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, such as limestone to lime for cement) and to transform many other materials.
According to the Oxford English Dictionary, kiln was derived from the words cyline, cylene, cyln(e) in Old English, in turn derived from Latin culina ("kitchen"). In Middle English, the word is attested as kulne, kyllne, kilne, kiln, kylle, kyll, kil, kill, keele, kiele. [1] [2] In Greek the word καίειν, kaiein, means 'to burn'.
The word "kiln" was originally pronounced "kil" with the "n" silent, as is referenced in Webster's Dictionary of 1828 [3] and in 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." [4] Bowen was noting that "kill" and "kiln" are homophones. [5]
Pit fired pottery was produced for thousands of years before the earliest known kiln, which dates to around 6000 BCE, and was found at the Yarim Tepe site in modern Iraq. [6] Neolithic kilns were able to produce temperatures greater than 900 °C (1652 °F). [7] Uses include:
Kilns are an essential part of the manufacture of almost all types of 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.
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: [9]
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 BCE. 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. [10]
Both Ancient Roman pottery and medieval Chinese pottery could be fired in industrial quantities, with tens of thousands of pieces in a single firing. [11] 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.
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 unless accelerated by use of a kiln. A variety of kiln technologies exist today: conventional, dehumidification, solar, vacuum and radio frequency.
Conventional wood dry kilns [13] are either package-type (side-loader) or track-type (tram) construction. Most hardwood lumber kilns are side-loader kilns in which fork trucks are used to load lumber packages into the kiln. Most softwood kilns are track types in which the timber is loaded on kiln/track cars for loading the kiln. Modern high-temperature, high-air-velocity conventional kilns can typically dry 25-millimetre-thick (1 in) green wood in 10 hours down to a moisture content of 18%. However, 25-mm-thick green red oak requires about 28 days to dry down to a moisture content of 8%.[ citation needed ]
Heat is typically introduced via steam running through fin/tube heat exchangers controlled by on/off pneumatic valves. Humidity is removed by a system of vents, the specific layout of which are usually particular to a given manufacturer. In general, cool dry air is introduced at one end of the kiln while warm moist air is expelled at the other. Hardwood conventional kilns also require the introduction of humidity via either steam spray or cold water misting systems to keep the relative humidity inside the kiln from dropping too low during the drying cycle. Fan directions are typically reversed periodically to ensure even drying of larger kiln charges.[ citation needed ]
Most softwood kilns operate below 115 °C (240 °F) temperature. Hardwood kiln drying schedules typically keep the dry bulb temperature below 80 °C (180 °F). Difficult-to-dry species might not exceed 60 °C (140 °F).
Dehumidification kilns are similar to other kilns in basic construction and drying times are usually comparable. Heat comes primarily from an integral dehumidification unit that also removes humidity. Auxiliary heat is often provided early in the schedule to supplement the dehumidifier.
Solar kilns are conventional kilns, typically built by hobbyists to keep initial investment costs low. Heat is provided via solar radiation, while internal air circulation is typically passive.
Vacuum and radio frequency kilns reduce the air pressure to attempt to speed up the drying process. A variety of these vacuum technologies exist, varying primarily in the method heat is introduced into the wood charge. Hot water platten vacuum kilns use aluminum heating plates with the water circulating within as the heat source, and typically operate at significantly reduced absolute pressure. Discontinuous and SSV (super-heated steam) use atmosphere pressure to introduce heat into the kiln charge. The entire kiln charge comes up to full atmospheric pressure, the air in the chamber is then heated and finally a vacuum is pulled as the charge cools. SSV run at partial-atmospheres, typically around 1/3 of full atmospheric pressure, in a hybrid of vacuum and conventional kiln technology (SSV kilns are significantly more popular in Europe where the locally harvested wood is easier to dry than the North American woods.) RF/V (radio frequency + vacuum) kilns use microwave radiation to heat the kiln charge, and typically have the highest operating cost due to the heat of vaporization being provided by electricity rather than local fossil fuel or waste wood sources.[ citation needed ]
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 of 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.[ citation needed ]
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 (with optimal moisture levels of under 20% being much easier to achieve). [14] [15] [16] [17] [18]
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 mass unit of water removed). This is especially true in the drying of thin veneers and high-temperature drying of softwoods.[ citation needed ]
Pottery is the process and the products of forming vessels and other objects with clay and other raw materials, which are fired at high temperatures to give them a hard and durable form. The place where such wares are made by a potter is also called a pottery. The definition of pottery, used by the ASTM International, is "all fired ceramic wares that contain clay when formed, except technical, structural, and refractory products". End applications include tableware, decorative ware, sanitary ware, and in technology and industry such as electrical insulators and laboratory ware. In art history and archaeology, especially of ancient and prehistoric periods, pottery often means only vessels, and sculpted figurines of the same material are called terracottas.
