The Vikings are often viewed as brutish, destructive village-pillagers, but their knack for innovation is perhaps overlooked. Viking-age Scandinavia was kind of the Silicon Valley of shipbuilding in the early Medieval period. Their iconic longboat designs, advanced navigational skills, and perhaps even legendary sunstones gave them the ability to raid, trade and establish settlements as far away as Russia, Italy and North Africa. A new study adds another bit of technology to the list of things that gave Vikings a leg up on their adversaries: they may have been capable of making industrial scale quantities of tar, according to a new paper published in the journal Antiquity.

Tar was probably essential to the Vikings’ lifestyle since each longship would have required about 130 gallons of tar to coat all of its wooden elements, the study suggests. Tar was also needed to coat the ships’ wool sails, and the boats would need to be regularly re-tarred between voyages as well. Multiply all that to fit the needs of a fleet and we're talking about a lot of tar here.

However, little was previously hypothesized about how they would have been able to produce the sticky substance en masse. The new study, authored by Andreas Hennius, an archaeologist from Uppsala University in Sweden, proposes a possible outline of how small scale tar production in the early centuries of the first millennium gave rise to potentially industrial use of tar by Vikings.

“I suggest that tar production in eastern Sweden developed from a small-scale household activity in the Roman Iron Age to large-scale production that relocated to the forested outlands during the Vendel/Viking Period,” Hennius writes in the paper. “This change, I propose, resulted from the increasing demand for tar driven by an evolving maritime culture.”

Several small tar kilns located in eastern-central Sweden dating from between 100 and 400 A.D. were first found in the early 2000s. The size of the kilns and proximity to homesteads indicate that they were probably made for household use and the tar was not produced for trade.

In 2005, archaeologists found similar kilns further north in Sweden, but these were much larger and dated between 680 to 900 A.D., which coincides with the rise of the Vikings. They could produce 50 to 80 gallons of tar in one burn, which is 10 times that of the smaller kilns. These kilns were also much closer to pine forests, which were the source for tar making materials of that time, according to Nature. No villages or gravesites have been found anywhere near the larger kilns, suggesting that they were not part of settlements, but rather industrial sites focused solely on mass production of tar.

It may seem strange that such a huge part of Viking life was not understood until recently. Most of these tar production sites were only uncovered in the last 15 years during road construction, but there may be more that were wrongly classified by archaeologists in the past. Many previously discovered pits in Scandinavia were likely misinterpreted as “charcoal production pits, trapping pits for animal hunting, and numerous other purposes,” Hennius tells George Dvorsky at Gizmodo.

Hennius says that the presence of barrels full of tar found at some Viking sites outside Scandinavia suggest that tar was a trade product shipped around the Viking world, though that is not yet confirmed. Hennius, of course, would like to amass more archaeological evidence to support the concept of a large-scale tar trade.

In the meantime, perhaps we should change our notion of Vikings to include not only fierce warriors grasping battle axes, but fierce engineers with tar brushes in their hands.