kiln - Ceramic or Pottery kilns, Wood-Drying Kilns

An oven for baking clay for bricks or pottery, or the clay and lime for cement, usually constructed of fireclay or resistant alloys. The term is also used for ovens operating at low temperatures for the drying of hops or grain.

A kiln is required to come to a controlled temperature, often very high, and so the design of the ovens normally focuses on insulation, and the ability to add fuel over a course of time. Care must be taken not to heat the kiln too rapidly or to too high a temperature.

Kilns are used to harden, burn or dry materials. Specific uses include:

To dry green lumber so that the lumber can be used immediately Drying wood for use as firewood Heating wood to the point of pyrolysis to produce charcoal For annealing, fusing and deforming glass For cremation (at high temperature) Drying of tobacco leaves Firing of certain materials to form ceramic materials Drying malted barley for brewing

See also:

Brick kiln Cement kiln Limekiln Oast house

Ceramic or Pottery kilns

Kilns are an essential part of the manufacture of all ceramics, which, by definition, require heat treatment, often at high temperature. In the case of pottery, clay materials are shaped, dried and fired in a kiln.

Type of Kilns

In the broadest terms there are two types of kiln, both sharing the same basic characteristics of being an insulated box with controlled inner temperature and atmosphere.

Intermittent – The ware to be fired, is loaded into the kiln. The kiln is sealed and the internal temperature increased according to a schedule. After the firing process is complete, both the kiln and ware are cooled. From the cool entrance, ware is slowly transported through the kiln, and its temperature is increased steadily as it approaches the central, hottest part of the kiln. From there, its transportation continues and the temperature is reduced until it exits the kiln at near room temperature. A specialty type of kiln, common in tableware and tile manufacture, is the Roller-hearth Kiln, in which ware placed on bats is carried through the kiln on rollers.

Kiln technology is very old. The development of the kiln from a simple earthen trench filled with pots and fuel (pit firing) to modern methods, happened in several simple stages. 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.

Anagama kiln - the Asian anagama kiln has been used since medieval times and is the oldest style of kiln in Japan. This kiln usually consists of one long firing chamber, pierced with smaller stacking ports on one side, with a firebox at one end and a flue at the other. Traditional anagama kilns are also built on a slope to allow for a better draft.A style of kiln known as Kazegama, "Wind kiln" in Japanese, was developed by Steve Davis of the USA in 1997 as an alternative to traditional wood firing practices such as those achieved in an Anagama. Bottle kiln - a type of intermittent kiln, usually coal-fired, formerly used in the firing of pottery; Top-hat Kiln - an intermittent kiln of a type sometimes used in the firing of pottery. Modern kilns - with the advent of the industrial age, kilns were designed to utilize electricity and more refined fuels, including natural gas and propane. Modern kilns can be fitted with computerized controls, allowing for refined adjustments during the firing cycle. Microwave-assisted firing offers significant economic benefits

Wood-Drying Kilns

A variety of wood drying kiln technologies exist today: conventional, dehumidification, solar, vacuum and radio frequency.

Conventional wood dry kilns are either package-type (sideloader) or track-type (tram) construction. Most hardwood lumber kilns are sideloader kilns in which fork trucks are used to load lumber packages into the kiln. Most softwood lumber kilns are track types in which lumber packages are loaded on kiln/track cars for loading the kiln. Small lumber drying plants typically use the package (sideloader) lumber kiln.

Modern high-temperature, high-air-velocity conventional kilns can typically dry 1 inch thick green lumber in 10 hours down to a moisture content of 18%. 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 lumber kilns operate below 240 degrees F temperature. Hardwood lumber kiln drying schedules typically keep the dry bulb temperature below 180 degrees F.

Dehumidification kilns are very similar to conventional kilns in basic construction.

Solar kilns are conventional kilns, typically built by hobbyists to keep initial investment costs low. 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 to introduce heat into the kiln charge. Discontinuous technology allows the entire kiln charge to come up to full atmospheric pressure, the air in the chamber is then heated, and finally vacuum is pulled. 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 versus species found in North America). 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 total (harmful) air emissions produced by wood kilns, including their heat source, can be significant. Typically, the higher the temperature the kiln operates at, the larger amount of emissions are produced (per pound of water removed).