Sweetening the land

Ellie Leary – Monuments for the Future Project Officer

Not all of the archaeology within the North York Moors is as ancient and enigmatic as the standing stones or rock art (see previous posts). Once a ubiquitous presence within the landscape, you can still stumble across the remains of a more industrial feature – lime kilns. These structures were part of an industry that has shaped and changed the landscape of the area from the extraction of the limestone from quarries to its end use as a building material and soil improver.

Over 400 lime kiln sites are recorded within the National Park’s Historic Environment Record, with the evidence coming mainly from historic maps, but also earthworks and standing remains. Only three of these kilns have the benefit of protected status as Scheduled Monuments, and in all cases they are included as part of a wider monument rather than in their own right. Another three have protected status as Grade II Listed Buildings.

Grade II Listed Building (1149198) - lime kiln, Hawnby (HER 5946). Copyright NYMNPA.

The use of lime has a long history in Britain dating back to at least the Roman period and over time it has had a wide range of practical uses from forming the base of plasters, mortar and concrete; as lime-wash for waterproofing walls and lightening interiors; in the bleaching of paper and preparing hides for tanning; as a disinfectant; and as a soil improver for agriculture.

During the Roman period it was used particularly for lime-mortar, plaster and lime wash; while during the mediaeval period the need for quantities of lime hugely increased with the construction of large stone-built buildings and bridges. From the 17th century onwards however the main use of lime has been in agriculture, with it being added to soil to improve acidic soils or as a top dressing to pasture to “sweeten” the land.

In most cases in order to turn raw limestone into a useable product it has to be fired in a kiln, creating a process called calcining where calcium carbonate is converted into calcium oxide. This process was both labour and fuel intensive and the trade was known as lime burning – those working at the kilns, were lime burners.

Most of the kilns known of within the North York Moors date to the 18th and 19th centuries, although earlier examples do exist. Excavations at Ayton Castle, for example, revealed a lime kiln dated to the 14th century, which may have produced lime mortar and cement for the construction of the castle’s tower house, the ruins of which still stand.  (This is one of the three kiln sites included within a wider Scheduled Monument – see above).

The earliest kilns were simple clamp kilns which consisted of a circular or rectangular hollow within which the limestone and fuel were layered, covered with clay or turf, and left to burn for a few days. Often clamp kilns leave little obvious trace, however the remaining protected kiln sites in this area (as mentioned above) include two clamp kilns built into the bank of a scheduled prehistoric cross dyke and another cut into the edge of a scheduled Bronze Age barrow – the actual kilns are all thought to be 18th or 19th century in date. Their remains can be seen as horse shoe shaped mounds of earth and stone rubble.

As the demand for lime increased kilns became more substantial in size although the transformation process remained the same. Kilns were generally circular or square stone structures, about 3m in height, with a bowl lined with sandstone or firebricks and at least one draw hole located at the bottom of the kiln. As the contents burnt through the lime was extracted through the draw holes at the bottom. Additional layers of stone and fuel could be added to the top if necessary, otherwise one-off firings were carried out as needed. A good example of this kind of kiln can be found at Old Byland where the remains of four lime kilns stand next to a road (see image below). They are located on the edge of a quarry to the south west of the village and some parts survive to 5m in height, with two of the kilns having the roof and flue surviving.

Old Byland roadside lime kilns (HER 2680). Copyright NYMNPA.

The end product removed from a kiln was called ‘lump lime’, ‘burnt lime’ or ‘quicklime’ and in order to convert this for use it has to be ‘slaked’ – a process involving adding water to cause a reaction which produces heat and steam. By then adding enough water, putty is produced, which, mixed with sand, produces a mortar. Over time this reverts back to calcium carbonate and hardens.  When used in agriculture the ‘lump lime’ was left in heaps, covered in earth and left to slake, eventually creating a powder that could be ploughed into the soil. Other methods were used too, including leaving the lime uncovered and occasionally turning to produce the same result.  ‘Lump lime’ is a volatile material and there were inherent dangers if it started to ‘slake’, producing heat, before it arrived at the final destination.  By the late 19th century, hydration plants were introduced that could grind the lime, sprinkle it with water, dry it and then bag it for transporting.

The location of kilns largely depends on the final use for the ‘quicklime’, so that if it were needed for building construction the kilns would most likely be located close to the building site. They could then either be dismantled and moved or left to decay once they were no longer needed.

Field kilns were sometimes built by farmers and land owning estates from the 17th century. Smaller kilns would have been built by farmers for occasional use to improve their land but estates often built larger kilns to serve the whole estate and wider area, providing a profitable source of income.

Another common location for kilns was close to or within limestone quarries. Many of these quarries are still obvious on the ground now as large excavated pits; historic mapping helps to identify the full extent of the quarries and the location of kilns. The 1893 Ordnance Survey map (below) shows Sour Ley Quarry near Helmsley with up to 20 lime kilns within the quarry.

Extract from 1:2,500 Ordnance Survey Map 1893

Easy access to transportation was also another consideration for the location – for fuel to be brought in and for the final product to taken away for sale.  Colonel Sir Joshua Crompton, 19th century owner of the Kepwick Estate on the western edge of the North York Moors, built a railway line in the early 1820s which carried limestone from a quarry on Kepwick Moor down to the lime kilns and stone yard to the west. Fuel for the kilns could be easily brought in and the final product taken away on the Thirsk to Yarm turnpike road (now the A19). With a very steep incline up to the quarry the railway used gravity; as the full wagons were sent down slope they pulled the empty ones up towards the quarry, whilst horses pulled the wagons along the flat plain to the west. The quarry and the start of the now dismantled railway line lie with the National Park boundary and the lime kilns themselves are a short distance outside the boundary and are protected as a Scheduled Monument.

Lime kiln north of Sinnington (HER 4981). Copyright NYMNPA.

As the demand for quicklime grew the process became industrialised, with new kilns designed with efficiency in mind as well as a higher quality lime product. As a result most of these smaller local kilns were abandoned by the 20th century, with some being dismantled and others left to decay, remaining in the landscape as a reminder of this chapter of industry.

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