How Wood Burns
It doesn’t – at least not directly. Here’s what happens to a log after it’s thrown into a fire:
- Drying – that log is full of water. Green wood has ~80% (of dry weight) moisture; a 9 lb log = 5 lbs wood + 4 lbs water. Seasoned wood has ~20%, so the same log split and dried for a year will weigh 6 lbs. There’s still a pound (pint) of water that must boil out before the log’s temperature will rise over ~230F. Dessication makes lots of steam that helps keep your meat moist; it also consumes a lot of heat from the fire.
- Decomposition – once the log is dry, heat-up resumes. The cellulose and lignin structure of wood breaks down at high temperatures and boils-off much like the water did. Instead of steam, we get smoke – a cornucopia of volatile organic compounds (VOCs) and particulates. Smoke is a gaseous fuel. At 800F, the log has been reduced to its carbon skeleton, which we call charcoal. Charcoal is a solid fuel.
- Combustion – our two fuels burn differently, each governed by The Fire Triangle. Charcoal burns as a red-hot ember (a surface oxidation). Smoke burns as an orange/yellow flame. Charcoal and smoke split the fire’s energy production roughly 50/50.
That simple, 3-step process has huge implications for barbecuers:
- The charcoal-only fire – charcoal is just wood that’s been taken through steps 1 and 2 at a factory. Having no volatiles, this fire is simple to control – just throttle the air supply. This allows the pit boss to load up a surplus of fuel, dial in the temperature with dampers, and get a long burn with minimal attention. The downside is that a charcoal-only fire is flavorless, making CO, CO2, and barbecue that tastes like pot roast.
- Smoke and creosote – the aromatics in smoke determine barbecue’s flavor. Cleanly-burned smoke makes good barbecue – that’s why pit bosses strive for clear or thin blue stack emissions. Unburned (chalky or yellowish) smoke is the source of bad barbecue – its volatiles will condense as creosote on your cold meat, making it black, bitter, and likely to cause gastronomic disturbances if consumed in quantity.
- Power control of a wood fire – choking a wood fire to reduce its power produces creosote. This is the grandest conceptual error in barbecue – pit makers design elaborate dampers and throttles, practitioners debate the merits of inlet vs. outlet throttling, novices get frustrated and give up. The only way to make clean smoke from a typical wood fire is to maintain good, hot geometry and control pit temperature by rationing fuel, rather than throttling combustion air.