Before modern industry, tar was made by heating wood, bark, roots, or similar material with limited oxygen so that tarry liquids could be collected. The exact method varied by region and period.
Pre-industrial tar-making was a skilled rural craft, not just burning wood. The maker needed heat without ordinary open combustion. If the material burned freely, the useful tar would be lost in flame and smoke. If the heat was controlled, the wood or bark could decompose and release liquids that could be guided or collected.
Different places used pits, clamps, kilns, covered heaps, retorts, pots, or other arrangements. Pine regions could focus on resinous roots and stumps. Birch-bark tar required a different scale and purpose. Later industrial systems made collection more controlled, but the older principle was still destructive distillation by heat.
This article should remain descriptive, not instructional. Making tar involves fire, smoke, hot sticky liquid, fumes, and possible contamination. It is a historical subject worth understanding without encouraging improvised experiments.
The important point is that pre-industrial tar was labour-intensive. It required woodland knowledge, fuel, timing, containers, transport, and markets. A barrel of pine tar represented far more than a black liquid in a cask.