Limekilns
During one of our recent walks, led by Kathy and Norman Wakefield, along the Northumberland coast at Bamburgh, we stopped for a short break near some limekilns. In the expectation of being able to write a short note on limekilns I asked Richard Palmer to take some photographs while we puzzled over aspects of the limekiln construction.
What are limekilns?
Limekilns were used to convert the raw material limestone into burnt lime, which was used in building mortar and as a fertiliser (or conditioner) for land. Limekilns were used to heat limestone rock at between 900˚C and 1100˚C, converting limestone to burn lime, which is an alkaline inorganic compound of calcium. Burn lime used to be known as quicklime because of its chemical reaction when added to water. The optimum period of the kiln was probably 1750 to 1850 although it does have a long use going back several hundred years. There were two kinds of kilns – a pye kiln (or pit) and a running kiln. The running kiln uses a structure popularly know as the limekiln or field kiln. The commonest form of field kiln is a sturdy structure of dry limestone walling forming a circular or square tower, about 15ft or more in diameter and about the same in height. The core of the structure is a circular bowl 8ft to 10ft in diameter, lined with sandstone and tapering to a bottom diameter of 3ft or so. At the bottom of the bowl a grate was inserted through which burn lime and ashes could be raked out.
The grate was at the end or top or a short tunnel, the mouth of which forms the archway which gives the kiln its unmistakable character. Air would be drawn up through the kiln base and layers of peat or coal and limestone would be placed in the kiln with some wood in the bottom to start the fire.
Subsequent layers of peat or coal and limestone would be added to the top as the burn lime was removed at the bottom.
The limekilns at Bamburgh.
These field limekilns would have been built to provide lime for mortar used in the renovation of Bamburgh Castle by Lord Crewe in the 1750s. The first Lord Armstrong bought the castle at the end of the 19th century and carried out further restoration. The castle remains in the ownership of the Armstrong family. I puzzled over the obvious missing bottom aperture where the grate would be located and of course it will be under the rubble you can see in Richard’s photograph. It looks as if the kiln had more than one bottom aperture, possible to catch the prevailing winds to aid combustion and removal of the lime ashes.
What is limestone?
Limestone is an organic sedimentary rock, made up of calcium carbonate, and formed from the remains of tiny shells and micro-skeletons deposited on the sea bed. Over millions of years the particles were buried by other sediment and under great pressure, formed solid rock. Limestone is formed in layers called bedding planes that contain vertical cracks (called joints) which make the rock permeable (allowing water to pass through).
Field limekilns used for farming.
The rainfall in Upper Teesdale falls in the range 1000mm to 2000mm per annum and rises to over 2000mm on Cross Fell. This means that the soils of the area tend to be leached of plant nutrients and rather acid in character so that peat forms on the higher areas. In order to be able to grow grass successfully (for sheep grazing) and avoid reverting to heather, farmers needed drainage and fertiliser. In the mid 19th century many fields in the Teesdale area were provided with drainage schemes and these can be seen today in the dry summer months when the wetter soil around the drains form green lines etched in the fields of buttercups. To counteract this leaching effect, farmers needed frequent applications of lime to neutralise the acidity in the soil and increase plant nutrients, organic matter and trace elements.
There are the remains of a large number of field limekilns in the Pennine Dales – lead miners were encouraged to develop smallholdings by the London Lead Company in Alston Moor and the Beaumonts in Allendale and Weardale and many farmers built their own limekilns.
The health of miners was benefited by having a proportion of outdoor work, and poor wages were offset by the milk of a cow, the rearing of a few sheep or the over-wintering of a few hill cattle.
A kiln would be strategically positioned near a limestone outcrop so that it required the minimum of effort to carry the bulky raw material. The position would be lower than the supply of coal and above the fields to be limed so that the carriage of material would always be downhill.
Out and about on the landscape.
It is hoped that some small interest may be aroused in the part that limekilns played in our industrial heritage after reading this short introduction. You may view our landscape with renewed interest. If you come across one then take a close look and check its position relative to the surrounding landscape. Is there a limestone quarry nearby? Where did the coal or peat come from?
Grykes, Clints and Gorges.
Dry valleys are evidence that rivers once flowed on the top of limestone thousands of years ago and rivers travelling at great speed would gradually wear away the surrounding rock causing drastic changes to the landscape. The valleys are usually very steep sided as a result. At the same time limestone mountains are formed where much the same thing happens. The vertical sides are formed by water which deepens the cracks where the freeze-thaw cycle takes place forcing apart the limestone. Limestone areas often have a flat, plateau like appearance. Where there is no soil in the landscape, the top bedding plane will be exposed as a limestone pavement. Many joints reach the surface along this pavement. However they are widened and deepened by solution to form grykes. The flat topped blocks between them are called clints and both together form limestone pavements. Other surface landforms result from limestone having collapsed. Where limestone collapses over an underground river it creates a gorge.
By Ronnie Teasdale
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