HOME

TheInfoList



OR:

A kiln is a thermally insulated chamber, a type of
oven upA double oven A ceramic oven An oven is a tool which 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 us ...
, 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 Clay is a type of fine-grained natural soil material containing clay minerals (hydrous aluminium phyllosilicates, e.g. kaolin, Al2 Si2 O5( OH)4). Clays develop plasticity when wet, due to a molecular film of water surrounding the clay par ...
into
pottery 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 and durable form. Major types include earthenware, stoneware and por ...
, tiles and
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 cons ...
s. Various industries use
rotary kiln A rotary kiln is a pyroprocessing device used to raise materials to a high temperature (calcination) in a continuous process. Materials produced using rotary kilns include: * Cement * Lime * Refractories * Metakaolin * Titanium dioxide * Al ...
s for
pyroprocessing Pyroprocessing (from Greek Πυρος = ''fire'') is a process in which materials are subjected to high temperatures (typically over 800 °C) in order to bring about a chemical or physical change. Pyroprocessing includes such terms as ore-ro ...
—to calcinate ores, to calcinate
limestone Limestone ( calcium carbonate ) is a type of carbonate sedimentary rock which is the main source of the material lime. It is composed mostly of the minerals calcite and aragonite, which are different crystal forms of . Limestone forms whe ...
to
lime Lime commonly refers to: * Lime (fruit), a green citrus fruit * Lime (material), inorganic materials containing calcium, usually calcium oxide or calcium hydroxide * Lime (color), a color between yellow and green Lime may also refer to: Botany ...
for
cement A cement is a binder, a chemical substance used for construction that sets, hardens, and adheres to other materials to bind them together. Cement is seldom used on its own, but rather to bind sand and gravel ( aggregate) together. Cement mi ...
, and to transform many other materials.


Pronunciation and etymology

According to the
Oxford English Dictionary The ''Oxford English Dictionary'' (''OED'') is the first and foundational historical dictionary of the English language, published by Oxford University Press (OUP). It traces the historical development of the English language, providing a com ...
, kiln was derived from the words cyline, cylene, cyln(e) in Old English, in turn derived from
Latin Latin (, or , ) is a classical language belonging to the Italic branch of the Indo-European languages. Latin was originally a dialect spoken in the lower Tiber area (then known as Latium) around present-day Rome, but through the power of the ...
''culina'' ("kitchen"). In
Middle English Middle English (abbreviated to ME) is a form of the English language that was spoken after the Norman conquest of 1066, until the late 15th century. The English language underwent distinct variations and developments following the Old English p ...
the word is attested as kulne, kyllne, kilne, kiln, kylle, kyll, kil, kill, keele, kiele. For over 600 years, the final "n" in kiln was silent. It wasn't until the late 20th century where the "n" began to be pronounced. This is due to a phenomenon known as
spelling pronunciation A spelling pronunciation is the pronunciation of a word according to its spelling when this differs from a longstanding standard or traditional pronunciation. Words that are spelled with letters that were never pronounced or that were not pronounc ...
, where the pronunciation of a word is surmised from its spelling and differs from its actual pronunciation. In James A. Bowen's book ''English Words as Spoken and Written for Upper Grades'' (1915), he refers to the 'ln' in 'kiln' as a digraph, a combination of two letters that make one sound and made a humorous reference to "kill" and "kiln" as
homophone A homophone () is a word that is pronounced the same (to varying extent) as another word but differs in meaning. A ''homophone'' may also differ in spelling. The two words may be spelled the same, for example ''rose'' (flower) and ''rose'' (p ...
s: "The digraph ln, n silent, occurs in kiln. A fall down the kiln can kill you."


