This is one of those things that I’ve always wondered about: how fast does wood darken, and how? The central stripe here, with three grains of wood, is a bass bar in a violin made in 1941. I had cut it halfway down (it was being replaced) and noticed that the soft grains to the outside were darkened, but the central grain retained some of its original brightness. The hard grain lines on either side of the central one limited it’s exposure to air, preventing the central section from darkening!