The purpose of firebricks is two-fold. Firstly, they act as insulation, holding the heat within the firebox and secondly, by protecting the outer casing of the stove from overheating, which could cause it to crack if it is made of cast iron, or distort if it is made of steel.e can supply firebricks for most models of stoves, but if you have a very old stove, or one that you do not know the make and model, you can use our custom cut firebrick. These are made of a light material called Vicuclad which has very good insulating properties.