Wood after burning

[jadyn]

Just been watching "Wild At Heart" on TV, which got me thinking about something. In the program, a wildfire is featured which burns a lot of property around something similar to a ranch. Much of the wood that gets burnt is bare, dry tree branches etc
During the clear up, this wood, which is almost charcoal by now, shows a distinctive pattern of small, raised black, cracked areas which are brittle to the touch.
I've also noticed this effect on timber my dad has burnt in bonfires over the years.
Can anyone explain what happens to the wood under these extremes of temperature to cause this distinctive effect for some depth into the wood?

Thanks Jadyn

[Postdog]

I am no expert, but this sounds logical. A lot of the make up of wood is actually moisture in one form or another, like water or sap. As that evaporates through the heat of burning, what remains is obviously less than what was started with, and so "gives" in the only way it can by cracking apart.

[gen2]

Yes, as Postdog says, it is shrinkage caused by the loss of water and other volatiles. If the wood was to dry out slowly (as when it is seasoned, or kiln-dried) then the whole piece of wood shrinks evenly to smaller dimensions. The heat of a fire however has the greatest effect on the outside of the wood whilst the insulating properties of the wood largely protect the interior. As the outside shrinks, it no longer fits around the still intact core, so it splits much like mud does when a reservoir dries up in summer.

[jadyn]

You've both provided absolutely brilliant answers.

Thank you both very much!

[beso]

The wood itself doesn't actually burn but is converted to acetylene gas by pyrolysis of the cellulose. The heat penetrates the wood releasing the gas from the below the surface. As the cellulose turns to gas areas which have pyrolysed are more susceptible producing a positive feedback which forms grooves.

The combustion takes place in the yellow part of the flame where it reaches oxygen. The deeper sections of the flame are tongues of unburnt gas. You can see this quite clearly when observing any flame.