The unsurprising answer is TAR. The Vikings knew how to produce it on an industrial scale, making batches of 200-300 litres, ten times the amount normally made at that time. Viking ships needed 500 litres of tar -EACH! And they had hundreds of ships in a fleetWe know this because archaeologists have found huge funnel-shaped pits. Tar was made in a similar way to charcoal:wood was buried then burnet in a low-oxygen environment. A container id buried underground to collect the dripping resin, giving tar pits their giveaway cone-shape with a cavity below.
All this production required a long time to be spent in the forest and a different form of social organisation, other than just farming.
