General information on the history of the brick
Brick
Brick is a ceramic structural material that, in modern times, is made by pressing clay into blocks and firing them to the requisite hardness in a kiln. Bricks in their most primitive form were not fired but were hardened by being dried in the sun. Sun-dried bricks were utilized for many centuries and are used even today in regions with the proper climate. Examples from approximately 5,000 years ago have been discovered in the Tigris-Euphrates basin, and the ancient races occupying this region may have been the first users of brick. In Babylonia, there was a lack of both timber and stone, and the thick clay deposited by the overflowing rivers was the only material adaptable to building. The Persians and the Assyrians used sun-dried blocks of clay for walls of great thickness, facing them with a protective coating of fired bricks. The Egyptians and the Greeks used bricks only to a limited extent, as they had access to plentiful supplies of stone and marble. The Romans manufactured fired bricks in enormous quantities and gave them an important role as a basic structural material in buildings throughout the Roman Empire. Bricks played an important part in early Christian architecture until the decline of the empire. Whereas the Romans had usually concealed their brickwork beneath a decorative facing of stone or marble, the Byzantines devised a technique for exposing the bricks and giving them a full decorative expression. This technique influenced the Romanesque style and brought especially good results in Lombardy and in Germany, where bricks came to be arranged in immensely varied patterns. Since the Middle Ages, brickwork has been in constant use everywhere, adapting itself to every sort of construction and to every change of architectural style. At the beginning of the 19th cent. mechanical brick-making processes began to be patented and by the latter half of the century had almost entirely replaced the ancient hand-fashioning methods. Contemporary American building bricks are rectangular blocks with the standard dimensions of about 5.7 x 9.5 x 20.3 cm. Good bricks are resistant to atmospheric action and high temperatures and are more durable than stone. Where heat resistance is especially important, fire bricks are used; these are made of special refractory clays called fire clays and are fired at very high temperatures.
Brick making
Bricks have been around for a long time – in fact, they were introduced by the Romans into this country, and by the Middle Ages, they began to be used in churches and other important buildings. After a lot of wooden houses were burnt down in the Great Fire of London in 1666, many houses were rebuilt in brick. They became even more important during the Industrial Revolution and it is only in the last 50 years that use has declined because of the increasing importance of concrete and other cement-based materials in building factories, bridges and houses.
Most clays will make reasonable bricks. Once the clay has been dug out, it is ground and mixed with enough water to allow it to be shaped to form “green” bricks. These are then dried slowly and then fired in a kiln at somewhere between 1000 and 1200°C.
At these high temperatures, the clay is “metamorphosed”. All the water is driven off, and new anhydrous minerals are formed, which are stable at high temperatures. Kaolinite breaks down to form aluminosilicates such as the mineral mullite and quartz. This is a kind of dehydration reaction. Once they have been fired, the bricks contain an interlocking network of long, thin mullite crystals, quartz, and some supercooled liquid (glass) which make the brick hard and strong. Where does the colour come from? Most natural clays contain iron minerals, either oxides or hydroxides. Iron can exist as ferrous iron in the reduced state (iron II), in which case it forms dark grey oxides and hydroxides, and this causes some sediments to be dark in colour. After firing, however, this iron may be oxidised to the iron III state, or ferric state, which forms the red-brown iron oxide hematite.
Other minor metal constituents in natural clay include Ca+, Na+ and K+, and these can cause melting by forming silicate liquids which speed up the alteration of the clay minerals. This silicate liquid cools to give a glassy coat to the particles and helps to make the brick hard. The amount of liquid has to be kept low to stop the bricks from going a funny shape or fusing together. On the other hand, if high strength bricks are needed, e.g. for building railway bridges, a relatively high proportion of the brick is allowed to melt under reducing conditions (giving a blue-grey brick), so that the pores are sealed by glass, giving an impermeable, much stronger brick. This type of brick used to be used in the nineteenth century for damp-proof courses in houses. Old bricks fired at low temperatures may remain porous and permeable, and let damp come in through the walls.
