Kiln
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A kiln is a thermally insulated chamber or oven in which a controlled temperature regime is produced. 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:
- Converting wood into charcoal or to dry green lumber so that the lumber can be used immediately
- For annealing, fusing and deforming glass
- For cremation (at high temperature)
- Drying of tobacco leaves
- Firing of certain materials to form ceramic
Ceramic or Pottery kilns
Kilns are an essential part of the manufacture of all ceramics, which, by definition, require high temperature treatment. During this process stage chemical and physical reactions occur which causes the material to be permanently altered. In the case of pottery, types of clay based formulations, the final characteristics are determined by the composition of the clay body and the way it is prepared; the temperature at which it is fired; and any glazes used. Satisfactory firing of a clay body converts a relatively weak and porous material consisting of innumerable particles into a strong, single mass composed on a glassy phase interspersed with pores and crystalline material. This crystalline material is a matrix of predominantly silicon and aluminium oxides, and results in a materials that is hard and durable. This process is simply referred to as "firing."Type of Kilns
Clay kiln technology is very old. The development of the kiln from a simple earthen trench filled with pots and fuel (pit firing) happened in several simple stages. Heat was conserved more efficiently around pots by building a firing chamber, baffles and a stoking hole. A chimney stack improves the draw of the kiln, and uses fuel more completely. Early examples of kilns found in the United Kingdom include those made for the making of 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.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. 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.Modern Kiln Design
With the advent of the industrial age, kilns were designed to utilize electricity and more refined fuels, including natural gas and propane. The majority of large, industrial pottery kilns now use natural gas, as it is generally clean, efficient and easy to control. Modern kilns can be fitted with computerized controls, allowing for refined adjustments during the firing cycle. 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.Kilns operated by electricity were developed in the 20th century, primarily for smaller scale use such as in schools, universities, and hobby centers. As these electrical appliances improved in dependability, they became a valuable tool for artists as well. 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.
Electric kilns are manufactured in round, oval and square shape designs, and are available in varying sizes. In general, an oval shaped kiln design allows for a more effective heating and a larger volume.
Wood-Drying Kilns
A variety of wood drying kilns exist today. Lumber 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. Of the two types the track type is far superior in lumber drying quality control (temperature and air flow rate through the lumber packages) when drying large amounts of lumber. Small lumber drying plants typically use the package (sideloader) lumber kiln.Modern high-temperature, high-air-velocity southern pine kilns can dry 1 inch thick green lumber in 10 hours down to a moisture content of 18%. However, 1 inch thick green oak requires about 28 days to dry down to a moisture content of 8%. Hardwood kilns also require special humidity sprays to keep the relative humidity inside the kiln from dropping too low during the drying cycle.
Most softwood lumber kilns operate below 240 degrees F temperature. Most hardwood lumber kilns operate below 180 degrees F temperature. Many hardwood kilns never exceed 140 degrees F temperature. Some modern lumber kilns are also partially depressurized to speed the drying process and reduce fuel usage.
See also
External Links
- [Young International Woodfirer Association, an international association of people building and using woodfired kilns]
- [The Log Book, the magazine about kiln woodfiring]
References
- Hamer, Frank and Janet. The Potter's Dictionary of Materials and Techniques. A & C Black Publishers, Limited, London, England, Third Edition 1991. ISBN 0-8122-3112-0.
- Smith, Ed. Dry Kiln Design Manual. J.E. Smith Engineering and Consulting, Blooming Grove, Texas. Available for purchase from author[link].
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