Brick manufacture

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The argillaceous materials used to manufacture bricks must be easily workable and not contain large hard components or lumps of lime. The latter can cause splitting of the brick when it is exposed to damp. The clay can contain lime, but it has to be evenly distributed. It is an advantage if the clay is well mixed with sand. Clay with too little sand is not easy to shape, but has the advantage of not shrinking so much when drying or being fired. Sand can be added to clays that are too 'fatty'. The quality of a clay mix can be found through doing some simple tests; it must easilyform intoa ball, and keepanyfine line impressions made by a hand. During drying it must become hard, and without too many fine cracks.

One thousand square metres of clay can produce about 650 000 bricks per metre of depth. The clay does not usually lie too deep in the ground, so it is relatively easy to extract. This is usually done by first scraping away the soil, then extracting the clay and, after re-planning the area, replacing the topsoil.

After the clay has been extracted it is covered with water. At one time, the resulting mixture would have been worked by hand with a special hoe or by ramming. This operation is now carried out by a machine which grinds the clay down to a fine consistency. Additives to reduce its fattiness can be put in the clay and the mixture is then well kneaded. If the clay is stored for between one and three months it becomes more workable and produces a better quality final result.

Sand can be used to make the clay leaner, but slag, fly ash and pulverized glass are also suitable. These not only reduce the amount of shrinkage but make the clay easier to form. The porosity of brick can be increased by adding materials which burn away when the stone is fired, leading to higher insulating values and often bette moisture regulation properties. Materials that can be used for this are sawdust, dried peat, chopped straw, pulverized coal or polystyrene pellets. Porosity can also be increased by adding 1520% of mineral materials such as ground lime, dolomite or marble; however, these produce carbon dioxide when fired, but at the same time bind the released sulphur and fluorine into harmless compounds such as gypsum.

Insulating materials such as fossil meal can also be added in parts of up to 90%. Fossil meal is a form of earth that consists of air-filled fossils from silica algae. The resulting block has good insulating properties and high porosity. Around Limfjorden in Denmark there is a naturally occurring clay containing nearly 85% fossil meal. It is called molere, and a complete brick industry is based around it. The resources, however, are very limited.

Forming

Clay needs a water content of approximately 25% in order to be formed. Forming is carried out mechanically by forcing the clay through a die or just by knocking the clay by hand into a mould. Mechanical hand presses are also used.

The industrial die presses the clay through a mouthpiece into a long 'sausage' with a cross-sectional area allowing for shrinkage (see Figure 8.6). Different sizes of mouthpiece and square or round pegs form holes in the clay sausage. Roof tiles can also be produced in this way. The sausage is cut into blocks on a bench. Mobile dies also have equipment to prepare the clay before pressing, and are used where there are smaller deposits of clay.

Handmade bricks are made by filling wooden or metal moulds in the same way as earth blocks, and striking with a piece of wood. The moulds are sprinkled with sand or dipped in oil or water between strikings. A 'brickstriker' and two assistants can produce 2000 ordinary bricks, 1200 flat roof tiles, or 600 profiled tiles in a day. Even if machinecut bricks are considerably more economical, the handmade brick with its rustic character is more attractive as a facing brick. As recently as 1973 it was estimated that 99% of all bricks produced in India were handmade (Spence, 1974).

Drying

The unfired brick products are stacked for drying under an open roof for one to two months. If manufacturing is done in winter, bricks may need to be stacked inside heated rooms. This increases the energy consumption a good deal. In modern brick factories special drying houses are kept very hot for two to five days.

The industrial die with a mouthpiece.

The industrial die with a mouthpiece.

Colour of bricks vary with composition of clay and firing temperature.

Colour of bricks vary with composition of clay and firing temperature.

Firing

When clay is heated up to boiling point the water in the pores evaporates, and at 200-300 °C the hydrate water evaporates. After this change, the clay will not revert to soft clay with the addition of

Small-scale open charcoal kiln In India. Source: Pratheeps.

Small-scale open charcoal kiln In India. Source: Pratheeps.

water, unlike an air-dried earth block. In the Roman Empire, bricks were not fired in temperatures higher than 350-450 °C; as can be seen in the case of a great many buildings that still stand today, such as the Roman Forum.

