A 2,200-year-old Roman shipwreck is offering an unusually sticky clue to ancient seafaring. Pollen trapped in the vessel’s waterproof coatings has helped researchers trace how Roman sailors sealed their hulls and repaired them over time.

The study, published in Frontiers in Materials, examined the Roman Republic shipwreck Ilovik-Paržine 1, which sank about 2,200 years ago off the coast of what is now Croatia.

Researchers in France and Croatia analysed the ship’s protective coating and found two types. One was made of pine tar, also called pitch. The other was a mixture of pine tar and beeswax.

“In archaeology, little attention is paid to organic waterproofing materials. Yet they are essential for navigation at sea or on rivers and are true witnesses of past naval technologies,” said first author Dr Armelle Charrié, an archaeometrist at the Laboratory of Mass Spectrometry of Interactions and Systems in Strasbourg.

“Studying the coatings, we found two different kinds on this vessel: one made of pine tar, also called pitch, and the other of a mixture of pine tar and beeswax. Analysis of pollen in the coating made it possible to identify the plant taxa present in the immediate environment during the construction or repairs of the ship.”

The wreck was discovered in 2016. This study is the first to combine molecular and pollen analysis to identify the ship’s coatings and reconstruct the vegetation present when those materials were produced and applied to the hull.

The work was carried out through a collaboration between the Department for Underwater Archaeology of the Croatian Conservation Institute and the ADRIBOATS program of the Centre Camille Jullian at Aix-Marseille University in France.

“Some regions throughout the Adriatic have particular characteristics that led local populations to develop a specific shipbuilding style,” Charrié said.

“Only studies like ours offer an overview of these traditions which bear witness to genuine know-how and diverse traditions.”

The team used structural, molecular and pollen analyses, including mass spectrometry, to study 10 coating samples and identify the biological origins of the materials used on the vessel.

Molecular fingerprinting found compounds linked to pine trees and showed every sample was dominated by heated coniferous resin or coniferous tar, also called pitch. One sample contained a different formulation made from beeswax and tar.

The study said that mixture was known to Greek shipbuilders as zopissa and made the adhesive more flexible and easier to apply when hot.

Because pitch is naturally sticky, it can trap pollen from nearby vegetation and preserve it for long periods. By examining the pollen types in the coatings and their relative abundance, the researchers narrowed down where some of the pitch may have been produced or reapplied during repairs.

The pollen pointed to a wide range of environments, including Mediterranean and Adriatic coasts and valleys. It included signs of holly oak and pine forests, as well as matorral, a Mediterranean shrubland where olive and hazel trees grow.

Alder and ash pollen pointed to vegetation linked to riverbanks and seashores. Small amounts of fir and beech were also found, trees associated with mountainous terrain on the north-eastern Adriatic coast, where the mountain ranges of Istria and Dalmatia lie relatively close to the sea.

The analysis also suggested the vessel received four to five separate batches of coating over time. The stern and central section appeared to have been covered with the same material, while researchers identified three distinct coating batches at the bow.

The study said that pattern may point to repeated repairs using materials obtained from different locations around the Mediterranean.

Earlier research on the vessel’s ballast pointed to Brundisium, today Brindisi, on the south-eastern coast of Italy as the place where the ship was built. The new pollen evidence suggests some of its coatings may also have been applied in that area. Other layers may have been added along the north-eastern Adriatic coast, close to where the wreck was found.

“While it seems obvious that ships sailing long distances need repairs, it’s simply not easy to demonstrate this,” Charrié said. “Pollen has been very useful in identifying different coatings where the molecular profiles were identical.”

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