Ancient Mortars Reveal Their Secrets Through Infrared Light: FTIR Peak Ratios Trace Calcite Origins

A humble lump of ancient mortar can hold the chronological blueprint of an entire building, but coaxing that information out of a crumbly, centuries-old sample has always been one of archaeometry’s most delicate challenges. Now, a team of researchers from the Czech Academy of Sciences has refined a spectroscopic technique that promises to make that task faster, cheaper and far less destructive. In a study published in Archaeological and Anthropological Sciences, Paola Pizzo, Petra Mácová, Jan Válek and Petr Kozlovcev demonstrate that Fourier-transform infrared spectroscopy, or FTIR, can do far more than previously assumed when it comes to untangling the origins of calcium carbonate in archaeological materials. Their work moves the technique beyond its traditional role as a simple pre-screening tool and positions it as a genuine analytical instrument for characterizing the binders that held ancient walls together.

The central problem the researchers tackled is deceptively simple to state but notoriously difficult to solve. When archaeologists want to radiocarbon date a lime mortar, they are essentially trying to date the moment the binder was prepared, because the carbon dioxide absorbed by the lime as it hardened captures atmospheric carbon of the construction era. The trouble is that ancient builders often used limestone aggregates, or the raw stone itself contained unburned geological carbonate. This geogenic carbon, potentially millions of years old, contaminates the sample and skews the radiocarbon age. Distinguishing the anthropogenic calcite, formed when lime plaster carbonated, from the geogenic calcite inherited from the parent rock is therefore the crucial first step in any reliable mortar dating campaign.

FTIR has long been the go-to method for a first pass at this problem. The technique works by shining infrared light through or across a sample and recording which wavelengths are absorbed. Each mineral absorbs at characteristic frequencies corresponding to the vibrations of its chemical bonds, producing a spectrum that acts like a molecular fingerprint. For calcite, the calcium carbonate polymorph that dominates most mortars, the most informative features are the absorption bands arising from the vibrations of the carbonate anion. Crucially, calcite formed by burning limestone at high temperatures and then slaking and carbonating it has a subtly different crystalline structure from calcite that crystallized slowly in geological deposits, and those structural differences leave measurable traces in the infrared spectrum.

The key insight, building on work pioneered by researchers such as Regev, Poduska, Weiner and Boaretto, is that the ratio between the heights of two specific absorption peaks, labelled ν2 and ν4, encodes information about the calcite’s formation history. The ν2 band reflects an out-of-plane bending vibration of the carbonate group, while the ν4 band corresponds to an in-plane bending mode. Anthropogenic calcite, which forms rapidly at ambient temperatures during the carbonation of lime, tends to be structurally disordered and rich in nanoscale defects, and this disorder shifts the relative intensities of these bands compared with well-ordered geological calcite. By calculating the ν2/ν4 ratio, researchers can gauge where a given calcite sits on the spectrum from freshly made, disordered binder to ancient, well-crystallized limestone.

What the new study adds is a systematic exploration of how this ratio behaves in practice, including the use of both peak height and peak area measurements, and a careful verification of how the ratio changes when aragonite, a second common calcium carbonate polymorph, is present in the sample. Aragonite shares calcium carbonate’s chemistry but differs in crystal structure, and it frequently appears in archaeological contexts, whether as biogenic material such as shell fragments incorporated into mortars or as a product of alteration processes. The researchers confirmed that the presence of aragonite modifies the ν2/ν4 ratio in ways that must be accounted for, a finding that guards against misinterpretation of spectra from mixed-phase samples, which are extremely common in real archaeological assemblages.

To test the methodology, the team applied it to a set of archaeological samples from the Romanesque church of Budeč in the Czech Republic, one of the most important early medieval sites in the country. The FTIR approach proved effective not only at distinguishing the origin of the calcite but also at characterizing binder-related particles, such as underburned or overburned lime lumps, and at estimating the proportion of pyrogenic calcite, formed through fire and human intervention, relative to geogenic calcite within the binders. This quantitative dimension matters enormously for radiocarbon dating, because knowing how much of the carbonate in a sample is genuinely anthropogenic allows laboratories to correct or reject samples that would otherwise yield misleading ages.

