Recent research published in the Archaeological and Anthropological Sciences traces the evolution of waterproofing technology in ancient Sicily between the sixth century BCE and the first century CE. By analyzing mortar samples from the ancient settlements of Selinunte and Agrigento, scientists have mapped how crushed terracotta and organic ash were incorporated into building materials as different Mediterranean cultures mingled. The chemical evidence suggests that as trade and conflict brought Greek, Carthaginian, and Roman populations into contact, local builders adapted foreign techniques to improve their water systems while keeping construction costs manageable.
Selinunte and Agrigento were founded as Greek colonies on the southern coast of Sicily, positioning them at the crossroads of the ancient Mediterranean. The inhabitants of these cities constructed monumental temples, busy residential quarters, and elaborate water infrastructure. This infrastructure included public cisterns, swimming pools, and ritual baths. Their surrounding environment sat atop porous calcarenite rock, a type of limestone that easily allowed rainwater to filter down into underground aquifers. Tapping into and storing this periodic water supply required highly durable, water-resistant linings for any artificial channels and basins.
Over the centuries, both cities oscillated among periods of independence, Carthaginian rule, and eventual incorporation into the Roman Republic. Tracing exactly how and when advanced waterproofing materials reached these cities has presented an open challenge for archaeologists. Crushed-terracotta mortar, sometimes called cocciopesto, appears early on in the eastern Mediterranean and the Levant, later becoming a staple of Roman engineering. Historical reconstructions have regularly proposed that Phoenician and Punic settlers spread these techniques to the western Mediterranean. Tracking the exact timeline of these building materials in Sicily helps explain how different communities exchanged technical ideas across borders.
The architectural investigation was led by Rory McLennan of The University of Queensland, alongside Duncan Keenan-Jones of The University of Manchester. The team surveyed sections of Selinunte and Agrigento to locate water features built during different eras, collecting 22 mortar specimens from nine specific structures. These locations included a bathtub complex, parallel water channels, deep domestic cisterns, and a large swimming pool.
To understand what the ancient builders were mixing together, the researchers prepared ultra-thin slices of the mortar and examined them under polarized light microscopes. This process involves slicing the rocky material so thin that light can pass upward through it, allowing specific minerals to be identified by their distinctive shapes and colors. The researchers utilized a digital imaging process to measure the exact size and roundness of thousands of individual sand grains and terracotta pieces within the samples.
The team also mapped the elemental makeup of the mortar using micro X-ray fluorescence microscopy and scanning electron microscopes. These analytical techniques shoot a focused beam of X-rays or electrons at the sample, causing the material to emit secondary energy that reveals its underlying atomic structure. This approach allowed the researchers to see exactly where chemical reactions had taken place between the lime binder and the added fragments over two thousand years ago.
The physical analysis indicates that the earliest waterproof linings in Selinunte and Agrigento, dating to the sixth and fifth centuries BCE, were relatively simple. Builders mixed local calcareous sand and seashore pebbles with pure lime. While effective as a basic physical barrier, this combination lacked the chemical properties needed to create a highly durable, water-resistant bond. The unspecialized materials functioned well enough but were prone to cracking and required periodic patching, as evidenced by layers of calcium deposits sandwiched between multiple ancient repair jobs.
By the fourth and third centuries BCE, builders in Selinunte began experimenting with new ingredients. The chemical maps reveal the intentional addition of crushed terracotta roof tiles, ceramics, and occasionally organic plant ash. Terracotta is highly porous and rich in silica and alumina. When mixed with lime, the fired clay triggers a pozzolanic reaction, a chemical process that produces new binding compounds and allows the mortar to harden significantly even in wet environments. The addition of plant ash in some layers points to a specific building tradition linked to Carthaginian and Punic settlements, showing how the local Greeks were importing technical knowledge from their incoming neighbors.
The varied sizes of the crushed pottery suggest that builders were thinking practically about the different stresses placed on their water systems. For wall linings that needed to be highly impermeable, they used finely ground terracotta. The smaller grains provided more total surface area for the hardening chemical reactions to occur. For floors, they used much larger chunks of broken ceramics to resist the physical crushing weight of people and standing water. The incorporation of both local shards and imported volcanic rock within the crushed pottery shows that masons recycled whatever materials were available, turning broken trade goods into architectural filler.
By the second century BCE in Agrigento, construction methods became highly standardized under growing Roman influence. Builders consistently used terracotta-rich layers for the outer surfaces touching water, while reverting to cheaper natural sand for the hidden preparatory layers underneath. This demonstrates that as the city grew wealthier and built larger athletic and bathing complexes, local engineers balanced the adoption of superior imported technology with strict cost management. Lime was expensive to produce, requiring high-heat fires to burn limestone for days, so builders bulked up the hidden base layers with locally accessible sand.
Dating the water features presented specific challenges throughout the study. Both ancient cities experienced repeated cycles of destruction, rebuilding, and leveling. Many structures were built directly on top of older foundations or resurfaced over several generations, making it difficult to attach a specific decade to the arrival of a new waterproofing technique. In some cases, a single pool contained mortar applied during its original Greek construction as well as a newer sealing layer added during Roman renovations hundreds of years later.
Additionally, analyzing the exact water resistance of these ancient recipes proved complicated. The spot-checking of the energy dispersive X-rays returned variable results strictly depending on which micrometer of the binder was targeted. This variability indicates that ancient mixing practices were not always uniform across a single batch of mortar. Future research may focus on directly recreating and testing the strength of these specific historical recipes to better understand the physical improvements the Punic and Roman additions provided over the original Greek methods.
The paper, “Connectivity and change in the waterproofing technology of ancient sicily: evidence from Selinunte and Agrigento,” was authored by Rory McLennan and Duncan Keenan-Jones.