New evidence for medieval copper alloy casting in the City of London

Rachel Cubitt
05.06.2016

Rachel Cubitt recently joined MOLA as a Trainee Registered Finds Specialist. In this blog she outlines the findings of one of her first assignments, researching a large group of ceramic mould fragments…

Medieval visitors to the area around Leadenhall Street in the City of London would have seen a hive of industrial activity.  A group of metalworkers were casting copper alloys, making large vessels such as bells and domestic cauldrons. Each one of these castings was made in a mould, formed by encasing a wooden pattern in clay. The pattern was carefully removed and the clay fired before the metal was poured in. Once the metal casting had solidified, the mould had to be broken open to remove the finished object. The smashed fragments of mould were of no further use and discarded as rubbish.

Fast-forward to 2013, when a team of our archaeologists started to excavate a new site in this area, and went on to find over 17kgs of discarded mould material along with other evidence for casting, such as crucibles and copper alloy waste. This material has now been studied by our experts and gives a good impression of the industrial activity in the area.

The mould fragments are generally small but many still have the characteristic curving profile that indicates large open vessels. The overall form of these vessels is more difficult to determine. The shape of some of the mould fragments suggests some were made with short legs, and possibly handles or spouts. Some of the mould fragments that may represent handles are decorated with a ring design incised into the mould, which would have appeared as a raised decoration on the cast object. For these fragments, comparison to existing medieval cast vessels in museum collections is a must, in order to look for parallels with similar decoration.

The next step will be to determine the composition of the alloys used. Few of the mould fragments have traces of metal on their surfaces but among the once molten waste, experts have identified part of a failed casting (an object that had broken because of a large air bubble trapped within the metal), and a sprue cup (molten metal that solidified in the funnel that fed the mould during pouring). Alloy composition is important because the alloy used for bells, which had to make a pleasing sound when struck, was different to that used for domestic vessels. In fact Billiter Street, close to the excavation site, shares its name with the occupational name for a bell-founder, which may provide a further clue to the origin of the waste.

Further research of the mould fragments and metalworking debris from this excavation will continue in the coming months. Although metalworking is already known to have taken place in this area of London, this new material includes a substantial number of diagnostic mould fragments and other items with the potential for scientific analysis. These fragments offer opportunities to better characterise the industry and will prove important in developing our understanding of medieval metalworking in the City.

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