Exploitation of Campi Flegrei Pozzolana for Hydraulic Works

Great works need great pits. In the last century BCE, the low, fuming craters west of Naples—Campi Flegrei—became a mining district for a material that looked like dust and behaved like a miracle. Crews cut into ashy banks near Baiae and Cumae, filling sacks with the powder that Vitruvius placed “between Cumae and the promontory of Minerva.” Barges slid along the shore to Puteoli, their decks grayed by the fine grit [2, 15].

Pliny, writing under the early empire, would celebrate the same earth: pulvis from the hills about Puteoli that, “as soon as it is submerged turns into a single mass of stone,” and then, more boldly, grows “every day stronger.” The claim sounded like a boast; harbors from Pozzuoli to Misenum served as demonstration tanks [3].

The ash had flavors. Some seams were glassier, some richer in the minerals that yielded hydraulicity. Builders learned the quirks by performance on site. In the yards at Puteoli, men measured two parts of this powder to one part lime for maritime work. The color difference in the mix—chalk white against soot gray—helped foremen judge consistency even before water touched it [2].

The soundscape was industrial. Picks struck pumice‑rich banks with a hollow clack. Sacks dragged across wooden ramps, leaving pale trails. At the port, the wind lifted ribbons of dust into the air like ghostly banners as stevedores tipped ash into holds bound for Ostia, Portus, or farther afield. The powder became Rome’s most consequential export you could measure in handfuls [1–3, 15].

By 1 CE, the supply was a system. Pozzolana started in the scarred earth of Campania, moved along the Bay of Naples by lighter, and left Puteoli for works across the Tyrrhenian. Where similar cementitious earths existed—Pliny notes Cyzicus and Cassandrea—builders could lean less on Campanian shipments. But for first‑class harbors, clients still asked for the good dust from the fuming fields [3, 15].