The heat-resistant tunnel kiln cars form a long queue ‘waiting’ to enter the kiln. Before they actually do so, however, they must first pass through the preheater, which heats the bricks to approx. 180°C. This preheating causes the bricks to expel their last bit of residual moisture (approx. 2 %). From the pre-heater tunnel, the fully laden tunnel kiln cars proceed through the 235-metre-long tunnel kiln very gradually, at a rate of ¼ section per thirty minutes.
The firing process
The temperature gradually rises from the mouth of the tunnel, until it reaches maximum level in the middle. Each brick colour calls for its own particular firing temperature range. For example, yellow bricks are heated to 1080°C, red ones to 1060°C, and manganese bricks to 1050°C. This implies that only one colour of brick can be fired per cycle. An even temperature is maintained across the entire breadth of the kiln, with a view to avoiding colour differences in the bricks. Once the maximum firing temperature has been reached, then rapid cooling follows. Ventilation swiftly reduces the temperature to approx. 620° C. The hot air extracted at this point it conveyed back to the dryers. After rapid cooling, the bricks proceed along a 25-metre stretch of tunnel where no further processing occurs; the bricks have now reached a stage where they have to cool gradually. Were they to cool down too quickly, then the bricks might crack or break. After a few days, the car emerges from the tunnel kiln. A complete cycle comprises 40 cars, which amounts to 1 million bricks.