still at least part of wood though, a part that's edible, as far as I'm aware, mercury has none of those (adding the bread, and extra ingredient, is cheating).
If something is part edible and part not, then it really depends on the nature of that not edible bit. If it's inert, then great. If it's not, then you could be kinda fucked.
The fact that something is 45% edible says precisely nothing about whether or not it is edible.