Lime Kilns
by Jane Firth
What was lime used for?
Lime, produced from limestone, has been used to make mortar for building since Roman times. In the Anglo-Saxon period, every church, monastery, bridge or manorial hall would have needed lime.
Many rural trades such as leather-tanning, soap-making and the bleaching and dying of yarn, also needed lime. And when farmers enclosed fell land, they often applied lime to it to reduce the acidity of the soil. It was also used extensively as a fertiliser to add calcium to arable land.[1]
Kilns transform limestone rock to usable lime
Limestone is a very hard rock so difficult to crush into the powder required to make mortar or fertiliser. However, if heated it is transformed into a usable form - lime.
Early lime kilns were “sow kilns’ which were a bowl shape in the ground. Layers of limestone and fuel were built up until they were level with the top of the bowl, and were then sealed with turfs. A flue for ventilation and a stoke hole were also built into the structure. Once burning was complete, the turves were removed and the lime raked out. [2] Sow kilns are hard to spot because, even if they had been lined with stone, as construction improved the stones were removed and recycled when the kiln was no longer needed.[3]
The lime kilns that we see today date from 1670 onwards. They were built into the side of a hill so that they could be loaded from the top. A vault of limestone was built over the furnace. It would take a whole day to load and once the fire was lit, it was kept stoked for about three days until all the lime had been burned. After a couple of days of cooling, the lime and ash were raked out from the bottom.
If the lime was to be used for mortar it would then be mixed with water to make ‘slaked lime’ and then mixed with sand to make mortar.
Ullswater’s Lime Kilns
Limestone was formed from deposits of shells and coral when the Cumbria of today was to be found south of the equator and under the sea. There are small outcrops of these limestone rocks throughout the Ullswater Valley but they are mainly found in the northern part, for example on Barton and Askham fells.
Lime kilns were generally built either near the source of the limestone, or near the area where the lime was needed, or where there was a good source of fuel. We know, for example, that limestone was brought down the lake by boat to lime kilns at Blowick and also at Glenridding. These were used to produce “20 loads of lime” to carry out repairs to Hartsop Hall in 1702.
In the 1780s, itemised payments from the Dalemain Estate include lime, leading lime, walling and hair. The lime seems to have been sourced from a kiln owned by the estate because other items in the accounts include: payments to stone breakers; cutting and leading peat and limestone to the kiln: and burning lime. As was common practice, “Robert Shaw was paid 6d. a pint for ale consumed at the kiln - lime-burning was a hot and unpleasant job"[4]. The “hair” referred to in this quote is horse hair which was used in mortar to make a stronger mix. Stone-breakers broke the limestone into pieces the right size for burning.
Lime to improve agricultural land
In 1800 no less than 79% of Cumberland’s land was classed as unproductive ‘waste’[5]. The enclosure movement, at the end of the 18th century and the first half of the 19th, meant increased demand for lime to improve the recently enclosed land. The enormous scale of lime burning along the Pennine edge suggests that lime was being produced in large quantities for dispatch to lower ground, either in the Eden valley or newly taken-in enclosures[6]. In the early nineteenth century, where limestone was plentiful, many farmers burned their own lime for agricultural use. However, by the 1860s, imported guano and chemical fertilisers were used instead of lime.
Use of lime in mortar all but ceased in the 1960s, though it is still used for renovating vernacular buildings.
Chemistry of Lime Burning
The limestone (Calcium carbonate CaCO3) is heated to around 900 0C. The fuel source depended on what was available, from slow-burning peat, to fast-burning gorse. Wood and later coal were also used. The burning turned the Calcium Carbonate into Calcium Oxide with Carbon dioxide as the waste product (CaCO3 + heat —> CaO + CO2). Calcium Oxide or ’quicklime’, when mixed with water (‘slaking’) becomes a stable powder or ‘lime’ (CaO + H2O = Ca(OH)2)7
By Jane Firth
1 Excavation of sow (lime) kiln at Pendragon Castle
by Hannah Kingsbury, https://www.yorkshiredales.org.uk/excavation-of-sow-lime-kiln-at-pendragon-castle/
2 Lime Burning and the Uses of Lime in the Historic County of Westmorland and along the Pennine Edge of Cumberland, David S. Johnson, in Transaction of C&WAAS, CW3, xiii, 2013, pp191-213 p195
3Lime Burning and the Uses of Lime in the Historic County of Westmorland and along the Pennine Edge of Cumberland, David S. Johnson, in Transaction of C&WAAS, CW3, xiii, 2013, pp191-213 p202
4 Lime Burning and the Uses of Lime in the Historic County of Westmorland and along the Pennine Edge of Cumberland, David S. Johnson, in Transaction of C&WAAS, CW3, xiii, 2013, pp191-213 p208
5Lime Burning and the Uses of Lime in the Historic County of Westmorland and along the Pennine Edge of Cumberland, David S. Johnson, in Transaction of C&WAAS, CW3, xiii, 2013, pp191-213 p202
6Lime Burning and the Uses of Lime in the Historic County of Westmorland and along the Pennine Edge of Cumberland, David S. Johnson, in Transaction of C&WAAS, CW3, xiii, 2013, pp191-213 p208
7The Lime Cycle https://britishlime.org/education/lime_cycle.php