Brick production and uses of bricks
The output of bricks has changed enormously between 1948 and today. For example, the total output of bricks in 1990 was about 4 billion, whereas, in 1965, the output was 8 billion. The main use for bricks is house-building, which has dropped dramatically since the 1960s. This is probably because of the substitution of other materials (e.g. cement-based blocks), and because building regulations dictate higher standards of thermal insulation than before, so lightweight blocks are made from a variety of other materials. These have a high porosity, which gives better insulation, and they are also, therefore, lighter, so they can be manufactured into much bigger volumes than a standard brick. Walls made of these bigger blocks are cheaper and quicker to build than ones made with conventional bricks.
Past and future use of bricks
As well as being used for house building, the Victorians used enormous quantities of bricks for other purposes, many of which were engineering bricks, e.g. warehouses, factories, mill chimneys, and railway viaducts. These bricks were often made from Carboniferous clays and shales, which were plentiful around the industrial centres of the British coalfields. One of the reasons for the decline in the number of bricks being used is because of substitution (prior to the 1950s, bricks were still used for load-bearing walls, but high rise flats and offices now tend to use a frame made out of steel or concrete to carry the structural load, and quite often even the walls are made out of other materials, either concrete, glass or other non load-bearing material. Where bricks are used, they usually form a layer only 1 brick thick. Whereas the Victorians used bricks for their railway viaducts, motorways built today use concrete for the motorway itself, the bridges and retaining walls. Airport runways also use vast amounts of concrete.
Another example of substitution in the use of brick-making clays is the manufacture of roof tiles. Nowadays, few houses have roofs that are made of clay tiles, or slate for that matter, because they have been replaced by cement-based tiles. Underground, the Victorian sewers were largely brick-lined, and smaller pipes were often made of earthenware. Large diameter pipes are made of concrete or cast iron, and smaller diameter pipes are made of plastic.
The biggest user of bricks remains in housing, and reserves of brick clay are not a problem in the foreseeable future.
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De Hoop Steenwerwe – The History of Bricks
Bricks dated 10,000 years old were found in the Middle East, and the earliest mention of brick making was found in the Bible, Exodus 1:14; 5:4-19. These records showed the Israelites made bricks for their Egyptian rulers with earth and straw. Examples of the civilizations who used mud brick are the ancient Egyptians and the Indus Valley Civilization, where it was used exclusively. In particular, it is evident from the ruins of Buhen, Mohenjo-daro and Harappa.
The first sun-dried bricks were made in Mesopotamia (what is now Iraq), in the ancient city of Ur in about 4000 BC, although the arch used for drying the bricks was not actually found. In Sumerian times offerings of food and drink were presented to “the Bone god”, who was “represented in the ritual by the first brick”. More recently, mortar for the foundations of the Hagia Sophia in Istanbul was mixed with accompanied by prayers, placed between every 12 bricks.
The Romans made use of fired bricks, and the Roman legions, which operated mobile kilns, introduced bricks to many parts of the empire. Roman bricks are often stamped with the mark of the legion that supervised their production. The use of bricks in southern and western Germany, for example, can be traced back to traditions already described by the Roman architect Vitruvius.
In pre-modern China, brick-making was the job of a lowly and unskilled artisan, but a kiln master was respected as a step above the latter. Early traces of bricks were found in a ruin site in Xi’an in 2009 dated back about 3800 years ago. Before this discovery, it is widely believed that bricks appeared about 3000 years ago in the Western Zhou dynasty since the earliest bricks were found in Western Zhou runes. These bricks are the earliest bricks discovered that were made by a fired process. Early descriptions of the production process and glazing techniques used for bricks can be found in the Song Dynasty carpenter’s manual Yingzao Fashi, published in 1103 by the government official Li Jie, who was put in charge of overseeing public works for the central government’s construction agency. The historian Timothy Brook writes of the production process in Ming Dynasty China (aided with visual illustrations from the Tiangong Kaiwu encyclopedic text published in 1637):
“…the kilnmaster had to make sure that the temperature inside the kiln stayed at a level that caused the clay to shimmer with the colour of molten gold or silver. He also had to know when to quench the kiln with water so as to produce the surface glaze. To anonymous laborers fell the less skilled stages of brick production: mixing clay and water, driving oxen over the mixture to trample it into a thick paste, scooping the paste into standardized wooden frames (to produce a brick roughly 42cm long, 20cm wide, and 10cm thick), smoothing the surfaces with a wire-strung bow, removing them from the frames, printing the fronts and backs with stamps that indicated where the bricks came from and who made them, loading the kilns with fuel (likelier wood than coal), stacking the bricks in the kiln, removing them to cool while the kilns were still hot, and bundling them into pallets for transportation. It was hot, filthy work.
