All right, I’ll admit . . . this is a matter of semantics. But when you write, words are important. And when you write Crime Fiction making sure you understand the terminology of the science involved is super important. So, what DOES wood do when it sits in our fireplace flaming or when a house is destroyed by fire? Technically it undergoes thermal decomposition. The word for that is Pyrolysis (from the Greek for fire and separating).
Here’s a quick and dirty explanation of wood’s Pyrolysis: Wood itself isn’t flammable, but when exposed to high enough temperatures it gives off flammable resin vapors. Those vapors burn. Meanwhile the structure of the wood itself “decomposes” into simpler compounds. That decomposition process yields yet more flammable vapors to keep the flames going.
So when you see dancing yellow flames on and around a piece of wood (hopefully in your fireplace or a bonfire as opposed to the walls of your home) you are actually seeing the volatile compounds that “out-gas” from the wood eagerly reacting with oxygen and burning. The wood continues just hanging, doing its decomposition thing. That decomposition/pyrolysis process is extremely intricate and dependent on many variables like the type of wood, pressure, water content, surrounding environment, exposed surface area . . . Are your eyes glazing over yet? Mine are, which is why I am SO not going into the chemistry of this).
Why does it matter that flammable gases/vapors are burning, and the wood itself is just decomposing (yep I heard you thinking it)? The stage of wood’s decomposition can help arson experts “read” a fire, giving them a picture of how long something burned, how hot, and even the direction a particular fire travelled. This data is valuable in determining the cause of a fire—and whether it was accidental or arson.
Reading studs (the wooden kind): For an example of how wood’s decomposition can help map a fire and locate the all-important point of origin, we need look no further than the humble wooden studs remaining in a burnt wall. Fire investigators use tire gauges or other measuring tools to determine the depth of char on these wooden structural remains. Studs with the deepest char mark the spot on the wall nearest the fire’s point of ignition—because deep char means further decomposition and a longer burning of the vapors off-gassed from the wood. The direction the fire moved from that point of ignition can be mapped by taking additional measurements. Less char (decomposition) in a wooden stud = a shorter burn. So, the fire moved from the area of the wall exhibiting the deepest char towards the area (or areas) with shallower char. And the wall stud with the shallowest char marks the furthest point the fire reached before going out.