Thuburbo Majus lies roughly sixty kilometers southwest of Tunis, near the modern town of El Fahs, in a fertile agricultural zone that helped the city flourish from the first through the seventh centuries CE. Granted civic status probably under Augustus and later elevated to the rank of honorary colony under Commodus in 188 CE, the city accumulated temples, a forum, a Capitolium and public baths. The Winter Baths, covering approximately 1,600 square meters in the urban core, grew through an asymmetrical layout born of repeated additions and renovations. An inscription studied in earlier scholarship allowed archaeologists to attribute the original construction to the first half of the third century CE, with major restoration phases in the late fourth to early fifth centuries and again in the early sixth century, before the building’s eventual abandonment.

The research team, led by Aida Zaddem and Karima Zoghlami with colleagues including Rafael Fort of the Institute of Geosciences in Madrid, sampled nine distinct stone lithotypes and thirty-two mortar specimens from walls across the complex. Petrographic thin sections, stained to distinguish calcite from dolomite, revealed the full petrographic identity of each rock: a nummulite-rich Oligocene packstone, travertine, ferriferous recrystallized micrite, a dense Jurassic limestone marketed even today as Aziza-type marble, a porous bioclastic calcarenite, speleothem calcite, an Eocene biomicrite, a Jurassic pelsparite and a glauconitic grainstone. Each of these was matched to a specific architectural role, and the match turned out to be strikingly deliberate.

The analysis exposed a procurement strategy of remarkable economy and precision. Nummulitic limestone and travertine, quarried within a radius of less than twenty kilometers, formed the monumental framework, minimizing transport costs while providing the mechanical strength the main structures required. The heating system told an even sharper story. The vault of the main furnace, the praefurnium, was built exclusively of fired bricks bonded with cocciopesto mortar, a technical imperative given that flames there could reach between 800 and 1,000 degrees Celsius, hot enough to decarbonate and disintegrate any carbonate stone. Yet the support pillars and the cheeks of the flue channels, made of glauconitic limestone and ferriferous micrite, remained intact, demonstrating that builders understood empirically that temperatures at those positions stayed below the limestone destruction threshold of roughly 600 to 800 degrees.

Specialization extended to the prestigious elements. Highly porous calcarenite, with porosity reaching 25 to 27 percent, was reserved for arches and voussoirs where lightness mattered, while marbles were chosen for columns, where hardness and visual prestige dominated. The mortars, meanwhile, fell into two great families: lime-based and gypsum-based. The lime mortars split into three subgroups that map directly onto Roman hydraulic science. The first type, moderately hydraulic, mixed a lime binder with siliceous and carbonate aggregates likely drawn from the sands of the nearby Oued Miliane and Oued Medjerda rivers; their hydraulicity came from clay impurities in marly limestone calcined into the binder itself, forming calcium silicate and aluminate hydrates without any added ceramics.

The second type was entirely different in purpose: non-hydraulic air-lime coatings built in multiple layers, their aggregates composed of crushed marble and vein calcite, with grain size progressively decreasing from the wall inward toward the finished surface. These white to beige plasters carried the highest calcium oxide contents of the entire assemblage, averaging around 57 percent, and their chemistry shows almost no silica, alumina or iron, confirming a purely aerial binder. Fascinatingly, one intermediate layer in some samples shows a faint reddish hue and weak hydraulic character, hinting that builders may have sprinkled finely ground ceramic powder, an artificial pozzolana, into that single stratum to fine-tune its properties. This kind of marble-dust coating echoes the plaster recipes that Vitruvius described for polished wall finishes.

The third lime type is the showpiece: highly hydraulic cocciopesto, loaded with ceramic fragments up to a centimeter across, used to waterproof the frigidarium pool and a decantation basin. Thermal analysis sealed the classification. The ratio of carbon dioxide to water lost during heating, a standard proxy for hydraulicity, dropped below 5 percent for these mortars, the signature of strongly hydraulic systems in which pozzolanic reactions between lime and fired clay generate abundant calcium silicate hydrates. Trace elements told the same story from another angle: cocciopesto samples carried the highest concentrations of niobium, rubidium, zirconium, zinc, chromium and vanadium, the geochemical fingerprint of silicate-rich ceramic aggregate. A fourth family of gypsum-based mortars, dominated by sulfate phases with sulfur trioxide averaging over 63 percent, came from the abandonment phase, when builders opportunistically exploited abundant local Triassic gypsum deposits even in structural roles that gypsum was traditionally never asked to play.

