Scientists undertaking a molecular analysis of a Roman shipwreck have uncovered the complex mix of materials that was used to waterproof boats over 2,000 years ago.

Discovered in 2016 off the coast of Croatia, the Ilovik-Paržine 1 shipwreck was thought to be a trade ship that was transporting wine and timber around the Mediterranean.

While the boat’s cargo has been studied before, a new analysis published in Frontiers in Materials is the first to explore the vessel’s unique combination of coatings that were applied to its hull. A team from the Croatian Conservation Institute, Aix-Marseille University and others analysed the pollen and molecular structure to better understand its production and application on the hull. 

“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. “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.”

Using 10 coating samples, the team identified the biological origin of natural substances used for the ship’s coating. They found molecules characteristic of pine trees, indicating that the main component of all coating samples was heated coniferous resin or coniferous tar, also called pitch. One sample showed that at least some of the coating was made from a different composition of materials, namely beeswax and tar. This mixture – known to Greek shipbuilders as zopissa – improves the adhesive’s flexibility and is easier to apply when hot.

“Some regions throughout the Adriatic have particular characteristics that led local populations to develop a specific shipbuilding style,” said Charrié. “Only studies like ours offer an overview into these traditions, which bear witness to genuine know-how and diverse traditions.”

Pitch is adhesive by nature and can trap and preserve pollen from the surrounding landscapes. Analysing these traces allowed the researchers to narrow down possible regions where the pitch could have been produced and re-applied during refurbishments.

The pollen that was identified included those characteristic of the Mediterranean and Adriatic coasts and valleys, with forests of holly oak and pine as well as matorral – a kind of Mediterranean shrubland – where olive and hazel trees grow. 

The presence of alder and ash points to vegetation growing close to river- and seashores, which can be found near the coast or in the nearby hinterland. Fir and beech were present in small proportions too. This vegetation is found in mountainous regions and typical of the north-eastern coastal regions of the Adriatic Sea, with the mountain ranges of Istria and Dalmatia not far.

The team’s findings also indicated that the ship likely underwent four to five distinct batches of coatings. The ship’s stern and central part was covered by the same coating, whereas three batches at the bow were distinct from one another. This, too, could indicate that the ship was patched up successively using materials sourced from various locations throughout the Mediterranean.

“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.”