The reliability of the method received an especially convincing demonstration on two Roman samples from the site of Nea Paphos in Cyprus, a UNESCO-listed ancient city famous for its Hellenistic and Roman remains. For these samples, the pyrogenic origin of the calcite had been disputed, making them an ideal stress test for the technique. The FTIR peak ratio analysis successfully characterized the binders and binder-related particles in a way that resolved the ambiguity, underscoring the method’s value as an independent line of evidence when the provenance of carbonate material is contested. The Cypriot samples were collected as part of the PlaCe-ITN project, funded through the European Union’s Horizon 2020 programme under a Marie Skłodowska-Curie grant, with cooperation from Cypriot heritage institutions including the Department of Antiquities and the Cyprus Museum.

Part of the appeal of the approach lies in its practicality. FTIR is micro-invasive and minimally destructive, requiring only tiny amounts of powdered material, which means that precious archaeological samples can be analysed without compromising their preservation or their availability for other tests. Compared with techniques such as X-ray diffraction with Rietveld refinement, cathodoluminescence, laser-induced fluorescence or Raman micro-spectroscopy, all of which have been proposed as screening tools for mortar dating, FTIR instruments are relatively inexpensive, widely available and fast to operate. The authors also had to contend with well-known confounding factors, including the effect of particle size on attenuated total reflection spectra and on the fundamental vibrations of the carbonate anion, effects documented in earlier studies of mineral spectroscopy. By combining peak height ratios with peak area ratios, the methodology gains robustness against some of these sources of variability.

The broader implications reach well beyond a single site or a single analytical trick. Radiocarbon dating of mortars has matured into one of the most powerful chronometric tools available for buildings archaeology, offering construction dates where no organic material exists for conventional dating. Yet every mortar date stands or falls on the quality of sample selection, and the ability to rapidly quantify the pyrogenic-to-geogenic calcite proportion with a benchtop spectrometer could transform how laboratories and field archaeologists triage material. The Budeč study itself connects to a wider research programme by the same group, which has previously assessed residual geogenic carbon in mortars and applied mortar characterization and radiocarbon analysis to establish the chronology of construction phases at the St. Peter and Paul Rotunda at Budeč.

For a technique that has spent decades in the analytical shadows, FTIR is now stepping into a leading role. The work by Pizzo and colleagues shows that a ratio between two absorption peaks, measured on a sample too small to see across a fingertip, can distinguish the fire of a medieval lime kiln from the slow chemistry of a Cretaceous seabed. As laboratories worldwide grapple with the challenge of dating the built heritage that cannot be dated any other way, methods like this one, which are fast, gentle and interpretable, are likely to become standard equipment in the archaeologist’s toolkit. The stones of Budeč and the ruins of Nea Paphos, it turns out, have been broadcasting their origins in infrared all along; researchers have simply learned to listen more carefully.

Subject of Research: Using FTIR peak height and area ratios to distinguish anthropogenic from geogenic calcium carbonate in archaeological mortars

Article Title: New perspectives on the use of FTIR for studying the relative proportions of anthropogenic and geogenic calcium carbonate: the application of peak height and area ratios

Article References: Pizzo, P., Mácová, P., Válek, J., & Kozlovcev, P. (2026). New perspectives on the use of FTIR for studying the relative proportions of anthropogenic and geogenic calcium carbonate: the application of peak height and area ratios. Archaeological and Anthropological Sciences, 18(10), Article 201. https://doi.org/10.1007/s12520-026-02566-7

Image Credits: AI Generated

DOI: 10.1007/s12520-026-02566-7

Keywords: FTIR spectroscopy, calcite origin, anthropogenic calcite, geogenic calcite, calcium carbonate, archaeological mortars, radiocarbon dating, lime plasters, aragonite, Budeč, Nea Paphos, mortar characterization