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In the middle ages, both the Blacksmith and the Smelter needed extreme heat for their work, much hotter than that produced by wood, so how can these people obtain this? The answer is charcoal.
First of all, charcoal and coal are two different things; coal is wood that was compressed and turned half to Sedimentary rock, which people mine, and charcoal is, well here’s the process:
Approximately 20 ft. of ground is cleared of vegetation and around 3 tons of wood are gathered and stacked in the shape of an igloo. The stack is then covered in dirt except for a small hole on top. The wood is then set on fire, well sort of on fire; the wood just smolders. The charcoal burners then must watch for a few days and make sure it doesn’t burst into flame. Inside, the wood smolders and smokes away all the moisture and other un-burnable stuff, and finally the charcoal burners rake away the dirt and ashes to reveal the charcoal, which is then sent away to the blacksmith, smelter, and rich people who can afford charcoal for heating.
I found this youtube video on making charcoal: