Color of the Flame

Until I’m able to build a shed around my gas-fired reduction kiln, I must wait for evening to arrive, when the breeze dies down and I’m able to see the flame from the venturi burners which power my kiln. It is a very beautiful thing, this gas and air mixture that combine to produce flame, this fascinating and dancing phenomenon of light and heat which cannot be seen to its fullest extent during the day.
I start two burners at a time, kitty-corner from each other, on very low, and I adjust the gas and air mixture every hour. Slowly, as the kiln and pots are warmed, the air, which is pulled in through the primary air ports of the burners, is increased to meet the demands of the increasing volume of gas, producing a luscious and fat blue flame, tinged with yellow. During the early hours of the firing I give the flame as much air as it needs, but later, about six hours into the firing, there comes a moment when the gas and air mixture must be controlled if the beautiful effects of reduction firing are to be realized. Long ago in Mesopotamia, about 4500 BC, potters learned how to control the atmosphere of a kiln, producing a red veneer on their wares by giving the flame full oxigen, or a black verneer on their pots by starving the flame slightly for air. Although our technology is vastly improved, we are using the very same basic technique 6500 years later. There is a very distinct color change to the flame when more gas than air is present, and a recognizable smell tells the potter that a reducing atmosphere is being created within the kiln. By giving the flame more gas than air, carbon monoxide is produced, which steals oxigen from the glaze and clay and replaces it with carbon, producing the wonderful and characteristic celadons, copper reds, subtle whites and stunning tenmoku glaze colors that are unique to high-fire reduction pottery. More gas than air makes for yet another and extremely beautiful color change for the flame, but it is also at this point that much heat is also needed to allow the kiln to rise in temperature, a point when the roar of the flame must be heard. It is a challenging time, these moments late at night when my ability and communication with the flame either make or break months of work.

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