What causes wood to change color when it combusts
While huddled around a campfire this weekend, the kids started commenting on how much wood changes when you burn it. Their attention was mostly on how logs were changing from brown to black and then white, totally overlooking the fact that some of the log was becoming transparent as it was transformed by combustion. So what’s in wood that causes the various shifts in color, while something like a candle burns without any ashen color changes?
The wood itself is mostly a collection of cellulose, less structural hemicellulose, lignin and other bits of nutrients, minerals and even metals. At as a piece of wood catches fire, the most important bit is the cellulose, which makes up the bulk of most plants, and is made of carbon, hydrogen and oxygen. Once the cellulose is heated to around 300° Fahrenheit, the bonds in these cellulose molecules start to break apart, releasing energy and elements that would really like to find some new buddies to bond with. The first round of new pairings usually gives you some charred wood and volatile gases that we usually just call smoke.
As more energy is added, more bonds are broken, and you start to get a range of new compounds. Carbon dioxide, water vapor, carbon monoxide and nitrogen all float into the air without you really noticing, since they’re released as transparent gases. Pure carbon remains as the blackened coals that are great to cook with, partially thanks to the fact that it’s no longer home to other ingredients that will be released as billowing smoke. The white bits of ash are then the components of the original wood that can’t be burned at these temperatures, such as potassium or calcium. This bouquet of non-wood components that aren’t really burning also make up the set of smells we associate with smoky fires, even through they’re more of a byproduct of the actual fire.
While nobody is about to call a campfire of burning logs ‘exotic,’ they are actually distinct from some of the other forms of combustion we regularly encounter these days. The two phases of combustion kick out heat over a longer period of time, which helps wood burn longer and coals stay hot longer. A candle, in contrast, burns in a single reaction, and must be paced by a wick to keep the light burning for more than just one quick puff.
Source: How Fire Works by Tom Harris, How Stuff Works