HOW CLAY BRICKS ARE MADE
Clay bricks are used in a wide range of buildings from housing to factories, and in the construction of tunnels, waterways, bridges etc. Their properties vary according to the purpose for which they are intended, but clays have provided the basic material of construction for centuries. Brick is the oldest manufactured building material, and much of its history is lost in antiquity. The oldest burnt or fired bricks have been found on the sites of the ancient cities of Babylonia, some of which are estimated to be about 6000 years old. Brick is, after all, virtually indestructible. The industry developed on traditional lines, using hand-making processes for the most part. The first patent for a clay-working machine was granted in the year 1619. Mechanisation, however, did not begin to take the place of manual methods until the middle of the nineteenth century. The moulded products were fired in relatively inefficient intermittent or static kilns until about 1858, when Hoffmann introduced a continuous kiln, which enabled all processes connected with the firing to be carried out concurrently and continuously. Since the introduction of clay working machinery and the Hoffmann Kiln, the Industry has made great progress, particularly since 1930, the output of bricks in Great Britain was doubled between 1930 and 1938.
RAW MATERIALS What is clay?
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.
Clay being extracted from a quarry by a mechanical digger.
TECHNICAL INFORMATION SHEET 16
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....