[1] Polymers like thermoset, or most solid organic compounds like wood or biological tissue, exhibit charring behaviour.
[1] Charring can result from naturally occurring processes like fire; it is also a deliberate and controlled reaction used in the manufacturing of certain products.
This is accomplished by either burning the parent fuel (wood or coal) in a low-oxygen environment or by heating it to a high temperature without allowing combustion to occur.
A "coal burning" blacksmith's forge actually produces the heat necessary for high-temperature metalworking by the continuous production and consumption of coke within a carefully managed fire.
This layer then serves to protect the remaining structurally sound core of wood, which can continue to carry the building loads if appropriately designed.