Brick
A brick is a type of block used to build walls, pavements and other elements in masonry construction. Properly, the term "brick" denotes a block composed of dried clay, but is now also used informally to denote other chemically cured construction blocks. Bricks can be joined using mortar, adhesives or by interlocking them. Bricks are produced in numerous classes, types, materials, and sizes which vary with region and time period, and are produced in bulk quantities. "Block" is a similar term referring to a rectangular building unit composed of similar materials, but is usually larger than a brick. Lightweight  bricks (also called lightweight blocks) are made from expanded clay aggregate. Fired bricks are one of the longest-lasting and strongest building materials, sometimes referred to as artificial stone, and have been used since circa 4000 BC. Air-dried bricks, also known as mudbricks, have a history older than fired bricks, and have an additional ingredient of a mechanical binder such as straw. Bricks are laid in "courses" and numerous patterns known as "bonds", collectively known as brickwork, and may be laid in various kinds of mortar to hold the bricks together to make a durable structure. History. Middle East  and South Asia. The earliest bricks were dried mudbricks, meaning that they were formed from clay-bearing earth or mud and dried (usually in the sun) until they were strong enough for use. The oldest discovered bricks, originally made from shaped mud and dating before 7500 BC, were found at Tell Aswad, in the upper Tigris region and in southeast Anatolia close to Diyarbakir. Mud brick construction was used at Catal Hüyük, from c. 7,400 BC. Mud brick structures, dating to c. 7,200 BC have been dated in Jericho, Jordan Valley. The South Asian inhabitants of Mehrgarh also constructed, air -dried mudbrick structures, between 7000 and 3300 BC. and later the ancient Indus Valley cities of Mohenjo-daro, Harappa, and Mehrgarh. Ceramic, or "fired brick" was used as early as 3000 BC in early Indus Valley cities like Kalibangan. China. The earliest fired bricks appeared in Neolithic China around 4400 BC at Chengtoushan, a walled settlement of the Daxi culture. These bricks were made of red clay, fired on all sides to above 600 °C, and used as flooring for houses. By the Qujialing period (3300 BC), fired bricks were being used to pave roads and as building foundations  at Chengtoushan. According to Lukas Nickel, the use of ceramic pieces for protecting and decorating floors and walls dates back at various cultural sites to 3000-2000 BC and perhaps even before, but these elements should be rather qualified as tiles. For the longest time builders relied on wood, mud and rammed earth, while fired brick and mud-brick played no structural role in architecture. Proper brick construction, for erecting walls and vaults, finally emerges in the 3rd century BC, when baked bricks of regular shape began to be employed for vaulting underground tombs. The oldest extant brick building above  ground is possibly Songyue Pagoda, dated to 523 AD. The carpenter's manual "Yingzao Fashi", published in 1103 at the time of the Song dynasty described the brick making process and glazing techniques then in use. Using the 17th-century encyclopaedic text "Tiangong Kaiwu", historian Timothy Brook outlined the brick production process of Ming Dynasty China: Europe. Early civilisations around the Mediterranean adopted the use of fired bricks, including the Ancient Greeks and Romans. The Roman legions operated mobile kilns, and built large brick structures throughout the Roman Empire, stamping the bricks with the seal of the legion. During the  Early Middle Ages the use of bricks in construction became popular in Northern Europe, after being introduced there from Northern-Western Italy. An independent style of brick architecture, known as brick Gothic (similar to Gothic architecture) flourished in places that lacked indigenous sources of rocks. Examples of this architectural style can be found in modern-day Denmark, Germany, Poland, and Kaliningrad (former East Prussia). This style evolved into Brick Renaissance as the stylistic changes associated with the Italian Renaissance spread to northern Europe, leading to the adoption of Renaissance elements into brick building. A clear distinction between the two styles  only developed at the transition to Baroque architecture. In Lübeck, for example, Brick Renaissance is clearly recognisable in buildings equipped with terracotta reliefs by the artist Statius von Düren, who was also active at Schwerin (Schwerin Castle) and Wismar (Fürstenhof). Long-distance bulk transport of bricks and other construction equipment remained prohibitively expensive until the development of modern transportation infrastructure, with the construction of canal, roads, and railways. Industrial era. Production of bricks increased massively with the onset of the Industrial Revolution and the rise in factory building in England. For reasons of speed and economy, bricks were increasingly preferred  as building material to stone, even in areas where the stone was readily available. It was at this time in London that bright red brick was chosen for construction to make the buildings more visible in the heavy fog and to help prevent traffic accidents. The transition from the traditional method of production known as hand-moulding to a mechanised form of mass-production slowly took place during the first half of the nineteenth century. Possibly the first successful brick-making machine was patented by Henry Clayton, employed at the Atlas Works in Middlesex, England, in 1855, and was capable  of producing up to 25,000 bricks daily with minimal supervision. His mechanical apparatus soon achieved widespread attention after it was adopted for use by the South Eastern Railway Company for brick-making at their factory near Folkestone. The Bradley & Craven Ltd 'Stiff-Plastic Brickmaking Machine' was patented in 1853, apparently predating Clayton. Bradley & Craven went on to be a dominant manufacturer of brickmaking machinery. Predating both Clayton and Bradley & Craven