Uses of kilns

Pit fired pottery Pit firing is the oldest known method for the firing of pottery. Examples have been dated as early as 29,000–25,000 BCE, while the earliest known kiln dates to around 6000 BCE, and was found at the Yarim Tepe site in modern Iraq. Kilns al ...
was produced for thousands of years before the earliest known kiln, which dates to around 6000 BC, and was found at the
Yarim Tepe Yarim Tepe is an archaeological site of an early farming settlement that goes back to about 6000 BC. It is located in the Sinjar valley some 7km southwest from the town of Tal Afar in northern Iraq. The site consists of several hills reflecting t ...
site in modern
Iraq Iraq,; ku, عێراق, translit=Êraq officially the Republic of Iraq, '; ku, کۆماری عێراق, translit=Komarî Êraq is a country in Western Asia. It is bordered by Turkey to Iraq–Turkey border, the north, Iran to Iran–Iraq ...
.
Neolithic The Neolithic period, or New Stone Age, is an Old World archaeological period and the final division of the Stone Age. It saw the Neolithic Revolution, a wide-ranging set of developments that appear to have arisen independently in several parts ...
kilns were able to produce temperatures greater than 900 °C (1652 °F).James E. McClellan III; Harold Dorn.
Science and Technology in World History: An Introduction
'. JHU Press; 14 April 2006. . p. 21.
Uses include: * Annealing, fusing and deforming
glass Glass is a non-crystalline, often transparent, amorphous solid that has widespread practical, technological, and decorative use in, for example, window panes, tableware, and optics. Glass is most often formed by rapid cooling (quenching) of ...
, or fusing metallic oxide paints to the surface of glass *Heat treatment for metallic workpieces *
Ceramics A ceramic is any of the various hard, brittle, heat-resistant and corrosion-resistant materials made by shaping and then firing an inorganic, nonmetallic material, such as clay, at a high temperature. Common examples are earthenware, porcelain ...
*
Brickworks A brickworks, also known as a brick factory, is a factory for the manufacturing of bricks, from clay or shale. Usually a brickworks is located on a clay bedrock (the most common material from which bricks are made), often with a quarry for ...
*Melting metal for casting *
Calcination Calcination refers to thermal treatment of a solid chemical compound (e.g. mixed carbonate ores) whereby the compound is raised to high temperature without melting under restricted supply of ambient oxygen (i.e. gaseous O2 fraction of air), gener ...
of ore in a
rotary kiln A rotary kiln is a pyroprocessing device used to raise materials to a high temperature (calcination) in a continuous process. Materials produced using rotary kilns include: * Cement * Lime * Refractories * Metakaolin * Titanium dioxide * Al ...
prior to
smelting Smelting is a process of applying heat to ore, to extract a base metal. It is a form of extractive metallurgy. It is used to extract many metals from their ores, including silver, iron, copper, and other base metals. Smelting uses heat and a ...
*Pyrolysis of chemical materials *Heating
limestone Limestone ( calcium carbonate ) is a type of carbonate sedimentary rock which is the main source of the material lime. It is composed mostly of the minerals calcite and aragonite, which are different crystal forms of . Limestone forms whe ...
with clay in the manufacture of
Portland cement Portland cement is the most common type of cement in general use around the world as a basic ingredient of concrete, mortar, stucco, and non-specialty grout. It was developed from other types of hydraulic lime in England in the early 19th ...
, the
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 ...
*Heating limestone to make quicklime or
calcium oxide Calcium oxide (CaO), commonly known as quicklime or burnt lime, is a widely used chemical compound. It is a white, caustic, alkaline, crystalline solid at room temperature. The broadly used term "''lime''" connotes calcium-containing inorganic ...
, the
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 + CO2 This reaction can take p ...
*Heating gypsum to make
plaster of Paris Plaster is a building material used for the protective or decorative coating of walls and ceilings and for moulding and casting decorative elements. In English, "plaster" usually means a material used for the interiors of buildings, while "re ...
*For
cremation Cremation is a method of Disposal of human corpses, final disposition of a Cadaver, dead body through Combustion, burning. Cremation may serve as a funeral or post-funeral rite and as an alternative to burial. In some countries, including India ...
(at high temperature) *Drying of
tobacco Tobacco is the common name of several plants in the genus '' Nicotiana'' of the family Solanaceae, and the general term for any product prepared from the cured leaves of these plants. More than 70 species of tobacco are known, but the ...
leaves *Drying
malted barley Malt is germinated cereal grain that has been dried in a process known as "malting". The grain is made to germinate by soaking in water and is then halted from germinating further by drying with hot air. Malted grain is used to make beer, whi ...
for brewing and other fermentations *Drying hops for brewing (known as a hop kiln or oast house) *Drying corn (grain) before grinding or storage, sometimes called a corn kiln, corn drying kiln *
Drying Drying is a mass transfer process consisting of the removal of water or another solvent by evaporation from a solid, semi-solid or liquid. This process is often used as a final production step before selling or packaging products. To be consider ...
green Green is the color between cyan and yellow on the visible spectrum. It is evoked by light which has a dominant wavelength of roughly 495570 nm. In subtractive color systems, used in painting and color printing, it is created by a combi ...
lumber Lumber is wood that has been processed into dimensional lumber, including beams and planks or boards, a stage in the process of wood production. Lumber is mainly used for construction framing, as well as finishing (floors, wall panels, wi ...
so it can be used immediately *Drying
wood Wood is a porous and fibrous structural tissue found in the stems and roots of trees and other woody plants. It is an organic materiala natural composite of cellulose fibers that are strong in tension and embedded in a matrix of lignin ...
for use as firewood *Heating wood to the point of
pyrolysis The pyrolysis (or devolatilization) process is the thermal decomposition of materials at elevated temperatures, often in an inert atmosphere. It involves a change of chemical composition. The word is coined from the Greek-derived elements ''py ...
to produce charcoal *extracting pine tar from pine tree logs or roots.


