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Collier—Don't burn the wood at the charcoal burn!
Charcoal, the fuel used for smelting iron from its ore in a blast furnace, was wood that had most of the moisture, gases, and impurities driven out of it by heat. What remained were chunks of nearly pure carbon. Although almost any wood could be used, hard woods such as hickory, ash, oak, beech and walnut were preferred because they yielded higher quality charcoal. Soft woods, like pine, contained tar and made poor charcoal (although it might be used if the byproduct of turpentine was desired). From late fall through the winter into early spring unskilled workers such as farmers’ sons cut most of the wood needed for the coming year’s charcoal. They cut the wood into four-foot lengths, split those lengths into halves or quarters, then stacked them in cords (a measure 8 feet long, 4 feet wide and 4 feet high) in the forest. To operate one blast furnace for one day used all the charcoal that could be made from one acre of hardwood. This meant cutting about 300 acres of woodland to operate one furnace for a year, and all this wood had to be converted into charcoal first.
As soon as the spring rainy season ended, teams of one or two colliers came with unskilled helpers to convert the stacked wood into charcoal. Colliers were among the highest paid employees among ironworkers because it required great skill to properly build a charcoal mound and later to recognize the various stages of the burn by the color and smell of the smoke. First they selected a spot in the forest suitable for a “charcoal burn.” It had to be level, dry, and protected from the wind, yet as close as possible to the wood stacked during the winter. After clearing away all stones, leaves, and brush, the collier laid out a circle some 30 to 50 feet in diameter. At the center the team constructed a chimney of sticks six to eight feet tall. The helpers then carefully packed the cordwood on end around the chimney as tightly together as possible to avoid air pockets. Meanwhile, as teamsters hauled more cordwood to the area, the collier and his crew stacked additional layers atop the first layer until they had a mound about 25 feet in diameter at the bottom and 15 feet high. Next they covered the pile with a tight layer of sticks and wood-chips, a layer of leaves, and finally dry soil. This made the mound nearly air-tight except for a few small vent holes poked through the covering at intervals on the sides near the ground.
The "burn" began when the chimney was filled with kindling and lighted by adding a shovelful of live coals from another fire. Once well underway, the flame was smothered by capping the chimney with a board and covering it with leaves and dry soil. With the supply of oxygen limited, wood in the mound only smoldered rather than burned. In fact, a constant watch was kept to prevent any flaming during the 10-14 days required to complete the process. Had the wood been allowed to burn instead of smolder, a pile of ashes rather than charcoal would have been the result. Instead this process drove out all water and soluble minerals from the wood, leaving behind only the desired carbon. Because the mound would shrink by about one-third during the burn, workers several times each day climbed atop and tramped it down to prevent the formation of gas pockets. This was dangerous because the worker risked breaking through the earth on top and ending up badly burned (if not killed). The only precaution taken was to use light-weight men or teen-aged boys with ropes tied around their waists for jerking them to safety if the worst happened.
It was the skilled collier who judged from experience and studying the changing color of the smoke when the coaling was finished and the charcoal ready. The fire was put out by cutting off all air by piling more dirt on the vent holes. The mound then cooled for ten to twelve days before workers removed the outer covering. The still-warm charcoal was taken out and raked into small piles to reduce loss of the whole batch in the event that sparking or a breeze might restart the fire. Finally, when sufficiently cool, teamsters hauled the "coal" by horse- or mule-drawn wagons to the furnace site and stored it in charcoal sheds near the blast furnace for future use.