Perhaps the most consequential findings are chronological. Mortar fingerprinting forced a re-evaluation of the building’s accepted construction sequence. A joint mortar from the latrines, previously assigned to the first renovation phase of the late fourth century, proved compositionally indistinguishable from the original third-century mortars, raising the possibility that the latrines were part of the initial design and only internally modified later. A coating from the corridor wall, attributed to the original phase on architectural grounds, instead matches the crushed-marble plasters of the late-fourth-century renovation. Most strikingly, the bottom of the decantation basin, built with original-phase cocciopesto, contrasts with its upper walls of gypsum-bonded rubble, suggesting the basin’s base belongs to the first construction campaign while its upper portion was added during abandonment, a functional repurposing of the water system. The authors caution that these reinterpretations remain preliminary until confirmed by absolute dating methods such as optically stimulated luminescence or isotopic analysis.

The study also carries a warning for conservation. Soluble salts, including halite, syngenite and secondary gypsum, pervade the mortars. The gypsum largely results from the dissolution and recrystallization of older gypsum binders migrating through the masonry; the halite, at an inland site, most likely rose capillary-ward from saline groundwater or came from unwashed, evaporite-rich local sands; and syngenite forms when gypsum reacts with potassium from atmospheric deposition or fertilizers. These salts crystallize and swell with humidity, and their behavior directly explains the poor state of preservation visible across the complex today. Any modern repair mortar that ignored this chemistry would accelerate rather than arrest the decay.

In the end, the Winter Baths emerge as a case study in how Roman builders married imperial engineering knowledge to local geology, balancing logistics against performance with a precision that thermal modeling has only now made legible. By treating mortars not merely as glue but as chronological and technological documents, the research provides both a corrected timeline for the monument’s evolution and a rigorous template for designing compatible restoration materials, one based on the very recipes Roman craftsmen mixed nearly eighteen centuries ago. For a site whose excavated area still covers only a fraction of its ancient footprint, the stones and mortars of Thuburbo Majus are proving to be its most eloquent historians.

Subject of Research: Archaeometric characterization of Roman building stones and mortars from the Winter Baths of Thuburbo Majus, Tunisia

Article Title: Multi-analytical characterization of construction materials from the ‘Winter Baths’ of Thuburbo Majus (Tunisia): Implications for architectural chronology and conservation

Article References: Zaddem, A., Zoghlami, K., Fort, R., Romdhane, H. B., Coralini, A., & Gasmi, M. (2026). Multi-analytical characterization of construction materials from the ‘Winter Baths’ of Thuburbo Majus (Tunisia): Implications for architectural chronology and conservation. Archaeological and Anthropological Sciences, 18(10), Article 205. https://doi.org/10.1007/s12520-026-02563-w

Image Credits: AI Generated

DOI: 10.1007/s12520-026-02563-w

Keywords: Thuburbo Majus, Roman baths, archaeometry, mortars, hydraulic lime, cocciopesto, petrography, X-ray diffraction, Tunisia, conservation, building chronology, gypsum mortar

The team, led by Eren Şahiner of Ankara University’s Earth Sciences Application and Research Center, together with Tamer Koralay, Bilal Söğüt, Tunç Sezgin, and Yusuf Kağan Kadıoğlu, dated five specimens drawn from the Frigidarium’s fabric. Optically stimulated luminescence, or OSL, works by measuring the trapped charge accumulated in mineral grains such as quartz since they were last exposed to sunlight or heat. When a brick is fired or mortar quartz is exposed during preparation, its luminescence clock resets; from that moment, background ionizing radiation from the surrounding sediments, cosmic rays, and the sample’s own radioisotopes steadily winds the clock forward. In the laboratory, controlled optical stimulation releases the stored signal, and the intensity reveals the equivalent dose, the total radiation absorbed since the resetting event. Dividing that dose by the environmental dose rate yields an age.

What distinguishes this study is the rigor of its dose-rate protocol. The researchers documented every step, including grain-size-dependent attenuation of beta radiation, a carbonate water-equivalence correction specifically designed for lime-bound mortars, and an explicit assessment of gamma dose-rate heterogeneity. Lime mortars are notoriously difficult subjects for luminescence dating because their carbonate content alters the way water content and radiation attenuation interact, and the team applied the Carb-style modeling approach to handle this. Gamma dose-rate heterogeneity is a further subtlety: in a wall of compound masonry, where bricks and mortar alternate, the radiation field is not uniform, and a sample’s dose rate depends on the geometry of its immediate surroundings. Rather than treating this uncertainty as a nuisance to be ignored, the authors modeled the gamma-field geometry as a Bayesian nuisance parameter, propagating the dosimetric uncertainty of compound masonry directly into the final chronological posteriors.

The five OSL ages were then synthesized in a constrained Bayesian phase model, a framework adapted from the radiocarbon dating community, carrying only stratigraphic information and terminus post quem priors. In plain terms, the model was told only the relative order of events and the earliest possible dates, and then asked to reconcile the luminescence measurements into coherent phases of the building’s life. Three phases emerged. Primary construction of the Frigidarium falls in the late first to early second century CE, with a posterior median of 162 CE and a 95.4 percent highest-posterior-density interval spanning 60 to 323 CE. This is consistent with the flourishing of Roman bath architecture in Caria during the Antonine and Severan periods, when cities of western Anatolia competed to monumentalize their civic centers.