Ceramic kilns

Kilns are an essential part of the manufacture of all
ceramic A ceramic is any of the various hard, brittle, heat-resistant and corrosion-resistant materials made by shaping and then firing an inorganic, nonmetallic material, such as clay, at a high temperature. Common examples are earthenware, porcelain ...
s. 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 Electronic may refer to: *Electronics, the science of how to control electric energy in semiconductor * ''Electronics'' (magazine), a defunct American trade journal *Electronic storage, the storage of data using an electronic device *Electronic co ...
control systems, although
pyrometric device Pyrometric devices gauge heatwork (the combined effect of both time and temperature) when firing materials inside a kiln. Pyrometric devices do not measure temperature, but can report ''temperature equivalents''. In principle, a pyrometric device r ...
s 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 Clinker nodules produced by sintering Sintering or frittage is the process of compacting and forming a solid mass of material by pressure or heat without melting it to the point of liquefaction. Sintering happens as part of a manufacturing ...
. 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 Silicon is a chemical element with the symbol Si and atomic number 14. It is a hard, brittle crystalline solid with a blue-grey metallic luster, and is a tetravalent metalloid and semiconductor. It is a member of group 14 in the periodic tab ...
and
aluminium Aluminium (aluminum in American and Canadian English) is a chemical element with the symbol Al and atomic number 13. Aluminium has a density lower than those of other common metals, at approximately one third that of steel. I ...
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: *
Hoffmann kiln The Hoffmann kiln is a series of batch process kilns. Hoffmann kilns are the most common kiln used in production of bricks and some other ceramic products. Patented by German Friedrich Hoffmann for brickmaking in 1858, it was later used for lime ...
* Bull’s Trench kiln *Habla (Zig-Zag) kiln *Roller kiln: A special type of kiln, common in tableware and tile manufacture, is the ''roller-hearth kiln'', in which wares placed on bats are carried through the kiln on rollers. In the intermittent kiln, the ware is placed inside the kiln, the kiln is closed, and the internal
temperature Temperature is a physical quantity that expresses quantitatively the perceptions of hotness and coldness. Temperature is measurement, measured with a thermometer. Thermometers are calibrated in various Conversion of units of temperature, temp ...
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: * Clamp kiln *Skove kiln *Scotch kiln *Down-draft kiln *Shuttle kilns: this is a car-bottom kiln with a door on one or both ends. Burners are positioned top and bottom on each side, creating a turbulent circular air flow. This type of kiln is generally a multi-car design and is used for processing whitewares, technical ceramics and refractories in batches. Depending upon the size of ware, shuttle kilns may be equipped with car-moving devices to transfer fired and unfired ware in and out of the kiln. Shuttle kilns can be either updraft or downdraft. A shuttle kiln derives its name from the fact that kiln cars can enter a shuttle kiln from either end of the kiln, whereas a tunnel kiln has flow in only one direction. 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 Chinese ceramics show a continuous development since pre-dynastic times and are one of the most significant forms of Chinese art and ceramics globally. The first pottery was made during the Palaeolithic era. Chinese ceramics range from construc ...
, 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 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 kil ...
of hilly southern China, usually fuelled by wood, long and thin and running up a slope, and the horseshoe-shaped
mantou kiln The mantou kiln () or horseshoe-shaped kiln was the most common type of pottery kiln in north China, in historical periods when the dragon kiln dominated south China; both seem to have emerged in the Warring States period of approximately 475 to 2 ...
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 Porcelain () is a ceramic material made by heating substances, generally including materials such as kaolinite, in a kiln to temperatures between . The strength and translucence of porcelain, relative to other types of pottery, arises main ...
. In the late Ming, the
egg-shaped kiln Jingdezhen porcelain () is Chinese porcelain produced in or near Jingdezhen in Jiangxi province 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 Zhenzon ...
or ''zhenyao'' was developed at
Jingdezhen Jingdezhen is a prefecture-level city, in northeastern Jiangxi province, with a total population of 1,669,057 (2018), bordering Anhui to the north. It is known as the " Porcelain Capital" because it has been producing Chinese ceramics for at le ...
