A kiln 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" was originally pronounced "kil" with the "n" silent, as is referenced in Webster's Dictionary of 1828.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 1915: “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. Despite its origins, the modern pronunciation of this word, where the "n" is pronounced, has become more widely accepted than the original pronunciation. This is most likely due to a phenomenon known as spelling pronunciation, where the pronunciation of a word is derived from its spelling and differs from its actual pronunciation. This is common in words with silent letters.
Kiln descends from the Old English cylene (/ˈkylene/), which was borrowed from Old Welsh 'Cylyn' which was then borrowed from the Latin culīna 'kitchen, cooking-stove, burning-place.
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. °C (1652 °F). Uses include:Neolithic kilns were able to produce temperatures greater than 900
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.
Kiln dried firewood was pioneered during the 1980s, and was later adopted extensively in Europe due to the economic and practical benefits of selling wood with a lower moisture content.
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.
Pottery is the process and the products 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. 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.
Raku ware 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.
Earthenware is glazed or unglazed nonvitreous pottery that has normally been fired below 1200 °C. Porcelain, bone china, and stoneware, all fired at high enough temperatures to vitrify, are the main other important types of pottery.
An oven is a thermally insulated chamber used for the heating, baking, or drying of a substance, and most commonly used for cooking. Kilns and furnaces are special-purpose ovens used in pottery and metalworking, respectively.
Stoneware is a rather broad term for pottery or other ceramics fired at a relatively high temperature. A modern technical definition is a vitreous or semi-vitreous ceramic made primarily from stoneware clay or non-refractory fire clay. Whether vitrified or not, it is nonporous ; it may or may not be glazed. Historically, across the world, it has been developed after earthenware and before porcelain, and has often been used for high-quality as well as utilitarian wares.
The anagama kiln is an ancient type of pottery kiln brought to Japan from China via Korea in the 5th century. It is a version of the climbing dragon kiln of south China, whose further development was also copied, for example in breaking up the firing space into a series of chambers in the noborigama kiln.
Bizen ware is a type of Japanese pottery traditionally from Bizen province, presently a part of Okayama prefecture.
Shino ware is Japanese pottery, usually stoneware, originally from Mino Province, in present-day Gifu Prefecture, Japan. It emerged in the 16th century, but the use of shino glaze is now widespread, both in Japan and abroad. It is identified by thick white glazes, red scorch marks, and a texture of small holes. Some experts believe it should not treated as distinct from Oribe ware but described as "white Oribe", with the pottery usually called just Oribe described as "green Oribe" instead.
Shigaraki ware (信楽焼) is a type of stoneware pottery made in Shigaraki area, Japan. The kiln is one of the Six Ancient Kilns in Japan. Although figures representing the tanuki are a popular product included as Shigaraki ware, the kiln and local pottery tradition has a long history.
Wood drying 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.
Paper clay is any clay body to which processed cellulose fiber has been added.
Ceramic glaze is an impervious layer or coating of a vitreous substance which has been fused to a ceramic body through firing. Glaze can serve to color, decorate or waterproof an item. Glazing renders earthenware vessels suitable for holding liquids, sealing the inherent porosity of unglazed biscuit earthenware. It also gives a tougher surface. Glaze is also used on stoneware and porcelain. In addition to their functionality, glazes can form a variety of surface finishes, including degrees of glossy or matte finish and color. Glazes may also enhance the underlying design or texture either unmodified or inscribed, carved or painted.
This is a list of pottery and ceramic terms.
Catawba Valley Pottery describes alkaline glazed stoneware made in the Catawba River Valley of Western North Carolina from the early 19th century, as well as certain contemporary pottery made in the region utilizing traditional methods and forms.
A wood-burning stove is a heating appliance capable of burning wood fuel and wood-derived biomass fuel, such as sawdust bricks. Generally the appliance consists of a solid metal closed firebox, often lined by fire brick, and one or more air controls. The first wood-burning stove was patented in Strasbourg in 1557, two centuries before the Industrial Revolution, which would make iron an inexpensive and common material, so such stoves were high end consumer items and only gradually spread in use.
Cizhou ware or Tz'u-chou ware is a term for a wide range of Chinese ceramics from between the late Tang dynasty and the early Ming dynasty, but especially associated with the Northern Song to Yuan period in the 11–14th century. It has been increasingly realized that a very large number of sites in northern China produced these wares, and their decoration is very variable, but most characteristically uses black and white, in a variety of techniques. For this reason Cizhou-type is often preferred as a general term. All are stoneware in Western terms, and "high-fired" or porcelain in Chinese terms. They were less high-status than other types such as celadons and Jun ware, and are regarded as "popular", though many are finely and carefully decorated.
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.
Iga ware is a style of Japanese pottery traditionally produced in Iga, Mie, former Iga Province, central Japan.
Dick Lehman is a U.S. ceramics artist based in Indiana. Dozens of articles and photos featuring his techniques and insights have appeared in periodicals and books on ceramic art since 1985, including 34 articles in U.S.-published Ceramics Monthly, the largest circulating magazine in the field, plus articles in 11 other international periodicals.
A dragon kiln or "climbing kiln", is a traditional Chinese form of kiln, used for Chinese ceramics, especially in southern China. It is long and thin, and relies on having a fairly steep slope, typically between 10° and 16°, up which the kiln runs. The kiln could achieve the very high temperatures, sometimes as high as 1400°C, necessary for high-fired wares including stoneware and porcelain, which long challenged European potters, and some examples were very large, up to 60 metres long, allowing up to 25,000 pieces to be fired at a time. By the early 12th century CE they might be over 135 metres long, allowing still larger quantities to be fired; more than 100,000 have been claimed.
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