In brick-making terms, clay covers a range of naturally occurring raw materials which are used to make a product. The clays vary considerably in physical properties, colour, hardness etc, and mineralogical content. They do, however, have certain properties in common. They have the ability to be crushed and mixed with water to form a plastic material which can be moulded into various shapes. This can then be fired to a high temperature during which process it attains a hard, weather resistant characteristic. The key, in geological terms, is the mineral content of the raw material. This is common to all clay types. Pure clay mineral is formed from the erosion and weathering of primary igneous rocks. The clay mineral is transported away by the action of water, wind, ice etc., and re-deposited elsewhere. In the process it picks up a number of impurities, Quartz, mica, Calcium Carbonate (lime), Iron Oxide etc, etc. The subsequent deposit becomes a sedimentary rock. Due to variances in the age of the deposit, the conditions of its deposition and the impurities involved there will be variations between different clay types and even on occasions within the same deposit. These variations may affect the brick making process and the properties of the finished product.
The choice of method of clay winning will depend on the depth, thickness, hardness and physical geology of the clay beds. The usual method for winning clay (extracting from the quarry) is once or twice a year by heavy plant machinery, whether it be excavators, back actors etc, to stockpile large amounts. The advantages of bulk winning are that it can take place during good weather, a large reserve close to the factory means that breakdown of quarry plant is not critical to the production schedule. The layering of the stockpile from large reserves helps to eliminate localised variations in the clay strata. Laboratory testing of the clays from different parts of the quarry determine the likely characteristics of the layers and clay is mixed according to the required properties of the finished item. Particular attention is given to environmental factors both during the clay win and when restoring the landscape after excavations are complete.
Clay preparation methods may have to accommodate the physical characteristics of the raw material and special provision may have to be made to deal with certain impurities. Preparation consists of transforming the clay rock into plastic mouldable material by a process of grinding and mixing with water. A typical factory might have a Primary crusher, these are used to break down large lumps of rock to manageable size, which can then be fed to a Secondary crusher, for example Pan mill, where the clay is reduced in size further. Water can be added here or if it is a dry pan the clay is reduced to dust and water added later. Further crushing takes place through conveyor rollers reducing the clay particles to about 1-2mm.
Before the bricks can be fired, as much moisture as possible must be removed or they will explode in the kilns. Drying involves the removal of water from the wet brick in such a way as to dry them out evenly from inside out. If the outer skin of the brick dries first it becomes impossible for the moisture inside to escape. In the kiln the extreme temperatures will force out this moisture and some cracking may occur. To prevent this happening the dryers are kept at temperatures of about 80 –120 degrees centigrade and the atmosphere is very humid keeping the exterior of the brick as moist as possible. This is monitored very closely to reduce surface cracking. The bricks will shrink in the dryers as the clay particles come together and they become strong enough to be stacked, but at this stage they have no weather resistant qualities. Drying schedules vary but between 18 to 40 hours is typical for an automated plant. Special shapes and large units can take up to a week or more. The dry bricks are then set onto kiln cars ready to be fired.
Firing temperatures vary considerably between different clay types and are often quite critical. During firing, bricks undergo a physical change. Clay particles and impurities are fused together to produce a hard durable and weather resistant product. This is called vitrification. This is usually accompanied by further shrinkage and a colour change. Temperatures vary greatly depending on clay type but are generally in the range of 900 – 1200 degrees centigrade.