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 Pottery was produced in enormous quantities in ancient Rome, mostly for utilitarian purposes. It is found all over the former Roman Empire and beyond. Monte Testaccio is a huge waste mound in Rome made almost entirely of broken amphorae used fo ...
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 Britain most often refers to: * The United Kingdom, a sovereign state in Europe comprising the island of Great Britain, the north-eastern part of the island of Ireland and many smaller islands * Great Britain, the largest island in the United King ...
include those that made roof-tiles during the
Roman Roman or Romans most often refers to: *Rome, the capital city of Italy *Ancient Rome, Roman civilization from 8th century BC to 5th century AD *Roman people, the people of ancient Rome *''Epistle to the Romans'', shortened to ''Romans'', a letter ...
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: *
Dragon kiln 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 kil ...
of south China: thin and long, climbing up a hillside. This type spread to the rest of
East Asia East Asia is the eastern region of Asia, which is defined in both geographical and ethno-cultural terms. The modern states of East Asia include China, Japan, Mongolia, North Korea, South Korea, and Taiwan. China, North Korea, South Korea and ...
giving the Japanese
anagama kiln The ''anagama'' kiln (Japanese Kanji: 穴窯/ Hiragana: あながま) 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 dev ...
, arriving via
Korea Korea ( ko, 한국, or , ) is a peninsular region in East Asia. Since 1945, it has been divided at or near the 38th parallel, with North Korea (Democratic People's Republic of Korea) comprising its northern half and South Korea (Republic o ...
in the 5th century. This kiln usually consists of one long firing chamber, pierced with smaller ware stacking ports on one side, with a firebox at one end and a flue at the other. Firing time can vary from one day to several weeks. Traditional anagama kilns are also built on a slope to allow for a better draft. The Japanese noborigama kiln is an evolution from anagama design as a multi-chamber kiln where wood is stacked from the front firebox at first, then only through the side-stoking holes with the benefit of having air heated up to from the front firebox, enabling more efficient firings. *Khmer Kiln: quite similar to the
anagama kiln The ''anagama'' kiln (Japanese Kanji: 穴窯/ Hiragana: あながま) 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 dev ...
; however, traditional Khmer Kilns had a flat roof. Chinese, Korean or Japanese kilns have an arch roof. These types of kiln vary in size and can measure in the tens of meters. The firing time also varies and can last several days. * Bottle kiln: a type of intermittent kiln, usually coal-fired, formerly used in the firing of pottery; such a kiln was surrounded by a tall brick hovel or cone, of typical bottle shape. The tableware was enclosed in sealed fireclay saggars; as the heat and smoke from the fires passed through the oven it would be fired at temperatures up to . *Biscuit kiln: The first firing would take place in the biscuit kiln. *Glost kiln: The biscuit-ware was glazed and given a second
glost firing Ceramic glaze is an impervious layer or coating of a vitreous substance which has been fused to a pottery body through firing. Glaze can serve to color, decorate or waterproof an item. Glazing renders earthenware vessels suitable for holding ...
in the larger glost kilns. *
Mantou kiln The mantou kiln () or horseshoe-shaped kiln was the most common type of pottery kiln in north China, in historical periods when the dragon kiln dominated south China; both seem to have emerged in the Warring States period of approximately 475 to 2 ...
of north China, smaller and more compact than the dragon kiln *Muffle kiln: This was used to fire over-glaze decoration, at a temperature under . In these cool kilns the smoke from the fires passed through flues outside the oven. *Catenary arch kiln: Typically used for the firing of pottery using
salt Salt is a mineral composed primarily of sodium chloride (NaCl), a chemical compound belonging to the larger class of salts; salt in the form of a natural crystalline mineral is known as rock salt or halite. Salt is present in vast quant ...
, these by their form (a
catenary In physics and geometry, a catenary (, ) is the curve that an idealized hanging chain or cable assumes under its own weight when supported only at its ends in a uniform gravitational field. The catenary curve has a U-like shape, superfici ...
arch) tend to retain their shape over repeated heating and cooling cycles, whereas other types require extensive metalwork supports. *Sèvres kiln: invented in Sèvres, France, it efficiently generated high-temperatures to produce waterproof ceramic bodies and easy-to-obtain glazes. It features a down-draft design that produces high temperature in shorter time, even with wood-firing. *Bourry box kiln, similar to previous one


