From the November/December 1996 issue of Clay Times
To determine the amount of heat (BTUs) required for your kiln to reach a desired temperature in a certain time frame, you need to know your kiln's volume. To calculate the volume, measure and record its height, width and depth. When dealing with a flat top kiln with no arches or wall curves, simply multiply these measurements together to find out the volume. Kilns with arches or round kilns are a bit more complicated, yet each volume is still the product of a simple mathematical formula:
For example: We have a sprung arch kiln with a width of 30", a depth of 30", and a 25" tall wall with a 5" arch rise. The rise is from the peak to the top of the wall where the arch starts. Multiply the rise (5") by .66 (2/3). This equals 3.3. Add the 3.3 to the 25". This now gives us our revised height measurement (28.3) and the three dimensions we use for multiplication: 30 x 30 x 28.3 = 25,470 cubic inches. For cubic feet, divide cubic inches by 1728. Our cu. in. number 25,470 divided by 1728 = 14.74 cu. ft.
Once you've determined your kiln volume, you can figure out how many BTUs are necessary to fire your kiln to a certain cone or temperature over a specified amount of time. Three variables dictate the input for your kiln: the desired end temperature, the material the kiln is made of, and the time it will take to reach your end temperature.
There are other ways to determine BTU input, such as weighing the entire load, but I feel this is impractical for a studio setting. It is much easier to multiply your interior volume by a certain amount of BTU input-in other words, BTUs per cubic feet. To fire to cone 06, you'll obviously need fewer BTUs per cubic feet (BTU/CF) than for firing to cone 10. Ah...but there's the time factor. For a raku firing to cone 06 in 15 minutes, you'll need perhaps 2-3 times the BTU/CF input of a stoneware kiln. Below is a chart showing BTU/CF necessary to reach temperature in a kiln used for other than raku. Multiply your kiln's cubic feet by these figures to determine the total BTU input your kiln will need. Below is a range of BTU/CF and this range represents time. The higher number will give you a firing of between 6-8 hours, while the lower BTU/CF figure will give you a 14-16 hour firing. Somewhere in between means a 10-12 hour firing.
|materials||cone 06||cone 6||cone 10|
|9" soft brick||6,000-10,000||8,000-13,000||10,000-16,000|
|9" hard brick||12,000-17,000||14,000-18,500||16,000-20,000|
|6" ceramic fiber||4,000- 6,000||6,000- 9,000||7,000-11,000|
|4 1/2" hard brick||70,000|
|2 1/2" soft brick||35,000|
|4 1/2" soft brick||30,000|
|1" ceramic fiber||25,000|
|2" ceramic fiber||20,000|
For example, you have a 30-cubic-foot soft brick kiln that you want to fire to cone 10 in 16 hours. Multiply 30 by 10,000 for your answer of 300,000 BTUs. As mentioned before, because of its rapid firing schedules, a raku kiln needs considerably higher BTU/CF figures than regular studio kilns. The figures below will give you cycle times of 20-30 minutes.
These are simple numbers that assume the kiln is well built and well designed. There are plenty of other variables that can affect time/temperature performance. These numbers are a guide, not a guarantee! Once you've multiplied your cubic feet by the numbers above, you'll have total BTU input. Divide this number by the number of burners you want to use and you'll have the necessary BTU output per burner.
Marc Ward can be reached at Ward Burner Systems,
PO Box 333, Dandridge, TN 37725, (865) 397-2914.
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