Editing Kiln (section)

== Ceramic kilns ==
Kilns are an essential part of the manufacture of almost all types of [[ceramic]]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, [[ceramic glaze|glaze]]s 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 [[electronics|electronic]] control systems, although [[pyrometric device]]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]]. 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 [[thermal insulation|insulate]]d 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]]
*[[Brick#Bull.27s Trench Kilns|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]] 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:<ref>{{cite web|title=Small Scale Brickmaking|url=http://collections.infocollections.org/ukedu/uk/d/Jh2385e/9.3.html|access-date=2013-03-20|archive-date=2013-12-31|archive-url=https://web.archive.org/web/20131231000935/http://collections.infocollections.org/ukedu/uk/d/Jh2385e/9.3.html|url-status=dead}}</ref>
*[[Brick clamp|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 fired pottery|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&nbsp;°C before 2000 [[Common Era|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&nbsp;°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.<ref>Rawson, 364, 369-370; Vainker, 222-223; [http://www.novaesium.de/artikel/keramik.htm JP Hayes article from the Grove Dictionary of Art]</ref>

Both [[Ancient Roman pottery]] and medieval Chinese pottery could be fired in industrial quantities, with tens of thousands of pieces in a single firing.<ref>Vainker, 222-223; [http://www.novaesium.de/artikel/keramik.htm JP Hayes article from the Grove Dictionary of Art]</ref> Early examples of simpler kilns found in [[Great Britain|Britain]] include those that made roof-tiles during the [[Roman Britain|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:
*'''[[Dragon kiln]]''' of south China: thin and long, climbing up a hillside. This type spread to the rest of [[East Asia]] giving the Japanese '''[[anagama kiln]]''', arriving via [[Korea]] 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 {{convert|600|C|F|abbr=on|sigfig=2}} from the front firebox, enabling more efficient firings.
[[File:Ancient Cambodian kiln reconstitution.jpg|thumb|During the reconstitution of a traditional Cambodian kiln at Khmer Ceramics & Fine Arts Centre in [[Siem Reap]], [[Cambodia]]]]
*'''Khmer Kiln''': quite similar to the [[anagama kiln]]; 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 oven|'''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 {{convert|1400|C|F|abbr=on|sigfig=2}}.
*'''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]] in glost kilns.
*'''[[Mantou kiln]]''' 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 {{convert|800|C|F|abbr=on|sigfig=2}}. 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]], these by their form (a [[catenary]] 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 {{convert|1240|C|F|abbr=on|sigfig=3}} 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 ===
[[File:1. Alt forn del Port de Sagunt (País Valencià, 1951).jpg|thumb|[[Sagunto]] kiln, 1951]]
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:
*'''Retort kiln''': a type of kiln which can reach temperatures around {{convert|1,500|C|F|abbr=on|sigfig=2}} 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]] 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]], as there is no open flame to consume oxygen molecules. However, reducing conditions can be created with appropriate gas input, or by using [[saggar]]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.{{citation needed|date=February 2015}}
*'''Microwave assisted firing''': this technique combines [[microwave]] 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 {{cvt|1400|C|F|sigfig=2}}, 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 {{convert|8|in|cm|order=flip}} in diameter.<ref>{{cite web| url = https://depts.washington.edu/open3dp/2010/04/microwave-kilns/| title = Microwave Kilns}}</ref>
*'''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.