As described in our blog post on Charcoal, the process of intense heating alters the magnetic properties of organic materials, enabling them to be easily detected using magnetic geophysical survey techniques.
Pottery or brick kilns are an excellent example of archaeological features with thermoremanent magnetisation, and so are detectable by magnetic geophysical survey techniques. The purpose of a kiln is to intensely heat materials (clay) to irreversibly transformed their chemical composition into a fired object (pot, brick or tile). As part of this process, organic material placed into the kiln often obtains an induced magnetisation. For example, when clay is subjected to intense heat it initially becomes paramagnetic (i.e. spin of electrons is random) and during the cooling process becomes ferromagnetic, whereby the electrons align with the direction of the earth’s magnetic field. Often, fired materials, such as brick, pottery or other ceramic objects, appear as highly magnetic anomalies and so are easily be detected through magnetic techniques. Also, worth noting are the structural elements that kilns are often composed of that are also likely to have magnetic properties, such as clay lining and supports or bars that allow an oxygen flow and ensure an even distribution of heat.
Depending on the structural composition of the kilns and quantity of materials with thermoremanent magnetisation, kilns have the potential to appear in magnetic data sets as anomalies with high increases in magnetic readings or as bipolar anomalies. As with other archaeological features that have thermoremanent magnetisation, sometimes anomalies associated with kilns can be difficult to conclusively interpret as a result of their lack of coherent shape or form. Conversely, if the kiln comprises coherent structural elements and there is strong supporting information of settlement or industrial activity, then positive identification is possible.
Marton to Acomb Landing pipeline excavation
A previously unknown Roman roadside settlement with pottery kilns was identified to the east of Green Hammerton during archaeological work connected with the installation of a water pipeline between Marton and Acomb Landing in North Yorkshire in 2008–9.
Two adjacent kilns were oriented with the chamber to the north-west and the flue and stoke-hole to the southeast. The better preserved of the two was a semi-sunken kiln, with the circular firing chamber constructed of at least three courses of large cobbles and a base of flat stones. The stone structure would have been lined with clay, some of which survived in situ, and the kiln would probably have had a clay superstructure, which may account for some of the burnt clay found within it. Four near-complete, sand-tempered clay kiln bars were recovered from this kiln. The bars all had a square section and generally tapered towards the ends, which would have enabled them to rest securely on a ledge running around the inside of the kiln and on the central pedestal. Most of the bars had one flat side, which may have been used as the upper surface in the kiln, as it would have provided a more level surface (oven floor) upon which to set the pots, lifting them above the fire in the base of the combustion chamber.
The adjacent kiln was similar in form to the one described above, but the firing chamber was not lined with cobbles. Three distinct episodes of firing were evident within the kiln, which were characterised by successive layers of burnt clay and charcoal extending the full length of the flue and firing chamber. Two fragments of kiln bar were also recovered from this kiln.
A total of 40 sherds of pottery were recovered from the debris within the above two kilns, all of which were grey wares, indicating a date from the early 3rd to the mid-4th century AD. None of the sherds were wasters, but the site had been truncated and the discard zone may have been beyond the extent of the excavation. The pieces included a Dales ware jar and a Dales-type jar, which led to the conclusion that they formed part of the small-scale, dispersed rural potting industry producing Dales-type jars in gritty grey ware, an industry of which little is known in this part of Yorkshire.
A third possible kiln was located approximately 50m to the south-east of those described above. It was piriform in plan with the deep kiln chamber to the southeast and the shallower flue to the northwest. Four distinct deposits of grey silty clay were recorded, of which two contained fired clay potentially deriving from the collapse or destruction of a domed superstructure. Cereal remains were recovered from the primary fill of the kiln, which likely reflects the use of threshing residues as kindling and would also account for remains of burnt heather in the deposit. Alternatively, the cereal remains may suggest that the kiln was used as a rudimentary corn drier. No diagnostic finds were recovered from this feature but its proximity to the other two indicated that they may be associated.
As part of our excavations next to a Roman fort at Carkin Moor in 2016, a large portion of an up-draught kiln was excavated. The kiln dated to the Roman period and the remains included parts of the walls of the furnace made from fired clay, and some items of kiln furniture in the form of clay kiln bars and internal stone supports. Also, inside the kiln were many near-complete vessels from the final (mis)firing of the kiln sometime in the 4th century AD. The structure was highly significant in its regional importance as very few Romano-British pottery kilns have been found in the area. Also, the final misfiring of the kiln helps us to interpret how the kilns were used during this period. It is not always easy to identify kilns in the archaeological record, but due to the high concentration of fired clay in the ground, this example was easily identified through geophysics.