A Roman Limekiln Discovery in Northamptonshire
- DATE WRITTEN:
- READING TIME: 4 minutes
- AUTHORS: Dane Wright (Senior Project Officer)
In late 2022 the Milton Keynes fieldwork team made an unexpected discovery during excavation of an extensive Iron Age farmstead next to the Dallington Brook in Northamptonshire. Dug into an infilled Iron Age enclosure ditch was a large circular pit with a stone lined base and sides. It was initially thought to be an oven of some form, but it soon became clear that this was much larger than the examples we usually encounter. No artefactual or biological (plant remains, animal bone) material was present within this feature, other than charcoal, to help indicate its function. However, as our research progressed, we identified it as a probable limekiln!
Limekilns are used to produce quick lime (also known as unslaked lime, burnt lime or lump lime), that is used to form the basis of mortar, plaster and concrete. Limekiln technology first appeared in Britain during the Roman period and evidence for lime production is often found close to villas and towns where the main building projects occurred, such as the limekiln found at Weekley, Northamptonshire, situated close to a villa at Ringstead. During the post-Roman period their use dropped off but they made a return, on a large scale during the medieval period.
There are two types of limekiln: flare kilns (or intermittent/periodic kiln) and draw kilns. Flare kilns have a single charge of limestone that would be burnt, then the kiln partially deconstructed and emptied before being relined, rebuilt and recharged. Whereas a draw kiln was loaded in layers of fuel and limestone, which was continuously burnt. The charge chamber at the top was constantly replenished and the quick lime would be produced at the base.
The flare kiln produced a higher quality quick lime as the limestone charge and the hearth were not in direct contact, which prevented the mixing of wood ash and lime. However, as the intermittent name suggests, the flare kiln was labor intensive and expensive to run and as a result the draw kiln was developed. The draw kiln was able to continuously produce lime without interruption or dismantling the kiln itself, but it produced a poorer quality of lime as it was mixed with ash from spent fuel due to the mixing of fuel and charge layers.
Flare kilns were the type generally used during the Roman and medieval periods. A fragment of charcoal from our limekiln has produced a radiocarbon date of 234–361 AD, indicating that it was in use during the mid to late Roman period! Other contemporary examples have been found in Northamptonshire at Hardwick Park, near Wellingborough, and Priors Hall, Corby.
The Kiln exposed and excavated at Northampton was a substantial below ground structure and consisted of a single sub-circular charge chamber, around 4.2m in diameter, and while truncated by modern plough activity, it was still a whopping 2.35m in depth. An internal ledge averaging 0.43m in diameter was situated at approximately 1.1m below the top of the chamber.
Below this, the bottom of the chamber was conical and lined with large angular stone blocks, which is indicative of the flare kiln type. A single draw-hole, or flue, was located at the base of the chamber wall, facing south-west. It was 0.6m wide and consisted of ten courses of roughly hewn stone. This had either collapsed or had been pulled down after the kiln’s final firing. The stone structure around the flue had become a reddish pink that is indicative of stone subjected to heat.
Two thick clay deposits were identified within the stone-lined charge chamber during excavation, both seemed to have been exposed to heat and the lower clay deposit sealed a very dark probable ash deposit that was the result of spent fuel. Flare kilns once fired are taken apart in order to retrieve the quick lime produced, the clay deposit may be the remains of a clay lining associated with the kiln’s final stages of use.
Outside of the kiln chamber and flue was a triangular-shaped, gradually sloping stoke hole, approximately 7m in length and orientated south-west/north-east. There was a large spread of ash and soot in front of the flue, which was likely to have been the result of periodic raking out of the spent fuel, as the kiln would have been closely monitored over the course of its firing period.
The kiln was located on the south-west side of a low rise within a gently undulating shallow valley. The below ground structure was sheltered from the prevailing southern winds and was around 100m to the south of the Dallington Brook; a good water source that could be used in the process of slackening the lime.
The kiln was situated on an area of limestone geology (Blisworth and Wellingborough Member formations) that extends north-westwards from New Dunston towards Nobottle. Several Post-medieval limestone quarries are known in this area, most notably a quarry located approximately 500m to the immediate north-east of the kiln at Harlestone Lodge Farm, now Fleetland Farm.
Interestingly, a limekiln is also marked in this location on the 1883–84 Ordnance Survey map, indicating that lime was still being produced in this area during the post-medieval period!
This work has been done on behalf of Bloor Homes in consultation with Quercus Heritage Limited.
