Thursday, April 4, 2013
Bronze warship ram reveals secrets
Analysis of a bronze battering ram from a 2000 year-old warship sheds light on how such an object would have been made in ancient times.
Known as the Belgammel Ram, the 20kg artefact was discovered by a group of British divers off the coast of Libya near Tobruk in 1964. The ram is from a small Greek or Roman warship – a “tesseraria”. These ships were equipped with massive bronze rams on the bow at the waterline and were used for ramming the side timbers of enemy ships. At 65cm long, the Belgammel Ram is smaller in size and would have been sited on the upper level on the bow. This second ram is known as a proembolion, which strengthened the bow and also served to break the oars of an enemy ship.
Since the Belgammel Ram was discovered, other rams have been found, some off the coast of Israel near Athlit, and more recently, off western Sicily. The latter finds look to be the remains of a battle site.
Leading marine archaeologist, Dr Nic Flemming a visiting fellow of the National Oceanography Centre, co-ordinated a team of specialists from five institutes to analyse the artefact before it was returned to the National Museum in Tripoli in May 2010. Their results have been published in the International Journal of Nautical Archaeology.
Dr Flemming said: “Casting a large alloy object weighing more than 20kg is not easy. To find out how it was done we needed specialists who could analyse the mix of metals in the alloys; experts who could study the internal crystal structure and the distribution of gas bubbles; and scholars who could examine the classical literature and other known examples of bronze castings.
Dr Chris Hunt and Annita Antoniadou of Queen’s University Belfast used radiocarbon dating of burnt wood found inside the ram to date it to between 100 BC to 100 AD. This date is consistent with the decorative style of the tridents and bird motive on the top of the ram, which were revealed in detail by laser-scanned images taken by archaeologist Dr Jon Adams of the University of Southampton.
It is possible that during its early history the bronze would have been remelted and mixed with other bronze on one or more occasions, perhaps when a warship was repaired or maybe captured.
Micro-drilled samples show that the composition of the bronze was 87 per cent copper, 6 per cent tin and 7 per cent lead. The concentrations of the different metals vary throughout the casting. Scanning Electron Microscopy, SEM, reveals that the lead was not dissolved with the other metals to make a composite alloy but that it had separated out into segregated intergranular blobs within the alloy as the metal cooled.
These results indicate the likelihood that the Belgammel Ram was cast in one piece and cooled as a single object. The thicker parts cooled more slowly than the thin parts so that the crystal structure and number of bubbles trapped in the metal varies from place to place.
The isotope characterisation of the lead component found in the bronze (an alloy of copper and tin) can be used as a fingerprint to reveal the origin of the lead ore used in making the metal alloy. Up until now, this approach has only provided a general location in the Mediterranean. But recent advances in the analysis technique means that the location can be identified with higher accuracy. The result shows that the lead component of the metal could have come from a district of Attica in Greece called Lavrion. An outcome of this improved technique means that the method can now be applied to other ancient metal artefacts to discover where the ore was sourced.
Nic Flemming continued: “We have learned such a huge amount from the Belgammel Ram and have developed new techniques which will help us unpick future mysteries.
“We will never know why the Belgammel Ram was on the seabed near Tobruk. There may have been a battle in the area, a skirmish with pirates. It could be that it was cargo from an ancient commercial vessel, about to be sold as salvage. The fragments of wood inside the ram show signs of fire, and we now know that partsof the bronze had been heated to a high temperature since it was cast which caused the crystal structure to change. The ship may have caught fire and the ram fell into the sea as the flames licked towards it. Some things will always remain a mystery. But we are pleased that we have gleaned so many details from this study that will help future work.”