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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.



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