The burning of limestone to produce lime was probably first undertaken in Europe in the medieval period. The construction of castles and manor houses, monastic buildings and bridges, required copious quantities of lime mortar and limewash to be available close at hand. Lime kilns were erected to meet this demand.
Generally in any area/country where permanent and high quality masonry buildings were erected, lime mortar and therefore burnt lime was needed in large quantities. It was also an essential agricultural material in many parts of the world where soils were wet and acidic.
The use of lime gathered pace during the later 16th and through the 17th centuries as land began to be developed for more intensive agricultural usage.
In their natural state, even where the underlying bedrock is limestone, many soils can be acidic - soupy to the farmer - with low levels of nutrients but high moisture levels and heavy to work. Whatever might grow in these soils provided little benefit to sheep or cow, nor thus to people. To bring such soils up to productive pasture or cultivation involved draining the land, paring off existing growth so it could be burned off, with the resultant potash being slowly washed into the soil by rain. Only at the end of this lengthy process was marl or burned lime - quicklime - added to the soil.
Though there were local variations, the accepted way of doing this was to lay the quicklime in small heaps across the field, sometimes covered in soil. These were left to stand to allow rain water to slake the lime, making it more readily absorbed by the soil. Ever so slowly calcium in the slaked lime was released into the soil, sweetening it by reducing acidity levels. Lime in itself is not a fertiliser, but instead acts as a catalyst for fertilisers. Lime fixes ammonia in the soil and limits the negative effects of nitrates. It helps to aerate the soil, thereby increasing microbial activity in recycling nutrients; it makes the soil more easily worked; and it releases calcium, an essential element of plant growth, into the soil.
By the middle of the 18th century, lime was in widespread use for reclaiming waste and for improving existing land, as well as for mortar and limewash. In addition it was used for a host of small-scale industrial purposes, such as tanning, textiles, soap and paper making.
Up to this time lime kilns were small and simple in concept. Indeed, the design and technology of a late 17th century kiln were little different from a medieval or Roman kiln. They show up as low circular earthworks, usually with a diameter of about 2 metres, surrounded by a low bank, with a narrow neck or funnel leading into the central bowl. They are rarely more than 1 metre deep and most are built into the slope, below small surface quarries or a natural scree slope.
These are called sod, or sow kilns and they operated in much the same way as a traditional charcoal clamp. Alternate layers of fuel and small stone were stacked, the whole mass being covered over with turves, and allowed to slowly burn through. The clamp was then dismantled to retrieve the burnt lime.
In the 18th and early 19th centuries kilns became more sophisticated with a stone-built superstructure. .
All such kilns operated in much the same way. Kindling was laid on a grill at the base of the cylindrical, or oval, vertical bowl with alternate layers of fuel and stone being laid on top. Most kilns had a small quarry immediately behind and a ramp so that stone and fuel could easily be tipped into the bowl. Once a kiln had been filled to capacity, the rate of burn was regulated to allow actual burning in the central section of the bowl; hot air from here dried out and heated up fresh stone in the upper third; while that in the lower third slowly cooled down ready to be drawn through the draw hole - or eye - at the base. It was then packed in sacks or barrels or loaded on carts to be spread across nearby fields or despatched to areas lacking limestone. Some kilns were constantly being topped up and drawn in a continuous process. Others were filled up, fired and drawn in one discrete operation, such kilns were worked on an intermittent basis.
As the 19th century progressed demand for lime outgrew the capacity of field kilns, and industrial-scale kilns were built, associated with large quarries.
Text by David Johnson (UK) Used gratefully without permission.