The vast majority of bricks used in construction are made from the naturally occurring raw material of clay. A small proportion are made from calcium silicate (in other words, sand/lime) or concrete, but this chapter will focus on those manufactured from clay.
In the manufacture of bricks, selected clays are prepared, moulded into shape and burnt under a variety of manufacturing processes, each of which will produce a brick that possesses certain qualities or properties. Various colours of brick can be made by using iron oxides, iron sulphides, and other additives. The mineral content of the clay raw material will also have an effect on the physical properties, colour and hardness of the finished brick.
All clays used for brick manufacture have one common characteristic: they must be capable of being finely ground by machine and then mixed with water, so that they may be moulded, or ‘pugged’.
After excavation from clay pits, the clay must first be ‘weathered’ to wash out all of the impurities and soluble salts, which could later lead to efflorescence on the face of the finished brickwork. The most basic method is to leave piles of clay outdoors and open to the elements during the winter so that the rain simply washes out the soluble salts. Other clays are passed through a wash-mill first before being stored in large open storage areas.
The washed clay then undergoes a number of grinding processes (usually three) until the raw material is reduced to a particle size of 1-2mm, at which point water is added in order to enable the clay to be moulded into brick shapes by one of the following three basic methods.