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.
Cookware and bakeware is food preparation equipment, such as cooking pots, pans, baking sheets etc. used in kitchens. Cookware is used on a stove or range cooktop, while bakeware is used in an oven. Some utensils are considered both cookware and bakeware.
An oven is a tool that is used to expose materials to a hot environment. Ovens contain a hollow chamber and provide a means of heating the chamber in a controlled way. In use since antiquity, they have been used to accomplish a wide variety of tasks requiring controlled heating. Because they are used for a variety of purposes, there are many different types of ovens. These types differ depending on their intended purpose and based upon how they generate heat.
Firewood is any wooden material that is gathered and used for fuel. Generally, firewood is not heavily processed and is in some sort of recognizable log or branch form, compared to other forms of wood fuel like pellets. Firewood can be seasoned and heat treated (dry) or unseasoned (fresh/wet). It is generally classified as either hardwood or softwood.
Dielectric heating, also known as electronic heating, radio frequency heating, and high-frequency heating, is the process in which a radio frequency (RF) alternating electric field, or radio wave or microwave electromagnetic radiation heats a dielectric material. At higher frequencies, this heating is caused by molecular dipole rotation within the dielectric.
The anagamakiln 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 be treated as distinct from Oribe ware but described as "white Oribe", with the pottery usually called just Oribe described as "green Oribe" instead.
A vacuum furnace is a type of furnace in which the product in the furnace is surrounded by a vacuum during processing. The absence of air or other gases prevents oxidation, heat loss from the product through convection, and removes a source of contamination. This enables the furnace to heat materials to temperatures as high as 3,000 °C (5,432 °F) with select materials. Maximum furnace temperatures and vacuum levels depend on melting points and vapor pressures of heated materials. Vacuum furnaces are used to carry out processes such as annealing, brazing, sintering and heat treatment with high consistency and low contamination.
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.
This is a list of pottery and ceramic terms.
Industrial ovens are heated chambers used for a variety of industrial applications, including drying, curing, or baking components, parts or final products. Industrial ovens can be used for large or small volume applications, in batches or continuously with a conveyor line, and a variety of temperature ranges, sizes and configurations.
A wood-burning stove is a heating or cooking appliance capable of burning wood fuel, often called solid 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. This was two centuries before the Industrial Revolution, so iron was still prohibitively expensive. The first wood-burning stoves were high-end consumer items and only gradually became used widely.
A bottle oven or bottle kiln 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.
Three-phase firing or iron reduction technique is a firing technique used in ancient Greek pottery production, specifically for painted vases. Already vessels from the Bronze Age feature the colouring typical of the technique, with yellow, orange or red clay and brown or red decoration. By the 7th century BC, the process was perfected in mainland Greece enabling the production of extremely shiny black-slipped surfaces, which led to the development of the black-figure and red-figure techniques, which dominated Greek vase painting until about 300 BC.
Iga ware is a style of Japanese pottery traditionally produced in Iga, Mie, former Iga Province, central Japan.
Dick Lehman is an American 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.
| Authority control databases: National |
|---|
