Clay brick manufacture Part two
Secondary clay materials are compounds of alumina, silica with minor amounts of lime, magnesia, soda or potash. Iron compounds, usually the oxides, hydroxides or carbonates, are nearly always present as impurities in brick clays, and they account for most of the wide range of colours found in the finished product. Clays containing up to 3% of iron oxide give white to cream or buff colours, which change to pinks and reds as the iron oxide content rises to between 8 and 10%. By adding manganese dioxide in proportions from 1 to 4%, a range of grey and brown colours can be produced. More important than their chemical composition are the facts that:
- when mixed with water, the clay minerals give a plastic mass that can be shaped by pressure to form a brick;
- at economically practical temperatures ranging between 1 000˚ to 1 200˚C, the clay particles can be fused into a cohesive mass of great compressive strength;
- controlled evaporation of the free water surrounding the particles in plastic clay minimises excessive shrinkage and defects in the structure of the brick.
Modern brick manufacture involves high speed processing at extrusion rates of up to 25 000 bricks per hour. Solid bricks of the size traditional in South Africa (222 x 106 x 73 mm) weigh 3 kg to 3,5 kg. Therefore, 1 000 finished bricks weigh approximately 3,5 tonnes. In the wet state before firing, the clay is heavier. For every 1 000 bricks at least 4 tonnes of material must be dried, fired to a temperature of 1 000˚ to 1 200˚C (depending on the clay used) and cooled down.