My daughter was asking me why burnt wood always has a reasonably regular pattern of cracks in it. I had no idea, but according to google...
It's complicated!
(Technically it's charred wood, not burnt wood as well)
So the char layer protects the wood underneath from burning.
The cracks that form work against this protection though.
Cracks can also form in unburnt timber during the drying process, and the process (or at least the forces) may be similar - The surface layer dries, and tries to shrink, but the still-moist inner layers create tension which causes cracks in the outer layer.
The spacing and depth of cracks depend only on the heat flux and pressure of the fire, and the cracks tend to form perpendicular to the grain. More heat flux and pressure mean more cracks.