The idea of signing the worker’s name and birth date on the brick and the place where it was made was not new to the Ming era and had little or nothing to do with vanity. As far back as the Qin Dynasty (221 BC–206 BC), the government required blacksmiths and weapon-makers to engrave their names onto weapons in order to trace the weapons back to them, lest their weapons should prove to be of a lower quality than the standard required by the government.
In the 12th century, bricks from Northern-Western Italy were re-introduced to Northern Germany, where an independent tradition evolved. It culminated in the so-called brick Gothic, a reduced style of Gothic architecture that flourished in Northern Europe, especially in the regions around the Baltic Sea which are without natural rock resources. Brick Gothic buildings, which are built almost exclusively of bricks, are to be found in Denmark, Germany, Poland, and Russia.
During the Renaissance and the Baroque, visible brick walls were unpopular and the brickwork was often covered with plaster. It was only during the mid-18th century that visible brick walls regained some degree of popularity, as illustrated by the Dutch Quarter of Potsdam, for example.
The transport in bulk of building materials such as bricks over long distances was rare before the age of canals, railways, roads and heavy goods vehicles. Before this time bricks were generally made close to their point of intended use. It has been estimated that in England in the eighteenth century carrying bricks by horse and cart for ten miles (16 km) over the poor roads then existing could more than double their price.
Bricks were often used, even in areas where stone was available, for reasons of speed and economy.
The buildings of the Industrial Revolution in Britain were largely constructed of brick and timber due to the demand created. During the building boom of the nineteenth century in the eastern seaboard cities of Boston and New York City, for example, locally made bricks were often used in construction in preference to the brownstones of New Jersey and Connecticut for these reasons.
The trend of building upwards for offices that emerged towards the beginning of the 19th century displaced brick in favor of cast and wrought iron and later steel and concrete. Some early ‘skyscrapers’ were made in masonry, and demonstrated the limitations of the material – for example, the Monadnock Building in Chicago (opened in 1896) is masonry and just seventeen stories high; the ground walls are almost 6 feet (1.8 m) thick, clearly building any higher would lead to excessive loss of internal floor space on the lower floors. Brick was revived for high structures in the 1950s following work by the Swiss Federal Institute of Technology and the Building Research Establishment in Watford, UK. This method produced eighteen-story structures with bearing walls no thicker than a single brick (150–225 mm). This potential has not been fully developed because of the ease and speed in building with other materials; in the late-20th century brick was confined to low – or medium-rise structures or as a thin decorative cladding over concrete-and-steel buildings or for internal non-load-bearing walls.
Methods of manufacture
Bricks may be made from clay, shale, soft slate, calcium silicate, concrete, or shaped from quarried stone. Clay is the most common material, with modern clay bricks formed in one of three processes – soft mud, dry press, or extruded. In 2007 a new type of brick was invented, based on fly ash, a by-product of coal power plants.
Mud bricks
The soft mud method is the most common, as it is the most economical. It starts with the raw clay, preferably in a mix with 25-30% sand to reduce shrinkage. The clay is first ground and mixed with water to the desired consistency. The clay is then pressed into steel moulds with a hydraulic press. The shaped clay is then fired (“burned”) at 900-1000 °C to achieve strength.
Rail kilns
In modern brickworks, this is usually done in a continuously fired tunnel kiln, in which the bricks move slowly through the kiln on conveyors, rails, or kiln cars to achieve consistency for all bricks. The bricks often have added lime, ash, and organic matter to speed the burning.
