Kiln Architecture and Technology

    Lime may be calcined in a variety of kiln types, or even without use of a kiln at all. An increase in the sophistication of firing equipment generally has a corresponding decrease in the amount of fuel and labour required, and also the ability to produce larger quantities of high quality limes. Kilns may be temporary or permanent constructions, capable of only a single firing or built for continued usage over considerable periods of time. Kiln architecture is therefore related to the scale of the job for which the lime is required. Wingate (1985) reviews methods for lime burning used in rural communities throughout the world at the present day.
    The simplest technique of calcination, though the most uneconomical with fuel is to stack layers of wood with layers of lime as an uncovered heap. There is no control over the burning process, and predictably there will be underburnt lime in the outer parts of the heap. Covering the heap with clay daub and allowing air inlet holes will, to a certain extent, rectify this problem. Alternatively, limestone can be calcined in a pit, into which the lime will collapse and can be slaked in situ. These ëkilnsí will have a temporary period of use and leave little or no trace on the archaeological record, except for possibly a lime coat on the pit wall.
    Permanent structure kilns fall into two basic types; flare kilns and draw or running kilns. The two have similar construction, generally a broad chimney, often set into the side of a hill. The kiln is loaded from the top (the hill side) and fired from the bottom, from whence the lime is also removed. Flare kilns are loaded with a single charge of limestone. Firstly a vault of limestone blocks is built over the furnace, above which the rest of the limestone is stacked. The fire is lit and kept stoked for several days until all the limestone has been calcined. The kiln is then unloaded, the lime sent to the slaking pits, and the process repeated with the next batch of limestone. In contrast, draw kilns have a permanent grate fixed over the furnace and the limestone is stacked above this in layers alternating with layers of fuel. As the fuel burns the limestone is calcined and the lime drops through the grate from where it is removed through the stoke hole. As the fuel/lime layers drop through the grate, further layers can be added at the top, allowing for a continuous process to be operated. Both types of kilns have their advantages and disadvantages. The obvious advantage of a draw kiln is that large amounts of lime can be produced and it is efficient on fuel. However, the lime will be mixed with ash from the burned wood.
    In many regions lime is still burned using traditional methods and kilns. Adam (1994) discusses the characteristics of kilns presently operating in Greece, Italy and Tunisia, all of which operate using technology available to Hellenistic and Roman craftsmen. These kilns are of the flare kiln variety as described above. It is likely that this was the commonest variety of kiln operated by the Romans. Cato, writing in c. 160 B. C., in his treatise on agriculture describes the construction and operation of such a flare kiln.

1. Periodic or 'Flare" Kiln


2. Draw Tunnel or Running Kiln


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