Ancient mortar from Pompeii could help researchers determine when Roman buildings were actually constructed, but only if scientists can separate original material from contamination. A new study of Pompeii mortar and Roman buildings shows that carbon preserved in the ancient construction material can provide evidence of age.
The study, led by Sara Calandra, tested a method for radiocarbon dating mortar from public buildings across Pompeii. Researchers examined material linked to several construction periods, from the second century B.C. to rebuilding after a major earthquake in A.D. 62 or 63. The earthquake badly damaged Pompeii and led to widespread reconstruction before Mount Vesuvius buried the city in A.D. 79.
Scientists tested mortar from across Pompeii
Researchers initially examined 53 mortar samples from major public structures. These included the Temple of Apollo, the Eumachia building, the Temple of Jupiter, and other buildings around the Forum. The mortars were made by mixing lime with volcanic material known as pozzolana, an important ingredient in durable Roman construction. Research into why ancient Roman concrete lasted for thousands of years has highlighted the role of volcanic material and lime in its unusual properties.
Radiocarbon dating depends on identifying carbon that entered mortar when builders originally made it. As lime mortar hardens, it absorbs carbon dioxide and forms calcium carbonate. In theory, this human-made, or anthropogenic, carbonate records the construction period.
Carbonation has also helped scientists understand the durability of Roman concrete. For dating, however, researchers must distinguish carbonate formed during construction from much older geological material. If limestone fragments or other ancient carbon enter a sample, radiocarbon testing can make the mortar appear older than it really is.
Only a handful of samples passed screening
Researchers used a four-step process to identify reliable material. They studied each wall’s construction history before examining the mortar’s minerals, chemistry, and microscopic structure. The screening eliminated 48 of the 53 samples before radiocarbon dating because they showed signs that could produce unreliable results.
The remaining samples underwent additional non-destructive testing to distinguish carbonate created during mortar production from naturally occurring geological carbonate. Some lime lumps that initially appeared suitable contained geological material and magnesium, which could distort the results.
Some mortar dates matched Pompeii’s history
The strongest results came from samples associated with the Eumachia building and the Temple of Apollo. Their radiocarbon measurements matched construction periods established through archaeological evidence. The results suggest mortar can provide useful chronological evidence when researchers successfully isolate carbonate formed during construction.
Other samples produced dates significantly older than expected. Several contained geological carbonate that contaminated the radiocarbon signal. Pompeii’s geological setting may create additional problems. The city spent centuries buried beneath volcanic material after the A.D. 79 eruption.
Chemical and geological processes continued after burial, while volcanic systems around the Bay of Naples released carbon dioxide. Tiny carbonate minerals can also occur within volcanic material used in Roman mortar. Such sources can introduce ancient carbon that is difficult to detect.
Method could sharpen Roman building timelines
The results show that Roman mortar cannot simply be collected and radiocarbon dated without extensive screening. Most of the Pompeii samples proved unsuitable, while carefully selected samples produced dates consistent with archaeological evidence.
The approach could help archaeologists establish construction dates at Roman sites where inscriptions, written records, or other chronological evidence are missing. It could also provide an independent way to test existing archaeological timelines, provided researchers can identify and remove sources of ancient carbon contamination.
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