Modern kilns

With the industrial age, kilns were designed to use electricity and more refined fuels, including
natural gas Natural gas (also called fossil gas or simply gas) is a naturally occurring mixture of gaseous hydrocarbons consisting primarily of methane in addition to various smaller amounts of other higher alkanes. Low levels of trace gases like carbo ...
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 cone Pyrometric cones are pyrometric devices that are used to gauge heatwork during the firing of ceramic materials. The cones, often used in sets of three, are positioned in a kiln with the wares to be fired and provide a visual indication of when t ...
s—devices that begin to melt at specific temperatures. Modern kilns include: *Retort kiln: a type of kiln which can reach temperatures around for extended periods of time. Typically, these kilns are used in industrial purposes, and feature movable charging cars which make up the bottom and door of the kiln. *Electric kilns: kilns operated by
electricity Electricity is the set of physical phenomena associated with the presence and motion of matter that has a property of electric charge. Electricity is related to magnetism, both being part of the phenomenon of electromagnetism, as described ...
were developed in the 20th century, primarily for smaller scale use such as in schools, universities, and hobby centers. The atmosphere in most designs of electric kiln is rich in
oxygen Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as wel ...
, as there is no open flame to consume oxygen molecules. However, reducing conditions can be created with appropriate gas input, or by using
saggar A saggar (also misspelled as sagger or segger) is a type of kiln furniture. It is a ceramic boxlike container used in the firing of pottery to enclose or protect ware being fired inside a kiln. Traditionally, saggars were made primarily from ...
s in a particular way. *Feller kiln: brought contemporary design to wood firing by re-using unburnt gas from the chimney to heat intake air before it enters the firebox. This leads to an even shorter firing cycle and less wood consumption. This design requires external ventilation to prevent the in-chimney radiator from melting, being typically in metal. The result is a very efficient wood kiln firing one cubic metre of ceramics with one cubic meter of wood. *Microwave assisted firing: this technique combines
microwave Microwave is a form of electromagnetic radiation with wavelengths ranging from about one meter to one millimeter corresponding to frequencies between 300 MHz and 300 GHz respectively. Different sources define different frequency ra ...
energy with more conventional energy sources, such as radiant gas or electric heating, to process ceramic materials to the required high temperatures. Microwave-assisted firing offers significant economic benefits. *Microwave kiln: These small kilns are designed to be placed inside a standard microwave oven. The kiln body is made from a porous ceramic material lined with a coating that absorbs microwave energy. The microwave kiln is placed inside a microwave oven and heated to the desired temperature. The heating process is much less controlled than most modern electric kilns, as there is no built-in temperature monitoring. The user must monitor the process closely to achieve the desired results, adjusting time and power levels programmed on the microwave oven. A small hole in the lid of the kiln can be used to estimate the interior temperature visually, as hot materials will glow. Microwave kilns are designed to reach internal temperatures of over 1400°C, hot enough to work some types of glass, metals, and ceramics, while the outside of the kiln remains cool enough to handle with hot pads or tongs. After firing, the kiln should be removed from the microwave oven and placed on heat-proof surface while it is allowed to cool. Microwave kilns are limited in size, usually no more than 8 inches in diameter. *Top-hat kiln: an intermittent kiln of a type sometimes used to fire pottery. The ware is set on a refractory hearth, or plinth, over which a box-shaped cover is lowered.


