designs provided for each burner flame to pass through a
ceramic tube in the kiln, from bottom to top. These kilns
were referred to as "muffle kilns", since they muffled the
flame and used the heat that radiated through the ceramic
tubes to reach the needed temperature. The atmosphere of
these kilns was clear and clean, and oxidation firing the
only condition. However, with fuel kilns which are not
muffled, freer flame circulation within the stack area
allows more direct heat penetration from the flame, and
consequently less fuel is required, resulting in savings to
the potter. Direct flame exposure also changes the effect of
the glazes, creating warm, pleasant differences rarely
available with a muffle kiln.
In the last decade, wood Has become more and
more a kiln heating fuel. Not so much for the purpose
of reaching glaze temperatures, but to induce mood ash into
the kiln chamber which creates effects onto pots which would
otherwise be unobtainable.
Other kinds of heat producing materials;
coal, fuel oil, deep fry oils, ect. can and have been used
fro kiln firing. their availability is some what
restrictive or require custom built rigging or
fireboxes. (The kiln heat generating area).
With today's environmental thinking to
eliminate polluting conditions, clean fuels such as methane
and grain alcohol are certainly options to consider
seriously for kiln firing fuels.
NEXT :
Updraft Kilns
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