What is? Thermally treated wood.
I was at a yard buying lumber earlier today and they had some thermally
treated ash. Dark brown, through and through.
He had a small piece he had finished and it was drop dead gorgeous...black
in the "grain area", deep brown elsewhere.
I did a bit of Googling and it seems the treatment carmalizes the sugars in
the wood resulting in much increase stability, much more water resistance
and an outdoor life of 25 years without chemicals.
Click to see the full signature.