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 materials
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
In the broadest terms there are two types of kiln, both sharing the same basic characteristics of being an insulated box in which ware is exposed at controlled temperature and atmospheric regime:
Intermittent – ware is loaded into the kiln which is then sealed and the internal temperature increased according to a schedule. After the firing process is complete both the kiln and ware are cooled
Continuous, or sometimes called Tunnel. These are long structures in which only the central part is directly heated. From the cool entrance ware is slowly transported along the kiln to experience increasing temperature until it reaches the central, hottest, part from where its continued transportation leads into through falling temperature until it exits at near room temperature. A specialist type common in tableware and tile manufacture, is the Roller-hearth Kiln in which ware, placed on bats, is carried through the kiln on rollers.
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.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; such a kiln was surrounded by a tall brick hovel or cone, of typical bottle shape.
- Top-hat Kiln - an intermittent kiln of a type sometimes used in the firing of pottery. The ware is set on a refractory hearth, or plinth, over which a box-shaped cover is then lowered.
- 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. 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.
Modern kilns - 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.
Wood-Drying Kilns
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.
