A kiln (/kɪln/ or /kɪl/, originally pronounced "kill", with the "n" silent) is a thermally insulated chamber, a type of oven, that produces temperatures sufficient to complete some process, such as hardening, drying, or chemical changes. Kilns have been used for millennia to turn objects made from clay into pottery, tiles and bricks. Various industries use rotary kilns for pyroprocessing—to calcinate ores, to calcinate limestone to lime for cement, and to transform many other materials.
The word kiln descends from the Old English cylene (/ˈkylene/), which was adapted from the Latin culīna "kitchen, cooking-stove, burning-place". During the Middle English Period, the "n" was not pronounced, as evidenced by kiln having frequently been spelled without the "n". Another word, miln, a place where wheat is ground, also had a silent "n". Whereas the spelling of "miln" was changed to "mill" to match its pronunciation, "kiln" maintained its spelling, which most likely led to a common mispronunciation, which has now become commonly used. However, there are small bastions where the original pronunciation has endured. Kiln, Mississippi, a small town known for its wood drying kilns that once served the timber industry, is still referred to as "the Kill" by locals.
Unwittingly adding the "n" sound at the end of "kiln" is due to people seeing the written word before ever hearing the actual pronunciation. Linguists call this phenomenon spelling pronunciation, where an incorrect (or obsolete) pronunciation is read aloud, becomes widespread, eventually reported by dictionaries, and the "original pronunciation, passed from parent to child, mouth to ear, for many generations is lost."
Phonetically, the "ln" in "kiln" is categorized as a digraph: a combination of two letters that make only one sound, such as the "mn" in "hymn". From English Words as Spoken and Written for Upper Grades by James A. Bowen 1900: "The digraph ln, n silent, occurs in kiln. A fall down the kiln can kill you." Bowen was pointing out the humorous fact that "kill" and "kiln" are homophones.
Pit fired pottery was produced for thousands of years before the earliest known kiln, which dates to around 6000 BC, and was found at the Yarim Tepe site in modern Iraq. Neolithic kilns were able to produce temperatures greater than 900 °C (1652 °F). Uses include:
Kilns are an essential part of the manufacture of all ceramics. Ceramics require high temperatures so chemical and physical reactions will occur to permanently alter the unfired body. In the case of pottery, clay materials are shaped, dried and then fired in a kiln. The final characteristics are determined by the composition and preparation of the clay body and the temperature at which it is fired. After a first firing, glazes may be used and the ware is fired a second time to fuse the glaze into the body. A third firing at a lower temperature may be required to fix overglaze decoration. Modern kilns often have sophisticated electronic control systems, although pyrometric devices are often also used.
Clay consists of fine-grained particles that are relatively weak and porous. Clay is combined with other minerals to create a workable clay body. The firing process includes sintering. This heats the clay until the particles partially melt and flow together, creating a strong, single mass, composed of a glassy phase interspersed with pores and crystalline material. Through firing, the pores are reduced in size, causing the material to shrink slightly. This crystalline material predominantly consists of silicon and aluminium oxides.
In the broadest terms, there are two types of kilns: intermittent and continuous, both being an insulated box with a controlled inner temperature and atmosphere.
A continuous kiln, sometimes called a tunnel kiln, is long with only the central portion directly heated. From the cool entrance, ware is slowly moved through the kiln, and its temperature is increased steadily as it approaches the central, hottest part of the kiln. As it continues through the kiln, the temperature is reduced until the ware exits the kiln nearly at room temperature. A continuous kiln is energy-efficient, because heat given off during cooling is recycled to pre-heat the incoming ware. In some designs, the ware is left in one place, while the heating zone moves across it. Kilns in this type include:
In the intermittent kiln, the ware is placed inside the kiln, the kiln is closed, and the internal temperature is increased according to a schedule. After the firing is completed, both the kiln and the ware are cooled. The ware is removed, the kiln is cleaned and the next cycle begins. Kilns in this type include:
Kiln technology is very old. Kilns developed from a simple earthen trench filled with pots and fuel pit firing, to modern methods. One improvement was to build a firing chamber around pots with baffles and a stoking hole. This conserved heat. A chimney stack improved the air flow or draw of the kiln, thus burning the fuel more completely.
