The clinker-built boats and ships of the Viking Age were held together by both wooden and iron nails, a fact which is well-documented by the finds of numerous rivets, roves, and spikes.
The use of iron in the process means that blacksmiths had an essential function in the boatbuilding process of the time. Despite its obvious significance, iron ship fastenings have been widely overlooked within the broader field of archaeological research. The Viking Ship Museum in Roskilde offers an expert and detailed overview of the process.
As the museum points out, various typologies have been proposed, and attempts have been made at reaching a consensus in terms of a descriptive terminology of these fastenings.
However, little attention has been paid to the iron itself or its exact metallurgical composition - beyond the documentation of the size, shape, and form.
Iron ore from wetlands
Most experts agree that the bulk of the iron in use in Viking Age-Denmark was produced from bog ore – natural deposits of iron ore found in wetlands, which were then roasted, smelted, and forged into different objects.
The majority of Viking ship reconstructions – including the six Skuldelev Ship reconstructions at the Viking Ship Museum in Roskilde – have been fastened using rivets and roves made from modern, industrially-produced iron, primarily Siemens-Martin steel or ARMCO iron.
The choice of material was made over 30 years ago with the construction of Roar Ege, the first of the Skuldelev ships to be reconstructed. At the time, few ship's nails had been analyzed, and the assumption was that Viking Age boatbuilders would no doubt have gone after the best materials they could acquire when building their ships.
Therefore, the decision was made to use a very pure form of iron with low carbon content. Out of the industrially produced iron available at the time, ARMCO was the product that was the closest match for what was known about the metallurgical content of archaeological nails at the time.
This choice has had severe consequences for the longevity of the sailing reconstructions of the Viking Ship Museum in Roskilde. Pure iron has proven to be very susceptible to rust in maritime contexts, and it takes a heavy toll on the wooden hulls.
The rivets expand as they rust, leading to cracks in the ships' planking. In turn, those cracked planks, and the fasteners that caused the damage, must be regularly replaced.
In 2016, the Viking Ship Museum in Roskilde decided to halt this cycle of replacement and repair of Roar Ege, the eldest of its Skuldelev ship reconstructions.
By then, so much of the planking and other internal components had been already replaced that to carry out further repair would have been to break with the principles of archaeological experiment.
Simply put, the Vikings would more than likely have scrapped the ship and built a new one. So, after 32 years of active service, Roar Ege was retired on land.
The Viking Ship Museum in Roskilde hosts a fleet of sailing reconstructions. Source: zealot666 / Pixabay
The museum's experience with Roar Ege had demonstrated that it was the iron and not the wood which proved to be the weak point in the hull.
This realization also emphasized a gap in the understanding of Viking Age maritime technology - pure iron of the sort used to date in the museum's reconstructions bears little relation to the type of iron used in the Viking Age.
But what exactly was Viking Age iron like? And how would it perform in a maritime context?
These are the questions that the museum will now attempt to answer with experimental archaeological research.
An important decision
Until the new research can be carried out, the museum has decided to fasten its projects and reconstructions with copper rivets and roves and not rivets and roves made of modern iron.
Even though copper nails weren't used in the Viking Age, the advantage of using copper nails is that they will not, in the same way as modern iron, create cracks in the ship's planking and shorten the ship's life.
Using copper will reduce the need for maintenance and repair, acknowledging the fact that the number of suitable oaks for boatbuilding is dwindling and ensuring that the valuable resource isn't wasted, the museum accentuates.
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