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The Firing Process

When clay bricks are heated to a high temperature, a chemical reaction occurs in the clay which makes the brick permanently hard, durable and resistant to weather and water.

When a brick is heated to a temperature between 20°C and 150°C, it loses most of the water added to the clay during the preparation phase.

When heated from 150°C to 600°C, the clay brick loses its remaining water. When firing a kiln, you will see a white vapour or steam coming from the top of the kiln during these first two phases. If you were to stop the firing process at 600°C, the bricks in the kiln would be useless for building purposes since they would not be stabilized and would easily be worn down by wind and rain.

When the temperature starts to rise over 600°C, chemical changes begin to occur in the clay which give the brick colour, hardness and durability. Temperatures of 900°C and above cause vitrification to occur. This means that a small quantity of glass-like material forms which helps glue all of elements in the clay together. It is after this point of vitrification that the brick will be at its hardest and most resistant and will be ideal for construction.

The final quality of the brick will depend on the amount of vitrification which occurs. It therefore is essential that the temperature of the kiln rises to well above 900°C in order to obtain a well-fired brick.