Ltd. however was the brick making machine patented by Richard A. Ver Valen of Haverstraw, New York, in 1852. The demand for high office building construction at the turn of  the 20th century led to a much greater use of cast and wrought iron, and later, steel and concrete. The use of brick for skyscraper construction severely limited the size of the building – the Monadnock Building, built in 1896 in Chicago, required exceptionally thick walls to maintain the structural integrity of its 17 storeys. Following pioneering work in the 1950s at the Swiss Federal Institute of Technology and the Building Research Establishment in Watford, UK, the use of improved masonry for the construction of tall structures up to 18 storeys high was made viable. However, the use of brick has  largely remained restricted to small to medium-sized buildings, as steel and concrete remain superior materials for high-rise construction. Bricks are often made of shale because it easily splits into thin layers. Methods of manufacture. Three basic types of brick are un-fired, fired, and chemically set bricks. Each type is manufactured differently. Mudbrick. Unfired bricks, also known as mudbricks, are made from a wet, clay-containing soil mixed with straw or similar binders. They are air-dried until ready for use. Fired brick. Fired bricks are burned in a kiln which makes them durable. Modern, fired, clay bricks  are formed in one of three processes – soft mud, dry press, or extruded. Depending on the country, either the extruded or soft mud method is the most common, since they are the most economical. Normally, bricks contain the following ingredients: Shaping methods. Three main methods are used for shaping the raw materials into bricks to be fired: Kilns. In many modern brickworks, bricks are usually fired in a continuously fired tunnel kiln, in which the bricks are fired as they move slowly through the kiln on conveyors, rails, or kiln cars, which achieves a more consistent brick product. The bricks  often have lime, ash, and organic matter added, which accelerates the burning process. The other major kiln type is the Bull's Trench Kiln (BTK), based on a design developed by British engineer W. Bull in the late 19th century. An oval or circular trench is dug, 6–9 metres wide, 2-2.5 metres deep, and 100–150 metres in circumference. 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. Historically, a stack of unfired bricks covered for protection from the weather was called a "hack". Cooled finished bricks are removed from the other end for transport to their destinations. In the middle, the brick workers create a firing zone by dropping fuel (coal, wood, oil, debris, and so on) 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 the bricks), and finally out the chimney, where the rising gases create suction that pulls air through the system. The reuse of heated air yields savings in fuel cost. As with the rail process, the BTK process is continuous. A half-dozen labourers 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. Influences on colour. The colour of fired clay bricks is influenced by the chemical and mineral content of the raw materials, the firing temperature, and the atmosphere in the kiln. For example, pink 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; as the temperature is increased the colour moves through dark red, purple, and then to  brown or grey at around . The names of bricks may reflect their origin and colour, such as London stock brick and Cambridgeshire White. "Brick tinting" may be performed to change the colour of bricks to blend-in areas of brickwork with the surrounding masonry. 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. Chemically set bricks. Chemically set bricks are not fired but may have the curing process accelerated by the application of heat and pressure in an autoclave. Calcium-silicate bricks. Calcium-silicate bricks are also called sandlime or flintlime bricks, depending on their ingredients. Rather than being made with clay they are made with lime binding the silicate material. The raw materials for calcium-silicate bricks include lime mixed in a proportion of about 1 to 10 with sand, 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 three to fourteen 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 bricklayer. The bricks can be made in a variety of colours; white, black, buff, and grey-blues are common, and pastel shades can be achieved. This type of brick is common in Sweden, Belarus, Russia and other post-Soviet countries, especially in houses built or renovated in the 1970s. A version known as fly  ash bricks, manufactured using fly ash, lime, and gypsum (known as the FaL-G process) are common in South Asia. Calcium-silicate bricks are also manufactured in Canada and the United States, and meet the criteria set forth in ASTM C73 – 10 Standard Specification for Calcium Silicate Brick (Sand-Lime Brick). Concrete bricks. Bricks formed from concrete are usually termed as blocks or concrete masonry unit, and are typically pale grey. 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 bricks and blocks are manufactured in a wide range of shapes, sizes and face treatments – a number of which simulate the appearance of clay bricks. Concrete bricks are available in many colours and as an engineering brick made with sulfate-resisting Portland cement or equivalent. When made with adequate amount of cement they are suitable for harsh environments such as wet conditions and retaining walls. They are made to standards BS 6073, EN 771-3 or ASTM C55. Concrete bricks contract or shrink so they need movement joints every 5  to 6 metres, but are similar to other bricks of similar density in thermal and sound resistance and fire resistance. Compressed earth blocks. Compressed earth blocks are made mostly from slightly moistened local soils compressed with a mechanical hydraulic press or manual lever press. A small amount of a cement binder may be added, resulting in a "stabilised compressed earth block". Types. There are thousands of types of bricks that are named for their use, size, forming method, origin, quality, texture, and/or materials. Categorized by manufacture method: Categorized by use: Specialized use bricks: Bricks named for place of origin: 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 . In most cases, the length of a brick is twice its width plus the width of a mortar joint, about or slightly more. This allows bricks to  be laid "bonded" in a structure which increases stability and strength (for an example, 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. In fact, this wall is built in a variation of "English bond" called "English cross bond" where the successive layers of stretchers are displaced horizontally from each other by half a brick length. In true "English bond", the perpendicular lines of the stretcher courses are  in line with each other. 