Charcoal is not burnt wood. It is wood that has been cooked to death in the deliberate absence of oxygen, and a large steel retort is the machine built to do that cooking with control rather than luck. The process is pyrolysis, and it moves through distinct thermal regimes: - Below 150C the charge simply dries. Free and bound moisture leaves, and every kilogram of water carried into the vessel steals roughly 2.6 MJ of energy that will never become product. - Between 200C and 260C hemicellulose collapses first, shedding acetic acid, methanol and carbon dioxide. - From 240C to 350C cellulose depolymerises through levoglucosan into permanent gases and condensable tars. - Lignin decomposes lazily from 280C to 500C and supplies most of the fixed carbon skeleton that survives. Near 280C the reaction turns exothermic. The charge begins heating itself, releasing on the order of 1 MJ per kilogram, and the operator stops feeding the firebox and starts managing the vessel instead. A retort is...