How to measure the energy used to fire clay bricks
Practical Action
specific calorific value and a note should be made of the source of the information. From this
data the total energy used is calculated:
Total energy
= mass of fuel x net calorific value
And the firing energy is obtained by subtracting the energy used for drying
Firing energy
= total energy - drying energy
We need to know how well bricks are fired, or rather the heat-work done. Testing brick
properties such as compressive strength will not give useful data to compare kilns because
properties are affected by factors other than firing. The effectiveness of a firing process is a
function of time and temperature, but the relationship is not linear. Bricks fired at 1,000°C
for 10 hours would not be the same as bricks fired at 100°C for 100 hours! We could plot
temperature against time for various points in the kiln and present this graphically to qualify
energy use. However, thermometry to measure such high temperatures is expensive1.
A more affordable alternative is to use Buller's Bars2 placed strategically in the kiln. Buller's
Bars are about 57 mm long and are made from ceramic material which melts at set
temperatures. They come in a range from 590 to 1,525°C. Four bars which melt at different
temperatures can be placed on a refractory stand to form a 'thermoscope'. Thermoscopes are
placed as required in the kiln.
Once the kiln is fired and cooled, a representative sample of bricks - the same number as
before - should be weighed, and the average fired mass calculated. Now we can calculate the
total mass of fired brick and the specific firing energy:
Mass of fired brick =
mass of a fired brick x number of fired bricks
Specific firing energy =
Firing energy
Mass of fired brick
Presenting results
Ultimately, the results of a brick firing process would consist of:
• The specific energy used.
• The moulding process.
• The clay vitrification temperature categorisation.
• The Buller's Bar numbers for different locations, and/or an average number.
• A statement of qualifying information, comments and observations
This brief contains a photocopiable monitoring form for recording data and calculations.
There is also an example of a completed form for guidance, and a checklist to ensure all the
vital data has been recorded.
1At 1998 prices, thermometry to monitor a 20,000 brick kiln would cost approximately
£1,500 (obviously this hardware should be re-usable). Using Buller's Bars would cost about
£70 per kiln.
2 Buller's Bars can be obtained from Taylor Tunnicliff Ltd., Normacot Road, Longton, Stoke-
on-Trent, ST3 1PA, UK; tel. 44 (0)1782 501174; fax. 44 (0)1782 328807.
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