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 specialised insulation materials. The correct brick for a job can be selected 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 width of the common brick remained fairly constant from 1625 when the size was regulated by statute at 9 x x 3 inches (but see brick tax), but the depth has varied from about or smaller in earlier times to about more recently. In the United Kingdom, the usual size of a modern brick (from 1965) is , which , with a nominal mortar joint, forms a "unit size" of , for a ratio of 6:3:2. In the United States, modern standard bricks are specified for various uses; The most commonly used is the modular brick has the "actual dimensions" of   ×   ×  inches (194 × 92 × 57 mm). With the standard inch mortar joint, this gives the "nominal dimensions" of 8 x 4 x inches which eases the calculation of the number of bricks in a given wall. The 2:1 ratio of modular bricks means that when they turn corners, a 1/2 running bond is formed without needing to  cut the brick down or fill the gap with a cut brick; and the height of modular bricks means that a soldier course matches the height of three modular running courses, or one standard CMU course. Some brickmakers create innovative sizes and shapes for bricks used for plastering (and therefore not visible on the inside of the building) where their inherent mechanical properties are more important than their visual ones. These bricks are usually slightly larger, but not as large as blocks and offer the following advantages: Blocks have a much greater range of sizes. Standard co-ordinating 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 2000 kg/m: this is reduced by frogging, hollow bricks, and so on, but aerated autoclaved concrete, even as a solid brick, can have densities in the range of 450–850 kg/m. 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 "frog" can refer to the indentation or the implement used to make it. Modern brickmakers usually use plastic frogs but in the past they were made  of wood. The compressive strength of bricks produced in the United States ranges from about , varying according to the use to which the brick are to be put. In England clay bricks can have strengths of up to 100 MPa, although a common house brick is likely to show a range of 20–40 MPa. Uses. In the United States, bricks have been used for both buildings and pavements. Examples of brick use in buildings can be seen in colonial era buildings and other notable structures around the country. Bricks have been used in pavements especially during the late 19th century  and early 20th century. The introduction of asphalt and concrete reduced the use of brick pavements, but they are still sometimes installed as a method of traffic calming or as a decorative surface in pedestrian precincts. For example, in the early 1900s, most of the streets in the city of Grand Rapids, Michigan, were paved with bricks. Today, there are only about 20 blocks of brick-paved streets remaining (totalling less than 0.5 percent of all the streets in the city limits). Much like in Grand Rapids, municipalities across the United States began replacing brick streets with inexpensive asphalt concrete  by the mid-20th century. Bricks in the metallurgy and glass industries are often used for lining furnaces, in particular refractory bricks such as silica, magnesia, chamotte and neutral (chromomagnesite) refractory bricks. This type of brick must have good thermal shock resistance, refractoriness under load, high melting point, and satisfactory porosity. There is a large refractory brick industry, especially in the United Kingdom, Japan, the United States, Belgium and the Netherlands. In Northwest Europe, bricks have been used in construction for centuries. Until recently, almost all houses were built almost entirely from bricks. Although many houses are now built using a  mixture of concrete blocks and other materials, many houses are skinned with a layer of bricks on the outside for aesthetic appeal. Engineering bricks are used where strength, low water porosity or acid (flue gas) resistance are needed. In the UK a red brick university is one founded in the late 19th or early 20th century. The term is used to refer to such institutions collectively to distinguish them from the older Oxbridge institutions, and refers to the use of bricks, as opposed to stone, in their buildings. Colombian architect Rogelio Salmona was noted for his extensive use of red  bricks in his buildings and for using natural shapes like spirals, radial geometry and curves in his designs. Most buildings in Colombia are made of brick, given the abundance of clay in equatorial countries like this one. Limitations. Starting in the 20th century, the use of brickwork declined in some areas due to concerns about earthquakes. Earthquakes such as the San Francisco earthquake of 1906 and the 1933 Long Beach earthquake revealed the weaknesses of unreinforced brick masonry in earthquake-prone areas. During seismic events, the mortar cracks and crumbles, so that the bricks are no longer held together. Brick  masonry with steel reinforcement, which helps hold the masonry together during earthquakes, has been used to replace unreinforced bricks in many buildings. Retrofitting older unreinforced masonry structures has been mandated in many jurisdictions.