The process of charcoal making... required many colliers to 'coal' the wood. Henry Drinker estimated that as many as 12 colliers might be necessary to keep a single furnace going...A dry, level spot near the woodland, sheltered from the wind by a declevity or by trees, was selected. The ground had to be dry and fine, free from stones and gravel, but not loamy or sandy. A circular spot from 30 to 50 feet in diameter, called the 'pit' or hearth', was prepared. Prior to this, the trees had been felled, trimmed, cut into lengths, and bundled into cords by the woodchoppers. This wood was brought to the 'pit', and the colliers stacked it in conical shape, standing the sticks on end. This was done by first forming in the center a circle of sticks to compose the chimney, 3 or 4 inches in diameter and about 6 feet high. Around this chimney the cords of wood were arranged to form the cone, which was about 25 feet in diameter at the base.
After the cone was built, a layer of damp leaves and loose earth or a coating of turf was placed on the pile. Chips, dry leaves or other inflammable material was placed into the chimney and lighted from the top, which was then partly closed with turf. Holes were drilled in the side to draw air. After the pile was lit, it had to be carefully watched day and night, and any sign of flame smothered. It took from 3 to 10 days to char the entire pile, depending on the nature of the wood, the state of the weather, and the skill of the colliers...
After the pile was completely charred, the colliers raked the charcoal into small piles, so that if any should break out into a blaze through spontaneous combustion, the whole mass would not be ignited. Hot, dry charcoal absorbs oxygen so readily that this danger is always present...
Various woods were used in charcoal making, including hickory, white oak, black oak, ash, chestnut, and pine. Hickory was considered the best, but as black oak was more abundant, it was more generally used. The yield of charcoal varied considerably, depending upon the quality of the wood, the more compact and fine-grained giving the largest yield.
The amount of charcoal consumed by the furnaces was enormous. Oley Furnace in 1783 consumed about 840 bushels every 24 hours, for which about 21 to 22 cords of wood were necessary. This furnace produced a little more than 2 tons of iron a day, and therefore for every ton of iron more than 400 bushels of charcoal were required. Since an acre of land is necessary to produce about 25 cords of wood of 20-year to 25-year growth, Oley Furnace used every day almost the amount of wood grown on an acre...
(from Arthur Cecil Bining Pennsylvania Iron Manufacture in the 18th Century [1973], pp 62-63)