The Roman Empire was famous for its system of road-building. But as anyone who’s bumped over a pothole knows, with roads comes the need for constant repair, rebuilding, and the associated headaches. A new paper suggests that the people of Pompeii had a unique quick-fix for filling in wheel ruts and holes on their stone streets: they filled them with molten iron.
According to the paper in the American Journal of Archaeology, archaeologists conducted a survey of Pompeii’s streets in July, 2014, finding 434 spots of iron on the paving stones, suggesting that liquified iron was used in road repair.
Pompeii, in particular, had some pretty serious road problems. That’s because most of the streets in the bustling seaside city were paved with silex, a type of cooled lava stone that wore away relatively quickly, leaving ruts from wagon wheels. The narrow streets of the city were also used to deal with sewage, which didn’t help matters, causing pits and cavities to form in the stone.
But the disruption of full-on road repair or replacement probably wasn’t acceptable to the Pompeiians. “One option for repair, complete repaving in stone, was a difficult and expensive endeavor that might block important through-routes in a city for months,” the authors suggest.
Because of that, the team believes, the Romans came up with a novel solution: dripping molten iron into the ruts and pits. In some cases, it appears they added stone or ground up ceramics to the iron as well. But the iron is only found on main thoroughfares where roadwork would have been a major hassle. On smaller side streets, it appears crews replaced the stones over time.
One question is whether iron was plentiful and cheap enough to be used in such repairs. The researchers believe the answer is yes. By the late 1st century A.D., Rome was already producing 550 tons of iron annually from deposits in recently conquered Britain, from an area in the southeast of the island called the Weald. Large amounts of iron were being mined in other areas as well. And the paper suggests that traders may have been using iron slag as ballast in their ships. When they reached a port, they could sell the slag, which still contained a large percentage of iron.
Roman furnaces appear to be capable of reaching the high temperatures needed to liquify iron. “How the Romans introduced liquefied iron material into the streets at Pompeii remains a mystery,” the authors write.
But lead author Eric Poehler of the University of Massachusetts Amherst writes that stray iron drops found on the street suggest that the molten metal was carried from a furnace to the repair site. The team suspects that municipal slaves or slaves employed by local magistrates were tasked with carrying the hot metal to the work sites and pouring the metal patches.
Going forward, the team plans to analyze the iron to figure out where it came from and examine roads in other areas where similar techniques may have been used.
In a way, the idea of using iron to fix potholes has come full circle. Currently, researchers in Minnesota are experimenting with using tailings left over from processing taconite, a type of low-grade iron ore, into a durable road patch.