Bull’s Trench Kilns
In India, brick making is typically a manual process. The most common type of brick kiln in use there are Bull’s Trench Kiln (BTK), based on a design developed by British engineer W. Bull in the late nineteenth century. An oval or circular trench, 6-9 meters wide, 2-2.5 meters deep, and 100-150 meters in circumference, is dug. A tall exhaust chimney is constructed in the centre. Half or more of the trench is filled with “green” (unfired) bricks which are stacked in an open lattice pattern to allow airflow. The lattice is capped with a roofing layer of finished brick.
In operation, new green bricks, along with roofing bricks, are stacked at one end of the brick pile; cooled finished bricks are removed from the other end for transport. In the middle the brick workers create a firing zone by dropping fuel (coal, wood, oil, debris, etc) through access holes in the roof above the trench.
The advantage of the BTK design is a much greater energy efficiency compared with clamp or scove kilns. Sheet metal or boards are used to route the airflow through the brick lattice so that fresh air flows first through the recently burned bricks, heating the air, then through the active burning zone. The air continues through the green brick zone (pre-heating and drying them), and finally out the chimney where the rising gases create suction which pulls air through the system. The reuse of heated air yields savings in fuel cost.
As with the rail process above, the BTK process is continuous. A half dozen laborers working around the clock can fire approximately 15,000-25,000 bricks a day. Unlike the rail process, in the BTK process the bricks do not move. Instead, the locations at which the bricks are loaded, fired, and unloaded gradually rotate through the trench.
Dry pressed bricks
The dry press method is similar to mud brick but starts with a much thicker clay mix, so it forms more accurate, sharper-edged bricks. The greater force in pressing and the longer burn make this method more expensive.
Extruded bricks
With extruded bricks the clay is mixed with 10-15% water (stiff extrusion) or 20-25% water (soft extrusion). This is forced through a die to create a long cable of material of the proper width and depth. This is then cut into bricks of the desired length by a wall of wires. Most structural bricks are made by this method, as hard dense bricks result, and holes or other perforations can be produced by the die. The introduction of holes reduces the needed volume of clay through the whole process, with the consequent reduction in cost.
The bricks are lighter and easier to handle, and have thermal properties different from solid bricks.
The cut bricks are hardened by drying for between 20 and 40 hours at 50-150 °C before being fired.
The heat for drying is often waste heat from the kiln.
Calcium silicate bricks
The raw materials for calcium silicate bricks include lime mixed with quartz, crushed flint or crushed siliceous rock together with mineral colourants. The materials are mixed and left until the lime is completely hydrated, the mixture is then pressed into moulds and cured in an autoclave for two or three hours to speed the chemical hardening. The finished bricks are very accurate and uniform, although the sharp arrises need careful handling to avoid damage to brick (and brick-layer). The bricks can be made in a variety of colours, white is common but pastel shades can be achieved. It is very common in Sweden, especially in houses built or renovated in the ‘70s. Here it is referred to as “Mexitegel” (en: Mexi[can] Bricks). In India these are known as Fly ash bricks, manufactured using the Falg process.
Influence on fired colour
The fired colour of clay bricks is influenced by the chemical and mineral content of raw materials, the firing temperature and the atmosphere in the kiln. For example pink coloured bricks are the result of a high iron content, white or yellow bricks have a higher lime content. Most bricks burn to various red hues, if the temperature is increased the colour moves through dark red, purple and then to brown or grey at around 1,300 °C (2,372 °F). Calcium silicate bricks have a wider range of shades and colours, depending on the colourants used. The names of bricks may reflect their origin and colour, such as London Red and Cambridgeshire White.
Bricks formed from concrete are usually termed blocks, and are typically pale grey in colour. They are made from a dry, small aggregate concrete which is formed in steel moulds by vibration and compaction in either an “egglayer” or static machine. The finished blocks are cured rather than fired using low-pressure steam. Concrete blocks are manufactured in a much wider range of shapes and sizes than clay bricks and are also available with a wider range of face treatments – a number of which are to simulate the appearance of clay bricks.
An impervious and ornamental surface may be laid on brick either by salt glazing, in which salt is added during the burning process, or by the use of a “slip,” which is a glaze material into which the bricks are dipped. Subsequent reheating in the kiln fuses the slip into a glazed surface integral with the brick base.