Wood-drying kiln

Green wood Green wood is wood that has been recently cut and therefore has not had an opportunity to season (dry) by evaporation of the internal moisture. Green wood contains more moisture than seasoned wood, which has been dried through passage of time or ...
coming straight from the felled tree has far too high a moisture content to be commercially useful and will rot, warp and split. Both
hardwood Hardwood is wood from dicot trees. These are usually found in broad-leaved temperate and tropical forests. In temperate and boreal latitudes they are mostly deciduous, but in tropics and subtropics mostly evergreen. Hardwood (which comes from ...
s 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 are either package-type (side-loader) or track-type (tram) construction. Most
hardwood Hardwood is wood from dicot trees. These are usually found in broad-leaved temperate and tropical forests. In temperate and boreal latitudes they are mostly deciduous, but in tropics and subtropics mostly evergreen. Hardwood (which comes from ...
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 (US: "lumber") is loaded on kiln/track cars for loading the kiln. Modern high-temperature, high-air-velocity conventional kilns can typically dry
green wood Green wood is wood that has been recently cut and therefore has not had an opportunity to season (dry) by evaporation of the internal moisture. Green wood contains more moisture than seasoned wood, which has been dried through passage of time or ...
in 10 hours down to a moisture content of 18%. However, 1-inch-thick
green Green is the color between cyan and yellow on the visible spectrum. It is evoked by light which has a dominant wavelength of roughly 495570 nm. In subtractive color systems, used in painting and color printing, it is created by a combi ...
red oak requires about 28 days to dry down to a moisture content of 8%. 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. Most softwood kilns operate below temperature. Hardwood kiln drying schedules typically keep the dry bulb temperature below . Difficult-to-dry species might not exceed . 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. 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. 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. 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). 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.


Gallery

File:Mekong-kilns.jpg, Brickmaking kilns, Mekong Delta. The cargo boat in the foreground is carrying the rice chaff used as fuel for the firing. File:Wood-fired-kilns.jpg, A wood fired pottery kiln in
Hội An Hội An (), formerly known as Fai-Fo or Faifoo, is a city with a population of approximately 120,000 in Vietnam's Quảng Nam Province and is noted as a UNESCO World Heritage Site since 1999. Along with the Cu Lao Cham archipelago, it is part o ...
, Vietnam. File:Kiln in Vietnam in Chau Thanh District, 12.webm, Rice chaff being put to a brickmaking kiln in Mekong delta FIle:CatenaryArchAluminiumKiln.jpg, A catenary arch kiln used for firing high temperature electron tube grade aluminium oxide ceramics File:Porcelain kiln with furnaces á alandier.jpg, A two-story porcelain kiln with furnaces ''á alandier'' in
Sèvres Sèvres (, ) is a commune in the southwestern suburbs of Paris, France. It is located from the centre of Paris, in the Hauts-de-Seine department, Île-de-France region. The commune, which had a population of 23,251 as of 2018, is known for ...
, France circa 1880 File:Woodville Behive kiln.jpg, CAD representation of a beehive kiln File:Woodville tunnel kiln cutaway.jpeg, CAD representation of a tunnel kiln File:Kiln yard.jpg, A kiln yard with multiple kilns


See also

* * * * * *
Kiln furniture Kiln furniture are devices and implements inside furnaces used during the heating of manufactured individual pieces, such as pottery or other ceramic or metal components. Materials Commonly used materials are cordierite (up to 1275 °C), mull ...


Notes


References

*Hamer, Frank and Janet. ''The Potter's Dictionary of Materials and Techniques.'' A & C Black Publishers, Limited, London, England, Third Edition 1991. . * Smith, Ed. ''Dry Kiln Design Manual.'' J.E. Smith Engineering and Consulting, Blooming Grove, Texas. Available for purchase from autho
J.E. Smith
* M. Kornmann and CTTB, "Clay bricks and roof tiles, manufacturing and properties", Soc. industrie minérale, Paris, (2007) *


External links


Kilns and Firing Structures
entry in the
UCLA Encyclopedia of Egyptology
'
Information about the history of bottle ovens (kilns)
fro
Gladstone Pottery Museum
in Stoke-on-Trent, UK.

{{Authority control Industrial furnaces Pottery Firing techniques sv:Ugn#Ugnar inom tillverkningsindustrin