It looks like it is 1cm deep in to surfice, I Used the planning tool to take off as little as possible. when I had been at it for a while it became clear, that if I want to take it off this way, I have about 3h / disk, 20 discs made me ask here for tip.
This is rare wood now around here, I regret not taking it home when I left the freinds woods.
It seems to be some sort off black mold, it has some fungus too, but not bad.
I cut them from a dead tree, that had fallen 2years ealyer, so it was dry when I cut it up. It is not dry now
I have put it away properly, afraid it will crack since it now is wet.
I had them standing on a north side of a building, when he moved them he layd them down without anything between them, away from the building, so rain and sun could/has goten to it.
The lack off air between them is as far as I can understand the season for the black stuff.