Three centuries of Roman limescale reveals a dirty secret about Pompeii’s public baths

It’s tempting to suppose that archaeology is filled with glamour – that it’s all about recovering wonderfully wrought artifacts from ancient tombs, or finding riches in unlikely places, like the 1000-year-old sword recently pulled by construction workers, corroded but intact, from a Dutch river. But more often, researchers must be content with faint hints of the past deduced from subtle traces left in the most mundane relics. It is remarkable what, with enough ingenuity, can be gleaned from the most unpromising of materials – such as Roman limescale. 

That’s what a team based in Germany has been studying, and they have used the precise chemical composition of these calcium carbonate deposits in the water supply of Roman Pompeii to reconstruct the history of water sources used by the ancient city.¹  

Pompeii has long been the archaeologist’s dream for those seeking to understand Roman culture in the first century CE. The eruption of Mount Vesuvius in 79CE destroyed the city almost overnight but preserved the remains in a layer of volcanic ash. Famously, the ruins show us Roman life arrested in a gruesome snapshot, the shapes of human bodies frozen into the hardened ash – some in heart-rending poses of death. Many of the buildings of the city were also stunningly well preserved, some including coloured murals that reveal the materials and methods of Roman artists. 

How to fill a Roman bath

Among the remains of Pompeii are some fabulously intact bath complexes, showing how lavish and sophisticated these facilities were in Roman society at those times. There were three major public baths in Pompeii, called the Republican, Stabian and Forum Baths. However, Pompeii never had a good natural source of fresh water, so it was no easy matter to keep the baths supplied. Before the first century CE (Pompeii was founded in 6BCE) water was provided from wells and by collecting rainwater in cisterns. The wells could be as much as 40m deep, the groundwater level being between 20 and 37m below the surface. So the water had to be lifted a considerable distance by man-powered machines. Even then this water wasn’t good enough for drinking. 

However, around the turn of the millennium an aqueduct (the Aqua Augusta) was constructed to bring water to the city from natural springs 30–40 km away – a much more reliable and convenient solution, which also supplied potable water. At this point the wells were abandoned and the Republican Baths fell into disuse. 

Like all natural water supplies, these water sources contained dissolved minerals that were liable to create deposits of calcium carbonate. Such limescale has been preserved in the wells, the aqueduct and the baths, and it now provides a record of the origins and quality of the water supply over a period of almost three centuries. There’s no shortage of material to study: the carbonate crusts on walls of the wells and collection basins are up to 3cm thick. The researchers could also collect samples from the bath pools themselves and from bronze devices used to heat them. 

Information in the isotopes

The clues are in the water chemistry. By sampling present-day groundwater at Pompeii and spring water from the sources that fed the aqueduct, the researchers could deduce how they differ. Trace amounts of copper and zinc in the deposits probably come from metal devices used to heat the water. The most significant differences between the water sources that leave their imprint in the carbonate deposits are the ratios of stable isotopes of carbon (¹³ C/¹² C) and oxygen (¹⁸ O/¹⁶ O), respectively denoted δ¹³ C and δ¹⁸ O. These differences reflect the differing geology of the two source regions of water from the wells and the aqueduct – for example, groundwater close to Pompeii receives carbon dioxide from volcanic activity – as well as changes induced by microbial activity in the waters. 

The latter factor seems to show up in the carbonate deposits in the Republican Baths, which were filled by well water. Specifically, there is a decrease in δ¹³ C between the deposits in the wells and those in the bath pools and drainage system, which the researchers interpret as a sign that the bath water was contaminated with human waste (such as urine and sweat) and by microbial activity. In other words – yuck – the bath water was not regularly replaced (that is, not more than daily). Which stands to reason, given the challenges of getting it from deep wells, but all the same means you wouldn’t want to be taking a dip late in the day. 

The stable-isotope signatures of the Stabian Baths are rather similar to those of the Republican Baths in the older parts of the water network, which were also fed from wells. But later parts of the baths show signs that they were supplied instead by the aqueduct. This innovation probably made the baths more hygienic, because the water could be changed more often. All this from ancient limescale, which makes it just as well that the Romans never felt the urge to descale their baths.