I would never have thought of it as I would have thought the carbon
would burn out.
That's why the choice of the type of pencil lead you use is
important. Part of the lead's survival is because the "carbon" is in
the form of graphite, not charcoal or something, and graphite is a
bit more resistant to burning away (though it slowly does) As one
poster pointed out, a number of the easily found brands are quite
flexible. These have softer plastic binders, and they will not
survive burnout. You want to get older types which aren't so
flexible, and get as well, the "hardest" grade lead (like 4H or 6H,
etc) you can find as well. The harder ones have increased clay in the
binder, so that also helps the graphite to survive.
I cannot say I've done this much. But some 20 years ago, I had
occasion to do this while casting a whole bunch of bracelet links,
and needed the hinge tubes to be open enough after casting to allow
accurate and easy drilling. The pencil leads I used then were older,
somewhat brittle, and a harder grade, and worked like a charm. More
recently, (ten years ago perhaps) I had occasion to do it again, but
that time, the more recent leads, though the same "hardness" grade,
were the more common fairly flexible types, and a number of them
Take from that what you will.
When I need to do this now (not all that often) I skip the pencil
leads and just use copper wire or iron binding wire, steel wire, or
even the shanks of old broken drill bits of the right size, etc. High
enough melting point to survive casting with any gold or silver
alloy, they oxidize during burnout, so the cast metal doesn't much
fuse to it. Then, after casting, leave it in a suitable acid
overnight. In the morning, the wires are gone. Redrill to clean up
the holes, and you're set.