Bread and circuses may have placated ancient Romans, but perhaps not year-round. A new study of chemical deposits at the ancient Roman watermill Barbegal (pictured) in southern France, considered one of the very first large-scale industrial complexes in history, reveals the mill was only used to produce flour for breadmaking seasonally.

The mill, built onto a rocky hillside in the second century C.E., once diverted rushing, mineral-rich spring water downhill to spin 16 waterwheels that would have turned grindstones that crushed wheat into flour. Although the woodwork has long since decayed away, mineral deposits left by the water formed casts around some of the wood, preserving remnants of the organic material within.

Researchers identified isotopes—or forms—of carbon and oxygen within 142 such deposits, knowing that different isotope ratios occur during different parts of the year. By analyzing the isotopes in the progressive mineral layers—a bit like reading tree rings—the researchers noticed a seasonal pattern indicating the mill was only in use from winter through early summer , they report today in Science Advances . Millworkers apparently had late summers and autumns off.

That pattern matches up with the Roman shipping season, the researchers note. Although most scholars had thought the Barbegal watermill was used to produce flour for general Roman use throughout the year, the mill’s seasonality instead suggests it was used predominantly to make a sailor staple: “ship’s bread” or hardtack, a simple, nonperishable combination of water and flour.