To reduce woods natural affinity to catch fire, humans have been developing different techniques for improving its resistance for thousands of years. From plant based waxes and coatings to petroleum based lacquers and paints, we have developed many different methods of keeping our homes safe from fire.
Interestingly, one of the oldest and most effective methods of protecting wood from fire is fire. The process of applying heat and fire to the exterior of a wood product for a short period of time changes both the cellular structure and thermodynamic conductivity of the wood.
Chemically, you can think of wood as being made up of two components – cellulose and lignin. Cellulose is what the tree grows first; it is the compound that gives a young sapling its flexible strength, and the leaves and fresh shoots are primarily made up of cellulose. Lignin gets deposited into the tree tissue as it gets older, and the lignin is hard, tough and brittle. An old tree-trunk would have a much higher percentage of lignin than a fresh shoot at the tip of a branch.
When you burn wood, the softer, more reactive cellulose vaporizes and gets burned off, while the harder lignin takes a long time to burn. Furthermore, it takes a lot higher heat to get the lignin to ignite again. Then additionally the outer layer of charred wood acts as an insulator.
Therefore, when we char the surface of our cedar siding or fencing, what we are effectively doing is burning off the outermost cellulose and leaving only the blackened lignin behind. In order for the charred wood siding or fencing to re-ignite requires much higher temperatures and much longer in contact with a flame source.
The best way to think about this is if you were to light a campfire, and then when the logs are half-burned you put the fire out. If you come back later and try to re-light those logs again, it is actually much harder to get them to catch fire, because they don’t have any of the more easily ignited cellulose compounds left.