What are these "clinkers" that are left behind in the fireplace?
"Clinkers" is the word my dad came up with for them, because they make a glassy "clink" when you drop them. They aren't like regular embers, they're much denser, and they look kind of like coprolites
made of particle board
. Can't tell you what kind of wood makes them, because we use various types and they show up unpredictably.