South Devon has important industrial heritage. Quicklime was an important commodity in the 18th and 19th centuries. It was used to make mortar, plaster and limewash, and frequently used as a dressing for the land to reduce the acidity of the soil. It was made in lime kilns by burning locally quarried limestone. Most were built between 1700 and 1850, they are often substantial stone structures and many operated until the 20th century. There are over 100 known lime kilns in South Devon; they’re particularly common around the Kingsbridge and Dart estuaries with 22 known on the Kingsbridge Salcombe estuary alone.
Limestone is a sedimentary rock composed largely of the mineral calcite (calcium carbonate: CaCO3). The Romans developed the burning of limestone to make lime for use in building as a mortar, although there is little evidence of their kilns in the country.
During the Middle Ages, with the increase in building, the demand for lime again increased. However until the middle of the eighteenth century most lime kilns were temporary structures near to the site where the lime was required. These were either left to collapse after use or dismantled.
Burning limestone, which is calcium carbonate, gives you quick lime, calcium oxide. Mixed with water this produces slaked lime, calcium hydroxide. When slaked lime or quick lime was added to the land it raised its pH and so improved its fertility. Slaked lime was also used as lime putty for building. This is soft when first mixed, but with time absorbs carbon dioxide from the atmosphere and hardens as it reverts back to calcium limestone.
Lime kilns were used from medieval times right through to the 18th and 19th centuries. They were used in earlier times for the production of mortar for building purposes, and sometimes, as during the 18th and 19th centuries, for the production of lime for agricultural purposes.
Lime for building is derived from chalk or limestone (carbonate of lime).
The process is very simple. It consists in heating the stone in kilns constructed in the open air, in the vicinity of places providing the fuel and the raw limestone, for there is no point in transporting the untreated bulk material.
The limekiln is about eighteen feet in height and is lined with bricks able to endure the fire. An opening at the bottom gives access. Either an arch of limestone is built over the fuel and then the kiln filled above this; or alternate layers of limestone and fuel are packed into the kiln. The fuel is them ignited, and the heat can be prolonged by shutting the top opening of the kiln ‘chimney’ with sods of grass. The heat decomposes the limestone into pure lime (quicklime) and carbonic gas. After the process is complete, the lime is broken up and removed from the kiln, and shipped to where it is needed.
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 1 to 2.5 inch (20-60 mm) 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”.
Only lump stone could be used, because the charge needed to “breathe” during firing. This also limited the size of kilns and explains why kilns were all much the same size. Above a certain diameter, the half-burned charge would be likely to collapse under its own weight, extinguishing the fire. So kilns always made 25-30 tonnes of lime in a batch. Typically the kiln took a day to load, three days to fire, two days to cool and a day to unload, so a one-week turnaround was normal. The degree of burning was controlled by trial and error from batch to batch by varying the amount of fuel used. Because there were large temperature differences between the centre of the charge and the material close to the wall, a mixture of under-burned (i.e. high loss on ignition), well-burned and dead-burned lime was normally produced. Typical fuel efficiency was low, with 0.5 tonnes or more of coal being used per tonne of finished lime (15 MJ/kg).
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.