The Vikings of the eighth century AD perfected the large-scale production of tar, which helped to preserve their ships on voyages of trade and plunder.

Funnel-shaped pits discovered in Sweden have been identified as tar kilns, in which wood was heated to extract tar that could be used to protect ships’ sails and planks. Andreas Hennius at Uppsala University in Sweden analysed the sites and ages of these pits and found that kilns dated to between 100 and 400 AD were small and located in settlements. But kilns dated at or after the end of the seventh century AD were much bigger and were situated near forests, providing easy access to timber.

These shifts in size and location took place as people in Viking lands adopted the sail and began making long sea expeditions. Growth of the Viking fleet probably prompted industrial tar manufacturing, which led to the development of kilns that could yield at least 200–300 litres of tar in one production cycle.