What causes us problems in dishwashers and the like can provide insights into archaeological secrets, a study has shown again: An analysis of lime deposits has provided clues to the development history of the Roman watermill complex of Barbegal. The results reflect technical adaptations, the way in which it was used and the decline of the factory-like facility, the researchers report.
The amazing complex in what is now southern France probably supplied tons of flour every day around 1800 years ago: the Barbegal mill complex is considered one of the oldest examples of industrial production in history. In contrast to the small units typical of the Roman Empire, it was a site for the mass processing of grain. There is evidence that the complex was used for around 100 years to supply the region’s Mediterranean ports with flour for the production of ship’s biscuits. From research into the remains, it is known that the complex, built into a hillside, comprised 16 mill wheels, arranged eight at a time in two parallel rows of buildings one above the other. The complex was supplied with water from above via an aqueduct, which was directed to the mill wheels one after the other.
Formed by the former water flow
For some time now, the team led by Cees Passchier from the Johannes Gutenberg University in Mainz has been researching the functionality and development history of the facility. Now, special finds from the facility have provided further clues: lime deposits. Together with international colleagues, Passchier and his team have examined carbonate pieces from the Barbegal mill complex that were kept in the Archaeological Museum in Arles. As they explain, these are fragments of deposits that once formed on the walls and floors of the facility’s wooden troughs as a result of the flow of water.
First, the team had to combine the approximately 140 carbonate pieces like puzzle pieces to form the once connected elements. The deposits, which were characterized by fine layers, were then analyzed using mass spectrometry. According to the researchers, the largest carbonate elements came from three wooden troughs, which apparently supplied three of the 16 mill wheels with water in the last phase of the plant’s use. As the team reports, the examination of these elements was initially able to shed light on a previously unanswered question: Was the plant always operated as a complete system or were the 16 water wheels also used independently of one another? It has now been shown that the layers of the three water troughs examined differ significantly from one another. This makes it clear: at least in the last phase of use, the mill wheels were apparently also operated separately.
Adjustments on the track
The analyses of the deposits also provided evidence of adjustments to the technical system. From one finding, the researchers deduced that at least one water wheel was replaced by a larger one. The clue was provided by the strange shape of a carbonate layer that had formed in one of the water channels that once supplied this wheel with water: While the lower, older layers suggested a low water level, the newer layers were deposited at a greater height in the channel. The team attributes this to the fact that the position of the inlet was adjusted: from originally steep and therefore with a low water level to a flatter orientation with a higher one. This change was apparently necessary due to an expansion, the researchers explain. “Such a change only makes sense if a larger water wheel is used,” says Passchier. A special piece of carbonate that apparently once formed on the water wheel itself confirmed this assumption: Because it does not have all of the carbonate layers, but only the last of the piece from the channel, the team reports.
The researchers were also able to gain information about the useful life of the last wooden structures in the complex by examining the carbonate layers. The results are based on an isotope analysis. The oxygen in the material contains isotopes that are present in different ratios depending on the water temperature. The results of the analysis enabled the researchers to assign certain layers to seasons. This in turn revealed that the deposits were deposited in the water channels over a period of seven to eight years.
The team also made discoveries that show that the carbonate pieces come from the final phase of the plant’s use: “The top and therefore youngest carbonate layer contains mussel shells and pieces of wood,” says Passchier. The researchers attribute this to a lack of maintenance and the fragments of decaying wooden structures in the plant. The findings therefore reflect the mill’s abandonment. “Although the water initially continued to flow, which meant that the carbonate continued to deposit, the inlet was no longer cleaned regularly,” explains Passchier.
The study has thus made an important contribution to our knowledge about the astonishing “industrial facility” from the 2nd century AD. Passchier concludes: “We were able to show that the development history of a water mill can be reconstructed using carbonate,” says the scientist.
Source: Johannes Gutenberg University Mainz, specialist article: Geoarchaeology, doi: 10.1002/gea.22016