Chinese kiln technology has always been a key factor in the development of Chinese pottery, and until recent centuries was the most advanced in the world. The Chinese developed kilns capable of firing at around 1,000 °C before 2000 BC. These were updraft kilns, often built below ground. Two main types of kiln were developed by about 200 AD and remained in use until modern times. These are the dragon kiln of hilly southern China, usually fuelled by wood, long and thin and running up a slope, and the horseshoe-shaped mantou kiln of the north Chinese plains, smaller and more compact. Both could reliably produce the temperatures of up to 1300 °C or more needed for porcelain. In the late Ming, the egg-shaped kiln or zhenyao was developed at Jingdezhen and mainly used there. This was something of a compromise between the other types, and offered locations in the firing chamber with a range of firing conditions.
Both Ancient Roman pottery and medieval Chinese pottery could be fired in industrial quantities, with tens of thousands of pieces in a single firing. Early examples of simpler kilns found in Britain include those that made roof-tiles during the Roman occupation. These kilns were built up the side of a slope, such that a fire could be lit at the bottom and the heat would rise up into the kiln.
Traditional kilns include:
With the industrial age, kilns were designed to use electricity and more refined fuels, including natural gas and propane. Many large industrial pottery kilns use natural gas, as it is generally clean, efficient and easy to control. Modern kilns can be fitted with computerized controls allowing for fine adjustments during the firing. A user may choose to control the rate of temperature climb or ramp, hold or soak the temperature at any given point, or control the rate of cooling. Both electric and gas kilns are common for smaller scale production in industry and craft, handmade and sculptural work.
The temperature of some kilns is controlled by pyrometric cones—devices that begin to melt at specific temperatures.
Modern kilns include:
Green wood coming straight from the felled tree has far too high a moisture content to be commercially useful and will rot, warp and split. Both hardwoods and softwood must be left to dry out until the moisture content is between 18% and 8%. This can be a long process, or it is speeded up by use of a kiln. A variety of kiln technologies exist today: conventional, dehumidification, solar, vacuum and radio frequency.
The economics of different wood drying technologies are based on the total energy, capital, insurance/risk, environmental impacts, labor, maintenance, and product degradation costs. These costs which can be a significant part of plant costs, involve the differential impact of the presence of drying equipment in a specific plant. Every piece of equipment from the green trimmer to the infeed system at the planer mill is part the "drying system". The true costs of the drying system can only be determined when comparing the total plant costs and risks with and without drying.
The total (harmful) air emissions produced by wood kilns, including their heat source, can be significant. Typically, the higher the temperature at which the kiln operates, the larger the quantity of emissions that are produced (per pound of water removed). This is especially true in the drying of thin veneers and high-temperature drying of softwoods.
A bottle kiln, or bottle oven, is a type of kiln. The word 'bottle' refers to the shape of the structure and not to the kiln's products, which are usually pottery, not glass.
Bottle kilns were typical of the industrial landscape of Stoke-on-Trent, where nearly 50 are preserved as listed buildings. Their association with Stoke-on-Trent reflects the fact that the British ceramic industry was mainly based in that city. Bottle kilns are found in other locations; for example for Coalport porcelain, and the Fulham Pottery in London.
Despite being very inefficient (supposedly 70% of the energy of the fuel was wasted), bottle kilns were constructed until the mid-twentieth century, after which they were replaced by other types of kiln, as the industry ceased to be coal-fired.Cement kiln
Cement kilns are used for the pyroprocessing stage of manufacture of Portland and other types of hydraulic cement, in which calcium carbonate reacts with silica-bearing minerals to form a mixture of calcium silicates. Over a billion tonnes of cement are made per year, and cement kilns are the heart of this production process: their capacity usually defines the capacity of the cement plant. As the main energy-consuming and greenhouse-gas–emitting stage of cement manufacture, improvement of kiln efficiency has been the central concern of cement manufacturing technology.Coal Kiln Crossing, Virginia
Coal Kiln Crossing is an unincorporated community in Accomack County, Virginia, United States.Fire brick
A fire brick, firebrick, or refractory brick is a block of refractory ceramic material used in lining furnaces, kilns, fireboxes, and fireplaces. A refractory brick is built primarily to withstand high temperature, but will also usually have a low thermal conductivity for greater energy efficiency. Usually dense firebricks are used in applications with extreme mechanical, chemical, or thermal stresses, such as the inside of a wood-fired kiln or a furnace, which is subject to abrasion from wood, fluxing from ash or slag, and high temperatures. In other, less harsh situations, such as in an electric or natural gas fired kiln, more porous bricks, commonly known as "kiln bricks" are a better choice. They are weaker, but they are much lighter, easier to form, and insulate far better than dense bricks. In any case, firebricks should not spall, and their strength should hold up well during rapid temperature changes.Grove Lime Kiln
Grove Lime Kiln is a disused 19th century lime kiln on the Isle of Portland, Dorset, England. It is located close to HM Prison Portland and The Grove village. Owned by the prison service, the lime kiln has been Grade II Listed since 2009.The Grove Lime kiln is one of a few remaining lime kilns on the island. A 19th-century lime kiln at Easton Lane, now a residence, is another Grade Listed II example. Another kiln is found opposite, while at New Ground, another well-preserved example exists, but is hidden by undergrowth. Finally at Avalanche Road, within the Southwell village, remains another surviving kiln.Jingdezhen porcelain
Jingdezhen porcelain (Chinese: 景德镇陶瓷) is Chinese porcelain produced in or near Jingdezhen in southern China. Jingdezhen may have produced pottery as early as the sixth century CE, though it is named after the reign name of Emperor Zhenzong, in whose reign it became a major kiln site, around 1004. By the 14th century it had become the largest centre of production of Chinese porcelain, which it has remained, increasing its dominance in subsequent centuries. From the Ming period onwards, official kilns in Jingdezhen were controlled by the emperor, making imperial porcelain in large quantity for the court and the emperor to give as gifts.
