Well, this is somewhat related....but the best bark-like material I ever saw/tested for firestarting was some newspaper that my wife had hit with flat black paint when she was spraypainting something in the backyard. (Newspaper used to be bark, or something closely related, right?) Anyway, when I saw a scrap of the paper laying around in the yard, I grabbed my hand magnifying lens and hit it with a spot of sunlight. Within about 10 seconds it was flaming away madly. I could hardly blow it out. The paint was dry, and promoted the burning process considerably over just newspaper alone. Not only that, but the flat black surface really made it easy for the paper to get hot enough to burst into flame quickly, not just smoulder or spot-burn without flame like usually happens when you use a handlens on regular newsprint.