Kilns are thermally insulated chambers, or ovens, in which controlled temperature regimes are produced. They 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, or fusing metallic oxide paints to the surface of glass
• For cremation (at high temperature)
• Drying of tobacco leaves
• Firing of material, such as clay, to form ceramics
• Drying malted barley for brewing
• Smelting ore to extract metal
• Heating limestone with clay to make cement

Ceramic kilns

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, although usually somewhat brittle.

Types 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 completed, both the kiln and ware are cooled.

Continuous, or sometimes called Tunnel. These are long structures in which only the central portion is directly heated. 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 continuous kiln is the most energy efficient because heat given off during cooling is recycled to pre-heat the incoming ware.

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.