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amazing medieval lime pit uncovered
Yesterday (13th February), Trust archaeologists Don Shimmin and Mark Baister started a watching brief on a trench on one of the current building sites in the former garrison at Colchester. The trench exposed a large medieval lime pit dating to the 12th or 13th century. The pit would have been used in the process of making mortar for building. The Trust has found two other medieval lime pits at or near the former garrison, in the vicinity of the site of St John’s abbey (one in 1973 and one more recently, in 2007), and these are associated with the building of the abbey itself. The sides of the lime pit excavated yesterday show evidence of intense burning, where loads of seashells (such as oyster shells) would have been roasted in burning charcoal in the base of the pit. Don also found some fragments of shell. The trench is about 1.5 metres deep but it does not cut the base of the pit. Don calculates that the lime pit would have been about 3.6 metres in diameter.
Medieval mortar was usually made from chalk but, in Colchester, it was more economic to make mortar from shells, as they were so readily available. In the medieval period, oysters were consumed on a massive scale and their shells were a by-product. The shells (calcium carbonate) were roasted to produce quick lime (calcium oxide). Lime mortar is made of lime and an aggregate such as sand, mixed with water. It is one of the oldest known kinds of mortar, dating back to the 4th century BC.
The Trust excavated a similar lime pit at the Sixth Form College (formerly the Gilberd School) in 1984, and this is described and illustrated in the Trust’s publication Colchester Archaeological Report 6, and there is a brief discussion of lime kilns in Colchester Archaeological Report 3.
The images show the trench on our current garrison site; being investigated by Mark and observed by a site worker, and the evidence of the lime pit in the trench, including the pit’s burnt (reddened) sides.
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