Sorry to chime in so late.
It is possible to find heartwood under the bark, but only of a dead tree. Moreover, this is from a tree that slowly died and turned its sapwood into heartwood.
I've recently started to coerce black locust into doing this, in order to have more heartwood and have lower crowns on small diameter trees. BL often has quite a lot of sapwood, which can amount to 1/3 of the wood in 15 cm diameter trees.
During late winter, I make two circular cuts at the base of the trunk with a chainsaw, ensuring that the phloem and cambium are completely cut through.
This doesn't stop upward flow of water from the roots (which happens through the sapwood), but it stops the downward flow of secondary metabolites, sugars, starches and so on from the leaves to the roots.
To make leaves and new shoots, a tree will use the starches and sugars stored in the roots and the sapwood of the trunk. When those in the roots aren't available anymore (flow is interrupted), it will need to bank on those from the sapwood. In the mean time, it will produce fewer leaves, it cannot make new roots (as there is no energy transfer to the roots anymore).
The interior sapwood rings will now start to die off (they are being emptied from starches and sugars), and the natural process of transformation from sapwood to heartwood (with deposition of secondary metabolites that darken the wood) will take place, but at an increased rate, whereas new sapwood is hardly produced anymore.
By the end of the summer, most such trees are dead, and their sapwood has been largely transformed into heartwood. You can now peel off the bark from a BL and use it as the back of your bow. It may still have one or two sapwood rings. which is fine.