Although apparently an unpromising location for potteries, being a remote town in a hilly region, Jingdezhen is close to the best quality deposits of petuntse, or porcelain stone, in China, as well as being surrounded by forests, mostly of pine, providing wood for the kilns. It also has a river leading to river systems flowing north and south, facilitating transport of fragile wares. The imperial kilns were in the centre of the city at Zhushan (Pearl Hill), with many other kilns four kilometres away at Hutian.It has produced a great variety of pottery and porcelain, for the Chinese market and as Chinese export porcelain, but its best-known high quality porcelain wares have been successively Qingbai ware in the Song and Yuan dynasties, blue and white porcelain from the 1330s, and the "famille rose" and other "famille" colours under the Qing dynasty.Kiln House
Kiln House is the fourth studio album by British blues rock band Fleetwood Mac, released on 18 September 1970 by Reprise Records. This is the first of the post-Peter Green Fleetwood Mac albums, and their last album to feature Jeremy Spencer. Christine McVie was present at the recording sessions and contributed backing vocals, keyboards and cover art, although she was not a full member of the band until shortly after the album's completion.Kiln Lane electric railway station
Kiln Lane electric railway station was situated at the eighth of eight passing loops on the otherwise single track central "country" section of the inter-urban Grimsby and Immingham Electric Railway when travelling from Corporation Bridge, Grimsby to Immingham Dock.Kiln Theatre
The Kiln Theatre (formerly the Tricycle Theatre) is a theatre located in Kilburn, in the London Borough of Brent, England. Since 1980, the theatre has presented a wide range of plays reflecting the cultural diversity of the area, as well as new writing, political work and verbatim reconstructions of public inquiries.
The theatre has produced original work by playwrights such as Lynn Nottage, Patrick Barlow, Richard Bean, David Edgar, Stephen Jeffreys, Abi Morgan, Simon Stephens, Roy Williams, Lolita Chakrabarti, Moira Buffini, Alexi Kaye Campbell, Florian Zeller and Ayad Akhtar.
The current artistic director is Indhu Rubasingham, who succeeded Nicolas Kent in 2012. The theatre's name was changed from the Tricycle to the Kiln in April 2018.Lime Kiln Point State Park
Lime Kiln Point State Park is a 42-acre Washington state park on the western shore of San Juan Island in the San Juan archipelago. The park is considered one of the best places in the world to view wild orcas from a land-based facility. Due to the unique bathymetric properties of the site, visitors on the shore can be within 20 feet of whales jumping out of the water (breaching and spyhopping). The park was the site of lime kilns beginning in 1860, and one kiln has been restored as a public exhibit.Lime kiln
A lime kiln is a kiln used for the calcination of limestone (calcium carbonate) to produce the form of lime called quicklime (calcium oxide). The chemical equation for this reaction is
CaCO3 + heat → CaO + CO2This reaction takes place at 900 °C (1650 °F; at which temperature the partial pressure of CO2 is 1 atmosphere), but a temperature around 1000 °C (1800 °F; at which temperature the partial pressure of CO2 is 3.8 atmospheres) is usually used to make the reaction proceed quickly. Excessive temperature is avoided because it produces unreactive, "dead-burned" lime.
