Iron is probably not the first material that springs to mind when you think of Viking Age boatbuilding. However, the clinker-built boats and ships of that era were held together using both wooden and iron nails, making the blacksmith’s craft an essential part of the boatbuilding process. Here at the Museum, the blacksmith works in close co-operation with the boatbuilders when producing the rivets and roves that hold the hulls together. There have been numerous finds of iron rivets from archaeological contexts, and even when the rivets themselves have rusted away, the shape of the rivet hole and the dimensions of the planks they held together can provide the necessary information to allow the blacksmith to reconstruct them.
Before rivets can be made however, the iron itself has to be produced. During the Viking Age, the majority of the iron in Scandinavia was produced from bog ore. This naturally occurring material would have been gathered from boggy areas and smelted in order to extract the iron. This is a relatively long and complex process. The ore must first be roasted and then crushed. It is then placed in a smelting furnace, along with alternating layers of charcoal and the heat from the furnace will eventually cause the iron (known at this stage as bloom) to separate from the waste products (slag). The bloom must then be worked further to remove any remaining impurities before the actual process of producing iron bars and eventually, rivets can begin.
Smelting and blacksmithing are processes, which are heavy on materials. It takes approximately 20kg of bog iron ore to produce 3-4kg of iron. Many more tonnes of wood are required in order to produce the charcoal required to fuel the furnace and the fires of the forge. When you see the blacksmith producing rivets at the forge on the Museum Island, it is worth remembering that this is but the final stage of a long and complicated chain of production.
Use and research
A great many rivets are required to hold a hull together. The small 7.7m long Gislinge Boat has ca. 450 rivets while the 30m reconstruction of the longship, Skuldelev 2, is held together with just under 8,000 rivets. The longevity of the rivets used in the various ship and boat reconstructions is constantly documented and records are made each time nails have to be removed and replaced.