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Charcoal burning in a clamp. Charcoal is wood that has been heated strongly in the near absence of air. The water leaves. The more volatile tars and gases leave. What remains is a light, black, porous carbon that burns hotter and cleaner than the timber it came from. The old way of making it in the forest was the clamp, also called a pile, a myler, or a kiln of turf and earth. It had no chimney in the brick sense, it was a stacked heap, sealed and managed by a man who lived beside it until the burn was done. The wood was chosen and prepared first. Hardwoods, oak, beech, hornbeam, birch, chestnut, were preferred for a dense coal. Softwoods gave a lighter product and more smoke. The timber was cut to lengths that could be stood or laid in a regular heap, often a yard or so, sometimes shorter. It was barked or left in the bark according to custom and to the end use. It was dried in the air until the worst of the sap was gone. Green wood wastes heat and steam and makes a poor coal. The collier's year followed the fall of timber and the weather. A clamp started on wet ground, or in a week of rain without a good cover, was a clamp already in trouble. The site was a level stance, a collier's hearth, often used again, a slight dish in the earth helped. The center received a wooden chimney, a stake, or a cage of sticks, or a vertical hole left by a withdrawn pole. Around that flue the wood was stood on end in a circle, or laid in radiating courses, butts inward or outward, according to the school of stacking. A second course leaned on the first. The heap grew into a mound, a flattened dome, higher in the middle. Air spaces were even. A hollow or a tight knot would burn through or stay raw. The skill was in the packing. A well-built pile looked almost like masonry made of billets. Over the wood went a skin that must keep out free air and yet not be a stone vault. First a layer of bracken, leaves or straw, then turf, sods of earth or damp dust from earlier burns, the breeze or coomb that lay around an old hearth. The covering was padded thick, with vents left at the foot, and with the central flue still open. Cracks were a danger. A crack is a chimney in the wrong place. The collier's work, once the fire was in, was in large part the closing of cracks with more earth as they opened in the night. Lighting was done down the central flue, or from a prepared tunnel at the base, with kindling and a few dry sticks. The aim was not a blaze, the aim was a fire that would travel slowly through the mass, driving off water and volatiles, while the covering kept a rush of air from turning the whole heap into a bonfire. When the column of smoke showed that the fire had taken, the central flue was stopped or reduced. Smoke then found the foot vents, and later holes the collier opened and closed on purpose. White wet smoke meant water still leaving. Sharper smoke meant the tars. A faint blue haze and a settling of the pile meant the wood was going over to coal. The collier read the smoke as another man reads a sky. A clamp of ordinary forest size might burn for days, sometimes more than a week. Larger myers lasted longer. The heap shrank as water and gas left. The covering sagged. If it sagged into a hole, air poured in and a bright spot appeared. A break that would heat the charge to ash if it were not stopped with earth. If the fire died in a sector, that sector came out as brown, half-burned brands called brown charcoal or brands, and had to be stacked again. The collier walked the circuit with a shovel and a rake, listened, smelled, and patched. He slept in a hut within sight of the mound. A neglected clamp was a waste of a season's cutting. When he judged the burn complete, he stopped every vent and let the heap cool under its skin. Opening too soon started the charge. Charcoal and air at still high heat will glow and vanish to ash. The covering was pulled back in stages. The coal was raked out, quenched with earth or a little water if it showed fire, and stacked in baskets or sacks. Yield by weight was a fraction of the wood put in, often about a quarter, sometimes less, depending on species and on how far the burn had been pushed. Volume shrank less than weight, which is why a sack of charcoal is light for its size. The product was graded by eye. Good charcoal is black, sonorous if struck, and shows the grain of the wood. It is clean to handle once cool. It lights more readily than raw timber of the same hardness, and gives a steady, almost smokeless heat in a closed draft. That is why it served smiths. A forge needs a fire that can be raised with the bellows to the welding heat of iron without a cloud of tar. It is why it served smelters of lead and tin, and before Coke took the work, the older iron furnaces that were built for charcoal. It is why households in towns that forbade raw coal, or that had no coal cooked on it. The same piece of oak, burned in an open grate, is a different fuel. Byproducts left the pile whether anyone caught them or not. Water ran first, then pyroligneous liquor and tars. In a simple clamp they soaked the ground or went into the air. In later wood distillation works, with iron retorts and condensers, the same gases became commercial vinegar, pitch, and wood spirit. The forest clamp was not built for that recovery, it was built for the black residue. Places took their names from the work. Collier once meant charcoal burner as readily as coal miner. A collery and an old woodland account may be a stance in the trees. Circular platforms leveled and slightly terraced, still lie on hillsides where the trade ran for centuries. The English Weald, the Forest of Dean, parts of the lake district, the French forest that fed the forges, the German and Polish woods of the Myler, the Swedish barrackslag. A platform reused for many seasons is a patch of darker earth and bits of breeze. The trees around it are often younger than the trade. The clamp was not the only vessel. A stone or brick kiln with a door and arranged flues did the same chemistry, with less watching of cracks, and could be used again without restacking the walls. In some districts, pits were used, the wood buried in a trench and covered. The pit is older in look, and harder to control. The brick kiln is later and more urban. The turf-covered pile is the woodland type, moved to follow the felling so that the wood need not be hauled far. Charcoal is fragile, long cartage over rough ground turns it to breeze. It was cheaper to walk the fire to the timber than the timber to a distant hearth. A smith's test of the coal was practical. It should not spit as green wood does. It should hold a hollow fire. It should not coat the iron with dirty soot beyond what the work allows. Different woods were said to give different coals for different jobs, a hard, slow oak coal for one heat, a quicker, lighter coal for another. Much of that lore was local and not written. What is constant is the exclusion of air. Give the same billets a free draft and you have a campfire and ash. Deny them air in a sealed oven at the same heat, and you have coal. A field oven whose walls are dirt. Quantities help keep the subject on the ground. A cord or a stack of wood cut and piled to a known measure went into a clamp of such and such diameter. The collier counted days of smoke and days of cooling against the wage or against the iron master's contract. An iron furnace of the charcoal age ate stacks in a hungry rhythm and wanted a ring of woodlands and burners around it. When coat from pit coal proved reliable in the blast furnace, that ring broke. It shrank to the forge, the farm shop, the gunpowder works that wanted a particular willow or alder coal, and the districts where wood was still cheaper than mineral fuel. Gunpowder's mead is a narrow branch of the same tree. The charcoal for powder was often made from light woods and smaller, more careful batches, sometimes in retorts rather than in a crude pile, because the grain of the coal affects how the powder burns. That is a specialized end. The ordinary clamp existed to turn cord wood into Smith's coal and household coal. The method is the same reduction of air. Faults in the burn have names in the trade even when the names vary. Brands are pieces not fully converted. Ashy holes are places where air one, a run or collapsed side, is a covering failure. Overburning makes a coal that is too slight and wastes yield. Underburning leaves raw wood in the core. The remedy in every case is attention during the days when the heap is a sealed smoking hill, and nothing about it looks like work to a passerby. Taken in order, the process is a short list. Cut and dry the wood, level the stance, raise a flue, stack the billets in a dome, cover with green stuff and earth, light the flue, close the flue, manage the vents by the color of the smoke, patch the cracks. When the smoke thins and the pile has settled, seal all and wait. Open, quench what still glows, and fill the sacks. Leave the breeze on the hearth for the next covering, move to the next fall of timber. The list is the craft, the chemistry underneath it is only heat plus too little air. The rest is stacking, watching, and not being in a hurry to pull the cover.