Natural stone bricks are of limited modern utility, due to their enormous comparative mass, the consequent foundation needs, and the time-consuming and skilled labour needed in their construction and laying. They are very durable and considered more handsome than clay bricks by some. Only a few stones are suitable for bricks. Common materials are granite, limestone and sandstone. Other stones may be used (e.g. marble, slate, quartzite, etc.) but these tend to be limited to a particular locality.
Optimal dimensions, characteristics, and strength
For efficient handling and laying bricks must be small enough and light enough to be picked up by the bricklayer using one hand (leaving the other hand free for the trowel). Bricks are usually laid flat and as a result the effective limit on the width of a brick is set by the distance which can conveniently be spanned between the thumb and fingers of one hand, normally about four inches (about 100 mm). In most cases, the length of a brick is about twice its width, about eight inches (about 200 mm) or slightly more. This allows bricks to be laid bonded in a structure to increase its stability and strength (for an example of this, see the illustration of bricks laid in English bond, at the head of this article). The wall is built using alternating courses of stretchers, bricks laid longways and headers, bricks laid crossways. The headers tie the wall together over its width.
A bigger brick makes for a thicker (and thus more insulating) wall. Historically, this meant that bigger bricks were necessary in colder climates (see for instance the slightly larger size of the Russian brick in table below), while a smaller brick was adequate, and more economical, in warmer regions. A notable illustration of this correlation is the Green Gate in Gdansk; built in 1571 of imported Dutch brick, too small for the colder climate of Gdansk, it was notorious for being a chilly and drafty residence. Nowadays this is no longer an issue, as modern walls typically incorporate specialized insulation materials.
The correct brick for a job can be picked from a choice of colour, surface texture, density, weight, absorption and pore structure, thermal characteristics, thermal and moisture movement, and fire resistance.
In England, the length and the width of the common brick has remained fairly constant over the centuries, but the depth has varied from about two inches (about 51 mm) or smaller in earlier times to about two and a half inches (about 64 mm) more recently. In the United States, modern bricks are usually about 8 × 4 × 2.25 inches (203 × 102 × 57 mm). In the United Kingdom, the usual (“work”) size of a modern brick is 215 × 102.5 × 65 mm (about 8.5 × 4 × 2.5 inches), which, with a nominal 10 mm mortar joint, forms a “coordinating” or fitted size of 225 × 112.5 × 75 mm, for a ratio of 6:3:2.
Some brickmakers create innovative sizes and shapes for bricks used for plastering (and therefore not visible) where their inherent mechanical properties are more important than the visual ones. These bricks are usually slightly larger, but not as large as blocks and offer the following advantages:
• a slightly larger brick requires less mortar and handling (fewer bricks) which reduces cost
• ribbed exterior aids plastering
• more complex interior cavities allow improved insulation, while maintaining strength.
Blocks have a much greater range of sizes. Standard coordinating sizes in length and height (in mm) include 400×200, 450×150, 450×200, 450×225, 450×300, 600×150, 600×200, and 600×225; depths (work size, mm) include 60, 75, 90, 100, 115, 140, 150, 190, 200, 225, and 250. They are usable across this range as they are lighter than clay bricks. The density of solid clay bricks is around 2,000 kg/m³: this is reduced by frogging, hollow bricks, etc.; but aerated autoclaved concrete, even as a solid brick, can have densities in the range of 450–850 kg/m³.
Bricks may also be classified as solid (less than 25% perforations by volume, although the brick may be “frogged,” having indentations on one of the longer faces), perforated (containing a pattern of small holes through the brick removing no more than 25% of the volume), cellular (containing a pattern of holes removing more than 20% of the volume, but closed on one face), or hollow (containing a pattern of large holes removing more than 25% of the brick’s volume). Blocks may be solid, cellular or hollow
The term “melfrog” for the indentation on one bed of the brick is a word that often excites curiosity as to its origin. The most likely explanation is that brickmakers also call the block that is placed in the mould to form the indentation a frog. Modern brickmakers usually use plastic frogs but in the past they were made of wood. When these are wet and have clay on them they resemble the amphibious kind of frog and this is where they got their name. Over time this term also came to refer to the indentation left by them.[Matthews 2006]