How Industrial-Scale Tar Production Powered The Viking Age

How Industrial-Scale Tar Production Powered The Viking Age

Vikings acquired the capacity to produce tar at an industrial scale as early as the 8th century AD, according to new research. The protective black goo was applied to the planks and sails of ships, which the Vikings used for trade and launching raids. Without the ability to produce copious amounts of tar, this new study suggests, the Viking Age may have never happened.

Tar sounds like a relatively modern invention, but it’s actually been around for quite some time. By the 16th century, Europeans had developed a technique whereby piles of wood, placed in funnel-shaped pits, were burned slowly under an oxygen-constricting layer of an earth-clay mixture and charcoal. Dripping tar from the burning wood fell into an outlet pipe, from which the precious material was collected.

Hundreds of years earlier, however, Vikings were also making and using tar, but their production methods were a mystery to archaeologists and historians. New research published today in the journal Antiquity is shedding new light on this unanswered question, revealing a unique method of tar production previously unknown to scientists.

The lone author of the new study, Andreas Hennius from the Department of Archaeology and Ancient History at Uppsala University in Sweden, documents the discovery of large tar-producing pits in the Swedish province of Uppland. Archaeologists have uncovered a surprising number of these extra-large pits over the past 15 years, which have been carbon dated to between 680 and 900 AD. This both pre-dates and coincides with the Viking Age (roughly 750 to 1050 AD). The Vikings used this tar to seal and protect structures made of wood, such as boats and longships, and to waterproof sails.

Photo and diagram of a funnel-shaped Viking tar pit. (Image: A. Hennius, 2018/Antiquity)

These tar pits were located several kilometres away from villages, likely due to their closer proximity to an essential ingredient of tar production: forests filled with wood. Like the tar pits of Early Modern Europe, the Viking pits were funnel-shaped, but instead of using an outlet pipe, the Vikings placed a 0.91m-wide (1 meter) container at the bottom of the pit to collect the drippings. This technique required the Vikings to dig out the entire pit to remove the container and its mucky contents.

These pits were enormous in size, capable of producing 200-300L of tar during each production cycle. The discovery shows that Vikings — as early as the 8th century AD — had acquired the capacity to produce tar at industrial-scale levels. Hennius refers to these facilities as “forest factories for tar production.”

Given this scale of production, it’s hard to believe that archaeologists are only discovering this now. Hennius says it’s only been during the last 15 years that archaeologists have been able to investigate, interpret, and scientifically analyse these types of tar pits.

“There are many more pits in Swedish forests, for now interpreted as charcoal production pits, trapping pits for animal hunting, and numerous other purposes,” Hennius told Gizmodo. “Many of these interpretations are probably wrong and there are a large number of tar pits hiding out there. But it will be quite a time-consuming effort to inspect all of these.”

Diagram and photo of a tar pit. (Image: Hennius, 2018/Antiquity)

Building, operating, and maintaining the larger pits in the forest required considerable work, said Hennius, involving such tasks as forest management, the cutting down of trees, chipping and stacking wood, and monitoring the firing. Afterwards, a supply of barrels would have to transported to various locations.

This level of tar production seems excessive, and it’s certainly more than a single household would ever need, but Hennius said it’s consistent with developments in Viking Age shipbuilding and maritime expansion that were happening at the time.

To construct a single longship, the Vikings needed an estimated 500L of tar, about 190m³ of wood, and 1600 hours of manual labour, according to Hennius.

“Tar is very useful for protecting wood from decay in house construction but especially within shipping,” he said. “Tar was used in enormous amounts within shipping up until boats were made from steel. For the Viking ships it was not only used for the wood in the boards but also for the caulking between the planks, the rigging, and the sails. Viking sails were made from wool and needed a coating to be effective.”

But the demand for tar didn’t just end there; the Vikings had to constantly replenish tar coatings on Viking vessel timbers and sails, creating further demand for the black material.

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Vikings managed fleets ranging from dozens to possibly even hundreds of ships. These ancient Scandinavians had an intimate connection with their maritime vessels, even burying high ranking individuals inside of ships.

It’s within this context that the Viking need for industrial-scale tar production starts to make sense. And in fact, Hennius goes so far as to suggest that the Viking use of voluminous amounts of tar, and their reliance on the forested outlands of Scandinavia, is what made the Viking Age possible.

With their tar-coated ships, the Vikings travelled along Russian rivers, conducting trade with the wealthy regions of the East Roman Empire and the Middle East. And as Hennius points out, the Vikings also traded their sticky goo; Viking tar barrels dating back to this time period have been found in Lower Saxony, now Germany.

A preserved Viking longship. (Image: University of Oslo)

“This paper presents a production feature for tar production that is unknown to most people,” Hennius told Gizmodo. “It also highlights a change in the mode of production from small-scale household production during the Roman Iron Age to large-scale production in the forests just before the Viking Age. I interpret these changes and the increased production as fundamental for the Viking maritime culture but also as an indication of more specialised handicraft and production during this time.”

It’s important to point out that this study is based on limited evidence from a limited geographical area in Sweden. Further excavations and investigations will likely provide a clearer picture of Viking tar production and its effect of Viking society.

Humans have been inventing useful materials for quite some time. Neanderthals made glue as far back as 200,000 years ago, Palaeoamericans in California produced tar-like bitumen some 5000 years ago (which sadly exposed them to dangerous toxins), and Romans developed highly durable concrete 2000 years ago. Humans, as history shows, are good at making the stuff that makes even better stuff.