... I don't understand a need for lead inserts (graphite) at
all since the plaster will fill in the hole of the wax tube.
Yes, but with fine guages of hole, the resulting plug of
investment is quite thin, and if the tube is more than a very
short section, that suspended plug is quite prone to break off
when the molten metal hits it. Then, not only does the tube cast
solid, but the little plug is floating around in the motlen
metal, and will cause a nasty defect in the casting.
by the way, if you cannot find a suitable graphite that will
withstand burnout, you can always use plain iron wire (like
binding wire, or even broken drill bits) in the same way. Don’t
use stainless, it’s harder to remove later. After casting, which
it withstands just fine, you treat the casting in the same way
you’d treat an item in which you’d broken off a drill bit: Let
it soak in some hot sparex for a while (kept seperate from the
pickle you normally use, of course), and the iron will dissolve
out. Depending on the depth of the tube, it can take a couple
hours or more, but it’s still easy enough.
Hope this helps.