Excavation of sow (lime) kiln at Pendragon Castle

Dr David Johnson, along with archaeologists from YDNPA, have recently led an excavation at a suspected sow kiln at Pendragon Castle. Fourteen volunteers took part over the first two weeks of September. The excavation was an exciting start to the Westmorland Dales Landscape Partnership’s Monuments at Risk project.

Before the excavation started the working hypothesis was that the kiln most likely dated to the Lady Anne Clifford restoration in the 1660s. This was due to the well-preserved condition of the earthwork. However, as the excavation developed the hypothesis was reworked.

Pendragon Castle dates to the late 12^th century and is situated in Mallerstang (Cumbria). There were a number of building phases. It received a licence to crenellate in 1309. (During the medieval period a licence to crenellate granted the holder permission to fortify his property; this was usually granted by the king.) Pendragon was reputedly slighted during a raid by the Scots in 1341. It was “rebuilt” in the 1360s. It then fell out of use, until Lady Anne Clifford restored it in 1660-2, returning it to its former glory. Following her death, it was partially dismantled by her successors due to the high upkeep costs of several castles. As a result, the castle gradually deteriorated into a ruinous state. For more information about the fortified tower-house, check out the Site of the Month blog from November 2017.

So what is a sow kiln? The overriding image of a lime kiln is a free-standing masonry structure with a squared or rounded front face with an arch at the base and a bowl or chamber within which limestone was converted to lime.  These date from about 1670, and can be seen throughout the landscape of the Westmorland (and Yorkshire) Dales. Lime kilns were built for a number of reasons – however, it was principally for lime mortar for construction or for spreading on fields for agriculture. Sow (or clamp or sod) kilns date from the Anglo-Saxon period (450-1066) and remained in use until the 19^th century. From the Anglo-Saxon period stone buildings needed mortar. These kilns converted limestone into quick lime for lime mortar. The nearest source of limestone to Pendragon Castle was Birkett Knott. This is quite a distance from the site. However, burnt lime is very caustic so you would build the kiln as close as possible to the building site and transport the stone to it.

The earthwork consists of a curving bank which is penannular in shape. [It is a bit like an Iron Age torque, with an opening at the front. This opening would have been closed by large stone lintels when the sow kiln was in use, removed after abandonment to be recycled as building material.] The sow kiln at Pendragon Castle is on the larger size for this type of kiln and as such was clearly designed for a major building project.

Within the bowl of the kiln the fuel (at Pendragon they were mainly using carbonaceous shale and coal) was layered with the limestone until a flattened dome level with the banks. This was then sealed with turves. There was usually a flue through the middle of the entrance of the kiln, which let oxygen in to feed the fire. Very often, there was also a stone-lined stoke hole, which extended to the back of the kiln. This allowed the air to circulate. The length of time of the process depended on a number of factors, including the time of year, weather, and how wet the fuel and stone was. However, it simmered slowly, and a kiln of this size would take about a week to 10 days for each episode of lime burning. As a rough estimate, the kiln had a capacity of 17m³ of stone and fuel. Once finished they dismantled the kiln by taking the turves off, the lime out, and cleaning the bowl. It would then have been used again, for the number of times needed to complete the job.

Over the years the construction of these kilns differed. For instance during the 17^th century, the banks of the kiln were lined with coursed stone. Whereas earlier kilns were “cruder” with the stone impressed into the banks. As mentioned earlier the size and preservation of the site pointed to the 1660s. However, the construction of the banks was “cruder” than expected and may point to an earlier date. It could have been connected to the building phases of the 14^th or 15^th century or even the castle’s construction in the late 12^th century.

During the course of the excavation four sherds of medieval pottery, a fragment of glass from a rim of a vessel, a small piece of pony shoe and 14 pieces of charcoal were recovered. These have been sent to experts to be analysed. They will attempt to date the charcoal by radiocarbon dating, however this will depend on the tree species. We will hopefully receive the final results in the New Year. The excavation will help to add to our understanding of the site.

During the excavation we led 2 guided walks. These walks explored some of the wider heritage of Mallerstang, including its special medieval landscape, as well as its mining and farming past. The walk ended at Pendragon where they could find out more about the dig and possible sow kiln.

The Westmorland Dales scheme is a four year landscape partnership funded by the National Lottery Heritage Fund. 10 of the 21 projects are focussed on exploring and uncovering the cultural heritage of the Westmorland Dales area. For more information about the scheme take a look at their website here. You can also subscribe to their newsletter.