From Distillers Wiki
Charring is the heating of wood with a flame until the wood becomes black. Charring forms new chemicals in the wood while destroying others. Charring also increases the surface area of the wood so that maturation process occurs more rapidly. Many whiskies are aged in charred oak barrels.
|Here is a cross section of a peice of maple that has been freshly cut and quickly charred over a flame without drying the wood first. Note how little of the wood is actually charred. The slightly red area right below the dark black char is the section of the wood in which carmelization of the woods sugar has occured. This region is primarily responsible for the flavour inparted to a bourbon when it is aged in wood.|
|Here is a sample of wood that has been quickly charred after being dried at 150 F for about 6 hours. Note that there is a broader carmelization layer.|
Here is sample of wood that has been dried for 6 hours at 150 F and then was slowly charred. Note how much larger the carmelization layer is compared to the other two layers.