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Tar production required specially constructed kilns—structures that allowed heating wood in oxygen-poor environment, preventing complete combustion while reaching temperatures that caused pyrolysis.
The Pit Kiln:
The simplest design was pit kiln—hole dug in slope, funnel-shaped, lined with clay or stones to prevent tar from soaking into ground. The wood was stacked in pit, covered with turf or soil to exclude most air, hole left at top for minimal ventilation and smoke escape, channel at bottom allowed tar to drain to collection vessel.
The pit’s size varied—from small personal-use kilns to substantial operations processing tons of wood. Larger kilns were more efficient—better heat retention, more stable temperature control, economies of scale—but required more labor to load, maintain, unload.
The Barrel Kiln:
More sophisticated design used iron barrel—wood loaded inside, barrel sealed except for drain hole at bottom and vent at top, heat applied externally. The barrel provided better temperature control, prevented tar loss, allowed reuse of structure for multiple batches.
The barrel design appeared later than pit kilns but became standard for commercial operations—efficiency advantages justified capital investment in metal barrel, improved yield made the equipment cost-effective.
The Temperature Control:
The critical skill was maintaining proper temperature—hot enough to cause pyrolysis (typically 400-500°C), not so hot that everything burned to ash. The temperature was judged by smoke color and amount, sound of process, smell, various sensory cues learned through experience.
Too cool and wood charred without releasing tar. Too hot and tar itself burned, wasted as smoke and flame. The balance was delicate, required constant attention, distinguished skilled tar-burner from amateur who wasted valuable material producing minimal yield.
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