Slaked lime (calcium hydroxide) can be formed by mixing water with quicklime.Nanfeng Kiln
Nanfeng Kiln (Chinese: 南风古灶; pinyin: Nánfēng gǔ zào; Jyutping: naam4 fung1 gu2 zou3) is a tourist attraction in the Ancient Nanfeng Kiln Cultural and Creative Zone, which is located in Shiwan Town, Chancheng District, Foshan city, Guangdong province of China. It was built in the Ming Dynasty Zhengde period (1506–1521), and has been continuously firing Shiwan ware for over 500 years so far.Nilak, Sistan and Baluchestan
Nilak (Persian: نيلك, also Romanized as Nīlak; also known as Kīln) is a village in Sand-e Mir Suiyan Rural District, Dashtiari District, Chabahar County, Sistan and Baluchestan Province, Iran. At the 2006 census, its population was 72, in 15 families.Oast house
An oast, oast house or hop kiln is a building designed for kilning (drying) hops as part of the brewing process. They can be found in most hop-growing (and former hop-growing) areas and are often good examples of vernacular architecture. Many redundant oasts have been converted into houses. The names oast and oast house are used interchangeably in Kent and Sussex. In Surrey, Hampshire, Herefordshire and Worcestershire they are always called hop kilns.
They consist of a rectangular one or two storey building (the "stowage") and one or more kilns in which the hops were spread out to be dried by hot air rising from a wood or charcoal fire below. The drying floors were thin and perforated to permit the heat to pass through and escape through a cowl in the roof which turned with the wind. The freshly picked hops from the fields were raked in to dry and then raked out to cool before being bagged up and sent to the brewery. The Kentish dialect word kell was sometimes used for kilns ("The oast has three kells.") and sometimes to mean the oast itself ("Take this lunchbox to your father, he's working in the kell."). The word oast itself also means "kiln".
The earliest surviving oast house is at Golford, Cranbrook near Tunbridge Wells. It dates from sometime in the 17th Century and closely mirrors the first documentary evidence on oasts soon after their introduction of hops into England in the mid 16th century. Early oast houses were simply adapted barns but, by the 18th Century, the distinctive tall buildings with conical roofs had been developed to increase the draught. At first these were square but around 1800 roundel kilns were developed in the belief that they were more efficient. Square kilns remained more popular in Herefordshire and Worcestershire and came back into fashion in the south east in the later 19th Century. In the 1930s, the cowls were replaced by louvred openings as electric fans and diesel oil ovens were employed.
Hops are today dried industrially and the many oast houses on farms have now been converted into dwellings. One of the best preserved oast house complexes is at The Hop Farm Country Park at Beltring.Overglaze decoration
Overglaze decoration, overglaze enamelling or on-glaze decoration is a method of decorating pottery, most often porcelain, where the coloured decoration is applied on top of the already fired and glazed surface, and then fixed in a second firing at a relatively low temperature, often in a muffle kiln. It is often described as producing "enamelled" decoration. The colours fuse on to the glaze, so the decoration becomes durable. This decorative firing is usually done at a lower temperature which allows for a more varied and vidid palette of colours, using pigments which will not colour correctly at the high temperature necessary to fire the porcelain body. Historically, a relatively narrow range of colours could be achieved with underglaze decoration, where the coloured pattern is applied before glazing, notably the cobalt blue of blue and white porcelain.
Many historical styles, for example Imari ware, Chinese doucai and wucai, combine the two types of decoration. In such cases the first firing for the body, underglaze decoration and glaze is followed by the second firing after the overglaze enamels have been applied.
The technique was first seen in Chinese ceramics in Cizhou stoneware from as early as the 12th century, with use on porcelain following within a century, though it did not become predominant until later, and the full possibilities were not realized until the 17th century. Some techniques use thin metal leaf as well as the more usual pigments, which are typically applied in a liquid or paste form, painted by brush, or using stencils or transfer printing. Today overglaze decoration is much less commonly used, other than in traditionalist wares, as the range of colours available in underglaze has greatly expanded.Pottery
Pottery is the process of forming vessels and other objects with clay and other ceramic materials, which are fired at high temperatures to give them a hard, durable form. Major types include earthenware, stoneware and porcelain. The place where such wares are made by a potter is also called a pottery (plural "potteries"). The definition of pottery used by the American Society for Testing and Materials (ASTM), is "all fired ceramic wares that contain clay when formed, except technical, structural, and refractory products." In archaeology, especially of ancient and prehistoric periods, "pottery" often means vessels only, and figures etc. of the same material are called "terracottas". Clay as a part of the materials used is required by some definitions of pottery, but this is dubious.
