Kiln
A kiln is a thermally insulated chamber, a type of oven, that produces temperatures sufficient to complete some process, such as hardening, drying, or chemical changes.
Various industries use rotary kilns for pyroprocessing (to calcinate ores, such as limestone to lime for cement) and to transform many other materials.
[5] Pit fired pottery was produced for thousands of years before the earliest known kiln, which dates to around 6000 BCE, and was found at the Yarim Tepe site in modern Iraq.
Ceramics require high temperatures so chemical and physical reactions will occur to permanently alter the unfired body.
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.
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.
Kilns developed from a simple earthen trench filled with pots and fuel pit firing, to modern methods.
[11] Early examples of simpler kilns found in Britain include those that made roof-tiles during the Roman occupation.
Many large industrial pottery kilns use natural gas, as it is generally clean, efficient and easy to control.
Both electric and gas kilns are common for smaller scale production in industry and craft, handmade and sculptural work.
Modern kilns include: Green wood coming straight from the felled tree has far too high a moisture content to be commercially useful and will rot, warp and split.
A variety of kiln technologies exist today: conventional, dehumidification, solar, vacuum and radio frequency.
Modern high-temperature, high-air-velocity conventional kilns can typically dry 25-millimetre-thick (1 in) green wood in 10 hours down to a moisture content of 18%.
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.
[citation needed] The economics of different wood drying technologies are based on the total energy, capital, insurance/risk, environmental impacts, labor, maintenance, and product degradation costs.
[14][15][16][17][18] 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 mass unit of water removed).
