The second century CE Roman watermill complex of Barbegal, France, is regarded as one of the first industrial complexes in human history. The 16 water wheels are no longer extant as all woodwork has decayed. However, carbonate deposits precipitated from water during operation of the mills forming casts on the woodwork. These casts are preserved in fragments and provide unique insights into the frequency of use and maintenance of the mills, and even into the structure of the water wheel chambers. Stable isotope time series of carbonate deposits reveal that the mill activity was regularly interrupted for several months. This strongly suggests that the mill complex was not used for a steady supply of flour to a major population center, as previously thought, but likely served to produce nonperishable hardtack for the nearby harbors.