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Image Credit:
Alessandri et al., PLOS ONE (2026)
Ancient Salt-Production Site Near Rome Gains a More Precise Date
A new study shows that archaeomagnetic dating can help refine the chronology of archaeological sites where radiocarbon dating is difficult, using the ancient salt-production site of Piscina Torta near Rome as a case study.
Piscina Torta lies near the central Tyrrhenian coast of Italy and covers at least 20 hectares. The site belongs to a wider group of specialised settlements interpreted as centres of ancient salt production, where brine was heated and evaporated in large coarse-ware jars through a technique known as briquetage.
Since 2022, excavations at Piscina Torta have focused on understanding the site’s production activities and chronology. The new study examines materials from Area 1, where archaeologists identified a kiln, deep stratigraphic deposits, ceramic fragments, and evidence associated with salt-processing activity.
Dating the site has been challenging for two main reasons. First, the ceramic assemblage consists almost entirely of coarse jars, which changed little in form over time and are therefore difficult to date precisely through typology alone. Second, the site’s occupation falls within the Hallstatt plateau, a period between roughly 800 and 400 BCE in which radiocarbon calibration often produces very broad date ranges.
The two radiocarbon dates obtained from Area 1 produced calibrated ranges of about 794–551 BCE and 796–567 BCE. These ranges are too broad to clarify the internal development of the site and cover much of the period already suggested by pottery comparisons.
To overcome this problem, the researchers applied archaeomagnetic dating. This method relies on the fact that heated archaeological materials can preserve a record of the Earth’s magnetic field at the time they were last fired. By comparing this magnetic signal with regional models of past magnetic-field variation, researchers can estimate when the heating took place.
The study used two types of evidence. The first came from two in-situ pillars belonging to kiln 1K3 in Area 1, which allowed the researchers to measure magnetic directions. The second came from selected pottery sherds from deep deposits, which were used to measure archaeomagnetic intensity.
The kiln was a rectangular structure measuring about 3.2 by 1.8 metres. Its exact function remains uncertain, and it may have been used for salt production, pottery firing, or another activity. However, its heated elements provided a suitable target for archaeomagnetic analysis.
The magnetic directions recorded in the two kiln pillars produced two possible date intervals: 930–755 BCE and 740–700 BCE. When combined with archaeological and typological evidence, the later interval proved more consistent with the site’s occupation history.
The pottery sherds also produced useful results, although their dates were broader because they relied only on magnetic intensity rather than full directional data. Two samples gave ranges compatible with the archaeological context, including 790–680 BCE and 715–545 BCE.
The researchers also re-examined pottery from Stratigraphic Unit 138 and Sounding II. Typological comparison suggests that SU 138 dates to around 725 BCE, while Sounding II likely falls between about 725 and 700 BCE.
By combining archaeomagnetic results with pottery typology and radiocarbon dating, the study refined the chronology of Area 1 into three phases. Phase A, from about 725 to 710 BCE, represents the active use of the area, probably for salt production. Phase B, from about 710 to 550 BCE, marks a change in function, when the area became a dumping zone for production debris and possibly material removed from a nearby canal. Phase C, from about 550 to 525 BCE, represents the abandonment of Area 1 before the complete abandonment of the wider Piscina Torta site.
The findings also suggest that activity in Area 1 may have begun earlier than previously indicated by surface survey, which had placed the site between about 675 and 525 BCE. The deeper stratigraphic layers show that the production area was already active in the late eighth century BCE.
The study is important not only for Piscina Torta, but also for archaeological dating more broadly. It demonstrates that archaeomagnetism can improve chronological resolution in periods where radiocarbon dating is limited by calibration problems.
Overall, the research provides a more precise timeline for one of central Italy’s ancient salt-production landscapes. It also shows how combining magnetic, ceramic, and radiocarbon evidence can produce a stronger chronology than any single method alone.
Published on: 09-07-2026
Edited by: Abdulmnam Samakie
Source: PLOS One
