Wood and bark dust.
The structure of the fibers of the sapwood does not make it stronger than heartwood. I've been trying to tell folks here and elsewhere that for years. There is no science that I've ever seen to back that claim up. There may be some empirical evidence for the tendency in SOME SPECIES, but as a general rule, it's a complete fallacy. The reason sapwood APPEARS to be stronger than heartwood, is not due to its structure, but its geography. Its position simply gives it more leverage to fight side lean than heartwood has. That huge difference is unquestionably supported by the laws of physics. When you think about it the main function of a hinge is to fight side lean. So when we talk about "stronger", we really mean better able to fight side lean. If you get my drift...I get it that the fiber is way stronger in the sapwood.
live or dead, decaying from inside or out, species etc is going to make a difference, but I have yet to see any science to support the theory. The physics on the other hand is so simple, it's unarguable. And I've seen it firsthand over and over in examining the fibers of broken hinges. Can we all agree that the longest fibers in a severed hinge show the area where the hinge held best? Based on that proposition, I've seen the center of the hinge hold as well or better than the sapwood in the majority of cases.
Normal face cut and nipping the sides, or sapwood a couple of inches, would push over a hell of a lot easier than say boring the hinge in the centre of the face by the same amount. SO by that logic the sapwood would be stronger, would it not?