The second phase captures a repair episode, dated with a mode of 375 CE within an interval of 365 to 499 CE. The coincidence is striking: the great Eastern Mediterranean earthquake of 365 CE, generated near Crete and felt across the entire region, devastated coastal cities and is one of the most extensively documented seismic events of antiquity. The repair date aligns closely with that catastrophe, suggesting that the Frigidarium was damaged and subsequently restored in its aftermath. Archaeoseismology, the discipline that reads earthquake damage in ancient structures, has long catalogued tilted walls, displaced masonry blocks, and collapsed colonnades across the Mediterranean, but assigning absolute dates to such damage has remained difficult. Here, the luminescence clock provides an independent chronological anchor for a restoration that historical sources alone could never have pinned to this provincial city.

The third phase concerns the building’s terminal collapse. Under dosimetric priors alone, the model placed the collapse at a mode of 545 CE within a broad window of 405 to 802 CE, a range too wide to discriminate among the known seismic events of the period. The Eastern Mediterranean experienced what some scholars describe as seismic clustering between the fourth and sixth centuries, with major earthquakes in 494 CE and in 554 to 558 CE, as well as events around 610 to 630 CE. Any of these could, in principle, have delivered the fatal blow. The Bayesian model alone could not choose between them, and the authors were careful not to overstate what the luminescence data could resolve.

The decisive breakthrough came from archaeology rather than physics. Coins minted in 613 CE were found sealed beneath the collapse debris, providing a hard terminus post quem: the building could not have fallen before those coins were deposited. When this numismatic evidence was incorporated into the Bayesian model, the collapse window narrowed dramatically to 613 to 850 CE, with a mode of 633 CE. Within that refined window, the circa 610 to 630 CE seismic anchor dominates, achieving an overlap probability of 0.31 at a 25-year tolerance. The statistical conclusion is that a seismic trigger is the most probable cause of the terminal loss of the Frigidarium’s monumental function in the early seventh century CE, a period of chronic instability for the Eastern Roman Empire marked by Persian wars, plague, and persistent tectonic activity in western Anatolia.

Stratonikeia itself is an ideal laboratory for such work. Located in the Yatağan district of Muğla Province, the city has been the focus of long-running excavations directed from Pamukkale University, and previous studies have documented earthquake relics in the ancient city and at the nearby sanctuary of Hekate at Lagina. Earlier archaeometric investigations of Stratonikeia’s bricks, mortars, and building stones, including luminescence dating of bricks from the Erikli Basilica and characterization of the theater’s architectural stones, laid the groundwork for the present study. The Frigidarium, the cold room where bathers began or ended their circuit of the Roman baths, is a structurally massive space whose thick walls preserve a legible record of construction, repair, and destruction.

The methodological implications extend well beyond one building. Mortar dating by luminescence has matured over the past two decades, and reviews of the field have emphasized the need for transparent, reproducible dose-rate assessment, particularly for carbonate-rich materials. By publishing a fully documented protocol, treating gamma-field geometry as an explicit model parameter, and releasing the R and Python scripts implementing the dose-rate calculation, equivalent-dose modeling, and Bayesian phase chronology, together with full Monte Carlo posterior samples, the team has set a benchmark for how archaeoseismic chronologies should be constructed and reported. The approach converts scattered, imprecise age estimates into a coherent probabilistic narrative of a building’s biography.

For the wider public, the study offers something rare: a precise, quantified answer to the question of when a great Roman monument was born, when it was wounded, and when it died. The Frigidarium of Stratonikeia rose in the high empire, was patched after the great earthquake of 365 CE, and stood for roughly another two and a half centuries before an early seventh-century tremor ended its life as a functioning bath. In the layered ruins of Caria, the marriage of luminescence physics and Bayesian statistics is now capable of reading that biography directly from the walls themselves, giving voice to buildings that survived their own civilization by more than a millennium.

Subject of Research: Bayesian archaeoseismic chronology of the Stratonikeia Frigidarium using optically stimulated luminescence dating

Article Title: Bayesian archaeoseismic chronology of the Stratonikeia Frigidarium from construction to seismic collapse

Article References: Şahiner, E., Koralay, T., Söğüt, B., Sezgin, T., & Kadıoğlu, Y. K. (2026). Bayesian archaeoseismic chronology of the Stratonikeia Frigidarium from construction to seismic collapse. Archaeological and Anthropological Sciences, 18(10), Article 203. https://doi.org/10.1007/s12520-026-02564-9

Image Credits: AI Generated

DOI: 10.1007/s12520-026-02564-9

Keywords: luminescence dating, OSL, Stratonikeia, archaeoseismology, Bayesian modelling, Roman architecture, frigidarium, Roman baths, earthquake chronology, mortar dating, Anatolia, 365 CE earthquake