Kilns and Kiln Firing
Technical Data

he following article by Mark Brabham on kilns and kiln firing was initially published on Claynet. Part 1 covers the following areas: Check List, Natural Gas vs L.P. Gas, Safe Kilns and Firings, Safety Devices for Kilns and Maintaining Burner Safety Controls. Part 2 covers Temperature Measurement and Control, Color Scale Chart and Industrial Firing Processes.

Check List

Please note: The following information relates to atmospheric gas burners only, i.e. burners that rely on atmospheric air, as distinct from air from an air fan, for combustion.

The chimney passage must not be restricted and the sizing should be checked carefully first. A rough rule-of-thumb ratio is 1 square inch of flue area to 8,000 BTU's of maximum gas input. Tight packing of the ware without a definite passage for the hot gasses can cause a problem. Increased firing times and reduction may indicate a problem here. Ensure the burners are mounted outside the burner port openings and the correct amount of secondary air (the air pulled in around the burner tip) is available. This can vary between burner types. Venturi type burners require less wasteful, secondary air other burners may need larger ports to enable complete combustion.

Make sure the gas supply components such as the feed line, gas regulator or gas bottles are of a sufficient size. Freezing of any of these indicates a problem. It is wasteful to undersize components and have large pressure drops. It is also important to make sure the burners are assembled correctly with adequate mixing length between the air inspirating section and the burner flame retaining tip.

It is important to keep the right ratio between the amount of gas fed to the burners and total air required for combustion (secondary and primary air) as close as practicable. This is approx. 24 parts air to 1 part gas for L.P. Gas and 10 parts air to 1 part gas for natural gas and will make sure the kiln fires to temperature as quickly as possible using minimum gas. This is only possible during those periods when reduction or excess air firing is not necessary (some excess air can be helpful in the earlier stages of a firing). Altering the chimney damper changes the pressure condition in the kiln, thereby controlling the amount of secondary and primary air. Primary air can also be controlled separately on venturi burners by altering the "wheel" or shutter on the inspirator casting.

The most common problem is the kiln not reaching temperature. Although there are many possible reasons, the first thing to check is the gas supply pressure. It is a benefit with L.P. Gas kilns to have a gauge on the burner as well as the bottle/tank as this acts as a check to ensure dirt particles have not been trapped in the line. The pressure on the cylinder is compared to the pressure at the burner, a large pressure drop indicates a restriction. Ensure the gas bottle outlet valve is absolutely clean before refitting the connection after filling.

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