Kiln Control

Temperature Rise in a Gas Kiln
is controlled by
the amount of gas + the amount of draft
available to combustion.

Draft in a Gas Kiln

Speed of the Draft

  • If the draft is too fast, heat escapes before the ware has time to absorb it.
  • If the draft is too slow, inadequate oxygen for combustion will be available, creating a reduction atmosphere (i.e. reduced oxygen content).

How Draft is Controlled

  • In a kiln fired with gas or wood, the draft is controlled by the damper, usually located on the opposite side of the ware chamber from the burners or fire box. It is usually a refractory shelf that slides over an opening to change the opening size.
  • Some gas kilns also have forced air burners (built in electric blowers ) that control the amount of oxygen available for combustion at the source of the flame.

Monitoring the Firing

By color, pyrometer, pyrometric cones, and oxygen analyzer

  • The color changes from cold darkness to red orange, orange, yellow, to white hot. Experienced firemen in the old days could tell by color what the temperature was. However, we now wear welding goggles to protect our eyes from ultra-violet light, and this makes it difficult to accurately see the color during a firing.
  • The pyrometer is a high-temperature thermometer that measures the heat in the kiln, but not the effect that the heat is having on the clay and glazes (the heatwork).
  • Pyrometric cones (or witness cones) are made of unfired ceramic raw materials that are formulated to melt at a certain temperature, provided that the heat has risen at a specified rate. They are a more accurate indication of the heatwork that has taken place in the ware being fired. Normally 3 consecutive cones are placed in a pack at the correct angle - a lower warning cone, the cone desired, and an “oops, it went too high” cone. When the desired cone has bent halfway over, it is time to turn off and close up the kiln.
  • An oxygen analyzer monitors the atmosphere in the kiln so that optimal oxidation and reduction atmospheres may be created when desired.