subject: The History Of The Early Kilns [print this page] Kilns are an essential part of the manufacture of all ceramics, which, by definition, require heat treatment, often at high temperatures. During this process, chemical and physical reactions occur which cause the material to be permanently altered. 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, by the temperature at which it is fired, and by the glazes that may be used. Although modern kilns often have sophisticated electrical systems to control the firing temperatures, pyrometric devices are also frequently 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. Part of the firing process includes sintering. This process 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. This crystalline material is a matrix of predominantly silicon and aluminium oxides, and is very hard and strong.
The common feature of early kilns was an egg-cup shaped burning chamber, with an air inlet at the base (the "eye"), constructed of brick. Limestone was crushed (often by hand) to fairly uniform 20-60 mm (1 to 2.5 inch) lumps - fine stone was rejected. Successive dome-shaped layers of coal and limestone were built up in the kiln on grate bars across the eye. When loading was complete, the kiln was kindled at the bottom, and the fire gradually spread upwards through the charge. When burnt through, the lime was cooled and raked out through the base. Fine coal ash dropped out and was rejected with the "riddlings".
Cement rotary kilns is used to calcine cement clinker, including dry process production and wet process. Metallurgy chemical kiln is used to magnetic roast poor iron ore in steel plants; oxidizing roast chrome, nickel iron ore; calcine high aluminum vanadium and soil ore in refractory plant, roast clinker or aluminum hydroxide in aluminum manufacturer and roast chrome ore in chemical plant. Lime kiln (active lime kiln) is used to roast active lime and light roast dolomite for steel works and ferroalloy works.
The main structure of rotary kiln body: the kiln shell, rim, thrust roller. The lime production was sometimes at an industrial scale. One example at Annery in North Devon, near Torrington, was made up of three kilns grouped together in an 'L' shape and was situated beside the Torrington canal and the River Torridge to bring in the limestone and coal, and to transport away the calcined lime in the days before properly metalled roads existed.
The large kiln at Crindledykes near Haydon Bridge, Northumbria, was one of more than 300 in the county. It was unique to the area in having four draw arches to a single pot. As production was cut back, the two side arches were blocked up, but were restored in 1989 by English Heritage.
The development of the national rail network increasingly made the local small-scale kilns unprofitable, and they gradually died out through the 19th century, replaced by larger industrial plants. At the same time, new uses for lime in the chemical, steel and sugar industries led to large-scale plants. These also saw the development of more efficient kilns.
