Okay so I had a little go with a couple of different trees last night. Both dead. One large standing and the wood itself very punky smelling of mushrooms once I'd peeled the bark off. The other smaller and fallen. Not punky, but the wood itself had black fungus lines occasionally running through it. Though the wood from this one made good tinder shavings. Both very warty silver birch. I tried scraping the outside bark into dust though even though the outside wasn't damp, they weren't bone dry and not really dusting up. So I pulled off plenty of super thin top layer curls and rubbed them all up. Gave them both enough ferro sparks to light the forest, and nothing! I put a lighter to them and to be honest they didn't instantly take from that. I have some of it which I'm going to get bone dry to test to hopefully scrape into dust. The other smooth white ones some of you mention with the perfect paper bark, I've actually seen in the local KFC carpark!