If fired at higher temperatures, the particles are pressed closer to each other and the brick becomes harder. Between 920 and 1070 °C the material begins to sinter. If the temperature is increased even further, the blocks will melt. However, higher temperatures are used in the production of fireproof bricks and porcelain, using special clay mixtures. To a well-trained ear, the temperature at which a brick was fired can be assessed by hitting it with a hammer; the higher and purer the sound, the higher the temperature of the firing. This is especially useful when recycling old bricks.

Clay containing iron turns red when fired, whereas clay containing more than 18% lime turns yellow. There are many different colour variations, also determined by the amount of oxygen used during the firing process. Red brick can vary from light red to dark brown.

Chamotte is produced from clay with a low iron and lime content. It can withstand temperatures of up to 1900 °C.

In certain products the brick can be glazed or coloured by the manufacturer using compounds such as oxides of lead, copper, manganese, cadmium, antimony and chromium. To set the glaze onto the brick requires a secondary firing until the glaze melts. The temperature of this firing should be well under the brick's firing temperature so that it does not lose its form.

Kilns

Many different types of kiln have been used over the years; there are three main types: the open charcoal kiln, the Hoffman kiln and the tunnel kiln. It is interesting to note that development of the brick kiln and the baking-oven has run parallel to each other.

The open charcoal kiln is the earliest type, used in smaller brickworks as late as the early twentieth century. It consists of two permanent, parallel kiln walls in brick. At the bottom of the walls or between them at the ends there are a series of openings for feeding in the fuel. Clay blocks to be fired are stacked up according to a very exact system. The top layer is a solid layer of ready-fired bricks with some openings for the smoke. They are then covered with earth. The firing takes about two days of intensive burning. The bricks are left in the kiln to cool slowly over a period of several days before the earth and the bricks are removed. A brick factory should therefore have two or three kilns to ensure continuous production. Firing in an open charcoal kiln is not very economical with regard to energy consumption. If production is local, the compensation for this is that transport energy is drastically reduced.

A small, unusual and totally new version of the open charcoal kiln has recently been developed in the Middle East (see Figure 8.9). The 'kiln' is in fact a whole house, which is fired. The clay blocks are stacked up into walls and vaults in their air-dried state. A thick layer of earth is placed over the whole building and a huge bonfire is then lit inside. A door or hole in the roof is required so that the fire can be loaded with wood. After a couple of days, firing is complete. The building then needs

another couple of days to cool down. The earth is removed, the windows are knocked out and any cracks in the walls are filled.

The Hoffman kiln, unlike the charcoal kiln that has to be cooled after each firing, can be kept in continuous use. The firing zone can be simply moved from chamber to chamber. Each chamber is fired for a set period before the heat moves to the next chamber. A complete rotation takes about three weeks. The bricks are fired with sawdust or fine coal-dust sprinkled down through small openings in the roof of the chambers. In modern brickworks where these circular kilns are still used, it is more usual to use oil as a fuel.

The tunnel kiln came into use after the Second World War (see Figure 8.10). This kiln can be up to 120 m long and is divided into a preheating zone, a firing zone and a cooling zone. The unburned clay bricks are placed on a truck that moves slowly through the kiln. The energy source can be coal, gas, oil or electricity. Dust exposure is much lower in the tunnel kiln as the operator does not need to enter the kiln. The roller kiln is a modification of the tunnel kiln where the bricks are transported on refractory rollers. In the roller kiln, clay with lower humidity levels can be used and the heating time is reduced. Today roller kilns are primarily used for sanitary stoneware products and ceramic tiles, but are also becoming common for brick production.

There is a big difference in the energy consumption of different kilns. The open charcoal kiln uses about twice as much energy as the Hoffman kiln, whilst the Hoffman kiln uses slightly more energy than large tunnel kilns. In roller kilns energy use is reduced by a further 30% (Gielen, 1997). Energy consumption during firing in the Hoffman kiln and the tunnel kiln varies a great deal depending upon the product being

Tunnel kiln. Source: RHI.

8.10

Tunnel kiln. Source: RHI.

fired, and falls considerably with lower firing temperatures, to about 60% for medium-fired products.

Sorting

There is an uneven distribution of heat in an open charcoal kiln. The bricks at the outside are usually less well fired than those in the middle. There is some wastage in the Hoffman kiln, but much less than that occurring in the open kiln. Tunnel kilns give the most even heat distribution and wastage is minimal, even if the outermost bricks have a tendency to sinter.

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