Pottery is one of the oldest human inventions, originating before the Neolithic period, with ceramic objects like the Gravettian culture Venus of Dolní Věstonice figurine discovered in the Czech Republic dating back to 29,000–25,000 BC, and pottery vessels that were discovered in Jiangxi, China, which date back to 18,000 BC. Early Neolithic pottery artefacts have been found in places such as Jōmon Japan (10,500 BC), the Russian Far East (14,000 BC), Sub-Saharan Africa and South America.
Pottery is made by forming a ceramic (often clay) body into objects of a desired shape and heating them to high temperatures (1000-1600 °C) in a kiln and induces reactions that lead to permanent changes including increasing the strength and solidity of the object's shape. Much pottery is purely utilitarian, but much can also be regarded as ceramic art. A clay body can be decorated before or after firing.
Clay-based pottery can be divided into three main groups: earthenware, stoneware and porcelain. These require increasingly more specific clay material, and increasingly higher firing temperatures. All three are made in glazed and unglazed varieties, for different purposes. All may also be decorated by various techniques. In many examples the group a piece belongs to is immediately visually apparent, but this is not always the case. The fritware of the Islamic world does not use clay, so technically falls outside these groups. Historic pottery of all these types is often grouped as either "fine" wares, relatively expensive and well-made, and following the aesthetic taste of the culture concerned, or alternatively "coarse", "popular" "folk" or "village" wares, mostly undecorated, or simply so, and often less well-made.Raku ware
Raku ware (楽焼, raku-yaki) is a type of Japanese pottery traditionally used in Japanese tea ceremonies, most often in the form of chawan tea bowls. It is traditionally characterised by being hand-shaped rather than thrown, fairly porous vessels, which result from low firing temperatures, lead glazes and the removal of pieces from the kiln while still glowing hot. In the traditional Japanese process, the fired raku piece is removed from the hot kiln and is allowed to cool in the open air. The familiar technique of placing the ware in a container filled with combustible material is not a traditional Raku practice.Tar
Tar is a dark brown or black viscous liquid of hydrocarbons and free carbon, obtained from a wide variety of organic materials through destructive distillation. Tar can be produced from coal, wood, petroleum, or peat. Production and trade in pine-derived tar was a major contributor in the economies of Northern Europe and Colonial America. Its main use was in preserving wooden sailing vessels against rot. The largest user was the Royal Navy of the United Kingdom. Demand for tar declined with the advent of iron and steel ships. It is hot to the touch, and can cause burns if one comes in contact with the substance.
Tar-like products can also be produced from other forms of organic matter, such as peat. Mineral products resembling tar can be produced from fossil hydrocarbons, such as petroleum. Coal tar is produced from coal as a byproduct of coke production.Wood drying
Wood drying (also seasoning lumber or wood seasoning) reduces the moisture content of wood before its use. When the drying is done in a kiln, the product is known as kiln-dried timber or lumber, whereas air drying is the more traditional method.
There are two main reasons for drying wood:
When wood is used as a construction material, whether as a structural support in a building or in woodworking objects, it will absorb or desorb moisture until it is in equilibrium with its surroundings. Equilibration (usually drying) causes unequal shrinkage in the wood, and can cause damage to the wood if equilibration occurs too rapidly. The equilibration must be controlled to prevent damage to the wood.
When wood is burned, it is usually best to dry it first. Damage from shrinkage is not a problem here, as it may be in the case of drying for woodworking purposes. Moisture affects the burning process, with unburnt hydrocarbons going up the chimney. If a 50% wet log is burnt at high temperature, with good heat extraction from the exhaust gas leading to a 100 °C exhaust temperature, about 5% of the energy of the log is wasted through evaporating and heating the water vapour. With condensers, the efficiency can be further increased; but, for the normal stove, the key to burning wet wood is to burn it very hot, perhaps starting fire with dry wood.
For some purposes, wood is not dried at all, and is used green. Often, wood must be in equilibrium with the air outside, as for construction wood, or the air indoors, as for wooden furniture.
Wood is air-dried or dried in a purpose built oven (kiln). Usually the wood is sawn before drying, but sometimes the log is dried whole.
Case hardening describes lumber or timber that has been dried too rapidly. Wood initially dries from the shell (surface), shrinking the shell and putting the core under compression. When this shell is at a low moisture content it will 'set' and resist shrinkage. The core of the wood is still at a higher moisture content. This core will then begin to dry and shrink. However, any shrinkage is resisted by the already 'set' shell. This leads to reversed stresses; compression stresses on the shell and tension stresses in the core. This results in unrelieved stress called case hardening. Case-hardened [wood] may warp considerably and dangerously when the stress is released by sawing.