Dave: Yes, I use centrifugal casting for jewelry and gravity
for sculptures. Spruing method the same for both. And, you're
right, it's a very old fashioned technique but I found it works
for me, using gold, silver or bronze. (Gets a little pricy
for gold though).
A drawback using this method for jewelry would be the limited
space in flasks and the need for more sprues can make it crowded.
The advantage is that you have metal from which the piece can
draw. To help this further, if you have areas which are thick
and thin together, sprue directly to the thicker area for
needed metal during cooling (same as you try to do now with your
single sprue method). Otherwise it will draw from your thin area
and cause problems. Another way would be to add a diameter of wax
to thick areas to provide a reservoir and save on space. A little
more clean up though.
You will have a gradation of sizes: in order from large to small:
pouring cup, gate, feeder sprues, jewelry piece. You
would not have a single sprue directly into your piece. The main
gate accepts the hot metal first and therefore has your pouring
cup attached to it. I always fluxed the hot metal will before
pouring to have cleaner castings using pure borax.