THERE IS NOTHING more basic than making
bricks. Man scoops soil from the earth's crust, mixes with
water moulds the brick and fires it using energy originally
from the sun. Bricks thus made are used to build protection
against the elements.
Clay is a mineral soil fraction composed of silica and aluminium containing silicates and aluminates. Clays that additionally contain calcium carbonate (chalk) are called calcareous, and they burn to form a yellow-coloured brick. Non-calcareous clays contain feldspar and iron oxide. Depending on the level of iron in the clay the fired bricks burn to a brown, pink or classic red-brick colour. These compounds are crucial, because they promote formation of the glassy phase, known as vitrification, in the clay during firing.
Bricks must be heated to temperatures of at least 1,000[degrees] C for the glassy phase to form. Such temperatures enable molten compounds to spread within the inert crystalline phases and to act as bonding agents. They join the crystalline particles together and fusion takes place to make a strong vitreous and ceramic product.
Bricks are typically fired to a temperature between 900 and 1200[degrees]C. At the lower end, bonding will be poor and weak bricks produced. Ideally bricks should be completely vitrified. In practice this rarely occurs outside of ceramics factories. For the majority of small enterprises, vitrification sufficient to bond the inert crystalline materials is achieved. This will produce bricks with enough strength to carry out their function over the time required.
In spite of many advances in technology the basic principles and methods are little changed from 5,000 years ago. Four main steps are involved: extraction of the clay, preparation of the clay, moulding and drying, and firing of the 'green' bricks.
The soil used must be clay-based and non-saline, and of a clay type which ensures the sought after plasticity and tensile strength for the forming stage. At the village level, brick makers can gauge whether soil is suitable for brick making by moulding a trial brick and leaving it in the sun. If it hardens without cracking this is a fair indication of suitability. Dedicated brick factories will require a variety of clay types and mixtures. Technicians will determine and identify exactly what they need by using a standard range of physical and chemical tests on clay samples.
Soil preparation is important too. This means careful sieving to extract all stones and adding sand at a 1:4 ratio, especially if the clay is prone to shrinkage and cracking. Tempering by soaking the soil in water for the appropriate time (depending on soil type) to produce a homogenous mixture is the next step. Tempering is carried out by a variety of methods from the village-level pit in which the mixture is made 'plastic' for moulding using a watering can, and hoes for mixing or even by trampling. Tempering facilities are used in brick making factories. In some areas a similar result is achieved by 'weathering' in which the soil is piled up well in advance (often years) of its requirement.
Next is the brick formation or moulding stage. Sand moulding in which the sand is used as a 'releasing agent' is now overtaking traditional slop moulding methods. Sand moulding allows the use of drier clay, retains brick shape and produces a much higher quality product. Slop moulding produces a very wet mixture.